Abstract

Coherence collapse is a dramatic spectral broadening that occurs in single-mode semiconductor lasers subject to weak (10^-3–10^-4) optical feedback well above threshold.¹ It is a particularly vexing and elusive phenomenon because it mimics the properties of deterministic chaos and noise-driven instabilities. Here we describe what we believe to be the first experimental observation of coherence collapse using phase-conjugate external feedback. A GaAs/AlGaAs index guided laser diode (Hitachi HLP-1400) is coupled by a variable attenuator—consisting of two polarizers and a half-wave plate—to a passive phase conjugate mirror (PPCM) that uses a barium titanate crystal.² An optical isolator is placed in the ring to make the PPCM undirectional. This avoids deleterious effects caused by counterpropagating beams. The line broadening was observed with a 16.5-GHz Fabry-Perot interferometer. External cavity modes were seen in the rf intensity noise spectra. This PPCM could produce reflectivites up to 15% at the laser facet. For these experiments, the reflectivity was varied from zero up to 2 × 10^-3. At the lower reflectivites (10^-4) the initially well-behaved laser experiences undamped relaxation oscillations which beat with the external cavity modes in the nonlinear laser medium to produce the broadband (~10 GHz) single-mode spectra characteristic of coherence collapse.

© 1991 Optical Society of America