Effect of external optical feedback on spectral properties of external cavity semiconductor lasers

The effects of external optical feedback on the spectral properties of single-mode external cavity semiconductor lasers that use a graded-index (GRIN) lens in the optical cavity have been investigated. Mode rejection ratio and intensity drop-out rate of the dominant mode as a function of optical strength have been measured. These measurements show that a laser with a short (160 ¿m) GRIN-lens external cavity can tolerate optical feedback as large as ¿20 dB without significant penalty. This minimum optical feedback can be larger when lasers with shorter cavity length are used.