In the early days of laser manufacture, there was a failure reject rate of 99% since there were so many possible failure mechanisms that were not understood and manufacturing control was much poorer. Nowadays, although laser manufacture has a much higher yield, the consequences of a premature laser failure can be very expensive since they are incorporated into such a wide variety of complex (and often very expensive) equipment.
Under ideal conditions, laser diodes can demonstrate excellent reliability, with lifetimes exceeding 2100,000 hours. However, they are extremely sensitive to electro-static discharge, excessive current levels, and current spikes, or transients. Symptoms of damage include reduced output power, shift in threshold current, change in beam divergence, difficulty focusing to previously-attained spot sizes, and ultimately failure to lase (LED-like output only).
Failure mechanisms of laser diodes
Semiconductor lasers have degradation process common to all semiconductors, such as defect migration, stress due to high voltage fields, etc. Since lasers interact with light, there are some extra mechanisms that occur that cause failure of lasers. Lasers deteriorate over time by reducing the amount of light emitted for a constant current and temperature, either catastrophically or more usually, very slowly. A laser is considered to have reached the end of life when they emit less than a certain fraction of the light at the start (commonly specified as 50%).
It can be argued that there are two major mechanisms that results in a failed laser.
1. One mechanism is that lasers emit light at very high intensity from a very small area (brightness equivalent to the surface of the sun). This means the emitting surface can also be damaged by the high electric fields present, which appears as a drop in emission (like a window getting darker). One cannot detect the likelihood of the laser experiencing facet damage other than through subjecting the laser to high emissions for an extended time - preferably at an elevated temperature.
2. The other major failure mechanism is due to the device overheating. This is usually caused by either the laser chip not being properly soldered onto its heat sink (there may an air void under the chip) or the laser heat sink itself is not properly bonded onto its larger heat sink. In either case, the deterioration can be very slow or catastrophic.
In summary, semiconductor lasers can be damaged by a variety of mechanisms. Many factors, including wafer growth procedures, device fabrication, and operating conditions determine which mechanism is dominant. The primary concern of this application note, however, is catastrophic facet damage (CFD) and latent damage resulting from electrical transients or other mechanisms related to handling and operating conditions, as well as overheating due to manufacturing faults.