Thermomechanical issues of high power laser diode catastrophic optical damage

Abstract

Catastrophic optical degradation (COD) of high power laser diodes is a crucial factor limiting ultra high power lasers. The understanding of the COD process is essential to improve the endurance of the high power laser diodes. The COD is observed as a process in which the active part of the laser diode is destroyed, forming characteristic defects, the so called dark line defects (DLDs). The DLDs are formed by arrays of dislocations generated during the laser operation. Local heating associated with non-radiative recombination is assumed to be at the origin of the COD process. A summary of the methods used to assess the COD, both in real time and post-mortem is presented. The main approaches developed in the last years to model the heat transport in the laser structures under non homogeneous temperature distribution are overviewed. Special emphasis is paid to the impact of the low dimensionality of QWs in two physical properties playing a major role in the COD process, namely, thermal conductivity and mechanical strength. A discussion about the impact of the nanoscale in both physical properties is presented. Finally, we summarize the main issues of the thermomechanical modelling of COD. Within this model the COD is launched when the local thermal stresses generated around the heat source overcome the yield stress of the active zone of the laser. The thermal runaway is related to the sharp decrease of the thermal conductivity once the onset of plasticity has been reached in the active zone of the laser.

Catastrophic Optical Damage in Semiconductor Lasers: Physics and New Results on InGaN High‐Power Diode Lasers

“…The phenomenon originates in a tiny region of the active zone of the laser in which the temperature or the absorption are initially slightly increased, but enough to start the feedback loop, which sometimes is called the “thermal runaway.” The reasons for locally increased absorption or elevated temperatures at microscopic “weak” points, which then serve as COD seeds, can be manifold, as discussed, for example, in other studies. [ 18,19 ] In general, one could distinguish between “intrinsic and extrinsic” causes for the buildup of temperature and absorption. A classical intrinsic scenario would be, for example, a slightly increased facet temperature, which causes a local bandgap reduction.…”

“…"weak" points, which then serve as COD seeds, can be manifold, as discussed, for example, in other studies. [18,19] In general, one could distinguish between "intrinsic and extrinsic" causes for the buildup of temperature and absorption. A classical intrinsic scenario would be, for example, a slightly increased facet temperature, which causes a local bandgap reduction.…”

“…[20] In general, one could even say that any inhomogeneity in the optically active region could potentially be the starting point of the COD, if only the light output is high enough. A COD-generating mechanism that could be ranked between extrinsic and intrinsic nature was described in papers by Souto et al [18,21] It has been explained that under certain conditions in modern quantum-well (QW) structures, such high-temperature gradients in the growth direction could occur such that the limit for elastic deformation of the involved materials could be exceeded and the stress relaxation would be realized by defect generation. These defects would then act then as starting points for the COD process.…”

From Two- to Three-Dimensional Model of Heat Flow in Edge-Emitting Laser: Theory, Experiment and Numerical Tools

“…The unwavering interest in this subject can be explained by expectations of obtaining greater and greater optical powers. Thus, the researchers design very sophisticated techniques of facet temperature reduction [9,10], investigate degradation mechanisms [11] or look for more efficient cooling systems [12].…”

Effect of thermal lensing and the micrometric degraded regions on the catastrophic optical damage process of high-power laser diodes

“…Understanding the physical mechanisms driving the COD should provide a step forward to increase the laser power and lifetime. In spite of the many efforts carried out to unravel the details of the COD [3,4], there are still many uncertainties about how it starts, how it propagates, and what are the driving forces leading to COD. COD consists of the sudden quenching of the laser diode optical power after many hours of regular operation.…”