Catastrophic Optical Damage in 950-nm Broad-Area Laser Diodes Due to Misaligned Optical Feedback and Injection
Improper back-coupling of optical feedback is a known cause for accelerated gradual degradation as well as catastrophic optical mirror damage of GaAs-based high-power semiconductor lasers. For those material systems emitting below the GaAs bandgap (870 nm at room temperature), such damage is induced by feedback-light absorption in the non-transparent cap layers and substrate, which intensifies the heating of the area close to the emission facet. For InGaAs/AlGaAs-based diode lasers in the spectral range from 880 to 1060 nm, however, all passive layers as well as the substrate are, with respect to interband absorption, transparent for the lasing wavelength so that, in this case, the degrading mechanism of misaligned optical feedback could not be explained entirely. By use of an experimental multi-device study of 950-nm broad-area lasers, we demonstrate that the threshold of catastrophic optical damage is decreased the most by positioning the feedback return spot such that it covers the p-side solder and the highly p-doped semiconductor layers. By means of thermoreflectance microscopy in conjunction with numerical simulations we find that such low damage thresholds are caused by a strong localization of the absorption of feedback light close to the laser front facet.