Abstract

Semiconductor injection lasers are frequently operated in external cavities for spectral narrowing and mode stabilization, although the result can also be self-pulsations and chaos. Single external mirror cavities can select a single longitudinal mode (cavity length L_ext ~ laser diode length L_d) or narrow the modal linewidth (L_ext ≫ L_d). For greatest coherence, grating external cavities are used for low-power (~1 mW) semiconductor lasers, and double external cavities or intracavity etalons are best for medium-to-high power devices.¹ Here we describe experimental and theoretical studies of the properties of double external cavity semiconductor lasers. Our experiments have used a GaAs/GaAlAs index-guided laser (Hitachi HLP-1400) coupled to two long (10–60 cm) linear external cavities, without frequency-selective elements such as gratings or etalons. Our theory rests on rate equations for the carrier density, electric field amplitude and phase, including two coherent optical feedback terms. These are solved for the steady-state modes, then a linear stability analysis is performed to study the self-pulsations observed experimentally. Finally, the rate equations are integrated numerically with optional Langevin noise terms, showing self-pulsations leading to period-doubling or quasiperiodic routes to chaos. The observed behavior is similar to that of the single external cavity laser with off-axis tilt asymmetry.² Stability conditions and spectral narrowing in the double external cavity laser are also discussed.

© 1991 Optical Society of America