Solitary Spectral Linewidth and Its Reduction with External Grating Feedback for a 1.55 µm InGaAsP BH Laser

Spectral linewidth measurements as a function of normalized pumping rate were performed on a 1.55 µm BH laser using the delayed self-heterodyne method. The solitary laser operated in a single longitudinal mode up to 1.8 times threshold, where the linewidth was measured to be 45 MHz. The experimental results are compared with theoretical calculations of the linewidth contributions due to spontaneous emission and carrier FM noise. Linewidth reduction was achieved by means of optical feedback with an external grating. With a 1 m cavity length the linewidth could be reduced by three orders of magnitude to a minimum of 40 kHz.