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.