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.

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.