We studied 2 types of polarization insensitive semiconductor optical amplifier (SOA) gates for use in wavelength division multiplexing (WDM) applications: 1) a low operation current SOA gate with a small and square bulk active region but without spot-size converters and 2) a multi channel SOA gate array with tapered waveguide spot-size converters (SS-SOA) on both sides. The low operation current SOA gate provided a very low current for fiber-to-fiber loss-less operation (5. 4-7. 0 mA) and a high extinction ratio (>30 dB) over a wide wavelength range (1530-1580 nm). For multi channel array assembling, the SS is indispensable. The 4-channel SS-SOA gate array was assembled on a planar lightwave circuit (PLC) platform for the first time. The gain characteristics of each channel were very similar and a low fiber-to-fiber loss-less current of 33 mA and a high extinction ratio of nearly 40 dB were achieved in all channels. The polarization dependence was less than 1 dB. Using the fully packaged 4-channel hybrid gate array module (a 4 channel SS-SOA on PLC platform), an ultra-wide-band (1530-1600 nm) high speed wavelength selector was successfully demonstrated. Both rise- and fall-times were less than 1 ns, which makes the wavelength selector suitable for high-speed optical packet switching. Electrical and optical interference between channels were negligible.

This paper describes the design, fabrication, and performance of a novel Ti:LiNbO3 optical modulator with a two-stage coplanar waveguide electrode for 40 Gbit/s optical transmission systems. The structure consists of a thin lower electrode and a thick upper electrode in conjunction with a ridge structure. The lower electrode ensures low voltage and the upper layer provides good microwave characteristics. Based on simulation results, a fully-packaged module was fabricated. The measured 3-dB electrical bandwidth is 30 GHz with a half-wave voltage of 2. 9 V.

We examined a Ti-diffused optical waveguide formed on a thin X-cut LiNbO3 substrate for a lower-drive-voltage modulator. Under the single-mode condition, optical mode-size decreases with LiNbO3 substrate thickness below 10 µm. A thin-sheet LiNbO3 modulator could achieve a low-drive-voltage of 1.3 V with a bandwidth of 15 GHz by adopting a narrow electrode-gap.

Simultaneous wavelength conversion of multi-WDM channels is expected to be a key technique in near-future networks. In this paper, 4-channel wavelength conversion using four-wave mixing (FWM) in a hybrid wavelength selector is successfully demonstrated. The wavelength selector consists of two four-channel spot-size-converter-integrated semiconductor optical amplifier (SS-SOA) gate arrays on a planar-lightwave-circuit (PLC) platform and two PLC-arrayed-waveguide-gratings (AWGs). As the wavelength selector has an individual SS-SOA for the wavelength conversion of each channel, there is negligible interference between channels. Four WDM channels with an 2.5 Gb/s modulation were converted from 1555 to 1575 nm. Clear eye openings and only a small power penalty of less than 0.5 dB were observed. The receiver sensitivity was -31 dBm at a bit error rate (BER) of 10-9.

We describe the design, fabrication, and characteristics of a push-pull type ridged Ti:LiNbO3 optical modulator with two electrodes. The structure keeps microwave propagation loss low and enables a large interaction between microwaves and optical waves under the condition of velocity and impedance matching, resulting in a large modulation bandwidth and low driving voltage. Using this structure, we have developed an optical intensity modulator with an optical 3-dB bandwidth of 45 GHz (an electrical 3-dB bandwidth of 30 GHz) and a half-wave voltage of 3.9 V in single-electdoe operation (a half-wave voltage of 1.95 V in push-pull operation)at a wavelength of 1.55µm.

An oblique-polishing tilted-coupling method for LD's and waveguides is proposed for suppressing feedback light from the endfaces. It is confirmed that the instability of coherence in DFB-LD is reduced with little excess loss.

We studied three types of lasers emitting narrow beam divergence of output light: 1) a spot-size converter integrated laser diodes (SS-LDs) with a vertically tapered waveguide, 2) one with a laterally tapered waveguide, and 3) one consisting of a small cross section of active region. We compared them with regard to their performance in coupling efficiency to a cleaved single mode fiber, threshold current, output power, and reliability. Both the spot-size converted integrated lasers with vertically and laterally tapered waveguide repeatedly provided low threshold currents of as low as 6 mA and low coupling loss to the fiber of 1.2 to 2.5 dB in two inch wafer processes. As a result of the aging test, the SS-lasers were predicted to have the same degradation rate as a conventional buried heterostructure laser. The laser having a small cross section of active layer also has low coupling loss and high efficiency up to 85.