From: bill.sloman@ieee.org   
      
   On 13/12/2025 9:03 am, Cursitor Doom wrote:   
   > Gentlemen,   
   >   
   > I have a bunch of red lasers I bought a while ago which perform best   
   > at 45mA. So I initially thought I'd knock up a current source for that   
   > much to keep them happy. However, I then remembered the temperature   
   > issue and wondered how best to compensate for that effect. There was   
   > some app note I found on the Mouser site which suggested that by far   
   > the best way to operate such laser LEDs is to drop the constant   
   > current approach and go for constant power instead, sampling the   
   > laser's output with a photodiode and rigging up a feedback loop, so   
   > whatever light level I get from 45mA at 20 degrees C can be kept   
   > stable regardless of the ambient temp or the device's junction temp.   
   > However, the app note didn't go into the practicalities. How is one   
   > supposed to sample the light output without interrupting the beam?   
   > There's just no appreciable spread within the confines of a housing to   
   > get any light from the edge region so how could this be done?   
      
   Serious lasers tend of come with a built-in photo-diode. Lasers are   
   double ended devices, and while the mirror at non-output end of the   
   cavity is even more reflective than the one that lets a bit of light out   
   to form your beam, there can still be enough leakage through it provide   
   a feedback signal.   
      
   If the part itself is unhelpful, putting the beam through a Brewster   
   angled surface - typically a micro-scope slide - can still give you   
   enough reflected light for amplitude control without wrecking the beam.   
      
   One thing to keep in mid with your constant current source is that the   
   lasing region is very compact, and even a very brief over-current can   
   burn it out.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)