Abstract
Semiconductor lasers are used as a coherent light source in a number of applications, including optical communication systems, optical data storage systems (compact discs (CDs), digital versatile discs (DVDs) etc.), optical measurement systems, printing, etc. The simplest type of semiconductor laser (also called “laser diode”) is the Fabry–Perot (FP) type semiconductor laser. A Fabry–Perot type laser diode uses the reflectivity of the facets (semiconductor-air interface) for providing necessary optical feedback to sustain laser oscillation. On the other hand, distributed feedback (DFB) semiconductor lasers use an internal Bragg grating within the laser cavity to provide optical feedback. The grating works as a wavelength-selective device to achieve highly stable narrow-linewidth laser operation. Distributed feedback lasers are particularly useful in wavelength-division multiplexed (WDM) optical communication systems where different DFB lasers transmit optical signals of different wavelengths that are very closely spaced. Because of their present and future technological significance, research activity on DFB lasers has increased manifold during the recent years. A major problem with semiconductor lasers, both FP and DFB types, is that these semiconductor lasers are highly sensitive to the laser light which re-enters the laser cavity after being reflected by an external reflector. External optical feedback of the laser light usually causes instability of operation of a laser diode and generates excessive noise in optical communication systems. A variety of optical elements, including lenses, fiber endfaces, etc. can be the source of unwanted optical feedback. Rayleigh backscatter from fiber can be another source of optical feedback. Packaged laser diodes may also receive optical feedback because of the external cavity formed between the laser diode chip and a transparent window of the package.