Abstract

Semiconductor injection lasers are frequently operated in external cavities for spectral narrowing and mode stabilization. However, the external cavity laser can exhibit coherence collapse, a catastrophic broadening of the spectral linewidth to tens of gigahertz under moderate external feedback.¹ For fundamental and practical reasons, one needs to know whether this behavior is stochastic (noise-driven) or deterministic. Here we describe experimental and theoretical studies of the coherence-collapsed semiconductor laser. Our experiments have used GaAs/GaAlAs index-guided lasers (Hitachi HLP-1400), operated from 1.1–1.5 times threshold and coupled to long (10–60 cm), weakly coupled (~10^-3–10^-4 in intensity) linear external cavities. The theoretical analyses of these experiments are based on rate equations for the carrier density and complex optical field; phase-amplitude coupling, gain saturation and coherent feedback terms are included. These equations are integrated for the free-running laser and the coherence-collapsed laser with and without noise terms: the time series, return map, autocorrelation function, power spectrum and correlation dimension are calculated in each case. The results show that the coherence-collapsed state is chaotic, with a correlation dimension of ~2.5, and the most likely explanation is mixing between external cavity mode partition fluctuations and feedback induced relaxation oscillations. The addition of noise tends to obscure the finer features of the chaotic attractor and makes the correlation dimension more difficult to determine.

© 1991 Optical Society of America