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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: laser diode operating range
Date: Thu, 21 Nov 2002 02:29:16 GMT
NNTP-Posting-Date: Wed, 20 Nov 2002 18:29:16 PST
Winfield Hill wrote:
> Generally laser diodes are used with light-feedback servo-mode circuits.
> That is mandated because of the rather dramatic increase in light level
> that occurs above the "threshold" voltage, where true lasing begins and
> takes over the light emission from simple LED action.
> Consider applications where a variable range of light is required. Using
> laser diodes with light servos tempts one to use the servo mode to achieve
> the needed variable light intensity. However, often the laser diode's
> threshold is a substantial fraction of the maximum power, 25% or more.
> This can create possible unpredictable operation. If one servos close
> to the threshold where coherent narrow-wavelength emission ceases, one
> might as well have purchased an inexpensive LED. :-)
> So one question is, what are typical lasing thresholds as a fraction of
> the full laser power? How about for different laser diode wavelengths?
I don't know if this is at all interesting in this context but late last
week I started work on a new optical unit for CD/DVD. Because DVD uses
visible light, I decided to get the DVD laser operational first. I can,
in principle, get all of the optical units I can eat but frustratingly,
the manufacturer of the complete mechanical assembly has not yet
provided a specification for the optical unit although it has been
promised. The optical unit has the usual Elantec chip for which
specifications are under NDA blah blah blah. Major PIA. The Elantec chip
is set up via a serial interface and has the opposite bit order from
everybody else in the industry. I suppose this was to keep me on my toes
in writing the basic primatives to talk to the chip.
At first we thought the Elantec contained an APC although it seems to
have too few pins for that. Thus we started out with that in mind. It
became clear shortly that there is no internal APC. There is, however,
an internal current DAC that I now use to set the laser power. We have
an APC on our main board but it looked like high risk to connect it to
the Elantec until we understood the power monitor signals (the
manufacturer of the assembly has not responded to questions about that
yet and I can't read Chinese on a bad FAX anyway - scream). I have,
however, gotten the laser running with about 0.4mw output from the
objective. Although I am still trying to figure out the power monitor
signals, the OPIC signals are very stable at this power level and I can
get very clean unnormalized focus (DC arithmetic) and tracking
(differential phase detector) signals. My guess is that my operating
point is about half way between threshold and damage and that the total
laser output power is around 2mw (these optical systems have a lot of
As it happens, we think our APC is compatible with Elantec so we intend
to use the APC in the long run but I believe that these particular
lasers are stable enough to use without APC for read power.
BTW, we have an interesting track record with optical units. The first
unit we did development on we destroyed about 20 units before we got all
of software, settings and adapter electronics optimized. I think we will
probably do that again. It's a PIA but the slightest slip in any of that
and we have a dead laser. Our most interesting failure was my focus
software apparently suffered a glitch in an overnight run and fried a
focus coil. In the process, it melted both objective lenses. The smoke
destroyed our only copy of "Planes, Trains and Automobiles." No loss. It
is a pretty bad movie.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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