Laser diode lifetime and reliability features highly in many applications. Laser diode failure mechanisms need to be accounted for so that the overall failure rate of a system containing a laser diode can be maximised.
In recent years much work has been carried out on laser diode failure mechanisms so that the laser diode lifetime has been improved.
Laser diode reliability and lifetime basics
There are a number of factors which affect the overall laser diode lifetime. While a catastrophic failure is one of the more obvious laser diode failure modes, it is also found that laser diodes degrade over time.
It is found that the forward current which does not contribute to the light generation causes the light emission characteristics to change. One aspect of this is that it takes more current to produce the same level of light. In many circumstances the laser diode life as the time when rises to a given value to attain the same light value - some have defined this as 1.2 times the initial value. This value may be chosen because there is a limit on any automatic power control circuitry in the equipment.
One of the key factors with the ageing process within laser diodes is the operating temperature. The degradation speed rises exponentially with the operating temperature.
Laser diode catastrophic optical damage, COD
Surge currents in laser diodes can lead to the rapid failure. It is found that in laser diodes that if the current is raised to increase the output optical power, a point is reached where the output power suddenly falls away and irreversible damage is entailed. This form of failure is known as Catastrophic Optical Damage, COD.
COD occurs when high output levels cause a short melting of part of the laser diode edge. At this point crystal defects form.
The very nature of laser diodes makes them susceptible to damage. Many operate at very high speeds - often in excess of 1GHz and in addition to this they are low voltage devices - around 2 volts. The combination of these two factors means that they are very susceptible to dame by surges.
In order to prevent the possibility of COD, it is necessary to protect the laser diode from surges. Turn on of general supply surges can provide short lived spikes that can easily damage the laser diode and cause failure.