From: jl@glen--canyon.com   
      
   On Sat, 13 Dec 2025 17:00:34 +0000, Cursitor Doom    
   wrote:   
      
   >On Sat, 13 Dec 2025 07:23:39 -0800, john larkin    
   >wrote:   
   >   
   >>On Sat, 13 Dec 2025 14:10:55 -0000 (UTC), Phil Hobbs   
   >> wrote:   
   >>   
   >>>Cursitor Doom  wrote:   
   >>>> On Sat, 13 Dec 2025 11:09:08 -0000 (UTC), Phil Hobbs   
   >>>>  wrote:   
   >>>>   
   >>>>> Gerhard Hoffmann  wrote:   
   >>>>>> Am 13.12.25 um 05:47 schrieb Bill Sloman:   
   >>>>>>   
   >>>>>>> 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.   
   >>>>>>>   
   >>>>>>   
   >>>>>> Yes, it is usually the frequency-selective optical reflector grid   
   >>>>>> that dies in ms. Faster than we are used from semiconductors.   
   >>>>>>   
   >>>>>> Gerhard   
   >>>>>>   
   >>>>>>   
   >>>>>   
   >>>>> A nice juicy carpet shock can blow the front facet off much faster than   
   >>>>> that (although the fragments will still be moving for awhile afterwards.   
   >>>>>   
   >>>>> Cheers   
   >>>>>   
   >>>>> Phil Hobbs   
   >>>>   
   >>>> No one seems to have come up with a viable solution, so I've been   
   >>>> giving it some extra thought. I considered the use of a semi-silvered   
   >>>> mirror at 45 degrees to the plane of the beam and sampling the light   
   >>>> level off that reflection, but then realized the loss through the   
   >>>> mirror would be unacceptable. The only other idea I can think of is to   
   >>>> use a mirror just off the plane of the beam which can swivel around   
   >>>> for a split second every few seconds to deflect the beam into the path   
   >>>> of the diode. That seems cumbersome and clunky but it's the only thing   
   >>>> I can come up with, not being a designer of any description.   
   >>>>   
   >>>   
   >>>You’re very likely to blow up more lasers trying to do constant power.   
   >>>Start with constant current, then wrap a constant power loop around it.   
   >>>   
   >>>Cheers   
   >>>   
   >>>Phil Hobbs   
   >>   
   >>I'm thinking that one would get a bettter optical output tempco by   
   >>driving a laser diode from a negative resistance source. Or, at least,   
   >>some proper impedance. It wouldn't take long to measure things.   
   >>   
   >>(Measuring IS a bit more difficult than talking)   
   >>   
   >>Or a current source twiddeled by a temp sensor.   
   >>   
   >>But why does the OP need extremely stable optical output?   
   >   
   >I could probably live without it, to be honest. I tend to over-think   
   >things massively. I could probably run those diodes fine at 10mA below   
   >their maximum constant current provided the current was reasonably   
   >stable. Hell, I've got dozens of the things; I'll blow a few up and   
   >find out empirically like everyone else does if they're honest. :)   
      
   It would be interesting to take a few measurements, like required   
   current to get constant optical output at a few temperatures, noting   
   laser voltage.   
      
   You'd need a photodiode or something.   
      
   Spritz some freeze spray on a laser diode or an LED. It's dramatic.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
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