Linewidth calculations in tunable semiconductor external-cavity lasers for coherent optical systems

Semiconductor lasers with an external cavity (SEC) employing strong frequency-selective feedback with a diffraction grating are being investigated extensively for use with DPSK and PSK coherent fiber-optic communication systems (COFOCSs) because of many advantages: (1) extremely narrow linewidths (0.5–40 kHz) from a compact cavity (5-10 cm); (2) fine tuning of a few gigahertz without servo control of cavity length; (3) coarse tuning over an extremely wide range of ~ 140 nm; (4) fair insensitivity to optical reflections; (5) precise temperature and current control not needed.