Low noise and narrow linewidth external cavity semiconductor laser for coherent frequency and time domain reflectometry

Abstract

A semiconductor laser (10) having an external cavity used for coherent frequency and time domain reflectometry is provided. The laser (10) provides a very stable single longitudinal mode characteristic, a very narrow lasing linewidth, and a very low noise optical output. The laser (10) includes a semiconductor optical amplifier (14) emitting a coherent light beam from one facet, a phase modulator (22) receiving the coherent light beam and linearly varying an optical frequency of the laser, and an etalon (32) selecting and stabilizing one longitudinal mode of the laser cavity for lasing, narrowing the linewidth and reducing the noise. A computer-controlled and piezoelectric actuated wavelength-selective grating (58) varies the length of the laser cavity and thereby tuning the optical frequency of the selected longitudinal mode to track the transmission frequency of the etalon by maintaining maximum power intensity thereby stabilizing the selected mode. A selected portion of the coherent light beam is directed back to the semiconductor optical amplifier (14) and further amplified. A reference portion of the light beam is coherently mixed with a backscattered signal portion of the light beam to yield reflectometry measurement or detection.