Why do semiconductor lasers become less bright as they are used for longer periods of time (assuming their battery is full)? Does it have something to do with the PN junction reaching some kind of equilibrium and becoming less effective? Does the laser induce some kind of temperature change reducing the efficiency of the semiconductor? Please explain!
The almost perfect crystal lattice when new begins to break down slowly over time and and this is accelerated by higher temperatures. Bonds break and reform non-symmetrically, impurities migrate in, other. Then the injected current has other paths to flow .... which in turn generates more heat and increases the aging rate. When a new laser diode is rated at 0.3 watt and say has 30% efficiency that means 0.3 watt of the 1 watt going is converted to light and 0.7 watt is heat, this ratio gets worse with time.