Millimeter wave fixed wireless systems in future backhaul and access network applications can be affected by weather conditions. The losses caused by rain attenuation and antenna misalignment due to wind-induced vibrations have greater impacts on the link budget reduction at E-band frequencies and higher. The current International Telecommunications Union Radiocommunication Sector (ITU-R) recommendation has been widely used to estimate rain attenuation, and the recent Asia Pacific Telecommunity (APT) report provides the model to estimate the wind-induced attenuation. This article provides the first experimental study of the combined rain and wind effects in a tropical location using both models at a frequency in the E band (74.625 GHz) and a short distance of 150 m. In addition to using wind speeds for attenuation estimation, the setup also provides direct antenna inclination angle measurements using the accelerometer data. This solves the limitation of relying on the wind speed since the wind-induced loss is dependent on the inclination direction. The results show that the current ITU-R model can be used to estimate the attenuation of a short fixed wireless link under heavy rain, and the addition of wind attenuation via the APT model can estimate the worst-case link budget during high wind speeds.

This Letter describes a high-speed yet simple fiber-wireless-fiber system in the 100-GHz band using a photonics-enabled receiver and optical phase modulator. At the antenna site, a millimeter-wave signal is down-converted to the microwave band using an electronic mixer with a local oscillator signal generated remotely using photonic technology. The down-converted signal is converted to an optical signal using an optical phase modulator. At the receiver, a simple direct detection of the phase-modulated signal is performed using optical filtering technology. To demonstrate the proof-of-concept, we successfully transmitted a 64-quadrature amplitude modulation orthogonal frequency-division multiplexing signal with a record line rate of 80 Gb/s (net data rate of 55 Gb/s) over a system consisting of two radio-over-fiber links and a 5 m wireless link in the 100-GHz band. The proposed system with simple antenna sites and optical transceivers can facilitate the deployment of ultra-dense small cells in high-frequency bands in beyond-5G networks.

A dual-wavelength digital RoF link was experimentally demonstrated by OSR-reduced 2-bit delta-sigma modulated 32-QAM OFDM. The optically generated data was transmitted over 25-km SMF at receiving power of -9.9 dBm.

We demonstrate a high-fidelity seamless fiber- wireless system in the W band for high-precision analog waveform transmission. The system is realized using a stable radio-over-fiber transmission and a direct receiver in the W band. Satisfactory performance was experimentally confirmed for 512- and 1024-quadrature amplitude modulation orthogonal frequency division multiplexing signals, showing that the seamless system can provide precise analog waveform transmission of radio-wave signals in future mobile networks.

We demonstrate the first fiber-radio-fiber system in beyond 90 GHz using low loss optical modulator and direct photonic down-conversion. We successfully transmitted 32-/64-QAM OFDM and single-carrier signal with a record line rate of 71.4 Gb/s and net data rate of 53.7 Gb/s.

We demonstrate the first 3times 3 full multiple-input multiple-output fiber-wireless seamless system in W band. Satisfactory performance is experimentally confirmed for a total capacity of approximately 119 Gb/s and 132 Gb/s using standard and flexible FBMC/OQAM signals, respectively.

A new structure of the electro-optic modulator to compensate the third-order intermodulation distortion (IMD3) is introduced. The modulator includes two Mach-Zehnder modulators (MZMs) operating with frequency chirp and the two modulated outputs are combined with an adequate phase difference. We revealed by theoretical analysis and numerical calculations that the IMD3 components in the receiver output could be selectively suppressed when the two MZMs operate with chirp parameters of opposite signs to each other. Spectral power of the IMD3 components in the proposed modulator was more than 15 dB lower than that in a normal Mach-Zehnder modulator at modulation index between 0.15 and 0.25 rad. The IMD3 compensation properties of the proposed modulator was experimentally confirmed by using a dual parallel Mach-Zehnder modulator (DPMZM) structure. We designed and fabricated the modulator with the single-chip structure and the single-input operation by integrating with 180. hybrid coupler on the modulator substrate. Modulation signals were applied to each modulation electrode by the 180. hybrid coupler to set the chirp parameters of two MZMs of the DPMZM. The properties of the fabricated modulator were measured by using 10 GHz two-tone signals. The performance of the IMD3 compensation agreed with that in the calculation. It was confirmed that the IMD3 compensation could be realized even by the fabricated modulator structure.

We proposed a variable linewidth light source using random walk phase noise. As a demonstration, the impact of the laser linewidth on the error-vector magnitude performance in the optical QPSK transmission was evaluated by using the proposed laser source experimentally, where the linewidth tunable range was from 40 kHz to 5 MHz.

This paper describes a simple secondary radar for detection of hidden pedestrians, by using a compact transponder which responds to a radio-wave transmitted from a car. The transponder doubles the frequency of the radio-wave for frequency-domain duplexing with a simple configuration. The distance between the car and the pedestrian can be measured by using a phase detection scheme, with no local oscillators at the pedestrian side.

Sample complexity, or equivalently the required number of photodetectors, of a carrier-less phase-retrieving coherent receiver is investigated numerically based on the experimental data
it can achieve comparable complexity to conventional coherent receivers.

We present a transmission of radio signals in high-frequency band over a seamless fiber-FSO system for ultra-dense radio access network. We successfully transmitted 80-Gb/s and 40-Gb/s 2×2 MIMO FBMC-OQAM signal in the 90-GHz band over the DL and UL direction.

We present a free space optics receiver with high robustness for optical alignment using a large active area, high-speed 2D-PDA, and its demonstration of 40-Gbps (PAM4) signal detection using a space diversity technique in DSP.

To alleviate the danger of foreign object debris on runways, Frequency Modulated Continuous Wave linear cell radar based detection systems have been evaluated in the literature. This radar uses a 90 GHz millimeter wave and radio-over-fiber (RoF) to achieve high detection accuracy and low cost. This paper reports the experimental results of interference mitigation experiments at Kuala Lumpur International Airport. False images formed due to interference can be suppressed by performing simple signal processing on more than one radar image in which the transmission delay in RoF links is discrepant.

A linear-cell-based radio-over-fiber (LC-RoF) system is proposed and demonstrated for efficient mobile communication in high-speed trains without hard handover processes. The configuration of the LC-RoF network and possible router architectures are discussed for the field trial test of the proposed system and for advanced systems in the future. Via the LC-RoF network, 90-GHz, millimeter-wave radio access between the ground and a train car is operated in a centralized manner. The resulting throughput of 1.5 Gbit/s is achieved in the field trial test, which is comprised of 250-Mbaud four-subcarrier differential quadrature phase-shift keying with a forward-error correction code implemented in field-programmable gate arrays to the Shinkansen train traveling at 240 km/h without any interruptions in the connections at the change of radio access units.

We present a high-speed integrated fiber-wireless system in the W band for transmission of high- frequency radio signals to ultra-dense small cells and moving cells. The system utilizes a wavelength-division-multiplexing intermediate- frequency-over-fiber system and a remote generation and transmission of local oscillator signals. We experimentally confirmed satisfactory performance for the transmission of 4 Ã- 25-Gb/s and 20-Gb/s OFDM signals over a fixed and a switchable system, respectively. The system is highly spectral efficiency and scalable for signal transmission in future mobile networks.

We have evaluated impact of plastic and glass optical multi-mode fiber to 28 GHz RF transmission. As the results, error vector magnitudes of 16-QAM and 64-QAM modulated signals over fibers are achieved reauired one.

We show the application of radio over fiber networks to high-speed train communication systems. Network control by optical path switching realizes handover-free uninterrupted signal transmission for the car traveling at 240 km/h.

We designed and fabricated an electro-optic modulator for optically compensating the nonlinear distortion. The single-chip structure and the single-input operation were realized by integrating a microwave 180-degree hybrid on the DPMZM substrate.

We experimentally demonstrate a seamless full-duplex fiber-wireless system in the W-band for Nyquist-SCM signal transmission. Satisfactory performance is confirmed for 45-Gb/s and 20-Gb/s signal transmission over the downlink and uplink directions.

We present a newly developed high-speed 16-pixel photodetector array device operated at 50-Gbuad which circumvents the need for fiber splicing, including its receiving performance when coupled to 16-multi-core fibers, assuming next-generation short-range optical fiber communications.

We propose an ultrahigh speed indoor communication network using a novel cross spectrum and cross medium of optical fiber, millimeter-wave (mmWave), and optical wireless communications. As a proof of concept demonstration, 40-Gb/s mmWave signal in the 100-GHz band was successfully transmitted over a seamless fiber-optical wireless system. The proposed system can provide low-latency, spectral and energy-efficient, and cost-effective solution for future beyond 5G or 6G network evolution.

We propose different fronthaul systems for facilitating future mobile networks based on the seamless convergence of fiber-optic and wireless systems in the millimeter-wave (mmWave) bands. First, a flexible and high-performance wireless fronthaul system is proposed through an encapsulation of radio signals onto a converged fiber-mmWave system. A simultaneous transmission of three radio signals over the system is successfully demonstrated. Second, a high-performance optical self-heterodyne system is proposed and demonstrated for the generation and transmission of radio access signals in high-frequency bands. Third, a high-spectral-efficiency optical fronthaul system for the simultaneous transmission of multiple radio signals in different frequency bands is proposed using a subcarrier-multiplexing intermediate-frequency-over-fiber system. Satisfactory performance is experimentally confirmed for the transmission of three different radio signals in the microwave and low- and high-mmWave bands. The proposed systems can overcome the challenges and bottlenecks of the current mobile fronthaul systems and can be useful in different usage scenarios of 5G and beyond networks.

We developed a high-speed and handover-free communication network for high-speed trains (HSTs) using an ultrafast and switchable wavelength-division multiplexing fiber-wireless backhaul system in the W band. We successfully transmitted approximately 20-Gb/s and 10-Gb/s signals over the switched fiber-wireless links in the downlink and uplink directions, respectively. An ultrafast radio-cell switching of less than 10 μs was experimentally demonstrated in both downlink and uplink directions. Moreover, the possibility of connecting a central station to many remote radio cells was evaluated, confirming that an uninterrupted communication network up to several tens of kilometers can be achieved for HSTs. The proposed system can overcome the current challenges in mobile networks and can provide a potential solution for the provision of advanced services to users on HSTs in future 5G and beyond networks.

We investigate the operation of a thin-film lithium niobite modulator with specific focus on the wavelength dependence. For a precise evaluation of the modulation distortion, we used an extinction-ratio-tunable-type modulator.

We propose and investigate Conjugated-Paired RoF (C-RoF) transmission with phase-shifted down-conversion, which simultaneously compensates for fiber nonlinearity and dispersive fading effects harmful in Radio-on-fiber (RoF) links, without digital signal processing.

We demonstrate 400-Gbps multi-core fiber based optical wireless communication using a newly developed two-dimensional photodetector array (2D-PDA). The 19-parallel beam from 19-core fiber can be successfully coupled with a 19-pixel 2D-PDA in free space.

A cell-less network for high-speed and non-interrupted communications to high-speed trains is proposed and demonstrated using a switched WDM fiber-wireless backhaul system. More than 10 Gb/s signal transmission with an ultra-fast cell switching is experimentally demonstrated in the uplink.

This paper reviews 90-GHz high-speed wireless links and high-resolution radars based on linear-cell systems consisting of radio-over-fibre for waveform distribution. We focus on applications for public transportation systems including airport runways, railways, etc.

We present a full-duplex fiber-wireless system in the $W$ -band for simultaneous transmission of multiple-input multiple-output (MIMO) signals in downlink and uplink directions. The proposed system uses wavelength-division multiplexing in the optical link and parallel-aligned/orthogonally aligned antenna polarizations in the wireless link for the MIMO and full-duplex signal transmission. Satisfactory performance is experimentally confirmed for 2× 2 MIMO OFDM and LTE-A signals in both directions. The system is highly spectral-efficient and scalable for large-scale MIMO signal transmission.

As fifth-generation (5G) technology is expected to offer super-broadband mobile services with new features such as low latency for critical applications and low-power operation for Internet of Things (IoT) applications, it will use wireless interfaces for small or spot cells with a new RF resource, the high band (frequencies above 6 GHz), in addition to the low band (below 6 GHz) on macro and small cells coexisting with conventional mobile technologies [1]-[4]. Another feature of 5G wireless interfaces is more precise control of radio waves in space division through the use of multi-antenna technologies, such as massive multiple-input/multiple-output (MIMO) or beamforming [5].

We propose and demonstrate spatial coherent matched detection using two-dimensional high-speed photodiode array. Capturing amplitude and phase distribution, all tributaries of 3×20-Gb/s MDM-QPSK are simultaneously demultiplexed and coherently demodulated without using optical modal fan-out devices.

We present a newly developed 32-pixel, high-speed 2D photodetector array operated at 25 Gbps (NRZ) with SMF and MCF, targeting direct coupling to MCF and FMF in dense SDM communications.

We propose a transfer-function-transformed optical modulator by introducing a branched asymmetric coplanar waveguide electrode. The structure has the potential to achieve low half-wave voltage and large bandwidth while maintaining its flexibility. We confirmed a linearity improvement with the fabricated modulator.

We report on the use of 30 and 100 GHz integrated millimeter-wave photoreceivers in high data rate photonic wireless communication. The role of the bandwidth of a narrow-band photoreceiver in increasing the data rate of wireless single carrier transmission is discussed. An 11-GHz bandwidth photoreceiver achieved a data rate of 44.6 Gb/s using an 8 GBd, 64-QAM (quadrature amplitude modulation) signal. A wider bandwidth (19 GHz) integrated photoreceiver was designed and fabricated for use in the 100-GHz range. High-frequency response photoreceivers have the following advantages: they operate in a wide bandwidth easily, use frequency resources efficiently, and are subject to low amounts of atmospheric attenuation. In addition to the single carrier wireless transmission study, a high data rate (12 Gb/s) multicarrier radio over fiber (16-QAM in orthogonal frequency division multiplexing) transmission experiment was demonstrated in order to confirm the ability to increase the data rate with a wide operational bandwidth.

We discuss and propose millimeter-wave linear cell systems for signal distribution on railway radiocommunication systems between train and trackside, and a foreign object debris detection system for airport runways using the radio-over-fiber network technology. Linear cell configurations based on wavelength-division multiplexing (WDM) and power-splitter-based distribution networks are discussed and demonstrated experimentally. A single-sideband modulation for data transmission in an intermediate frequency over a fiber system is successfully transmitted with an ultrafast wavelength-tunable laser diode for WDM routing to track the train car within 3 μs. Radar signal distribution by an optical double-sideband suppressed-carrier modulation mitigates the dispersion effect of the fiber cable to transmit a 32-GHz signal into the remote radar heads.

We propose and devise a digitized optical modulator for 4-ASK. The proposed device enables optical modulation and waveform shaping for multilevel amplitude shift keying simultaneously. The measured transfer function of the fabricated modulator shows good agreement with the designed function.

We present a newly developed 32-pixel high-speed 2D photodetector array operated at up to 39 Gbaud, and its application for WDM FSO communicat ions. 4-WDM 25-Gbaud FSO parallel beams could be directly detected by the 2D-PDA.

We present a scalable fiber-wireless system in W-band for the simultaneous transmission of MIMO signals in both downlink and uplink directions. Satisfactory performance is confirmed for 2 × 2 MIMO OFDM and LTE-A signals in both directions.

We present radio-over-fiber technologies for fiber and wireless convergence for small cell and linear cell networks, including a flexible and low-latency mobile fronthaul for ultra-dense small cells, and high-speed wireless backhaul system for high-speed trains.

A high-speed, handover-free communication for high-speed trains by a switched WDM fiber-wireless system in the W-band is presented. More than 20 Gb/s signal transmission with a cell switching time of less than 4 μs is experimentally demonstrated.

We analyzed the complex transfer function and modulation spectrum of the Mach-Zehnder (MZ) modulator with optical power and phase change imbalances. From the calculations, we found that the characteristic operation can be obtained in three cases: high extinction ratio, zero-chirp, and straight-line trajectory in the IQ plane. However, the results also indicate that we cannot achieve an operation equivalent to that of the ideal modulator, due to the small parasitic chirp in a single-drive modulator. To control the extinction ratio and chirp parameter individually, we propose an integrated modulator with some tunable Y-branches. By using the optical-power-imbalance tunable modulators, we experimentally demonstrate the characteristic operations and obtain the modulation spectrum equivalent to the ideal MZ modulator. However, compared to the ideal amplitude modulator, MZ modulators exhibit an intrinsic nonlinearity in the sinusoidal response. Therefore, we used a third-order nonlinearity compensation method using the superposition of electrical third-order harmonics. The measured spectrum consisted of only first-order modulation sidebands, which is equivalent to the ideal double-sideband suppressed-carrier modulation.

The W-band signal near the antenna surface is detected using a THz probing system, by assuming near/far electric field distribution measurement on a phased array antenna coupled to an RoF radar system. By using 10 GHz + 12.5 kHz optical sampling pulse with a ZnTe EO probing head, the 80-GHz signal generated by a UTC-PD could be successfully detected as a down-converted signal of 100 kHz. It suggests a potential to measure the electric field distributions of 1-D/2-D patch antenna arrays, used in advanced RoF systems.

Semiconductor quantum dots can be used for an optical gain-chip in an external cavity to construct a stable two- or few-mode laser. In this study, we demonstrate wide-range mode separation control of a few-mode laser by using a multistep etalon filter. Since the precise control of multistep thickness in one etalon plate was achieved, the frequency separation of the few-mode laser can be changed by choosing the etalon position. We can change the frequency separation from 83 to 104 GHz by using this method.

We present a 100-GHz integrated photoreceiver using an optical-to-radio converter and an enhancement-mode PHEMT amplifier driven by all optical resources without an external electric power supply. Technology for the simultaneous generation of radio and power was developed using a zero-bias operational uni-travelling carrier (UTC) high-speed photodetector. To improve the optical-to-electrical conversion efficiency, a 110-GHz enhancement-mode InP-PHEMT amplifier was also newly developed, which operates using an internal electric power supply in an optical-to-radio converter. Using a hybrid integration based on all optical resources, a +5.5 dBm RF output at 97 GHz was successfully achieved through the optical-to-radio conversion process. Herein, the two key devices and the integrated photoreceiver are discussed.

This paper describes the concept of sensor over fibres (SoF), consisting of many sensor heads connected by fibre networks. SoF transfer waveforms from the sensor heads and performs joint signal processing to achieve high-resolution and agile imaging, while wireless sensor networks are digital networks to collect measured data at small sensors. SoF can compile information from sensor heads coherently at central signal processing units. We will also show applications of SoF technologies, such as high resolution millimetre-wave radar for airport runway surveillance.

This paper reviews our recent research activities on seamless fiber-wireless systems for future mobile networks, including: radio-on-radio-over-fiber
signal processing-aided multiple signal transmission
high-capacity seamless system
and simultaneous power- and radio-over-fiber.

This presentation reviews radar systems with many antenna units connected by optical networks, which distribute waveforms of wideband frequency modulated millimeter-wave signals, where the range resolution can be a few centimeters.

We have developed a 100-GHz fiber-fed optical-to-radio converter for radio-and power-over-fiber transmission. A zero-bias operational high-speed photodetector (3-dB bandwidth >100 GHz) could be applied for the optical-to-radio converter, taking significant advantage of no-bias feed 100-GHz operation. We have successfully demonstrated simultaneous radio and power generation from one device through a bias-T circuit. A 100-GHz pseudomorphic high electron mobility transistor amplifier allowed a hybrid integrated photoreceiver to enhance the O/E conversion gain. Using the generated electric power, a gate bias in the 100-GHz amplifier was driven. The performance of each device and the integrated photoreceiver is described. High data rate photonic wireless transmission based on photonic power supply without external electric power supply is demonstrated.

We have developed a 100-GHz fiber-fed optical-to-radio converter for radio-and power-over-fiber transmission. A zero-bias operational high-speed photodetector (3-dB bandwidth >100 GHz) could be applied for the optical-to-radio converter, taking significant advantage of no-bias feed 100-GHz operation. We have successfully demonstrated simultaneous radio and power generation from one device through a bias-T circuit. A 100-GHz pseudomorphic high electron mobility transistor amplifier allowed a hybrid integrated photoreceiver to enhance the O/E conversion gain. Using the generated electric power, a gate bias in the 100-GHz amplifier was driven. The performance of each device and the integrated photoreceiver is described. High data rate photonic wireless transmission based on photonic power supply without external electric power supply is demonstrated.

© 2017 COPYRIGHT SPIE.In this paper, we present efficient solutions for simultaneous transmission of multiple radio signals over seamless fiber wireless systems, including radio signals in legacy microwave bands and in high frequency bands. At central stations, radio signals can be mapped onto the same optical transport channel using data mapping algorithms and/or subcarrier multiplexing technique. After the fiber transmission, the received signals can be down-converted, digitized, and de-mapped to recover the originally transmitted signals. We present and compare two different methods, including a radio-over-fiber system and optical up-conversion at remote sites and an intermediate-frequency-over-fiber system and an electrical up-conversion at remote sites. We experimentally confirm the suitability of both the transmission methods, and achieve satisfactory performance for all signals, including LTE-advanced, orthogonal frequency-division multiplexing, and filtered-orthogonal frequency-division multiplexing signals. In particular, the latter method can provide a high optical spectral efficiency and low fiber dispersion effect and is suitable for ultra-dense small cell deployment in future mobile networks.

We present a double cladding, high-mesa-type waveguide UTC photodetector with an improved the responsivity. In this device structure, an InGaAs thin core layer was sandwiched by p-InP/InGaAsP and n-InP/InGaAsP cladding layers, including a UTC structure, in order to obtain a good optical coupling between the waveguide and the fiber. By comparing the resulting mode field with that obtained with a single cladding layer structure, we confirmed that the vertical mode field was enlarged. Without a spot size converter, the measured responsivity was as high as 0.6 A/W at 1550 nm, which suggests a responsivity three times higher than that of back-illuminated structures, and higher responsivity than that of previous reports. A high frequency performance (f(3dB) = 100 GHz) was also measured. The device structure, including the layer, doping level conditions, and optical fiber coupling results are discussed, and its performance is characterized in detail.

© 2016 EuMA.We successfully designed and fabricated a high-conversion-gain 100-GHz photoreceiver integrated with a UTC-PD and a PHEMT amplifier for 92-GHz carrier photonic wireless communication. Then, in a demonstration of 92-GHz carrier, OFDM, 16-QAM photonic wireless communication, we confirmed that high data rate of 11.03-Gbps (bit error rate of 1× 10-3) could be achieved at a low optical input power, without post amplifiers for the photoreceiver. The designs and the results are discussed.

© 2016 EuMA.Multi-layered stacked patch-antennas on an electro-optic (EO) material are proposed for optical modulation. By using multi-layered structures, larger antenna gain can be obtained to induce stronger millimeter-wave electric fields. The induced millimeter-wave electric field can be used for optical modulation through the Pockels effects of the EO crystal. Since the antenna gain is enhanced by using multi-layered stacked patch-antennas, the modulation index improvement can be obtained.

The generation of high-frequency signals including those in the terahertz region has become indispensable in state-of-the-art communication systems. A synthesizer as a measurement device generates signals of desired frequencies with high-frequency resolution. Electric signals (microwaves or millimeter waves) can be generated by a photomixer performing optical-electrical energy conversion based on the phase difference between two coherent optical signals. Recently, the frequency bandwidth for photomixers has reached the terahertz region and photomixers are now more readily available. The main purpose of this letter is to provide an optical synthesizer with a high-frequency resolution (better than 1 Hz) and with seamless frequency coverage from the microwave to the terahertz-wave regions using one apparatus. The system is designed in such a way that the mean of the resolution frequency includes the frequency instability of the generated signal.

We developed a 100 GHz optical-to-radio converter module integrated with a 100 GHz amplifier, which was applied in radio (12-Gbps, OFDM, 16-QAM. IF = 92-GHz) and power over fiber transmission through a multi-core fiber.

We propose and demonstrate a simultaneous transmission of LTE-A, 25-GHz OFDM/FBMC radio access, and 96-GHz filtered-OFDM mobile fronthaul signals over a simple and low cost intermediate-frequency-over-fiber system. We confirm the successful transmission of all signals.

We developed a 100 GHz optical-to-radio converter module integrated with a 100 GHz amplifier, which was applied in radio (12-Gbps, OFDM, 16-QAM. IF = 92-GHz) and power over fiber transmission through a multi-core fiber.

© 2016 IEICE-ES.We investigated fundamental characteristics of a hybrid integrated photoreceiver module with a 1.5 μm-band quantum dot semiconductor optical amplifier (QD-SOA) and conventional pin-photodiode (pin-PD) as a first stage. The results indicated the photoreceiver integrated with a QD-SOA and QD-PD has the potential to be operated for signals over 100 Gb/s.

© 2016 IEICE-ES.A ridge-waveguide laser with highly stacked InAs quantum dot structure based on strain compensation technique was fabricated. The threshold current of this laser was decreased to approximately 75 mA without coating the facet mirror. A high characteristic temperature of over 100 K was obtained using this laser.

© 2016 OSA.We clarify that fiber nonlinearity cancellation in the conjugated-paired radio-on-fiber system is a phase sensitive process. The best nonlinearity tolerance is achieved if the dual sidebands are constructively superposed during photonic down conversion.

© 2016 OSA.We report a recent progress for performance of 90GHz electro-optic modulator with patch-antennas in high-power wireless radio irradiation. Optical sidebands to carrier ratio of -40dB can be achieved by wireless radio irradiation power of 33dBm.

© 2016 OSA.We demonstrate 100-GHz optical-to-radio converter without electric power supplies by combination of a newly developed bias-free-operational UTC-PD and an integrated bias-tee for effective regeneration of the supplied power to a 100-GHz amplifier.

© 2016 OSA.We fabricated 1.5 μm quantum dot waveguide photodetector, and characterized the electrical and optical properties. It was found that the avalanche photocurrent strongly depends on TE/TM polarization.

In this paper, the application of the ultraprecision ductile-mode cutting method to the fabrication of microring waveguides in lithium niobate crystal was investigated. Although it was difficult to apply a mechanical cutting method to the fabrication of microring waveguides with smooth sidewalls, it was confirmed that no harmful cutting traces on the machined surface occur with the appropriate movement of the cutting tool. The root-mean-square surface roughness of the resulting sidewall was 6.1 nm, which is sufficiently small to suppress the scattering loss of the circulating light. In addition, the conditions for the ductile-mode cutting of lithium niobate crystal were investigated. (C) 2016 The Japan Society of Applied Physics

In this paper, the application of the ultraprecision ductile-mode cutting method to the fabrication of microring waveguides in lithium niobate crystal was investigated. Although it was difficult to apply a mechanical cutting method to the fabrication of microring waveguides with smooth sidewalls, it was confirmed that no harmful cutting traces on the machined surface occur with the appropriate movement of the cutting tool. The root-mean-square surface roughness of the resulting sidewall was 6.1 nm, which is sufficiently small to suppress the scattering loss of the circulating light. In addition, the conditions for the ductile-mode cutting of lithium niobate crystal were investigated. (C) 2016 The Japan Society of Applied Physics

© 2016 IEEE.We have developed a stable two-mode laser by using a quantum dot optical gain chip in an external cavity. In this study, we demonstrate continuous change of frequency separation of a two-mode laser by using a birefringent etalon filter. Since the polarization of the quantum dot optical gain chip is TE-dominant, the frequency separation of the two-mode laser is controlled by rotating the birefringent etalon filter. We can change the frequency separation from 84 to 93 GHz by using this method.

© 2016 IEEE.We have demonstrated an orthogonal frequency division multiplexing (OFDM) radar experiment combined with millimeter-wave radio over fiber (RoF). In result, high flame rate and high space resolution of 30 cm in 5 m distance (theoretical space resolution of 1.52 cm) was successfully achieved.

Fiber-wireless transmission in the terahertz-wave band as well as in the millimeter-wave band is the key for a seamless integration of radio and optical networks in a physical layer. Advanced optical fiber communication technology enables a high precision direct waveform transfer between optical and radio domains. For realization of a seamless conversion, an optical-terahertz converter is configured with optical heterodyning for an optical-to-terahertz conversion. Analog optical modulation techniques by an input millimeter-wave signal and an intermediate frequency component after a frequency downconversion are utilized for a terahertz-to-optical conversion. A quadrature phase-shift keying signal transmission in an optical-fiber-to-radio-to-optical-fiber link (fiber-wireless bridge) is successfully demonstrated with the 90 and 300-GHz signals, simultaneously, using an optical frequency comb source. The possible transmission distance in severe weather conditions in these frequency bands is also discussed.

Fiber-wireless transmission in the terahertz-wave band as well as in the millimeter-wave band is the key for a seamless integration of radio and optical networks in a physical layer. Advanced optical fiber communication technology enables a high precision direct waveform transfer between optical and radio domains. For realization of a seamless conversion, an optical-terahertz converter is configured with optical heterodyning for an optical-to-terahertz conversion. Analog optical modulation techniques by an input millimeter-wave signal and an intermediate frequency component after a frequency downconversion are utilized for a terahertz-to-optical conversion. A quadrature phase-shift keying signal transmission in an optical-fiber-to-radio-to-optical-fiber link (fiber-wireless bridge) is successfully demonstrated with the 90 and 300-GHz signals, simultaneously, using an optical frequency comb source. The possible transmission distance in severe weather conditions in these frequency bands is also discussed.

© 2016 IEICE.We present an efficient mobile fronthaul system for co-transmission of radio signals in the microwave and millimeter-wave bands. We successfully transmit a WLAN and an LTE-A signal at 2.6 GHz and 33.6 GHz, respectively, over the system.

© 2016 IEICE.A 100 GHz photoreceiver was designed, fabricated, and integrated with a pHEMT amplifier, based on a photonic power supply. In this paper, a high data rate photonic wireless communication over 11 Gbps is demonstrated.

© 2016 IEICE.We propose a new measurement method of modulator response in phasor domain for Mach-Zehnder modulators embedded in IQ modulator using simple equipments. The modulation curve with extinction ratio of 14.7 and 22 dB were measured.

© 2016 IEICE.We designed and fabricated a LiNbO3 integrated optical modulator for mode-division multiplexing, where optical signals in two modes can be independently controlled and combined together. Mode crosstalk was smaller than -20dB in C-band.

© 2016 IEICE.We propose new electro-optic modulators using gap-embedded patch antennas with stacked structure. Utilizing stacked structure, the antenna gain, electric field for modulation, and interaction length increase, then modulation efficiency increases 3.65 dB at 60 GHz.

Mobile transport networks will play a vital role in future 5G and beyond networks. In particular, access transport networks connecting radio access with core networks are of critical importance. They will be required to support massive connectivity, super high data rates, and real-time services in a ubiquitous environment. To attain these targets, transport networks should be constructed on the basis of a variety of technologies and methods, depending on application scenarios, geographic areas, and deployment models. In this article, we present several technologies, including analog radio-over-fiber transmission, intermediate-frequency-over-fiber technology, radio-on-radio transmission, and the convergence of fiber and millimeter-wave systems, that can facilitate building such effective transport networks in many use cases. For each technology, we present the system concept, possible application cases, and demonstration results. We also discuss potential standardization and development directions so the proposed technologies can be widely used.

This paper proposes a high-speed multiband filtered-orthogonal frequency division multiplexing (F-OFDM) signal transmission over a seamless fiber-millimeter-wave (MMW) system at 92.5 GHz. The system employs a frequency and phase stabilized optical MMW signal generation at the transmitter and a simple direct detection at the receiver. We confirm a successful transmission of multiband F-OFDM signal over a seamless 20-km single mode fiber (SMF) and 1-m 92.5-GHz wireless system. Compared to the single-band and multiband OFDM signal transmission, the proposed system can provide a much better performance, particularly at the edges of each band.

© 2016 IEEE.The rat-race (RR) circuit, which operates as a 180-degree hybrid, with an asymmetric power split ratio was integrated with the dual electrode type electro-optic modulator on the same substrate. Thereby, frequency chirp modulation with a compact configuration was realized. The RR circuit was designed with 4.5 1 power split ratio on a z-cut LiNbO3 substrate. The operation of the modulator was confirmed by observing the output light spectra. Moreover, the chirp parameter was directly measured by using the chromic dispersion of the optical fiber, where the power penalty of detected modulation signals transmitted through optical fiber was observed as a function of the fiber distance. The chirp parameter of the modulated light at 10 GHz was confirmed to be 0.47 by the measurement.

An array of patch-antennas with meandering-gaps on an optical modulator is proposed for wireless millimeter-wave beam-steering through high-speed radio-over-fiber systems. Wireless millimeter-wave can be received by the array of patch-antennas and directly modulated to lightwave by the optical modulator. The wireless millimeter-wave can be steered using the meandering-gaps at the patch-antennas by controlling interaction between millimeter-wave and lightwave electric fields in electro-optic modulation. The basic operation and analysis of the proposed device are discussed. In the experiment, 5 x 5 antenna array in 40 GHz millimeter-wave bands was designed and realized for device characterization and demonstration to wireless millimeter-wave beam-steering. There were five variations of wireless millimeter-wave beam-steering for one-dimensional in xz- or yz-planes that can be obtained with wireless millimeter-wave steerable beams of about +/- 30 degrees. Additionally, 25 variations of wireless beam-steering can be obtained for two dimension in xyz-space through orthogonal optical modulation. The proposed device is promising to be applied in millimeter-wave/tera-hertz bands for future directional wireless communication and sensing with high-speed and high-resolution operation.

In order to adapt to the dynamics in the future optical networks, we propose and experimentally demonstrate two flexible high-order quadrature amplitude modulation (QAM) transmitter schemes: (a) a flexible transmitter using a tandem in-phase/quadrature modulators for generating 16QAM, 32QAM, 64QAM signals; and (b) a scheme based on monolithically integrated quad-Mach-Zehnder in-phase/quadrature with binary driving electronics for synthesizing minimum phase-shift keying (MSK), quadrature phase-shift-keying (QPSK), 8-ary phase-shift keying (8PSK), and 16QAM. These schemes provide different approaches to configure a flexible high-order QAM transmitter, either using cascaded off-the-shelf modulators or via highly integrated monolithic modulator. These flexible and reconfigurable transmitter schemes exhibit different characteristics in terms of system performance, which provides the network operators different flexible transmitter solutions suitable for different application scenarios in elastic optical networks.

© 2015 The Institute of Electronics, Information and Comm.We report current research progress on a metamaterial antenna integrated to an optical modulator for millimeter-wave-photonic links. The metamaterial antenna is composed by an array of electric-LC resonators on a LiNbO3 optical crystal. Large millimeter-wave electric field is induced across the capacitive gaps of the resonators due to free-space millimeter-wave irradiation. Optical modulation through Pockels effects can be obtained when light propagates along the capacitive gaps. The integrated device is operated effectively by considering interaction between millimeter-wave and lightwave electric fields along the capacitive gaps. Basic operations of the integrated device for 90GHz millimeter-wave bands are reported and discussed. Optical sidebands with carrier-to-sideband ratio of about ™60dB by millimeter-wave irradiation power of ∼20mW can be experimentally measured using optical spectrum analyzer.

A monolithically integrated quantum dot (QD) optical gain modulator (OGM) with a QD semiconductor optical amplifier (SOA) was successfully developed with T-band (1.0 mu m waveband) and O-band (1.3 mu m waveband) QD optical gain materials for Gbps-order, high-speed optical data generation. The insertion loss due to coupling between the device and the optical fiber was effectively compensated for by the SOA section. It was also confirmed that the monolithic QD-OGM/SOA device enabled >4.8 Gbps optical data generation with a clear eye opening in the T-band. Furthermore, we successfully demonstrated error-free 4.8 Gbps optical data transmissions in each of the six wavelength channels over a 10-km-long photonic crystal fiber using the monolithic QD-OGM/SOA device in multiple O-band wavelength channels, which were generated by the single QD gain chip. These results suggest that the monolithic QD-OGM/SOA device will be advantageous in ultra-broadband optical frequency systems that utilize the T+O-band for short-and medium-range optical communications. (C) 2016 The Japan Society of Applied Physics

A heterogeneous wavelength-tunable laser diode combining quantum dot and silicon photonics technologies is proposed. A compact wavelength-tunable filter with two ring resonators was carefully designed and fabricated using silicon photonics technology. The tunable laser combining the wavelength-tunable filter and an optical amplifier, which includes InAs quantum dots, achieved a 44.0 nm wavelength-tuning range at around 1250 nm. The broadband optical gain of the quantum dot optical amplifier was effectively used by the optimized wavelength-tunable filter. This heterogeneous wavelength-tunable laser diode could become a breakthrough technology for high-capacity data transmission systems. (C) 2016 The Japan Society of Applied Physics

We fabricate broad-area laser diodes consisting of 30-layer stacks of InAs quantum dots by using a strain-compensation technique on a vicinal (001)InP substrate. These laser diodes exhibit ground-state lasing at 1576 nm in the pulsed mode with a high characteristic temperature of 111K at around room temperature (20-80 degrees C). (C) 2016 The Japan Society of Applied Physics

<p>This paper proposes a high-speed multiband filtered-orthogonal frequency division multiplexing (F-OFDM) signal transmission over a seamless fiber-millimeter-wave (MMW) system at 92.5 GHz. The system employs a frequency and phase stabilized optical MMW signal generation at the transmitter and a simple direct detection at the receiver. We confirm a successful transmission of multiband F-OFDM signal over a seamless 20-km single mode fiber (SMF) and 1-m 92.5-GHz wireless system. Compared to the single-band and multiband OFDM signal transmission, the proposed system can provide a much better performance, particularly at the edges of each band.</p>

© 2016 SPIE.A seamless combination of fiber and millimeter-wave (MMW) systems can be very attractive for future heterogeneous mobile networks such as 5G because of its flexibility and high bandwidth. Analog mobile signal transmission over seamless fiber-MMW systems is very promising to reduce the latency and the required band-width, and to simplify the systems. However, stable and high-performance seamless systems are indispensable to conserve the quality of the analog signal transmission. In this paper, we present several technologies to develop such seamless fiber-MMW systems. In the downlink direction, a high-performance system can be realized using a high-quality optical MMW signal generator and a self-homodyne MMW signal detector. In the uplink direction, a cascade of radio-on-radio and radio-over-fiber systems using a burst-mode optical amplifier can support bursty radio signal transmission. A full-duplex transmission with negligible interference effects can be realized using frequency multiplexing in the radio link and wavelength-division multiplexing in the optical link. A high-spectral efficiency MMW-over-fiber system using an intermediate frequency-over-fiber system and a high-quality remote delivery of a local oscillator signal is highly desirable to reduce the costs.

© 2016 SPIE.A 100-GHz narrowband photoreceiver module integrated with a zero-bias operational uni-traveling-carrier photodiode (UTC-PD) and a GaAs-based pseudomorphic high-electron-mobility transistor (pHEMT) amplifier was fabricated and characterized. Both devices exhibited flat frequency response and outstanding overall performance. The UTC-PD showed a 3-dB bandwidth beyond 110 GHz while the pHEMT amplifier featured low power consumption and a gain of 24 dB over the 85-100 GHz range. A butterfly metal package equipped with a 1.0 mm (W) coaxial connector and a microstrip-coplanar waveguide conversion substrate was designed for low insertion loss and low return loss. The fabricated photoreceiver module demonstrated high conversion gain, a maximum output power of +9.5 dBm at 96 GHz, and DC-power consumption of 0.21 W.

Conversion devices between free-space microwave/millimeter-wave and lightwave signals are required in Fiber-Wireless (Fi-Wi) links. In this paper, we propose a free-space millimeter-wave Electro-Optic (EO) modulator using Quasi-Phase-Matching (QPM) gap-embedded-patch-antennas on a low dielectric constant substrate. Free-space millimeter-wave signals can be received and converted directly to lightwave signals using the proposed device. It can be operated passively with no external power supply and operated with extremely low millimeter-wave losses. The QPM array structure can be adopted for improving modulation efficiency by considering the transit-time effects. The proposed device is easy connected to optical fiber, therefore Fi-Wi can be realized using the proposed device. Basic structure, analysis, and experimental results of the proposed device are discussed and reported for 40 GHz operational millimeter-wave bands.

We propose a flexible and high-performance fronthaul system for future mobile networks using a seamless combination of fiber-optic and millimeter-wave (MMW) systems. The proposed system employs a high-quality optical MMW signal generation at the transmitter and a stable and high-performance MMW self-homodyne detection at the receiver. In the downlink direction, successful transmission of intraband and interband carrier aggregation (CA) long-term evolution-advanced (LTE-A) signals over the system is experimentally confirmed. We also confirm satisfactory performance for the transmission over a fiber-wireless bridge using a cascade of the seamless system and a conventional radio-over-fiber (RoF) link. In the uplink direction, we successfully demonstrate the transmission of an intraband CA LTE-A signal over a cascade of a radio-on-radio and a RoF system. In addition, we confirm that the full-duplex transmission of LTE-A signals over the proposed bidirectional system does not experience performance degradation compared to the case with half-duplex transmission. The proposed system is a potential solution for mobile-signal transmission in future networks.

A laser-phase-fluctuation-insensitive optical coherent transmission of a 10-Gbaud 16-quadrture-amplitude-modulation radio-over-fiber signal with an offset-frequency-spaced two-tone local light and an digital-signal-processing technique is experimentally demonstrated. Error-free symbol recovery after the 20-km-long fiber-optic transmission is shown and the transmission quality is evaluated.

A closed-form capacity expression for polarization-multiplexed coherent MIMO radio-over-fiber (RoF) links at millimeter-wave bands is derived and the impact of the fiber's polarization rotation and the wireless antenna misalignment on the MIMO channel capacity is revealed. The analytical results are validated via an indoor W-band (75-110 GHz) 10-Gbaud MIMO-RoF transmission experiment.

Single-carrier photonic wireless transmission at 31 GHz and 8.0 Gbaud with 64QAM is successfully achieved with error-free operation. A newly developed wide bandwidth high power photodetector and a pre-equalizing subsystem are presented.

This paper presents the simultaneous transmission of millimeter-wave and terahertz-wave radio signals over an optical fiber and air by a single optical frequency comb source with several types of receiver configurations. (C) 2015 Optical Society of America

To avoid the modal crosstalk-induced power fading, a mode assignment technique has been proposed and experimentally demonstrated for mode division multiplexed W-band radio-over-fiber links.

We propose a dual-hop network to high-speed trains by using a seamless fiber-millimeter-wave system for backhaul transmission and a high-speed in-train network. Transmission of a 64-QAM OFDM signal over the cascaded system is successfully confirmed.

This paper reviews train communication networks (TCNs) based on radio-over-fibre (RoF) technology. TCNs connect base stations (BSs) in trains to world wide networks through trackside links consisting of optical fibres and many radio access units (RAUs). Waveform for radio communications can be effectively delivered to each RAU by RoF. The TCN can be controlled by train location information to reduce overhead due to hand-over in mobile communication systems.

We report a 30-110 GHz, high conversion gain, and high output photodetector consisting of a hybrid integration of a high-speed photodetector and an RF amplifier with an additional gain emphasis circuit. High data-rate photonic wireless communication over 44 Gbps was demonstrated with a predistortion technique. The minimum optical input power level for an error free operation was also discussed.

This paper reviews seamless networks based on radio-over-fiber (RoF) technology. We focus on train communication networks (TCNs) consisting of optical networks and many radio access units (RAUs). Waveform for radio communications can be effectively delivered to each RAU by RoF. The TCN can be controlled by train location information to reduce overhead due to hand-over in mobile communication systems.

A photonic terahertz synthesizer is configured and demonstrated by100-GH, 300-GHz and 600-GHz signal generation. An optical frequency shifter in an amplified optical fiber loop technique configures an optical frequency comb generator with a resultant optical bandwidth up to 2 THz. An observed phase noise characteristic by the photonic synthesizer is considerable for application to the complex modulation formats even in the terahertz bands.

We propose an architecture of the radio-over-fiber systems for train communication networks. Prediction-based dynamic optical routing can track the train cars by train location information. High spectral-efficient millimeter-wave signal modulation and demodulation are also discussed with the modulation format and its configuration of the frequency conversions in the paper.

We present efficient mobile fronthaul systems for simultaneous transmission of radio signals in the microwave and millimeter-wave hands over fiber links. We compare the performance for signal transmission over intermediate frequency over fiber systems using an electrical and optical up-conversion to the MEW hand at remote sites. We experimentally confirm the successful transmission of an LTE-A signal at 2.6 GHz and a wideband OFDM signal at 35.4 GHz over the systems.

We experimentally demonstrate fiber-nonlinearity-tolerant conjugated-paired radio-on-fiber (C-RoF) system for the first time. A C-RoF signal is simply generated by dual-sideband modulation and fiber nonlinearity is cancelled through asymmetric photonic heterodyne down-conversion. A QPSK-CRoF signal is transferred over 20-km SMF fiber.

New communication system for high-speed train passengers is required to provide broadband services in the train. Radio over fiber in millimeter wave band is considered to provide a high throughput of the system. As a proof of concept, we generate 92 GHz 200 M symbol/sec 64QAM signals by optical frequency doubler. EVM of the signal is 7.14%. The result shows 1.2Gbps transmission is available in MMW band by radio over fiber.

We successfully designed and fabricated a highconversion-gain 100-GHz photoreceiver integrated with a UTC-PD and a PHEMT amplifier for 92-GHz carrier photonic wireless communication. Then, in a demonstration of 92-GHz carrier, OFDM, 16-QAM photonic wireless communication, we confirmed that high data rate of 11.03-Gbps (bit error rate of 1 x 10(-3)) could be achieved at a low optical input power, without post amplifiers for the photoreceiver. The designs and the results are discussed.

© 2016 IEEE. In wireless communications, the data traffic increases rapidly every year, as well as in fixed optical fiber communications, where the trend for data traffic is not saturated yet. In the next generation of advanced wireless communication services, a high data rate, high capacity, low latency, and low cost are desired. The large radio cover area for present macro-cell should be divided into plenty of small-cells in order to mitigate heavy data traffic in the one macro-cell [1]. Here, radio over fiber (RoF) is a promising technology for access networks such as mobile back-haul and front-haul in wireless communication. High data rate transmission through optical fibers should be introduced near end-user points and be converted to radio signals from simple antennas in small-cells. Here, a high speed photoreceiver is one of key components in RoF communications. On the other hand, power over fiber (PoF) technology, which provides electric power supply through optical fibers, is a compatible technology with a good affinity to RoF technology. The 90 to 110 GHz band is a very attractive frequency band, which involves not only low atmospheric attenuation [2] but also a wider bandwidth compared to that in the microwave region.

We discuss key device technologies for realization of an optical-fiber-network-connected distributed radar system in the millimeter-wave band using radio over fiber techniques. The radio over fiber technique is useful for precise signal generation and distribution over the fiber network. In the scenario, each millimeter-wave radar head is not necessary to be equipped with aprecision radar signal source; low cost and low energy consumption radar head will be realized. For connection to the optical network, advanced high-speed optical/electrical converters are developed for signal conversion from/to optical signals in 90-GHz band directly in a central office and each radar head. A signal-to-noise power ratio of generated 90-GHz optical signal was achieved larger than 30 dB: enough quality to use in the radar system. An optical switching technique is useful not only for routing the optical signal to the radar heads but also for filtering unnecessary optical components. These device technologies are applicable for distributed millimeter-wave radar systems.

We have proposed a new electro-optic signal converter which enables us to convert wireless millimeter-wave signals to Iightwave signals. This is based on a LiNbO3 guided-wave optical modulator combined with vertically-stacked planar patch antennas embedded with a narrow (similar to 5 mu m) gap. Utilizing the stacked structure, the antenna gain, antenna area, and electric field for optical modulation are increased, so that the signal conversion efficiency is increased by 3.65 dB at 60 GHz.

A terahertz-wave communication system directly connected to an optical fiber network is promising for application to future mobile backhaul and fronthaul links. The possible broad bandwidth in the terahertz band is useful for high-speed signal transmission as well as radio-space encapsulation to the high-frequency carrier. In both cases, the low-latency feature becomes important to enhance the throughput in mobile communication and is realized by waveform transport technology without any digital-signal-processing-basedmedia conversion. A highly precise optical frequency comb signal generated by optical modulation and the vector signal demodulation technology adopted from advanced optical fiber communication technologies help perform modulation and demodulation with impairment compensation at just the edges of the link. Terahertz wave, radio over fiber, waveform transport, coherent detection, multilevel modulation, radio on radio.

© 2015 IEEE.An optical frequency-interleaving full-duplex multi-channel transmission of W-band frequency-modulated continuous-wave downlink and 10-Gb/s on-off-keying uplink signals is investigated. Two-channel transmission with a flexible wavelength channel selector over 10-km-class fiber-optic network is experimentally demonstrated.

Quantum-dot nanotechnology is attractive for use in advanced photonic devices that will augment the available optical-frequency resources and will increase the number of wavelength channels usable by wired and wireless networks in short/middle-range communication systems. © 2015 OSA.

We discussed crosstalk issues on parallel photonics assuming high-density integration. RF crosstalk has become a more serious problem than optical crosstalk, and it is affected by the operating frequency (baud rate). A bias free UTC-PD was designed and fabricated. © 2015 OSA.

© 2015 IEEE.In this paper, we investigate the optical power fluctuation of the output in optical comb generator using an optical single sideband (SSB) modulator with a fiber loop. This fluctuation is caused by undesired sidebands of the SSB modulator. We show that the fluctuation can be suppressed by stabilizing the fiber loop length. In experiment, the averaged peak-to-peak power fluctuation of 23 comb outputs was suppressed from 0.86 dB to 0.13 dB.

© 2015 IEEE.300-GHz multilevel signal transmission over optical fibers and air is demonstrated using advanced optical communication technology including optical frequency comb generation. Local oscillator signal transport over optical fibers to optical-terahertz converters removes the need for setting any terahertz-capable oscillators in the converters. This fiber-remoted configuration with seamless conversion between the optical and terahertz signals is compatible with high-speed terahertz backhaul and fronthaul links.

© 2015 IEEE.In this study, we successfully fabricated a W-band high-power photoreceiver with high output power and low power consumption. This W-band photoreceiver was integrated with a bias-free uni-travelling carrier photodetector (UTC-PD) and an InP high electron mobility transistor (PHEMT) amplifier for the first time. An ultrabroad 3 dB bandwidth beyond 110 GHz in an UTC-PD and maximum oscillation frequency of 320 GHz in a PHEMT low-noise amplifier were used for this hybrid integration. At 109 GHz, a high radio frequency output power of +10 dBm, high transimpedance gain of 828 □, and low power consumption of 67 mW was achieved in an optical-electrical converting process.

© 2015 EuMA.We propose a flexible and low-latency cascaded fiber-wireless system for future mobile fronthauling using seamless combination of fiber-optic and millimeter-wave links. Successful transmission of intraband and interband carrier aggregation LTE-A signals over the system is experimentally confirmed. Satisfactory performance is achieved owing to the use of a high-quality optical millimeter-wave signal generation technique at the transmitter and a proposed high-performance self-homodyne detection at the receiver.

© 2015 IEEE.Envelope-detector-based transceiver configurations are useful for both digital signal transmission and radar systems in the terahertz band. A dual-purpose transceiver is demonstrated using a 1-Gb/s on-off keying signal and 12.5-GHz-bandwidth frequency-modulated continuous-wave radar.

© 2015 IEEE.We present a pseudo-delta-sigma modulator and demonstrate transmission using an advanced modulation. This digitally implemented transmitter helps in reducing the complexity at the receiver and realizing waveform transport over a digital optical network.

© 2015 IEEE.An optical frequency-interleaving full-duplex transmission of 96-GHz-band frequency-modulated continuous-wave downlink signal and 10-Gb/s on-off-keying uplink signals is investigated. The proposed full-duplexing transmission with a flexible wavelength channel selector over 10-km fiber-optic link is experimentally demonstrated.

A combination of fiber and radio links would be attractive for efficient fronthauling to small cells and moving cells. However, achieving connectivity by using wired-wireless media converters significantly increases system complexity, power consumption, and latency. A seamless combination of fiber and radio links in which photonics-based technologies are used to generate and transmit radiowave signals is more suitable. In this article, we present an overview of fiber-radio link combination for mobile signal transmission. We propose an efficient solution for future mobile fronthauling based on the analog transmission of mobile signals over a bidirectional seamless system. We discuss the potential as well as the challenges and perspectives of the systems applied for mobile fronthauling in small-cell-and moving-cell-based networks. We also present a proof-of-concept demonstration of the transmission of mobile signals over the system. Satisfactory performance is obtained for different high-speed advanced signals. The transmission range over fiber links and the estimated range of the radio links are sufficiently long to facilitate the practical implementation of the system. The system is scalable to include new deployments of small cells and wireless services using wavelength-division and subcarrier multiplexing techniques. It can provide a high-speed, low-latency, and flexible solution for fronthauling in new emerging wireless systems such as 5G networks.

We propose a new optical coherent detection scheme with a "two-tone" local light, which is also in principle insensitive to the laser phase fluctuation and is assisted by the digital signal processing technique for radio-over-fiber (RoF) systems. The main feature of our proposal is that the frequency separation of the two-tone local light is different from that of the RoF signal, which is called "offset-frequency-spaced" in this paper. First, we explain the principle of our new proposal and experimentally demonstrate the data recovery. Then, the influence of the frequency detuning between photo-detected modulated and unmodulated signals is discussed. Moreover, the transmission performance with an error vector magnitude (EVM) is evaluated for the optical coherent detection of a 10-Gbaud quadrature-phase-shift-keying RoF signal after a 20-km-long standard single-mode fiber transmission. (C) 2015 Optical Society of America

An electro-optic (EO) modulator integrated with the microwave planar circuit directly formed on a LiNbO3 (LN) substrate for low frequency-chirp performance and compact configuration is introduced. Frequency chirp of EO intensity modulators was investigated and a dual-electrode Mach-Zehnder (MZ) modulator combined with a microwave rat-race (RR) circuit was considered for the low-chirp modulation. The RR circuit, which operates as a 180-degree hybrid, was designed on a z-cut LN substrate to create two modulation signals of the same amplitude in anti-phase with each other from a single input signal. Output ports of the RR were connected to the modulation electrodes on the substrate. The two signals of the equal amplitude drive two phase modulation parts of the modulator so that the symmetric interference are realized to obtain intensity modulation of low frequency-chirp. The modulator was designed and fabricated on a single LN substrate for around 10 GHz modulation frequencies and 1550 nm light wavelength. The chirp parameters were measured to be less than 0.2 in the frequency range between 8 and 12 GHz. By compensating imbalance of the light power splitting in the waveguide MZ interferometer the chirp could be reduced even more.

Optical single sideband (SSB) modulation with the Mach-Zehnder (MZ) interferometer was realized by integrating the modulation electrode with the branch-line coupler (BLC) as a 90-degree hybrid onto the modulator substrate. In this paper, BLCs of the microsrtip-line structure were miniaturized on modulator substrates, LiNbO3 (LN), to realize more compact optical SSB modulators. We introduced two techniques of miniaturizing the BLC, one is using periodically installed open-circuited stabs and the other is installing series capacitors. Compared with a conventional pattern of the BLC, an area of the miniaturized BLC by using periodically installed open-circuited stubs was reduced to about 50%, and that by installing series capacitors was done to about 60%. The operation of these miniaturized BLCs was experimentally confirmed as the 90-degree hybrid at around 10GHz. Output ports of each miniaturized BLC were directly connected with the modulation electrode on the modulator substrate. Thereby, we fabricated two types of compact SSB modulators for 1550nm light wavelength. In the experiments, the optical SSB modulation was successfully confirmed by the output light spectra and the sideband suppression ratio of more than 30dB were observed.

In dynamic optical networking scenarios, it is desirable that the optical transmitter chooses the most suitable modulation format in order to achieve optimal transmission performance. Owing to the ability of switching among different modulation formats, flexible optical transmitters based on reconfigurable optical devices are becoming a key component for the implementation of future flexible optical networks. In this paper, we experimentally demonstrate a flexible 8-ary transmitter to achieve adaptive switching between 8-ary phase-shift keying (8PSK) and circular 8-ary quadrature-amplitude modulation (8QAM) through reconfiguration of two cascaded in-phase/quadrature (IQ) modulators with different driving signals and biasing conditions. An arbitrary binary quadrature-amplitude modulation (2QAM) with constant or non-constant amplitude is proposed and experimentally demonstrated using an IQ modulator. Then, optical 8PSK or 8QAM modulation formats are successfully synthesized when a standard squared QPSK modulator is cascaded with a constant-amplitude or non-constant-amplitude 2QAM, respectively. (C) 2015 Optical Society of America

A re-configurable wavelength de-multiplexer for wave-length-division-multiplexed (WDM) radio-over-fiber (RoF) systems, which is specially designed for delivering frequency-modulated continuous-wave (FM-CW) signals, is newly developed. The principle and characteristics of the developed de-multiplexer are described in detail. Then the de-multiplexing performances of 4-channel WDM 32-GHz-band, 8-channel WDM 48-GHz-band, and 5-channel WDM 96-GHz-band FM-CW RoF signals are evaluated with the de-multiplexer.

In this paper, we propose a flexible and high-capacity front-haul link for the uplink transmission of high-speed mobile signals using a cascade of radio-on-radio (RoR) and radio-over-fiber (RoF) systems. To emulate the cases that may occur in the uplink direction, we experimentally investigate the performance of superposing an uplink bursty LTE-A signal on the cascaded system using optical packet signal transmission. The performance of systems using different types of erbium-doped fiber amplifiers (EDFAs), including a high-transient EDFA, an automatic-gain-control EDFA, and a burst-mode (BM) EDFA is evaluated and compared. We confirm that the dynamic transience of the EDFAs has a significant influence on the signal performance. By using a BM-EDFA, we confirm successful transmission of the uplink packetized LTE-A signal on the cascaded system. Both the measured error vector magnitude and the received optical power range metrics exceed the requirements. We also estimate the maximum transmission range of the RoR link, and it is confirmed that a sufficiently long range could be achieved for the applications in mobile front-haul networks.

We report the adaptability of the burst-mode erbium-doped fiber amplifier (BM-EDFA) for uplink transmission of sharply rising analog radio-over-fiber (RoF) signals by using long-term evolution (LTE) -Advanced format on a mobile front-haul. Recent drastically increased mobile data traffic is boosting the demand for high-speed radio communication technologies for next-generation mobile services to enhance user experience. However, the latency become increasingly visible as serious issues. Analog RoF technology is a promising candidate for a next generation mobile front-haul to realize low latency. For the uplink, an RoF signal may rise sharply in response to a burst of in-coming radio signals. We propose that a newly developed BM-EDFA is applied for such a sharply rising RoF signal transmission. The BM-EDFA that we designed using enhanced intrinsic saturation power EDF to suppress the gain transient caused by received optical power fluctuations with optical feedback. The new BM-EDFA was designed for a wider linear output power range and lower NF than the previous BM-EDFA. The observed range of received optical power satisfying an error vector magnitude of less than 8%rms achieved over 16 dB. We consider that our BM-EDFAs with wide linear ranges of output power will be a key device for the LTE-Advanced RoF uplink signal transmission via optical access networks for the next-generation mobile front-haul.

A millimeter-wave radar receiver using a z-cut LiNbO3 optical modulator with orthogonal-gap-embedded patch-antennas on a low-k dielectric material is proposed. A millimeter-wave from a reflected radar signal can be received by the patch-antennas and converted directly to a lightwave through electro-optic modulation. A low-k dielectric material is used as a substrate for improving antenna gain. Additionally, an interaction length between millimeter-wave and lightwave electric fields becomes long. As a result, large modulation efficiency can be obtained, which is proportional to sensitivity of the millimeter-wave radar receiver. Optical millimeter-wave radar beam-forming can be obtained using the proposed device with meandering-gaps for controlling interaction between millimeter-wave and lightwave electric fields in electro-optic modulation. Analysis and experimentally demonstration of the proposed device are discussed and reported for 40GHz millimeter-wave bands. Optical millimeter-wave radar beam-forming in 2-D is also discussed.

© 2015 OSA. We demonstrate high spurious suppression ratio single-sideband modulation by a high extinction-ratio parallel Mach-Zehnder modulator. An electrical third-order nonlinearity suppression technique can enhance the suppression ratio as high as 47 dB.

This paper describes photonic technologies for millimeter-wave generation and distribution. External modulation would be useful for stable and high-quality radio-over-fiber signal generation, where frequency stability and spurious suppression are very important for radio services such as mobile communications, radio locations etc. This paper provides outline of radio-over-fiber signal generation techniques. Precise and high-speed light-wave optical modulation would play important roles for suppression of undesired spectrum components. Photonic harmonic generation would be useful for very high-frequency signals. Reciprocating optical modulators consisting of optical modulators and fiber Bragg gratings can provide optical modulation by sub-THz signals.

We propose and demonstrate pump-phase-noise-tolerant 1-to-7 wavelength multicasting for QAM signals using flexible coherent multi-carrier pump. Less than 0.8dB penalty is obtained after multicasting with the elimination of phase noise from pump using coherent pumping.

Through FWM in our fabricated QD-SOA and without additional pumps, three 1-to-3 WDM multicasts and three XOR logic gates are simultaneously realized for three 10 Gbps NRZ-DPSK signals. The error-free performance is achieved for all of the signals and the power penalties are &lt; 1.3 dB at BER of 10(-9).

A digital-signal-processing-assisted and laser-phase-fluctuation-insensitive optical coherent transmission of a 10-Gbaud 16-quadrture-amplitude-modulation radio-over-fiber signal with an offset-frequency-spaced two-tone local light is experimentally demonstrated. Error-free symbol recovery after the 20-km-long fiber-optic transmission is shown.

We designed a high-baud-rate UTC photodetector without a bias circuit for the first time. The photodetector's wideband frequency response over 100 GHz and clear eye diagrams up to 107 Gbaud were confirmed at 0 V. Bit error rate is discussed for 56 Gbaud and 107 Gbaud.

We have successfully developed a zero-bias operational ultra-broadband uni-travelingcarrier photodiode (UTC-PD) with a frequency bandwidth above 110 GHz using a low carrier concentration of 3 x 10(14) cm(-3) in the carrier collection layer.

We propose a novel high-capacity and low-latency front-haul system for small cells-based heterogeneous wireless networks. We experimentally demonstrate a proof-of-concept seamless convergence of radio-over-fiber (RoF) and millimeter-wave (MMW) system at 90 GHz for high-speed wireless signal transmission. Simultaneous transmission and demodulation of standard-compliant very-high-throughput WLAN IEEE 802.11ac and high-speed LTE-A signals over the system are successfully confirmed for the downlink direction. Satisfactory transmission performance over the converged 10-km RoF and a 5-m 90-GHz wireless link is achieved. The measured root-mean-square error vector magnitude is much better than the requirement defined by the standards.

A monolithically integrated quantum-dot (QD) optical gain modulator with semiconductor optical amplifier was developed for successful demonstration of error-free Gb/s-order high-speed, 10-km-long transmissions using 1.3-mu m multiple wavelength channels from a QD comb laser.

We experimentally evaluate the time-varying crosstalk frequency response of a multicore fiber. Our findings suggest that dynamically adaptive transmission would be able to exploit this behavior, to improve the performance of crosstalk-limited links.

We present a high-speed and low-latency seamless fiber-millimeter-wave system for future mobile front-haul networks. The bidirectional transmission of a downlink interband and an uplink intraband carrier aggregation LTE-A signal over the system is successfully demonstrated.

In this paper, we propose a high-speed wireless signal transmission over a simple analog intensity modulation/ direct detection (IM/DD) fiber-millimeter-wave (MMW) system at 92.5 GHz. The system employs a frequency and phase stabilized optical MMW signal generation at transmitter and a simple direct detection at receiver. We confirm a successful transmission of 10Gb/s 16-QAM OFDM-based wireless signal over a seamless 8-km SMF and 27-cm 92.5-GHz wireless system. To further increase the fiber transmission range, we use a simple adaptive modulation approach. Successful transmission over 10-km single mode fiber (SMF) system is confirmed for a 8.1 Gb/s wireless signal using both 16-QAM and QPSK modulation. We also estimate the maximum transmission range for the MMW link, and it is proved that a transmission over a 1-km MMW link can be achieved. Both the confirmed fiber length and estimated MMW range are sufficiently long for future mobile front-haul systems in next generation wireless networks. The system therefore can provide a high-speed, low-latency, and flexible solution for front-hauling in new emerging wireless systems such as 5G mobile networks.

We demonstrate loop-assisted multi-input-multi-output coherent matched detector, enabling ultra-wideband parallel optical sampling in time-frequency domain. 8-subchannels packed in 80 GHz in single polarization was simultaneously captured in parallel only using a single 10-Gbaud coherent receiver.

Wired and wireless seamless networks comprised of optical-fiber and radio-wave links can provide high-speed, resilient, and flexible broadband services, one of the important requirements for which is the precise and highspeed control of light waves based on advanced photonic technologies such as digital coherent detection, highspeed electro-optic conversion, and radio-over-fiber. In this paper, we focus on advanced photonic devices for optical and radio-wave links and on the expected performance and function of seamless networks.

We experimentally demonstrate generation of polarization-multiplexed phase-conjugated twin signals by using oppositely-biased dual-polarization IQ modulators, compensating for nonlinear signal distortion in fiber transmission. Bias condition is optimized investigating nonlinearity tolerance of the generated signals transmitted over SMF.

We present a high-spectral efficiency fronthaul system for the transmission of millimeter-wave and multiple-input multiple-output (MIMO) signals in future mobile networks. The transmission of an interband carrier aggregation and a 2 x 2 MIMO LTE-A signal over the system is successfully demonstrated.

We propose a flexible and low-latency cascaded fiber-wireless system for future mobile fronthauling using seamless combination of fiber-optic and millimeter-wave links. Successful transmission of intraband and interband carrier aggregation LTE-A signals over the system is experimentally confirmed. Satisfactory performance is achieved owing to the use of a high-quality optical millimeter-wave signal generation technique at the transmitter and a proposed high-performance self-homodyne detection at the receiver.

In this paper, we demonstrate a 300-GHz FM-CW radar using optical modulation technology. An optical frequency comb based on the optical modulation is utilized as a broadband frequency multiplier with a factor of 12. At a central frequency of 300 GHz, we generate a 12-GHz-bandwidth FM-CW signal with a pulse duration of 10 mu s, realizing a precise ranging with a resolution of several centimeters. We show that the optical frequency comb generator is capable of generating larger bandwidth FM-CW signals.

© 2015 IEEE.High-precision imaging technology and its application are highly demanded for the enhancement of civil security. They are used to help prevent major incidents in railways or airports. In these applications, both small object detection and high-range resolution are key features. As far as the detectable target size is concerned, a high-frequency carrier is indispensable in radar systems. However, a radio signal with millimeter-wave (high frequency) suffers from large atmospheric attenuation. For example, the attenuation coefficient at 90 GHz is estimated to be 1 dB/km, while it is less than 0.1 dB/km in microwave bands [1]. Thus, the radar signal will have to originate from an area close to the target. Setting up a radar system with a synthesizer close to the target might not be desirable because a high-precision electrical synthesizer is generally large and consumes high energy. Radio-over-fiber (RoF) technology, used to transport the signal over optical fibers, is a promising candidate to deliver the millimeter-wave signal to the suitable point over a low-loss optical fiber cable. Radars utilizing the RoF technology have already been reported; however, digital signal processing in remote radar heads was required to reduce large transmission data [2, 3].

In addition to new radio technologies, end-to-end transport networks will play a vital role in future 5G (and beyond) networks. In particular, access transport networks connecting radio access with core networks are of critical importance. They should be able to support massive connectivity, super high data rates, and real time services in a ubiquitous environment. To attain these targets, transport networks should be constructed on the basis of a variety of technologies and methods, depending on application scenarios, geographical areas, and deployment models. In this paper, we present several technologies, including analog radio-over-fiber transmission, intermediate-frequency-over-fiber technology, radio-on-radio transmission, and the convergence of fiber and millimeter-wave systems, that can facilitate building such effective transport networks in many use cases. For each technology, we present the system concept, possible application cases, and some demonstration results. We also discuss potential standardization and development directions so that the proposed technologies can be widely used.

A government-funded R&D program aiming at a resilient optical and millimeter-wave wireless link for a novel span protection scheme in a disaster recovery is presented. We also describe the key enabling technologies of both optical and wireless sub-systems and devices, including MIMO radio-over-fiber system, optical two-tone generation, and the transceiver and power amplifier in W-band.

In this paper, we experimentally demonstrate simultaneous multichannel wavelength multicasting (MWM) and exclusive-OR logic gate multicasting (XOR-LGM) for three 10Gbps non-return-to-zero differential phase-shift-keying (NRZ-DPSK) signals in quantum-dot semiconductor optical amplifier (QD-SOA) by exploiting the four-wave mixing (FWM) process. No additional pump is needed in the scheme. Through the interaction of the input three 10Gbps DPSK signal lights in QD-SOA, each channel is successfully multicasted to three wavelengths (1-to-3 for each), totally 3-to-9 MWM, and at the same time, three-output XOR-LGM is obtained at three different wavelengths. All the new generated channels are with a power penalty less than 1.2dB at a BER of 10(-9). Degenerate and non-degenerate FWM components are fully used in the experiment for data and logic multicasting. (C) 2014 Optical Society of America

We investigate a high-capacity, space-division-multiplexed (SDM) transmission system using self-homodyne detection (SHD) in multicore fiber (MCF). We first investigate SHD phase noise cancellation with both kHz and MHz range linewidths for both quadrature-phase shift-keyed (QPSK) and 16 quadrature-amplitude modulation (16QAM) signals, finding that phase noise cancellation in SHD enabled transmission with MHz linewidth lasers that resulted in error floors when using intradyne detection. We then demonstrate a high throughput SHD transmission system using low-cost, MHz linewidth distributed feedback lasers. We transmit a CW pilot-tone on a single core of a 10.1 km MCF span with the remaining 18 cores used to transmit 125 wavelength-division multiplexed (WDM) QPSK and polarization-division-multiplexed (PDM)-QPSK signals with 50 GHz channel spacing at 25 GBd. For PDM transmission and assuming a 7% forward-error correction overhead this is equivalent 210 Tb/s transmission with a SE of 33.4 b/s/Hz. High-capacity transmission is achieved despite high inter-core crosstalk, broad transmitter linewidth and narrow channel spacing, showing that combining SHD with MCF enables high throughput, low-cost transmission in next-generation optical networks. (C) 2014 Optical Society of America

We discuss the performance of Mach-Zehnder modulators with asymmetric power-splitting Y-branches theoretically and confirmed it experimentally. The output characteristics of the modulator depend on the relationship between the power splitting ratios of the front and the back Y-branches. Even if the modulator consists of two asymmetric Y-branches, we can achieve high-extinction-ratio operation, with an additional loss as compared with the ideal case, by tuning the splitting ratio of only one of the two Y-branches. The additional loss, however, is small in many cases because the asymmetry considered here is caused by fabrication errors. (C) 2014 The Japan Society of Applied Physics

Optical wavelength conversion (OWC) is expected to be a desirable function in future optical transparent networks. Since high-order quadrature amplitude modulation (QAM) is more sensitive to the phase noise, in the OWC of high-order QAM signals, it is crucial to suppress the extra noise introduced in the OWC subsystem, especially for the scenario with multiple cascaded OWCs. Here, we propose and experimentally demonstrate a pump-linewidth-tolerant OWC scheme suitable for high-order QAM signals using coherent two-tone pumps. Using 3.5-MHz-linewidth distributed feedback (DFB) lasers as pump sources, our scheme enables wavelength conversion of both 16QAM and 64QAM signals with negligible power penalty, in a periodically-poled Lithium Niobate (PPLN) waveguide based OWC. We also demonstrate the performance of pump phase noise cancellation, showing that such coherent two-tone pump schemes can eliminate the need for ultra-narrow linewidth pump lasers and enable practical implementation of low-cost OWC in future dynamic optical networks. (C)2014 Optical Society of America

Laser-phase-noise-insensitive optical coherent transmission of a radio-over-fiber signal with two-tone local light and digital signal processing technique is experimentally demonstrated. The estimated BER of less than 10(-9) is successfully achieved after 20-km-long fiber-optic transmission.

We experimentally demonstrate a 50-Gbps 16QAM wavelength conversion with &lt;0.2dB penalty, based on FWM in HNLF using coherent pumps. The coherent pumping makes it independent of the phase noise from pumps and enables the use of cost-effective pump laser.

We propose a novel method to measure chirp of an electro-absorption modulator at any frequency using optical homodyne quadrature demodulation technique. Chirp parameters are directly obtained in the small-signal vector space, enabling practical chirp assessment for advanced modulation optical transmission.

Millimeter-wave (MMW) signal generation devices using difference frequency generation (DFG) based on the second order nonlinear optical effect in a rectangular waveguide were studied in detail theoretically and experimentally. The temporal and spatial coupling process of a generated MMW signal to a TE10 mode in a rectangular waveguide embedded with a nonlinear crystal was analyzed using the finite-difference time-domain (FDTD) method. In the experiment, 60 GHz-band signals were successfully obtained from fabricated prototype devices using a rectangular waveguide embedded with z-cut LiTaO3 crystal.

The technology of encapsulating a microwave signal into a terahertz carrier and an optical signal is proposed and demonstrated experimentally. Broad bandwidth terahertz radio can encapsulate existing microwave services, including advanced mobile communication signals, by using photonics technology. The concept of radio-on-terahertz-over-fiber helps to realize the future design of mobile fronthaul links. These links will have seamless connectivity between radio and optical networks because of the low latency of the links, which results from the use of analog signal transmission technology.

This paper describes high-speed data transmission based on combination of optical fiber and radio-wave links, which would enable low-latency transfer and agile deployment capability, where broadband radio signal waveforms can be transferred by radio-on-fiber technology.

Seamless conversion from millimeter-wave radio signals to optical baseband signals was demonstrated using a direct IQ down-converter. Hybrid optical and millimeter-wave signal transmission was performed using a conventional digital-coherent receiver, without any special algorithms. (C) 2013 Optical Society of America

All-optical wavelength conversion (AOWC) plays an important role in the future transparent optical networks, in order to enhance the reconfigurability and non-blocking capacity. On the other hand, high-order quadrature amplitude modulations (QAMs) have been extensively studied for achieving the high-speed and high-spectral-efficiency optical transmission. Since high-order QAMs are more sensitive to phase and amplitude noise, to implement an AOWC sub-system suitable for highorder QAM signals with minimized power penalty, it is important to optimize the operation conditions in order to avoid extra nonlinear distortions co-existed in the AOWC process. Our experimental results show that, constellation monitoring provides a more intuitive and accurate approach to monitor the converted high-order QAM signals, especially in presence of nonlinear phase noise such as self-phase modulation (SPM). We experimentally demonstrate an AOWC of 64QAM signal through fourwave mixing (FWM) in highly-nonlinear (HNLF). The performance of the AOWC is optimized through the constellation monitoring of the converted signal, achieving a negligible power penalty (&lt; 0.3dB at BER of 10-3) for 60-Gbps 64QAM after conversion. (C) 2013 Optical Society of America

We demonstrate 12-Gbaud QPSK signal transmission in a fiber-wireless link and compare a modulator-based optical two-tone generator and an optical heterodyning with free-running lasers. The obtained frequency fluctuation of the two-tone system meets the requirements of the radio regulations.

We demonstrate time-interleaved carrier-suppressed return-to-zero (TI-CSRZ) signaling, aiming for high-spectral-efficiency dual-carrier transmission. Optical filter-less modulation and demodulation schemes are proposed, achieving 40-Gb/s TI-CSRZ-QPSK.

We propose an extinction-ratio and chirp tunable modulator in single-drive operation. Using the modulator, we demonstrated precise amplitude modulation without parasitic phase modulation. Extinction-ratio was larger than 52.2 dB, with low chirp (&lt;vertical bar 0.007 vertical bar) operation.

We propose and experimentally demonstrate optical data exchange scheme between 16QAM and QPSK with narrow spacing of 50GHz. The use of coherent pump makes it free of phase noise from pumps, and enables the use of low-cost DFB as pump source.

We discuss digital signal processing (DSP) techniques for self-homodyne detection (SHD), multi-core fiber (MCF) transmission links, and related technologies. We focus on exploiting the reduced phase noise of self-homodyne multi-core fiber (SH-MCF) systems to enable DSP resource savings and describe digital receiver architectures that mixes signal and local oscillator in the digital domain.

The two-mode lasing is obtained at the telecom-band using an InAs quantum dot gain chip. The wavelengths of the two modes were 1539.73 and 1540.44 nm corresponded to the frequency separation of 89.8 GHz which generated millimeter wave signal.

This paper describes high-speed optical modulation and detection devices for radio-over-fibre ( RoF) systems which can distribute millimetre-wave signals over optical fibres with low-loss, and RoF-based high-performance imaging systems which can be used for foreign object debris in runways or railways. High-speed and high-sensitivity photo-detection would be very important to achieve high-resolution imaging. We will share our recent research results on precise lightwave modulation techniques and on high-speed photodetectors using quantum dots.

本稿では、高周波信号により駆動された干渉型光スイッチのクロストークを評価する手法について述べる。光スイッチのドロップポートの出力に含まれるキャリア成分を利用するもので、時間領域測定における光検出器の利得帯域幅積の制約を回避することが可能である。この手法を用いて、干渉型光スイッチの各アームにマッハツェンダー構造を入れ子にした2x2光スイッチのクロストークを実験的に評価した結果も報告する。

本稿では、高周波信号により駆動された干渉型光スイッチのクロストークを評価する手法について述べる。光スイッチのドロップポートの出力に含まれるキャリア成分を利用するもので、時間領域測定における光検出器の利得帯域幅積の制約を回避することが可能である。この手法を用いて、干渉型光スイッチの各アームにマッハツェンダー構造を入れ子にした2x2光スイッチのクロストークを実験的に評価した結果も報告する。

本稿では、高周波信号により駆動された干渉型光スイッチのクロストークを評価する手法について述べる。光スイッチのドロップポートの出力に含まれるキャリア成分を利用するもので、時間領域測定における光検出器の利得帯域幅積の制約を回避することが可能である。この手法を用いて、干渉型光スイッチの各アームにマッハツェンダー構造を入れ子にした2x2光スイッチのクロストークを実験的に評価した結果も報告する。

本稿では、高周波信号により駆動された干渉型光スイッチのクロストークを評価する手法について述べる。光スイッチのドロップポートの出力に含まれるキャリア成分を利用するもので、時間領域測定における光検出器の利得帯域幅積の制約を回避することが可能である。この手法を用いて、干渉型光スイッチの各アームにマッハツェンダー構造を入れ子にした2x2光スイッチのクロストークを実験的に評価した結果も報告する。

本稿では、高周波信号により駆動された干渉型光スイッチのクロストークを評価する手法について述べる。光スイッチのドロップポートの出力に含まれるキャリア成分を利用するもので、時間領域測定における光検出器の利得帯域幅積の制約を回避することが可能である。この手法を用いて、干渉型光スイッチの各アームにマッハツェンダー構造を入れ子にした2x2光スイッチのクロストークを実験的に評価した結果も報告する。

This paper presents multi-tone parallel coherent matched detection that orthogonally detects superchannels without crosstalk between neighboring channels. The receiver consists of multiple sets of multi-tone coherent matched detector employed in parallel. In each detector, the received superchannels signal is homodyne mixed in multi frequency with locally generated multi-tone optical frequency comb; detected is a signal set that has the amplitude and phase exactly matched with the local comb. By launching orthogonal sets of local comb to the multiple parallel coherent matched detectors, the received superchannels are orthogonally downconverted to the baseband frequencies keeping the amplitude and phase information of all channels included. With an aid of n x n transform matrix, all channels are separately recovered from the downconverted signal sets. The system does not rely on any optical filters for channel demultiplexing and separation, with increased flexibility in wavelength arrangement. In addition, the parallel configuration equivalently enhance the bandwidth of the coherent matched detector keeping the speed in each tributary channel as high as possible. In this paper, it is experimentally demonstrated that even-odd interleaved 23 x 20-Gb/s QPSK superchannels are orthogonally demultiplexed and detected by two-tone coherent matched detection. (c) 2013 Optical Society of America

In order to adapt to the dynamics in the future optical networks, we propose a flexible high-order QAM transmitter using a tandem in-phase/quadrature (IQ) modulators to synthesize different high-order quadrature amplitude modulation (QAM) formats, such as 16QAM, 32 or 36QAM and 64QAM. To generate high-order QAMs, an offset-QAM is firstly generated using an IQ modulator driven by electronics with reduced modulation-level, and then mapped to other quadrants through another following IQ modulator configured as a standard quadrature phase-shift keying (QPSK) modulator. All of the embedded sub-Mach-Zehnder modulators are operated in push-pull configurations to avoid introducing excess phase chirp. In contrast with the schemes based on a single IQ modulator driven by multilevel electronics or a highly-integrated parallel modulator, by deploying commercially-available optical modulators and driving electronics with reduced modulation-level, the transmitter complexity in optics and electronics is well-balanced. In the case of generating optical 64QAM, different from another tandem scheme deploying dual-drive IQ modulator driven by independent four binary streams, less phase chirp is observed in our proposed scheme, and comparable implementation penalty is obtained even without applying additional specific compensation algorithm in the coherent receiver. Moreover, thanks to the tandem structure and the deployment of QPSK modulator, the obtained high-order QAM is naturally differentially coded, which is helpful to solve the phase ambiguity at coherent receiver. We experimentally demonstrate the generations of these high-order QAMs including 16QAM, 32/36QAM and 64QAM, and confirm the error-free operations with comparable BER performance to the "electrical" approach based on a single IQ modulator. (c) 2013 Optical Society of America

We propose a coherent optical and radio seamless network concept that allows broadband access without deployment of additional optical fibers within an optical fiber dead zone while enhancing network resilience to disasters. Recently developed radio-over-fiber (RoF) and digital coherent detection technologies can seamlessly convert between optical and radio signals. A millimeter-wave radio with a capacity greater than 10 Gb/s and high-speed digital signal processing is feasible for this purpose. We provide a preliminary demonstration of a high-speed, W-band (75-110 GHz) radio that is seamlessly connected to an optical RoF transmitter using a highly accurate optical modulation technique to stabilize the center frequencies of radio signals. Using a W-band digital receiver with a sensitivity of -37 dBm, we successfully transmitted an 18.6Gb/s quadrature-phase-shift-keying signal through both air and an optical fiber. Copyright © 2013 The Institute of Electronics, Information and Communication Engineers.

Instead of using eight-level electronics or highly-integrated optics, 64QAM generation is experimentally demonstrated using two tandem IQ modulators with four-level electronics. The complexity in electronics and optics is well-balanced. 16QAM and 36QAM are also synthesized.

In this paper, we present a photonic wireless system operating in the W-Band (75-110 GHz) featuring ultra-high bandwidth as well as simple NRZ-OOK modulation format. By using a cascaded optical RF and data modulation approach on the transmitter side, flexible adjustment of the wireless RF carrier in the W-Band is achieved, while direct electrical to optical conversion with advanced photonic components on the receiver side enables true photonic wireless bridging. In first experiments, data rates up to 10 Gb/s have been experimentally demonstrated. © 2013 IEEE.

We evaluated the single side-band phase noise of a 40 GHz beat signal generated by two free-running lasers. This allowed us to verify the utility of the two free-running lasers is verified as a light source for a next-generation radio-over-fiber system using frequency such as those in the millimeter-wave and terahertz bands. We also measured the phase noise of a frequency quadrupler using a Mach-Zehnder modulator for comparison. The phase noise of the two free-running lasers and the frequency quadrupler are - 63.85 and -95.22 dBc/Hz at a 10 kHz offset frequency, respectively. Copyright © 2013 The Institute of Electronics, Information and Communication Engineers.

Transmission of 3GPP LTE signal over a seamlessly integrated radio-over-fiber and millimeter-wave wireless link is theoretically analyzed and experimentally demonstrated. We successfully transmitted and demodulated standard LTE eNB conformance test signals using QPSK, 16-QAM and 64-QAM modulation format for user data over the proposed system. The measured EVM is well under the required limit defined for the test signals, showing the potential to use the link as a high capacity backhauling system for high speed mobile networks using small cells and/or high carrier frequency at millimeter-wave.

We discuss the recent progress of space division multiplexed transmission using a 19-core fiber. We find that high core-number multi-core fibers can both enable large transmission capacity and potentially reduce system costs and power consumption.

Multi-tone coherent-matched detection is demonstrated for multi-channel reception of superchannel/OFDM signals without optical filters for channel selection. Assisted with MIMO equalization, interleaved 23x20Gb/s QPSK are orthogonally demultiplexed.

This paper describes high-speed electro-optic device technologies for radio-on-fiber (RoF) systems. Digital and analog signals can be transmitted over a fiber, in RoF systems. Multi-level modulation plays important roles both in wired and wireless systems. Quadrature amplitude modulation signals can be synthesized by using parallel Mach-Zehnder modulators.

We propose a radio access network for railways employing millimeter-wave Radio-onRadio- over-fiber and distributed antenna system, and evaluate its energy and deployment efficiency. We show a trade-off relation between the metrics and drive optimum system design.

A frequency-reconfigurable radio-over-fiber transmitter is designed and demonstrated with an optical frequency comb source for millimeter-wave and terahertz signal transmission over a fiber-wireless hybrid link. Optical-to-radio and radio-to-optical converters with a high-speed photo mixer and frequency-heterodyne technique successfully perform coherent quadrature-phase-shift-keying signal generation and reception in the optical domain at 90 GHz and 300 GHz. A digital radio receiver also receives and demodulates the transmitted QPSK signal at 300 GHz with the same bit error rate behavior as in an optical link.

Crosstalk is one of the significant measures of an optical cross-bar switch. In this paper, we describe a configuration for crosstalk suppression of a balanced-bridge interferometric type optical switch and its application to an optical-signal routing device. Since crosstalk of the optical switch originates from imbalance in the amplitude of a lightwave propagating within the arm of the interferometer, additional Mach-Zehnder structures were embedded for trimming the amplitude of the light. By adjusting the transmittance of the trimmers, crosstalk of less than -56 dB is achieved for dc voltage. Also, for high-frequency voltages, a crosstalk estimation procedure was developed, and crosstalk at 10-GHz sinusoidal voltage was evaluated to be -48 dB. Furthermore, based on the transient response measurement, its switching time was evaluated to be 26 ps. For a demonstration utilizing extremely low crosstalk and quick response, a guard-time-free optical signal routing experiment is also described. © 2013 IEEE.

For precise chirp parameter control, we investigated x-cut LiNbO3 single-drive asymmetric Mach-Zehnder modulators with extinction-ratio adjustable active Y-branch structures, which can enhance on-off extinction ratio. The chirp parameter of the modulated output was precisely measured by using high extinction ratio operation. Experimental results show that it is possible to fabricate modulators with desired chirp parameters.

Multi-input multi-output (MIMO) transmission of two millimeter-wave radio signals seamlessly converted from polarization-division-multiplexed quadrature-phase-shift-keying optical signals is successfully demonstrated, where a radio access unit basically consisting of only optical-to-electrical converters and a radio receiver performs total signal equalization of both the optical and the radio paths and demodulation with digital signal processing (DSP). Orthogonally polarized optical components that are directly converted to two-channel radio components can be demultiplexed and demodulated with high-speed DSP as in optical digital coherent detection. 20-Gbaud optical and radio seamless MIMO transmission provides a total capacity of 74.4 Gb/s with a forward error correction overhead of 7%. (C) 2012 Optical Society of America

Polarization division multiplexing (PDM) and wavelength division multiplexing (WDM) are essential techniques for enhancing the capacity of photonic networks and facilitating the efficient use of optical frequency resources. 2 PDM x 2 WDM x 10 Gbps error-free simultaneous transmissions in the 1.0-mu m waveband and C-waveband are successfully demonstrated for the first time using an ultra-broadband photonic transport system over a 14.4-km-long holey fiber transmission line. (c) 2012 Optical Society of America

In this article, we propose a concept of an "adaptable access system" as future access system architecture. First, current issues and trends in communications networks are shown from various points of view, to discuss essential items for a future access system. Next, to set the basic direction of research and development for physical network infrastructure, three visions in the design of future access system architectures are shown. Based on the three visions, three technological targets (key concepts) for realizing the adaptable access system, active single star, servicearea-variable minimum cellular, and access system virtualization, are derived. © 2012 IEEE.

We propose a measurement method of photodetector frequency response. This technique is based on a heterodyne principle, where a high-extinction ratio Mach-Zehnder modulator is used to generate a two-tone lightwave as a stimulus signal up to millimeter-wave frequency bands. Our proposing method can provide a relatively short traceability chain to optical and electric power standards.

We fabricated a diode-pumped mode-locked Ti-diffused Er-doped LiNbO3 (Ti:Er:LN) laser with a SiO2/TiO2 dielectric multilayered mirror. The lasing threshold power was 70mW at 1484 nm, and the slope efficiency was 1.48%. Stable optical pulses with a width of 10.86 ps were measured by driving the phase modulator at 13.575 GHz.

Coherent optical two-tone generation using an optical frequency comb generator based on an amplified optical fiber loop is successfully demonstrated. The observed phase noises in the 100-GHz band are less than -80 dBc/Hz at 1 MHz. This method is suitable for high-speed radio communication based on advanced modulation techniques.

Quadrature phase-shift keying (QPSK) is usually generated using an in-phase/quadrature (IQ) modulator in a balanced driving-condition, showing a square-shape constellation in complex plane. This conventional QPSK is referred to as square QPSK (S-QPSK) in this paper. On the other hand, when an IQ modulator is driven in an un-balanced manner with different amplitudes in in-phase (I) and quadrature (Q) branches, a rectangular QPSK (R-QPSK) could be synthesized. The concept of RQPSK is proposed for the first time and applied to optical eight-ary phaseshift keying (8PSK) transmitter. By cascading an S-QPSK and an R-QPSK, an optical 8PSK could be synthesized. The transmitter configuration is based on two cascaded IQ modulators, which also could be used to generate other advanced multi-level formats like quadrature amplitude modulation (QAM) when different driving and bias conditions are applied. Therefore, the proposed transmitter structure has potential to be deployed as a versatile transmitter for synthesis of several different multi-level modulation formats for the future dynamic optical networks. A 30-Gb/s optical 8PSK is experimentally demonstrated using the proposed solution. © 2011 Optical Society of America.

We demonstrate high baud-rate DQPSK modulation with full-ETDM technique using a novel high-speed optical IQ modulator consisting of a ridge-type optical waveguide structure on a thin LiNbO3 substrate. Our fabrication technique achieves a drastic extension of the modulator's bandwidth and a reduction of half-wave voltage. Demonstration of 90-Gbaud NRZ-DP-DQPSK signal generation with the modulator successfully achieved a bit rate of 360-Gb/s under full-ETDM configuration.

We propose and demonstrate a reconfigurable multilevel transmitter using a monolithically-integrated quad Mach-Zehnder inphase/quadrature (QMZ-IQ) modulator with binary driving electronics. Different from previous parallel-integrated quadrature amplitude modulation (QAM) transmitter solutions, only one electrode is required to adjust the relative phase offset among embedded sub-Mach-Zehnder modulators in the proposed IQ superstructure. By feeding different RF driving electronics and operating the integrated modulator as different bias conditions, different advanced multilevel modulation formats, such as QAM and 8-ary phase-shift keying (8-PSK), could be synthesized. In this paper, a 40-Gb/s 16-QAM and a 30-Gb/s 8-PSK are generated using the proposed multilevel transmitter, respectively. Offline digital processing is employed for bit-error rates estimation and constellation reconstruction. (C)2011 Optical Society of America

Successful transmission of live (real time) video traffic is demonstrated using a prototype 100 Gb/s single-polarization differential quadrature phase shift keying (DQPSK) transmitter and receiver over an in-service 504 km link of the LambdaXtreme® optical transport platform. Teaming with Verizon Business, we demonstrate the feasibility of upgrading an existing, live traffic bearing network to 100 Gb/s per wavelength without any changes to the current hardware or software. The 107 Gb/s signal is added at a reconfigurable optical add/drop multiplexer (ROADM) at a network node in Tampa, Florida, and dropped at a ROADM in Miami, Florida, where both live video and pseudorandom test sequences are received. In addition, we discuss the steps leading up to this field trial that included several important precursor laboratory experiments. We detail the generation, detection, coding, and longhaul transmission of single-polarization DQPSK at a line rate of 53.5 Gbaud to support a net information bit rate of 100 Gb/s. © 2010 Alcatel-Lucent.

本研究では,光通信システムにおいて広く用いられているLiNbO_3光変調器の高効率・小型化を図ることを目的とする.その手法として,進行波型光変調器と比ペて小型であり狭帯域において高効率変調が可能である共振型LiNbO_3光変調器を用い,Auミラーによって光波を反射させ再び変調をかけることで高効率・小型化を図るものとする.今回,実際に作製し評価を行った結果,Auミラー反射を用いることによる高効率・小型化を確認した.

本研究では,光通信システムにおいて広く用いられているLiNbO_3光変調器の高効率・小型化を図ることを目的とする.その手法として,進行波型光変調器と比ペて小型であり狭帯域において高効率変調が可能である共振型LiNbO_3光変調器を用い,Auミラーによって光波を反射させ再び変調をかけることで高効率・小型化を図るものとする.今回,実際に作製し評価を行った結果,Auミラー反射を用いることによる高効率・小型化を確認した.

We propose a high extinction-ratio (ER) Mach-Zehnder modulator (MZM) with an active Y-branch optical intensity trimmer. In addition, we investigate a high ER integrated MZM for optical single-sideband (SSB) signal generation, where the active Y-branch trimmers are embedded in a dual-parallel MZM. Optical SSB generation with high suppression ratio of main carrier (47 dB) and undesired sidebands (&gt;42 dB) is achieved by optimizing the optical amplitude imbalances corresponding to the ER.

我々のグループでは,LiNbO_3光変調器の基本構造となるマッハツェンダ型干渉構造の光入射側のY分岐部分に新たな電極を設置することで,光変調器の消光比を改善する報告を行った.今回これらの方法を多値変調技術に応用することを考え,並列型マッハツェンダ構造を有するLiNbO_3光変調器でのバイアス制御方法についてシミュレーションを行い確認した.また新たな電極構造を設置すると,その電極から発生する電界により光の位相変化が生じるため,その位相変化を考慮したバイアス制御方法が必要となる.本研究では実際にその位相変化の影響を実験によって確認し,その構造を並列型LiNbO_3光変調器の構造に導入するためのバイアス制御方法について検討した.

我々のグループでは,LiNbO_3光変調器の基本構造となるマッハツェンダ型干渉構造の光入射側のY分岐部分に新たな電極を設置することで,光変調器の消光比を改善する報告を行った.今回これらの方法を多値変調技術に応用することを考え,並列型マッハツェンダ構造を有するLiNbO_3光変調器でのバイアス制御方法についてシミュレーションを行い確認した.また新たな電極構造を設置すると,その電極から発生する電界により光の位相変化が生じるため,その位相変化を考慮したバイアス制御方法が必要となる.本研究では実際にその位相変化の影響を実験によって確認し,その構造を並列型LiNbO_3光変調器の構造に導入するためのバイアス制御方法について検討した.

Broadband optical switches are required for a wave-length division multiplexed photonic network node. The wavelength dependency of the LiNbO3 optical switch characteristics are mainly attributed to the directional coupler section. An asymmetric X-junction was suitable for a broadband optical switch because it has flat spectral response over wide wavelength range and it is easy to fabricate. We have optimized the structure of an asymmetric X-junction and fabricated the 1×2 LiNbO3 optical switches using it. The optical switch showed broad band characteristics compared to the switch with a directional coupler. © IEICE 2010.

The broadband wavelength tunability of femtosecond pulse generation using a Mach-Zehnder-modulator-based flat-comb generator (MZ-FCG) and a dispersion-flattened dispersion-decreasing fiber (DF-DDF) was demonstrated. Near-Fourier-transform-limit picosecond pulses generated from the MZ-FCG were compressed into femtosecond pulses by adiabatic soliton compression. By tuning the wavelength of the input cw light, 200 fs, 10 GHz pulses were generated in the wavelength range of 1535 to 1570 nm. Such wide-range wavelength tunability was realized by both the independence of a comb-flattening condition from the inputted wavelength and the dispersion flatness of the DF-DDF. (C) 2009 Optical Society of America

We propose and demonstrate an optical minimum-shift-keying (MSK) transmitter using a single-quad Mach-Zehnder (MZ) in-phase and quadrature modulator, where four sub-Mach-Zehnder modulators are monolithically integrated on a main MZ superstructure. Using the proposed transmitter, a MSK signal at bit rate of B could be successfully generated by orthogonally superposing two low-speed (B/2) carrier-suppressed return-to-zero differential phase-shift-keying streams with a relative time offset, 1/B. MSK signals at bit rate of 20 and 80 Gbits/s were successfully generated using the proposed MSK transmitter. A continuous changed phase trajectory of the obtained MSK was confirmed using a complex spectrum analyzer. The filtering tolerance of the MSK was also characterized using a bandwidth-variable tunable filter. (C) 2009 Optical Society of America

We demonstrate tunable all-optical pulse compression and stretching via doublet Brillouin gain lines in an optical fiber. Tunable pulse compression or a stretching ratio of 1.47-0.43 accompanying a time delay is achieved by controlling the separation between two gain lines, for an input pulse train with a 40 ns width and a repetition rate of 5 MHz,ina 4km silica fiber with a fixed pump power of 88.1 mW. The limitation of this pulse compressor or stretcher is also discussed. © 2009 Optical Society of America.

We present recent progress of high-speed Mach-Zehnder modulator technologies for advanced modulation formats. Multi-level quadrature amplitude modulation signal can be synthesized by using parallel Mach-Zehnder modulators. We can generate complicated multi-level optical signals from binary data streams, where binary modulated signals are vectorially summed in optical circuits. Frequency response of each Mach-Zehnder interferometer is also very important to achieve high-speed signals. We can enhance the bandwidth of the response, with thin substrate. 87 Gbaud modulation was demonstrated with a dual-parallel Mach- Zehnder modulator. Copyright © 2009 The Institute of Electronics, Information and Communication Engineers.

Optical quadrature amplitude modulation (QAM) attracts considerable attention in the enhancement of communications capacity, due to its high spectral efficiency. However, this modulation format is more sensitive to signal distortion than conventional formats, making it more difficult to create, transmit, and detect signals. In this paper, we propose an adaptive scheme for demodulation of the distorted optical signals with paying attention to use of DSP-technology. The scheme is based on the adjustment of the thresholds against the signal distortion. Successful demodulation of optical QAM signals is demonstrated with a sufficiently low bit-error rate. (C) 2008 Optical Society of America

Optical quadrature amplitude modulation (QAM) has attracted considerable attention because of its high spectral efficiency. For high capacity, the modulation rate should also be enhanced, which requires the generation of high-baud-rate multilevel electric signals with high precision in amplitude. In this paper, we describe a method to reduce the amplitude fluctuation of the electric signal by adopting a return-to-zero (RZ) format. We also demonstrate 2.0-Gbaud optical QAM signal generation with a conventional dual-parallel Mach-Zehnder modulator (DPMZM) driven by the electric RZ data signals.

By combining a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) with a dispersion-flattened dispersion-decreasing fiber, femtosecond pulses have been generated from a cw light. Near-Fourier-transform-limit picosecond pulses generated from the MZ-FCG were compressed into femtosecond order by pulse compression. Our system enables flexible tuning of the repetition rate and pulse width, because those depend on the driving signal of the MZ-FCG. Pulse trains of 200 fs width were continuously and stably generated without mode hopping, with a repetition rate range from 5 to 17 GHz. Our system consists of a modulator and compression fiber; thus, the configuration is simpler and more stable. (C) 2008 Optical Society of America.

This Letter reports on picosecond pulse generation at a repetition of 10 GHz by using a single-stage electrooptic LiNbO3 Mach-Zehnder modulator, stressing the simplicity of its setup. It is analytically and experimentally proved that dual-arm modulation with in-phase sinusoidal signals having slightly different amplitudes generated a highly coherent optical comb with a great spectral flatness and a parabolic phase relationship in its spectrum. The generated comb was Fourier synthesized and shaped into an ultrashort pulse train with an optical bandpass filter and a dispersive fiber. The pulse source was highly stable, exhibiting an ultralow timing jitter of less than 130 fs. © 2008 Optical Society of America.

In optical packet switching (OPS) and optical code division multiple access (OCDMA) systems, label generation and processing are key technologies. Recently, several label processors have been proposed and demonstrated. However, in order to recognize N different labels, N separate devices are required. Here, we propose and experimentally demonstrate a large-scale, multiple optical code (OC)-label generation and processing technology based on multi-port, a fully tunable optical spectrum synthesizer (OSS) and a multi-wavelength electro-optic frequency comb generator. The OSS can generate 80 different OC-labels simultaneously and can perform 80-parallel matched filtering. We also demonstrated its application to OCDMA. © 2007 Optical Society of America.

We theoretically prove that a conventional Mach̶Zehnder modulator can generate an optical frequency comb with excellent spectral flatness. The modulator is asymmetrically dual driven by large amplitude sinusoidal signals with different amplitudes. The driving condition to obtain spectral flatness is analytically derived and optimized, yielding a simple formula. This formula also predicts the conversion efficiency and bandwidth of the generated frequency comb. © 2007 Optical Society of America.

We have first fabricated and characterized Ti-diffused channel waveguides on Er-doped LiNbO3 single crystal. We achieved an optical gain of 2.1 dB/cm at 1531.4 nm signal light (TM mode) by optical pumping at 1480 nm with pump power of 200 mW/port for a 40 mm long Ti-diffused Z-cut 0.5 mol% Er-doped LiNbO3 waveguide. The total gain of a pig tailed device was 3.8 dB.

A lithium mobate (LiNbO3)/silicon (Si) hybrid structure has been developed by the surface-activated bonding of LiNbO3 chips with gold (Au) thin film to Si substrates with patterned Au film. After organic contaminants on the An surfaces were removed using argon radio-frequency plasma, Au-to-Au bonding was carried out in ambient air. Strong bonding at significantly low temperatures below 100 degrees C without generating cracks has been demonstrated.

We fabricated an etched grating filter on a Ti-diffused waveguide in LiNbO3 using inductively coupled plasma etching with C 4F8/Ar as an etching gas, which has an etching rate of 85.1 nm/min. The etched grating filter had a reflectivity of 35% and a bandwidth of 0.02 nm. Maximum reflectivity was obtained when the electric field of an incident beam was perpendicular to the grating. © IEICE 2006.

A nested Mach-Zehnder (MZ) modulator, which consists of two sub MZ interferometers embedded in a main MZ interferometer, is a multifunctional modulator. It can work as an optical single side-band (SSB) modulator, a differential quadrature phase shift keying (DQPSK) modulator and so on. Despite difficulties due to their complex structure, in order to avoid the operation instability of such modulators the bias condition of each MZ interferometer must be controlled automatically according to the modulation formats. In this paper, we proposed a specific bias condition monitor technique to the nested modulator. © IEICE 2006.

We investigated an integrated optical modulator consisting of two Mach-Zehnder interferometers. The modulator can generate optical signals in various types of modulation formats, which have advantages for long-haul transmission, optical labeling, etc. By using a fabricated versatile optical modulator having traveling-wave electrodes designed for high-speed signals, we demonstrated generation of optical 40 Gb/s frequency-shift-keying signals, which can be demodulated by an optical filter. 80 Gb/s optical differential quadrature-shift-keying modulation was also demonstrated, where 40 Gb/s in-phase and quadrature data were, simultaneously, fed to the modulator. (c) 2006 Optical Society of America.

Optical frequency comb generation by using a novel optoelectronic oscillator (OEO) is proposed and demonstrated with the emphasis placed on self-oscillating operation. In the OEO, a wideband LiNbO3 phase modulator is driven with a large-amplitude radio-frequency (RF) feedback signal to generate a deeply phase-modulated light wave
accordingly, an optical frequency comb with a bandwidth greater than the RF signal is generated by self-oscillation. Although it generates multifrequency components, the OEO exhibits characteristics of a single-mode oscillator. Its operation is stable and self-starting. An optical frequency comb with a 120 GHz bandwidth and 9.95 GHz frequency spacing was successfully generated by self-oscillation at a single frequency. © 2006 Optical Society of America.

We demonstrated optical filter characterization using optical frequency sweep with optical single sideband modulation technique, where the frequency shift range was 2GHz-10 GHz. Undesired sidebands generated in the single sideband modulation due to electric phase errors can be compensated by automatic synchronized bias control. Fine pass band structures of a dual-section FBG were precisely measured, where the bandwidths of the pass bands were 440 MHz.

We propose an optical initial phase control technique for external modulation in the format of minimum-shift keying (MSK). In this scheme, the optical phase discontinuities inherent to external frequency-shift-keying (FSK) modulation are compensated by use of an initial phase controller in conjunction with a FSK modulator. We present the principle of the control method and then numerically and experimentally prove that the signal has a modulation spectrum more compact than that of conventional binary phase-shift keying. 10-Gbit/s external modulation in MSK format was experimentally demonstrated. © 2005 Optical Society of America.

We have realized a novel optical modulation format conversion using double-sideband suppressed-carrier modulation. An optical wideband frequency-shift-keying (FSK) signal, generated by an external FSK modulator, can be directly converted into an optical phase-shift-keying (PSK) signal, where the FSK signal having two spectral components was fed to an optical intensity modulator followed by an optical bandpass filter. Optical frequency of the FSK signal was mapped into optical phase of the bandpass filter output whose phase deviation depends on a chirp of the FSK signal. We demonstrated modulation format conversion from FSK to PSK at 10 Gbps, by using a high-speed optical FSK modulator and a dual electrode Mach-Zehnder intensity modulator. © 2005 Optical Society of America.

A novel pulse-carving technique for an externally-modulated wideband frequency-shift-keying (FSK) format is proposed. Synchronous intensity modulation on the FSK signal suppresses undesired transient components generated during the bit transition time. Numerical analysis for the pulse-carving technique employing a Mach-Zehnder modulator driven by a sinusoidal signal indicates that the suppression of the components is typically more than 20 dB.

We demonstrate high-speed optical frequency-shift-keying (FSK) transmission over a 95km single-mode fiber (SMF) at 10Gbit/s using a LiNbO₃ optical FSK modulator and an optical interleaver. The measured eye diagrams, bit error rate (BER) performances and optical spectrum indicate the FSK modulation is similar to zero chirp non-return-to-zero (NRZ) on-off-keying (OOK) transmission. Therefore, in view of dispersion tolerance, the FSK modulation can be treated as well as NRZ OOK.

We fabricated an integrated reciprocating optical modulator consisting of a phase-shifted fiber Bragg grating and an optical high-speed phase modulator. High-order double sideband components were effectively generated by reciprocating modulation process. We also demonstrated photonic millimeter-wave generation by feeding the double-sideband components to a high-speed photodetector. © 2005, The Institute of Electronics, Information and Communication Engineers. All rights reserved.

A novel balance detection scheme for high-speed optical frequency-shift-keying (FSK) transmission wasproposed. Fiber dispersion can be compensated by a balanced photodetector with an optical delay. 10 Gbit/s optical FSK transmission over a 130 km single mode fiber was demonstrated, where the FSK signal was generated byusing a LiNbO3 high-speed optical FSK modulator. © 2005, The Institute of Electronics, Information and Communication Engineers. All rights reserved.

A reciprocating optical modulator (ROM) generates a millimeter-wave signal in good efficiency. Internal bias modulation on a ROM is reported, in this paper. Owing to reciprocating modulation in the ROM, the driving voltage required for amplitude modulation on a millimeter-wave signal is highly decreased. In this paper, a millimeterwave signal in the 52.8-GHz frequency band is amplitude-modulated with a low-voltage (2.0 V) baseband signal, where half-wave voltage of the modulator embedded in the ROM is 6.4 V. © 2005, The Institute of Electronics, Information and Communication Engineers. All rights reserved.

This paper describes optical frequency comb generation using optical single-sideband modulation technique. We can obtain optical frequency shift using the singnal-sideband modulator consisting of four optical phase modulators, where the frequency shift is precisely equal to that of an rf-signal fed to the modulator. A series of optical spectral components can be generated by using an optical fiber loop having an optical single-sideband modulator. © 2004, The Institute of Electronics, Information and Communication Engineers. All rights reserved.

We demonstrate simultaneous optical modulation by frequency shift keying (FSK) and intensity modulation (IM) using a newly developed high-speed FSK modulator, for optical packet systems where payload and label signals are, respectively, in IM and FSK format. The modulator consists of four optical phase modulators, with travelingwave electrodes to obtain high-speed frequency switching and large frequency deviation in FSK modulation. © 2004, The Institute of Electronics, Information and Communication Engineers. All rights reserved.

The near fields of electromagnetic waves scattered from two-dimensional slightly rough surfaces are studied by using the stochastic functional approach. The correlation coefficient between the surface and the intensity of the scattered-wave field is investigated to estimate the fidelity of near-field intensity images. We show that the fidelity depends on both the polarization and the angle of incidence and that high fidelity can be obtained by a TM-polarized incident wave whose incident angle is not close to the critical angle of the total reflection.

We demonstrate a class of optimum detection strategies for extracting the
maximum information from sets of equiprobable real symmetric qubit states of a
single photon. These optimum strategies have been predicted by Sasaki et al.
[Phys. Rev. A{\bf 59}, 3325 (1999)]. The peculiar aspect is that the detections
with at least three outputs suffice for optimum extraction of information
regardless of the number of signal elements. The cases of ternary (or trine),
quinary, and septenary polarization signals are studied where a standard von
Neumann detection (a projection onto a binary orthogonal basis) fails to access
the maximum information. Our experiments demonstrate that it is possible with
present technologies to attain about 96% of the theoretical limit.