Semiconductor lasers at wavelength of 1.3 - 1.6 Km are the predominant light sources for
fiber-optic link. Distributed feedback (DFB) structure for the wavelength control and
strained quantum well structures for high gain and low threshold are the mature
technologies for both WDM and TDM applications. Although direct modulation capability
is the advantage of semiconductor lasers up to 2.5 Gbps systems, strong frequency chirp
characteristics at 10 Gbps or beyond put limitation in transmission stretch through fiber
dispersion. Integration of a cw operation DFB laser with an electro-absorptive modulator
is one solution to reduce the chirp constraints. For WDM application the precise control of
wavelength is necessary. Although a variety of tunable DFB lasers haye been studied, a
more practical procedure is simply to select lasers of appropriate wavelengths from a
number of laser chips. Recently a trimming technique for fine tuning the wavelength was
proposed.