During the laser operation, the most common aging symptom is that its efficiency will decline. At a given set of operating parameters, the light output will decline too. A very pragmatic way to define lifetime is therefore to determine the minimum still acceptable light output quantity. If the light output at the beginning of the operating time was 100%, an acceptable output to fulfill the intended purpose could still be 50%, 70% or 80% for instance. Laser diodes tend to degrade faster towards the end of its lifetime. We could for instance say that a laser diode at a fix current and ambient temperature will deliver at least 70% of its optical output power for at least xx hours.
Another way to define lifetime is to specify a set light output power and the maximum operating current increase to compensate for efficiency loss during the operating time at a constant ambient temperature. An example would be an statement such as: ‘within the next xx hours, the forward current of the laser must not be increased over 30% to sustain the original light output power at a constant ambient temperature’.
Both these exemplary definitions of lifetime must be understood as average projections in the future that are calculated using a model on a limited amount of lasers.
For very efficient lasers with very little amount of defects, the degradation of the laser is only visible after a couple of thousand hours operation. At industrial-grade quality it could take even years to actually see a degradation beyond the resolution of the measurement equipment involved.
In order to learn more about the degradation characteristics, lasers are operated at at least two or three different temperatures and operating currents well above their specification. This is often called overstress testing and it is an established method of efficiently validating degradation models. This kind of test is only performed on samples in quality labs and allows to observe a significant light output degradation in a reduced time frame.
By observing the different degrees of degradation at different temperatures above the standard operating conditions, estimates about the degradation rate at normal ambient temperature and nominal operating current can be issued with the help of a degradation model.