We achieved first dynamic all-optical signal processing with a bandgap-engineered MZI SOA all-optical switch. The wide-gap Selective Area Growth (SAG) technique was used to provide multi-bandgap materials with a single step epitaxy. The maximum photoluminescence (PL) peak shift obtained between the active region and the passive region was 192 nm. The static current switching with the fabricated switch indicated a large carrier induced refractive index change; up to 14 π phase shift was obtained with 60 mA injection in the SOA. The carrier recovery time of the SOA for obtaining a phase shift of π was estimated to be 250-300 ps. A clear eye pattern was obtained in 2.5 Gbps all-optical wavelength conversion. This is the first all-optical wavelength conversion demonstration with a bandgap-engineered PIC with either selective area growth or quantum-well intermixing techniques.

Waveguide-type optical amplifiers doped with Ytterbium and Erbium ions are theoretically studied. Sensitization of Er-doped amplifiers with Yb ion doping have many advantages such as the possibility of using broader pumping wavelength range and efficient pumping with smaller pumping power. Transient amplification characteristics of optical short pulses are numerically analyzed using FDTD method. The amplification characteristics are compared with the result of the steady state analysis using the rate equations.

We demonstrated the transmission over 80 km at 10 Gb/s by using the amplifier and electroabsorption-modulator integrated laser diode. Tilt-facet antireflection window is implemented, inside of which a monitor photodiode is monolithically integrated for accurate power regulation. To better control the amplifier-input power and to reduce the feedback of the amplified spontaneous emission, an attenuator is incorporated by means of the inner-window. By amplifying the modulated signal and compensating modulator-chirp by gain-saturation in the amplifier, high power optical transmission is achieved from a device with -10 dB attenuation at total laser and amplifier currents of 200 mA.

We present and demonstrate a novel method of generating a π phase-alternated return-to-zero (RZ) signal together with pulse-amplitude equalization in a rational harmonic mode-locked fiber ring laser, by using a dual-drive Mach-Zehnder modulator. By adjusting the voltages applied to both arms of the modulator, amplitude-equalization and π phase shift can be achieved successfully at a 9.95 GHz repetition rate. The generated alternate-phase RZ signals show enhanced transmission performance in the single-mode fiber (SMF) links without dispersion compensation.

This paper describes improvement of the gain in the Er-doped lithium niobate waveguide optical amplifiers. A new configuration is proposed, which is loaded with a high-index cladding for the purpose of realizing a larger overlap between the guided light fields and doped Er ion-concentration. It is shown by theoretical simulations and also by experiments that the clad is advantageous to shifts of the light fields toward the substrate surface, and that the overlap between the light field and Er ion-concentration becomes large. Improved optical gains are measured for a fabricated device with a thin-TiO2 clad in the LiNbO3 substrate.

For wavelength conversion based on cross-gain modulation (XGM) and cross-phase modulation (XPM) in semiconductor optical amplifiers (SOAs), a CW assist light is quite effective for acceleration of carrier recovery and reduction of pattern effects. We theoretically study assist light conditions both for XGM- and XPM-based wavelength conversion by numerically simulating eye-diagrams. Taking into account the spatial and temporal variations of carrier density along the SOA length, we successfully clarify the dependences of wavelength, power, and propagation direction of the assist light, and reveal the principal difference of response characteristics between XGM and XPM depending on carrier modulation.

Conventional WDM systems multiplex channels with different signal bandwidths using fixed and equal channel spacing. As a result, their spectral efficiency is rather poor. If the wavelength and the bandwidth of each channel in a WDM system could be freely changed as needed, a variety of services with different signal bandwidths could be accommodated efficiently. This is expected to yield high spectral efficiency. For this purpose, this paper proposes a WDM optically amplified system that combines optical power splitting with homodyne detection; its use in three configurations, point to point, ring (center to remote nodes), and peer to peer, is described. Coherent optical systems generally need a frequency stable local light source in addition to a sending light source in each WDM channel. We improve cost effectiveness by proposing that the output of one light source be divided to yield the local light for frequency selection by homodyne detection and the sending light source whose output is externally modulated by transmission signal. In this configuration, the local light level is low to permit high levels of sending power. The key problem is how to get high SNR with limited low-level local lights. This paper derives the optimum receiving loss condition that can maximize the SNR with local light levels as low as -20 dBm for the point to point configuration. For the ring configuration, the system overcomes the optical power loss created by splitting numbers over 1,000 even if the local lights are as low as 0 dBm. The ring configuration can, therefore, flexibly accommodate many users and services. We also elucidate the relation between SNR and BER for DPSK homodyne detection in a bandwidth-flexible system.

This paper presents 160-Gbit/s full channel time-division demultiplexing using a semiconductor optical amplifier hybrid integrated demultiplexer on a planer lightwave circuit. Error-free demultiplexing from a 160-Gbit/s signal to 8 channel 20 Gbit/s signals is successfully demonstrated. Results of a 160-Gbit/s optical time-division-multiplexed full channel OTDM signal transmission experiment using the circuit and successful 80-km transmission are presented.

Ultrafast all-optical switching was experimentally demonstrated using four-wave mixing in an SOA. Two pump pulses with different wavelengths and timings were used for 12 switching. The cross-correlation measurements of FWM signals using a short reference pulse show the high-speed switching capability for wavelength routing in OTDM networks.

We have proposed and theoretically verified a 2R (reshaping and regeneration) limiter circuit using continuous wave (CW) holding beam for cross-gain modulation (XGM) wavelength converter, through simulation. The gain clamping effect of semiconductor optical amplifier (SOA), which is caused by CW holding beam injected into SOA, was used to obtain the accurate optical gain and phase conditions for SOA's in 2R limiter circuit. XGM wavelength converter with the proposed 2R limiter circuit provides higher extinction ratio (ER) as well as more enhanced operation speed than any other wavelength converter. Our numerical results show that after the wavelength-converted signal from XGM wavelength converter passed the 2R limiter circuit, it was re-inverted with the improved ER of 30 dB at 5 Gb/s. In case of high-speed operation, great enhancement to decrease power penalty of about 12 dB was shown at 10 Gb/s.

Symmetric Mach-Zehnder (SMZ) type all-optical swit-ches are discussed. The SMZ type all-optical switches feature the so-called differential phase modulation scheme to achieve a speed unrestricted by efficient, thus usually slow nonlinearities. In these switches, semiconductor optical amplifiers (SOAs) are often used to realize low optical power switching. We discussed SOAs from a view point of all-optical switch applications, rather than amplifier applications. Finally, all-optical signal processing experiments are discussed with the SMZ type all-optical switches. These include ultrafast demultiplexing of 336 Gb/s signal pulses and random operations at 42 Gb/s for all-optical logic operation and wavelength conversion.

Simultaneous wavelength conversion utilizing four-wave mixing in optically-pumped GaN/AlN intersubband optical amplifiers has been investigated by means of a finite-difference time-domain (FDTD) model. The conversion efficiencies at a pump power of +7-+10 dBm were predicted to be -9-+6 dB depending on the frequency detuning (0.3-10.9 THz). The difference in efficiency among 18 channels of WDM signals with 100-GHz spacing was within about 3 dB.

All optical regenerations or wavelength conversions using SOA-based polarization discriminated switch injected by an assist light were investigated. First of all, cross gain modulation (XGM) and cross phase modulation (XPM) in a SOA injected by an external assist light were quantitatively analyzed. A simple measurement technique of XGM and XPM was shown to confirm that the injection of assist light could reduce a gain recovery time with some sacrifice for XGM and XPM efficiency. All-optical 3R regeneration using two-stage SOA-based polarization discriminated switch at 40 Gbit/s and its tolerances for some degradation against intensity deviation and optical signal-to-noise ratio (OSNR) were also shown. Finally, regeneration capability was evaluated through a dispersion shifted fiber (DSF)-based re-circulating loop transmission experiment. Those results indicate that the SOA-based polarization discriminated switch is a promising candidate for all-optical regenerator from the practical point of view.

We describe the characteristics of all-optical switching schemes based on semiconductor optical amplifiers (SOAs), with particular emphasis on the role of the fast carrier dynamics. The SOA response to a single short pulse as well as to a data-modulated pulse train is investigated and the properties of schemes relying on cross-gain as well as cross-phase modulation are discussed. The possible benefits of using SOAs with quantum dot active regions are theoretically analyzed. The bandfilling characteristics and the presence of fast capture processes may allow to reach bitrates in excess of 100 Gb/s even for simple cross-gain modulation schemes.

An upgrade scheme using identical sending lights and local lights for optical amplifier systems based on homodyne detection is described. Optimum receiving loss for maximizing the SNR is analytically derived for the proposed configuration where the local taped lights are low level to avoid a sending power decrease. Derived formulas describing the optimum loss condition and the maximum SNR show that 10 Gbit/s systems can be cost effectively upgraded to 40 Gbit/s systems with the same repeater spacing.

A new technique for optical generation of high-purity millimeter-wave (mm-wave) signals--namely, by synthesizing the outputs from cascadingly phase-locked multiple semiconductor lasers--was developed. Firstly, a high-spectral-purity mm-wave signal was optically generated by heterodyning the outputs from two phase-locked external-cavity semiconductor lasers. The beat signal was detected by a p-i-n photodiode whose output was directly coupled to a coax-waveguide converter followed by a W-band harmonic mixer. By constructing an optical phase-locked loop (OPLL), a high-spectral-purity mm-wave signal with an electrical power of 2.3 µW was successfully generated at 110 GHz with an rms phase fluctuation of 57 mrad. Secondly, the frequency of the mm-wave signal was extended by use of three cascadingly phase-locked semiconductor lasers. This technique uses a semiconductor optical amplifier (SOA) to generate four-wave-mixing (FWM) signals as well as to amplify the input signals. When the three lasers were appropriately tuned, two pairs of FWM signals were nearly degenerated. By phase-locking the offset frequency in one of the nearly degenerated pairs, the frequency separations among the three lasers were kept at a ratio of 1:2. Thus, we successfully generated high-purity millimeter-wave optical-beat signals at frequencies at 330.566 GHz with an rms phase fluctuation of 0.38 rad. A detailed analysis of the phase fluctuations was carried out on the basis of measured power spectral densities. The possibility of extending the mm-wave frequency up to 1 THz by using four cascadingly phase-locked lasers was also discussed.

Wavelength conversion using the cross-gain modulation (XGM) of amplified spontaneous emission (ASE) in a traveling-wave type semiconductor optical amplifier (TW-SOA) is theoretically studied. Taking into account the spatial and temporal variations of carrier density along the SOA length, output signal and converted ASE waveforms are analyzed. We also reveal the dependency of the signal and converted ASE waveforms on input signal power and repetition frequency, and confirm that numerical analyses well agree with the experimental results. Finally we qualitatively clarify the way to improve frequency response by simulating eye-diagrams for long SOAs and assist light pumping for the first time.

We propose a novel method of precisely measuring the polarization dependence of single pass gain (PDG) in a semiconductor optical amplifier integrated with spot-size convertors (SS-SOA). By averaging the signal gain of a SS-SOA over a wide wavelength range using the amplified spontaneous emission (ASE) of an erbium doped fiber (EDF), the PDG can be accurately estimated. This is because the influence of gain ripples on the measurement results are drastically reduced. We successfully evaluated the PDG of an angled-facet SS-SOA, even before the process of anti-reflection coating, within a small error of 0.5dB. The EDF-ASE technique is useful in sampling tests and selecting angled-facet SS-SOA chips from wafers. The polarization dependence of the coupling efficiency (PDCE) between a SS-SOA and optical fiber is also evaluated by measuring the photo-current of the active layer for TE and TM input signals. It is possible, therefore, to specify the polarization characteristics of the active region and spot-size converter region, which are indispensable parameters for the design of the SS-SOA.

An 80 Gbit/s conventional and carrier-suppressed return-to-zero optical time-division multiplexing signal transmission over a 208 km standard single-mode fiber was experimentally demonstrated. This was achieved by using mid-span optical phase conjugation based on four-wave mixing in semiconductor optical amplifiers. In addition, it was confirmed that the transmitted carrier-suppressed return-to-zero optical signal's carrier phase-relation was held.

Various types of wavelength-selectable light sources (WSLs) and wavelength-tunable laser diodes (LDs) have been developed, and the one based on an array of distributed feedback (DFB) laser diodes (LDs) has the advantage of tuning that is both simple and stable tuning. It requires only the selection of a DFB-LD and a temperature control. We report on monolithically integrated WSLs with a DFB-LD array, multimode interference (MMI) coupler, semiconductor optical amplifier (SOA), and electro-absorption (EA) modulator. To make them compact, we introduced microarray structures and to ensure that they were easy to fabricate, we used selective area growth. For the WSL with an integrated EA modulator, we developed a center-temperature-shift method that optimizes the detuning wavelength between the lasing wavelength and the absorption edge wavelength of the EA-modulator. Using this method, we obtained a uniform extinction ratio and were able to demonstrate error-free 2.5-Gb/s transmission over a 600-km fiber span. A CW-WSL without an EA-modulator should provide enough output power to compensate the loss caused by the external modulators, but the high-power operation of a CW-WSL is sensitive to optical feedback from the front facet. We therefore used an angled facet in our WSLs and eliminated a mode hop problem. More than 20 mW of fiber-coupled power was obtained over 23 ITU-T channels on a 50-GHz grid.