160 Gbit/s optical-time-division-multiplexing (OTDM) transmitter/receiver employing electroabsorption (EA) modulators are described. In the 160 Gbit/s OTDM transmitter, the optical multiplexer, which implemented four EA modulators, is used and the generation of authentic 160 Gbit/s OTDM signal is realized. The optical multiplexer also enables to generate the phase-coded OTDM signal such as carrier-suppressed return-to-zero (CS-RZ) signal at 160 Gbit/s by changing driving temperatures of the EA modulators. In the 160 Gbit/s receiver, the EA modulator is also used in an optical demultiplexer and a phase-locked-loop (PLL) for clock extraction. As both optical demultiplexer and PLL are insensitive to polarization state of incoming signal, highly stable operation is achieved. We also show some results of transmission experiment using the developed OTDM transmitter/receiver and discuss the advantage of a switching capability of modulation format in the 160 Gbit/s signal transmission.

The initial phase alternation of RZ pulses having duty cycle beyond 50% in dispersion-managed-link is found to help stabilize DM solitons transmissions. The stable soliton propagation of such wide RZ pulses should ease the difficulties designing soliton-based DWDM systems due to less spectral occupancy/channel. For the proof of concept, 40 Gbit/s WDM transmissions are numerically investigated and the initial phase alternation improved the transmission distance by the factor of 2 in the soliton-soliton interaction limited regime. The advantage of this concept has also been verified by conducting 40 Gbit/s single and 8 channels WDM transmission experiments using OTDM techniques with initial phase alternation.

40 Gbit/s optical transceiver using a novel OTDM MUX module has been developed. OTDM (Optical-Time-Division-Multiplexing) MUX module, the core component of the transmitter, consisted of a optical splitter, two electro-absorption (EA) modulators and a combiner in a sealed small package. As the split optical paths run through the "air" in the module, greatly stable optical phase relation between bit-interleaved pulses could be maintained. With the OTDM MUX module, the selection between conventional Return-to-Zero (conventional-RZ) format and carrier-suppressed RZ (CS-RZ) format is performed by slightly changing the wavelength of laser-diode. In a receiver, 40 Gbit/s optical data train is optically demultiplexed to 10 Gbit/s optical train, before detected by the O/E receiver for 10 Gbit/s RZ format. Back-to-back MUX-DEMUX evaluations of the transceiver exhibited good sensitivities of under -30 dBm measured at 40 Gbit/s optical input to achieve the bit-error-rate (BER) of 10-9. Another unique feature of the transceiver system was a spectrum switch capability. The stable RZ and CS-RZ multiplexing operation was confirmed in the experiment. Once we adjust the 40 Gbit/s optical signal to CS-RZ format, the optical spectrum would maintain its CS spectrum shape for a long time to the benefit of the stable long transmission characteristics. In the recirculating loop experiment employing the OTDM MUX transceiver, the larger power margin was successfully observed with CS-RZ format than with conventional-RZ format, indicating that proper encoding of conventional-RZ and CS-RZ was realized with this prototype transceiver. In the case of CS-RZ format, the error free (BER < 10-9) transmission over 720 km was achieved with the long repeater amplifier span of 120 km.

The reduction of Soliton-soliton interaction to stabilize the soliton pulse propagation in the periodic dispersion-compensated standard fiber system using optical bandpass filter has been investigated by numerical simulation, and experimentally 10 Gbit/s soliton transmission was realized without fine tuning dispersion management over 5700 km, using appropriate optical bandpass filters and polarization scrambler.

Polarization insensitive discrete electroabsorption modulators have been designed as an optical gating device. It reveals the first finding, to our knowledge, that the ratio of the optical confinement factor (Γ) to the differential of the values (ΔΓ) between TE and TM polarized lights decides polarization dependence of attenuation. The ratio ΔΓ/Γ is significantly reduced by increasing core thickness. Large optical confinement structures combining a thick InGaAsP bulk absorption layer and polyimide-buried mesa-ridge waveguide have fabricated. The ratio ΔΓ/Γ of the high-mesa structure was estimated to be less than 0.05 in the gain-region of an erbium-doped fiber amplifier (EDFA), which enable us extremely low polarization sensitivity less than 1 dB up to 20 dB extinction. Proper waveguide length of the structure allowed low insertion loss ( 9.3 dB), small loss-change ( 1.8 dB) and sufficient modulation depth ( 30 dB) simultaneously in the EDFA's gain region. The low-mesa structure provided low insertion loss around 7 dB with small deviation in the wavelength region. High modulation band-width and a polarization-insensitive optical gating waveform have also demonstrated.

A nonlinear optical fiber loop mirror (NOLM) adapted for all-optical 2R operation at ultrahigh bit-rates was experimentally and theoretically investigated. The proposed NOLM was created by adding inline/external fiber polarizers and also an inline optical phase-bias compensator (OPBC) to a standard NOLM. A theoretical investigation revealed that the operation of the standard NOLM became unstable due to residual polarization crosstalk of the polarization-maintaining optical components making up the NOLM, and that it could be dramatically improved with the inline/external polarizers. The NOLM with the polarizers ensured stable switching operation with high switching-dynamic-range (>30 dB) against the change of the wavelength of the input clock pulses, and the change of the environment temperature. We also experimentally verified that the OPBC played a dramatic role to ensure excellent dynamic switching performance of the NOLM, and to achieve signal-Q-recovery of the regenerated signals. All optical 2R experiments at 40 Gb/s and 160 Gb/s were performed with the modified NOLM. Signal regeneration with improved extinction ratio and signal Q value was successfully demonstrated. Q-recovery to the input of the control pulses degraded with ASE noise accumulation was also successfully achieved.