# Why do semiconductor lasers become weaker after extended use (not due to battery)?

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!

• A list of faliure modes of LEDs is available on Wikipedia. Semiconductor lasers are basically LEDs with cavities, so these failure modes are also relevant for them (although some laser-specific failure modes may also exist). – Ruslan Mar 16 at 17:14
• Could it have something to do with population inversion? The electrons need to get excited back to the conduction band because they are dropping down to the valence band at a faster rate than they are being promoted? – Failure123 Mar 16 at 18:26

## 1 Answer

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.

• Doesn't that refer to long term degradation of the lasers efficiency? I was more wondering why lasers become dimmer during long periods of use, and then, after resting for a period of time are able to achieve maximal brightness. I think it may be a different process.. – Failure123 Mar 16 at 17:15
• @Failure, how did you measure that the laser dimmed during use? How long did it take to dim (microseconds or hours)? Exactly what kind of laser was it? – The Photon Mar 16 at 18:11
• Yep, temperature destroys very thin N-P layer. N-P layer becomes thickier and its $E=U/d$ will fall. With falling $E$, laser degrades. – sanaris Mar 16 at 18:17
• Is it possible that it has to do with population inversion? That electrons are not being excited to the higher energy level faster than they are being depleted from it, so there is an accumulation of electrons at the lower energy level? – Failure123 Mar 16 at 18:27
• @ThePhoton Just a standard laser pointer, the type that you would use for a presentation. – Failure123 Mar 16 at 18:29