Reflection Noise In Index Guided InGaAsP Lasers.

We report a detailed study of the excess noise induced in index guided InGaAsP laser structures by reflection feedback. The phenomena of high frequency noise (1-5 GHz), low frequency noise ( 100 MHz) and intensity fluctuations are shown to have a common physical origin in the unusual instability of the coupled laser-external cavity system. After a randomly occurring light intensity drop, the light output recovers in 10-15 steps, each corresponding to an external cavity round-trip (high frequency noise); the total recovery time corresponds to the low frequency noise. The instability, and the subsequent noise, can be suppressed under conditions of very strong feedback such as obtained for lasers with anti-reflection coated facets. The reflection noise characteristics are shown to be largely independent of the laser structure and structure modifications such as distributed feedback.