Newsgroups: comp.robotics
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From: nagle@netcom.com (John Nagle)
Subject: Re: laser diode safety
Message-ID: <nagleCCK6nr.7I6@netcom.com>
Organization: NETCOM On-line Communication Services (408 241-9760 guest)
References: <1993Aug26.084151.2444@hemlock.cray.com> 	<1993Aug26.151805.28818@newshub.ariel.yorku.ca> 	<nagleCCEoF2.KzD@netcom.com> <BROWEN.93Aug27092853@lyapunov.aoc.nrao.edu>
Date: Mon, 30 Aug 1993 06:14:14 GMT
Lines: 31

browen@lyapunov.aoc.nrao.edu (Bruce Rowen) writes:
>The key to getting a large amount of power through a diode is to play
>with the thermal duty cycle limits. The 30mA you quote is most likely
>a *continious* current. If I ran 10A through my diode for more than a
>few milliseconds, the thing would explode!. This is true for most IR
>transmitters, you can safely up the current while reducing the duty
>cycle of on/off times. 

       Many laser diodes can fail at slight overloads in microseconds.  The
failure mechanism isn't thermal; it's mirror overload.  When there are
too many photons, the mirror's reflectance capacity is exceeded, the
population inversion near the mirror ceases, and lasing stops near the
mirror.  This causes local heating and fracture of the mirror face.
it's a wierd failure mechanism, different from that of other semiconductors.
It's peak power level that matters for these things, not average power level,
which complicates drive circuits considerably.

>My diode is basically a standard IR transmitter with a largeish heat
>sinking surface behind the substrate. The data sheet calls it a laser
>diode, but it is nothing like the diodes in semiconductor lasers that
>have all the extra goodies to focus the beam and not burn up. 

       Are you sure this is a real laser diode, not a big LED?  Is the
output coherent?  (If you focus it to a spot, do you see striation
patterns, or just a smooth dot?)  Can you modulate it fast (megahertz)?
Ordinary IR transmitters are just LEDs, and have frequency responses
limited to around 100KHz or so.

       What's the manufacturer and part number?

					John Nagle
