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We demonstrate a general technique for realizing fine control of optical pulse width via doublet gain or loss lines. The optimized doublet gain or loss lines for fine control of optical pulse width can be designed by using an inverse design method. The preliminary experimental demonstration of fine control of optical pulse width is performed by using doublet Brillouin gain lines in an optical fiber. Tunable pulse compression or 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 pulse width and a repetition rate of 5 MHz, in a 4 km silica fiber with a fixed pump power of 88.1 mW.

This paper presents an electrooptic vector digital-to-analog converter (EO-DAC) for coherent synthesis of optical multilevel signals, such as quadrature-amplitude modulation (QAM) signals. For generation of an optical multilevel/QAM signal, the inphase (I) and quadrature (Q) components of a carrier lightwave should be orthogonally modulated in multi-levels. Conventionally, inphase-quadrature (IQ) orthogonal modulators have been utilized for this purpose. In the scheme, however, the symbol rate available for multilevel/QAM signals is limited, and it is difficult to obtain a clear constellation because of the difficulty involved in handling with electrical multilevel signals. The EO-DAC employing multiparallel modulator, described in this paper, can be used to synthesize multilevel signals through the combination of binary data streams. Using this proposed technology, multilevel signals can be coherently synthesized at a high bit rate, and clearer constellations can be obtained. Further, because of its flexibility, and versatility, the EO-DAC can be used for the several types of multilevel modulation formats. In this paper, we describe the EO synthesis of optical multilevel signals especially focusing on the method based on the EO-DAC, along with comparisons with other conventional methods.

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.

In the present study, the remote field eddy current testing (RFECT) was used to detect back-side local corrosion in the bottom plates of an oil storage tank constructed of a ferromagnetic material. A phenomenon similar to a remote field for a tube-type test piece was confirmed in this measurement system with/without the U-type component. The appropriate distance between the excitation coil and the pick-up coil and the excitation frequency were experimentally examined to detect the back-side flaws of ferromagnetic plates. In addition, using the proposed method, back-side flaws of ferromagnetic plates were evaluated with the phase change of the measured signals of the pick-up coil.

We demonstrate record 25.6-Tb/s transmission over 240 km using 160 WDM channels on a 50-GHz grid in the C+L bands. Each channel contains two polarization-multiplexed 85.4-Gb/s RZ-DQPSK signals, yielding a spectral efficiency of 3.2b/s/Hz in each band.

In the recent progress in terahertz (THz) devices, various kinds of source devices, such as resonant tunneling diodes, quantum cascade lasers and so forth, have been developed. Frequency measurement of THz radiations, which can operate in high speed and at room-temperature, is important for development of high-performance THz source devices. Recently, frequency measurement using optical combs are demonstrated by several groups. In these techniques, modelocked lasers (MLLs) are used for optical comb source, so that phase-locking techniques are required in order to stabilize the repetition frequency of the MLLs. On the other hand, a modulator-based optical comb generator has high accuracy and stability in the comb spacing, which is comparable to that of microwave signal driving the modulator. Thus it is suitable for frequency measurement of THz waves. In this paper, we demonstrated frequency measurement of THz waves using a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG). The frequency measurement was carried out by an electro-optic (EO) sampling method, where an optical two-tone signal extracted from the optical comb generated by the MZ-FCG was used for the probe light. A 100 GHz signal generated by a W-band frequency multiplier and the probe beam collinearly traveled through an EO crystal, and beat signals between them were measured by a combination of a balanced photodetector and a spectrum analyzer. As a result, frequency measurement of the 100 GHz wave was successfully demonstrated, in which the linewidth of the beat signal was less than 1 Hz.

An important challenge for implementing optical signal processing functions such as wavelength conversion or wavelength data exchange (WDE) is to avoid the introduction of linear and nonlinear phase noise in the subsystem. This is particularly important for phase noise sensitive, high-order quadrature-amplitude modulation (QAM) signals. In this paper, we propose and experimentally demonstrate an optical data exchange scheme through cascaded 2nd-order nonlinearities in periodically-poled lithium niobate (PPLN) waveguides using coherent pumping. The proposed coherent pumping scheme enables noise from the coherent pumps to be cancelled out in the swapped data after WDE, even with broad linewidth distributed feedback (DFB) pump lasers. Hence, this scheme allows phase noise tolerant processing functions, enabling the low-cost implementation of WDE for high-order QAM signals. We experimentally demonstrate WDEs between 10-Gbaud 4QAM (4QAM) signal and 12.5-Gbaud 4QAM (16QAM) signal with 3.5-MHz linewidth DFB pump lasers and 50-GHz channel spacing. Error-free operation is observed for the swapped QAM signals with coherent DFB pumping whilst use of free-running DFB pumps leads to visible error floors and unrecoverable phase errors. The phase noise cancellation in the coherent pump scheme is further confirmed by study of the recovered carrier phase of the converted signals. In addition to pump phase noise, the influence of crosstalk caused by the finite extinction ratio in WDE is also experimentally investigated for the swapped QAM signals.

One of polarization diversity techniques in offset-frequency-spaced two-tone optical coherent transmission of radio-over-fiber signal is investigated. As the polarization diversity, successful combination of quadrature-phase-shift-keying constellations recovered from two orthogonal polarization components after a 20-km fiber-optic transmission is experimentally demonstrated.

We propose a frequency-stabilized multi-frequency terahertz (THz) source system, which is composed of THz sources frequency-locked to a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG). We report on phase-locking of a THz-QCL by using the MZ-FCG.

We propose dual coherent sampling technique, aiming for monitoring coherent multilevel signals by using slow-speed digital signal-processing. 10-Gbaud QPSK signals are experimentally captured at 500 MSa/s, statistically tracking laser phase drift.

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.

We demonstrate optical comb generation by cascaded Mach-Zehnder modulator based flat comb generators. Two modulators were non-uniformly operated for satisfying with both “higher-bandwidth” and “finer frequency-spacing” optical comb generation.

We experimentally investigate the phase-sensitive amplification characteristics in quantum-dot semiconductor optical amplifier. Phase-sensitive dynamic ranges (PSDRs) of ∼3dB and ∼11dB are obtained for degenerate and non-degenerate components, respectively, with 5-dBm total launch power.

Research into optical modulators has made remarkable progress in recent years. This paper discusses the possibility of applying the high extinction ratio optical modulator to a high-stability and high-frequency (over 100 GHz) optical reference signal generator. High-frequency reference signals are generated by a highly stable optical two-tone generator, which is used for high-rate communication and astronomical application. One method to generate two optical signals is producing them from a pair of laser sources using an optical phase-locked loop for feed back control; however, the optical phase-locked loop has a stability problem in its operation. A good alternative method to the optical phase-locked scheme is the LiNbO3 Mach-Zehnder (MZ) optical intensity modulator, which is capable of generating two highly stable optical signals (upper sideband and lower sideband components) by applying a sinusoidal microwave signal to an input laser signal. The two optical signals require phase stability better than 10-13 in the Allan standard deviation, vibration robustness, and polarization maintaining capability. The signal coherence loss estimated from the phase stability of the two optical signals generated by the MZ modulator shows that the optical MZ modulator has the ability to generate highly stable optical signals.

We demonstrate 100Gbit/s DQPSK transmission experiment over 50km SMF. Without resorting OTDM, 100Gbit/s transmission was enabled with DQPSK format and commercially available electronics. Possibility of DQPSK modulation for future 100G Ethernet transport is verified.

High-speed control of lightwave using electrooptic (EO) effect is investigated in this paper. Agile optical frequency shift can be achieved by optical single-sideband (SSB) and frequency-shift-keying (FSK) modulators, where high-speed optical phase-shift-keying (PSK) signals can also be generated by using FSK/SSB modulators. We also describe ultrahigh extinction ratio optical intensity modulation (IM) technique for two-tone lightwave signals with high spurious suppression, which is useful for photonic microwave and millimeter-wave generation. In addition, we investigated high-order optical sideband generation techniques: quadruple dual-sideband suppressed carrier (QDSB-SC) modulation and reciprocating optical modulation (ROM). Sub-tetrahertz signals can be obtained from lightwaves with high-order sidebands

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.

Orthogonal time-frequency domain multiplexing (OTFDM) enables ultra-wideband high-spectral-efficiency multicarrier transmission without relying on optical filters for multiplexing/demultiplexing of subchannels. Impact of tight filtering on OTFDM signal is investigated, assuming bandwidth narrowing in wavelength-selective switching 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.

Generation of cw-THz waves with high frequency accuracy was demonstrated by using a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG). A combination of the MZ-FCG and a highly nonlinear dispersion shifted fiber generated broadband optical combs, and cw-THz waves in the range of 3 THz were generated by photonic down-conversion of two-tone signals extracted from the broadband combs.

One of polarization diversity techniques in offset-frequency-spaced two-tone optical coherent detection scheme for radio-over-fiber systems is investigated. A combination of constellations recovered from two orthogonal-polarization components is proposed and the diversity effect is experimentally demonstrated.

Local signal generation with high phase stability is one of the key elements of an interferometer system. In the ALMA (Atacama Large Millimeter/submillimeter Array) project, we have plans to generate and transmit a micro/millimeter-wave LO (Local Oscillator) reference signal as an optical two-tone signal. To realize this, we introduce a method to generate an optical two-tone signal by double sideband suppressed carrier modulation with a lithium niobate (LiNbO3) Mach–Zehnder optical modulator. The phase instability of the LO signal causes a loss of coherence in the interferometer. We measured the phase instability and estimated the coherence loss while assuming the highest frequency of ALMA LO. The observed Allan standard deviation for 22GHz was 1.9 ×10 −14 at 1s. Additional noise was found at a longer time scale than 4s; however, the reason is not well understood at this stage. If this additional noise was classified as white frequency noise, the estimated coherence loss was 5.2% when 938GHz frequency and a 100-s integration time were assumed. On the other hand, if this additional noise was caused by some other effect, for example phase fluctuation of the amplifier, the coherence loss was estimated to be 0.33% at 938GHz frequency (100s).

A high-speed external modulation scheme for optical continuous-phase frequency-shift keying (CPFSK) is presented. In external frequency-shift keying (FSK) modulation using single-sideband modulation technology, an optical upper/lower sideband (USB/LSB) component with respect to a carrier frequency is selectively generated. However, the FSK-modulated signal has phase discontinuities at the timings of frequency shifts. To overcome the problem, we propose a synchronous control technique that enables continuous phase modulation. In the external CPFSK modulation, the USB/LSB is allowed to shift to the other state when their phases coincide with each other. It is shown that CPFSK with a zero-to-peak frequency deviation of 0.5times(bit rate) is achieved with this synchronous control. Occupant bandwidth is less than half that of the externally modulated incoherent FSK with phase discontinuities. At a modulation speed of 10 Gb/s, higher order sidelobes are highly suppressed by more than 20 dB at the 20-GHz offset, comparing to a conventional binary phase-shift keying (BPSK). By the use of a Mach-Zehnder interferometer with balanced photodetection, receiver sensitivity is 3-dB greater than that of an on-off keying, as well as a BPSK. In this paper, we report on the experimental demonstration of CPFSK modulation/demodulation. A six-channel wavelength-division-multiplexed, 10-Gb/s CPFSK modulation/demodulation was successfully demonstrated

Two key technologies were reviewed to propose advanced approaches to functional glasses. The first is combinatorial glass research system. The combinatorial glass research system consists of an automatic batch preparation apparatus, two types of automatic apparatuses for melting of glass batches in crucibles and two types of apparatuses for evaluation of thermal stability, namely a DTA apparatus equipped with sample changer and a temperature gradient furnace. The combinatorial system was effective in searching for new glass forming systems and to speed up preparation of glass samples. The second technology is a new technique based on anodization of thin film aluminum. The anodic oxidation of sputtered aluminum film was applied to preparation of alumina nanostructures possessing nanopore arrays on glass surfaces. Oxides or metallic compounds could be introduced into nanopores using sol–gel technique or electro deposition technique. The introduction of TiO 2 by sol–gel process yielded high performance photo catalysts and the impregnation of Ni metal or Fe–Pt alloy into nanopores by electro deposition gave high density magnetic rod array of about 1 Tbit/in. 2 . The new processes based on anodic oxidation of sputtered aluminum and titanium would be very effective to develop functional nanostructure layers on glass surfaces.

An 18-sample glass-melting furnace was newly developed and glass-forming compositional regions were found in lead-free B 2 O 3 -TeO 2 -[xBaF 2 /(100-x)BaO] ternary systems. The softening temperatures were measured in the systems to screen out low-melting compositions. The softening temperature ranges from 450 to 470 °C and the thermal expansion coefficient ranges (115-140)x 10 -7 °C -1 . These values were comparable to those of typical lead-containing glasses. It was also found that substitution of BaF 2 for BaO effectively lowers the melting temperatures without changing the thermal expansion coefficient.

We described a novel optical label swapping (OLS) technique for optical packet systems using frequency-shift-keying (FSK) optical labeling. High-speed optical FSK signal can be generated by using an external FSK modulator consisting of four optical phase modulators. The FSK modulator was based on optical single-sideband (SSB) modulation technique, and comprised of traveling-wave electrodes for high-speed frequency switching. We demonstrate 10 Gbps FSK transmission, and simultaneous modulation by FSK and intensity modulation (IM). OLS using double-sideband modulation was also demonstrated, where this technique can be used for a bundled wavelength-domain-multiplexing (WDM) channels without using an array of pumping light sources.

We have proposed a laser-phase-fluctuation-insensitive optical coherent detection scheme assisted by a digital signal processing (DSP) technique for radio-over-fiber (RoF) systems. In this system, a “two-tone” local light is used for an individual optical coherent detection of both the carrier and the modulated components of RoF signal, where a frequency separation of two-tone local light is the same as that of RoF signal. To distinguish the carrier and the modulated components in the process of optical coherent detection, they have to be separated in the optical domain. In this paper, we proposed a new optical coherent detection scheme with an “offset-frequency-spaced two-tone” local light, which is also in principle insensitive to the laser phase fluctuation and is assisted by the DSP technique. In the proposed scheme, it is not required to separate the carrier and the modulated components in the optical domain because they can be easily separated in the electrical domain after the photodetection. Therefore, the system configuration is expected to be simpler than that in the previous scheme. 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 also 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. As a result, it is shown that the EVM of less than 12.4 %rms is achieved.

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.

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 demonstrate a down-conversion-based twin photon source pumped by a 10 GHz repetition-rate-tunable comb laser at 1553 nm wavelength. We show high Hong-Ou-Mandel interference visibilities, which are free from the pump-power induced degradation.

We propose and investigate a digital processing technique for dispersion compensation in coherent matched detectors, by which ultra-wideband multi-carrier signals beyond electrical bandwidth are orthogonally demultiplexed and demodulated, fully electrically dispersion compensated.

We demonstrate a robust and modulation-format-transparent multicasting wavelength converter based on a Mach-Zehnder optical comb generator. All the wavelength-converted channels show maximum 0.6 dB and 1.1 dB penalties for QPSK and 16QAM signals, respectively.

We demonstrated rapid measurement of spectra of terahertz (THz) radiation from a quantum cascade lasers. THz waves generated by photonic down-conversion using Mach-Zehnder-modulator-based flat comb generator were used for local oscillator signals.

We investigate fiber nonlinearity tolerance of traceback equalization (TBE) for mitigating non-uniform distortion caused by unbalance of modulation electrodes in QAM transmitters. TBE outperforms conventional FIR-based adaptive equalizers in an adequately linear transmission regime.

Recent progresses in optical comb and communications technologies are realizing new synergies such as switching or frequency-standard distribution for coherent transmission. Both applications are quite useful for next generation infrastructure which will sustain enormous traffic.

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 < 1.3dB at BER of 10−9.

We propose and demonstrate pump-phase-noise-free 2-to-7 multichannel wavelength multicasting for QAM signals using flexible coherent multi-carrier pump.

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.

We demonstrate 75-km single-mode fiber transmission of a 40-Gb/s optical 16-level quadrature amplitude modulation (16QAM) signal generated by a quad-parallel Mach-Zehnder optical modulator (QPMZM), where four Mach-Zehnder structures are embedded in parallel monolithically on a lithium niobate substrate. Owing to the optical 16QAM signal generation method based on the coherent superposition in the QPMZM and improved coherent receiver utilizing digital filters, improvement of receiver sensitivity has been achieved as well. The experimentally obtained receiver sensitivity for near the forward-error correction limit and power penalty due to transmission are experimentally evaluated as 16.3 dB and about 2-3 dB, respectively.

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.

With the recent trend of employing spectrally efficient multi-level modulation formats in optical communication systems, it is highly desirable to exploit the all-optical wavelength conversion (AOWC) schemes for formats, such as 16- or 64- quadrature amplitude modulation (QAM) signals. Since the linear and nonlinear phase noise is transferred with the data to the converted wavelength in AOWC subsystem, it is imperative to optimize the operation condition in AOWC to suppress noise to allow compatibility phase-noise sensitive high-order QAM formats. We first investigate the performance optimization of AOWC for high-order QAMs like 64QAM through four-wave mixing in highly nonlinear fiber, and then discuss our proposed AOWCs schemes for high-order QAM signals such as 16/64-QAM using coherent pumps which is free of phase noise from pumps.

We developed evaluation of crosstalk of an optical switch integrated on a lithium niobate substrate, without limitation of gain-bandwidth product. Utilizing optical modulation and frequency-domain analysis, crosstalk is successfully evaluated to be less than −48 dB for 10-GHz rf sinusoidal signal.