Results 1 to 5 of 5

Thread: What is 'threshold'?

  1. #1
    Join Date
    Sep 2007
    Location
    Wiltshire
    Posts
    85

    Default What is 'threshold'?

    I assumed it was the point when ANY light was given off by a diode but looking at Sams faq I might well be wrong and that it is the point when actual lasing starts.

    If it is indeed the point when lasing starts it might well explain why I blew up a 5mW diode as Sams faq seems to be saying that the maximum current for the diode is only 10% past the threshold. I.e if the threshold is 30mA then the diode maximum current would be just 33mA. Is this correct?

    I hope the above makes sense and would very much appreciate it if anyone can clear this up for me to save a repeat disaster.

    One last thing. What sort of current should a 5mW red diode be taking? I have no spec sheets or part number nor do I know the make.

    Many thanks - Dave
    A lovely childhood. Just me my mother and the voices.

  2. #2
    Join Date
    Jan 2007
    Location
    Cupertino, California
    Posts
    2,130

    Default

    A 5mW diode should need no more then 20mA to opperate. A laser diode must reach a power level before it can begin lasing. This is usually directly determined by the doping of the lasing material. It will start lasing around the point where the voltage drop is the same as the transition voltage of the material.
    CLICKY!!!

    Admin: In the immortal words of Captain Planet: YOU HAVE THE POWER
    Admin: (To quit being a bitch)

  3. #3

    Default

    20 mA? Most 5 mW diodes I've used had thresholds around 30 to 45 mA and maxima around 40 to 60 mA. Oddly, most can be overdriven, and power yields vary a lot but most I had were safe up to 76 mA. That included the 650 nm one Maplin used to sell, code LE09, and the Philips OF4944 655 nm gain guided type (which is very generous in output), and a Samsung SLD6351826 635 nm diode.

    Threshold isn't very exact, but it's the point at which you can detect a sharp change from spontaneous to stimulated emission. It's the point at which enough atoms are pumped to a population inversion (most electrons in outer 'shells' of atoms, not inner ones as usual) before they can emit spontaneously, so that any photons already arriving (from either spontaneous or other stimulated emission) will stimulate them. The bandwidth narrows to little wider than a single line, and the output starts to look specular on a surface as the coherent wavefronts bounce off carrying clear information about it to your eyes.
    Last edited by The_Doctor; 09-20-2007 at 12:32.

  4. #4
    Join Date
    Sep 2007
    Location
    Wiltshire
    Posts
    85

    Default

    Hmmmmmmmmm. Get stranger. I got another red <5 mW diode today (&#163;3.99 ebay) and measured the current at which it started speckling which is 10 mA. (I am calling this the 'threshold'. However speckly output seems to peak at 33-35 mA. Does this sound correct? It doesn't agree with Sams faq at all.

    A lovely childhood. Just me my mother and the voices.

  5. #5

    Default

    Specular output happens before the current rises to specified threshold. That 'peak' you saw is probably the real threshold, the point at which output becomes strongly proportional to input.
    Last edited by The_Doctor; 09-21-2007 at 15:56.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •