COD monitoring of diode lasers

Catastrophic optical damage (COD) is a sudden degradation mechanism, which becomes effective at elevated emission powers of diode lasers. 1,2 Typically, it involves emission-power-degradation and often even alterations at the laser's facets. Although a large body of work has been reported on the analysis of this process, 3,4 the sequence of events is still not experimentally resolved and therefore not understood completely. COD involves three main phases: • First, a critical facet temperature must be reached. This temperature amounts to 120–160°C. It can be reached, e.g. during long-term cw operation, when defect accumulation indeed increases the facet temperature. 5 Alternatively, single high current pulses can lead to facet heating beyond this critical temperature within some 10 ns. 6 • After reaching the critical facet temperature, the thermal runaway starts. Within 1–10 ns very high temperatures on the order of the melting temperature of semiconductor material may be reached. 7 So far this phase has never been experimentally resolved. • Finally, there is propagation of a defect front along the cavity axis. Typical propagation velocities are on the order of some 10–30 m/s. 8 Such initial values slow (eventually to zero), when the emission power, which feeds this process by re-absorption of laser light, degrades. This phase (with a duration µs to ms) leaves the device with ‘dark bands’ of reduced spontaneous emission of the gain medium.