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 wavelength tunable DBR laser monolithically integrated with an EA-modulator as a WDM system light source was fabricated by selective area MOVPE growth. The lasing wavelength and band-gap energy were simultaneously controlled on the same epitaxial wafer by using a modulated grown thickness of InGaAsP/InGaAsP MQW layers. A wavelength tuning range of 3.5 nm, an output power of 3 mW, and an extinction ratio of 14 dB for 3 V were achieved. The measured 3 dB frequency bandwidth was 2 GHz. No significant change in modulation characteristics were observed when wavelength tuning by injecting the current into the DBR.

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.

1.5 µm-band LiNbO3 quasiphase matched (QPM) wavelength converters consisting of a periodical domain inverted structure and a proton exchanged waveguide, have been studied in detail both theoretically and experimentally. Optimum device fabrication conditions are investigated with respected to waveguide propagation loss, coupling loss to a single-mode fiber and wavelength conversion efficiency. A normalized conversion efficiency as high as 200 %/W (by a SHG measurement) and a fiber-to-fiber insertion loss of less than 3.5 dB (@1.55 µm) is obtained for a wavelength converter module with a device length of 40 mm. It is shown that a highly uniform periodical domain inverted structure and a uniform proton exchange waveguide are key to obtaining efficient wavelength conversion. The tolerance of the waveguide width fluctuation is found to be very critical and is less than 20 nm for a 40 mm-long device. Future optimization of LiNbO3 QPM wavelength converters and the possible device applications in future optical communication systems are also presented.

A 160 GHz colliding-pulse mode-locked laser diode (CPM-LD) was stabilized by injection of a stable master laser pulse train repeated at a 16th-subharmonic-frequency (9.873 GHz) of the CPM-LD's mode-locking frequency. Synchroscan steak camera measurements revealed a clear pulse train with 16-times repetition frequency of the master laser pulse train for the stabilized CPM-LD output, indicating that CPM-LD output was synchronized to the master laser and that the timing jitter was also reduced. The timing jitter of the stabilized CPM-LD was quantitatively evaluated by an all-optical down converting technique using the nonlinearity of optical fiber. This technique is simple and has a wider bandwidth in comparison to a conventional technique, making it possible to accurately measure the phase noise of ultrafast optical pulse train when its repetition frequency exceeds 100 GHz. The electrical power spectra measurements indicated that the CPM-LD's mode-locking frequency was exactly locked by the injection of the master laser pulse train and that the timing jitter decreased as the injection power increased. The timing jitter was reduced from 2.2 ps in free running operation to 0.26 ps at an injection power of 57 mW, comparable to that of the master laser (0.21 ps).