High-stability 1.5 μm external-cavity semiconductor lasers forphase-lock applications

Applications using phase-locked semiconductor lasers, such as homodyne detection, require lasers with narrow linewidth and high-frequency stability. The design and operating characteristics of two 1.5 μm external-cavity semiconductor lasers built for such applications are described. The measured beat linewidth is 4 kHz, and the spectral density of relative frequency noise deviates significantly from the intrinsic white spectrum only at frequencies below 4 kHz. It is estimated that this frequency jitter will induce approximately 1.1° RMS phase error in a second-order homodyne optical phase-lock loop that is optimized for the present beat linewidth