Accelerated Degradation of High Power Diode Lasers Caused by External Optical Feedback Operation

We present results of investigations on high power broad area lasers in the spectral range between 780nm and 1060nm with different antireflection coatings on the outcoupling facet. Comparative life tests on high power semiconductor laser bars and single emitters with and without external optical feedback delivered significantly different results. Depending on the emission wavelength, we found different degradation rates and failure modes such as accelerated gradual degradation and catastrophic optical mirror damage caused by different energy reabsorption scenarios with respect to the energy gap of the substrate material.

I. Introduction

High power semiconductor lasers are the most efficient narrow-band light sources for the pumping of solid-state and fiber lasers. The constant demand for cost reduction in terms of dollar per Watt forced the development of laser bars and single emitters with higher output powers and maximum wall-plug efficiencies. The easiest approach to achieve this goal was an increase of the cavity length up to 6 mm. The accompanied loss of power conversion efficiency (PCE) was compensated by a lowered reflectivity of the antireflection (AR) coating on the outcoupling facet down to 1% for an operation even without volume Bragg gratings. This led to a significantly increased susceptibility of several electro-optical diode laser parameters to external optical feedback (EOF) including the risk of feedback induced accelerated gradual or sudden degradation and catastrophic optical (mirror) damage [CO(M)D] [1]–[2].