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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
References: <3DD9B41E.A6D85CCB@webaccess.net> <3DDAE478.2FE930EF@webaccess.net>
Date: Wed, 20 Nov 2002 03:10:52 GMT
NNTP-Posting-Date: Tue, 19 Nov 2002 19:10:52 PST
Winfield Hill wrote:
> Chuck wrote...
> >Winfield Hill wrote:
> >> Chuck wrote...
> >>> There is no particular need for a laser diode if the application
> >>> does not require a narrow line source. The major need is in fiber
> >>> communication to reduce dispersion and in devices using holograms,
> >>> diffraction gratings, Bragg cells, interferometers and the like.
> >> An interesting use is fluorescence spectrometry, where the
> >> fluorescence emission wavelength may be near the excitation
> >> wavelength. Bleaching is a serious issue that dictates power
> >> control, but the easy servo-control solution to low power can
> >> mean a cessation of lasing. Resulting in line broadening, or
> >> a shift to longer wavelengths, interfering with fluorescence
> >> detection. One tries to avoid or minimize expensive filters.
> >> Thanks,
> >> - Win
> > I see what you are worried about. I have not looked at UV lasers
> > but for your application, you really want an APC (Automatic Power
> > Control). [ snip ]
> Thanks, Chuck but we do have APC (my own analog implementation,
> which works down to 2% light level if desired, where the laser
> first starts putting out light). That's not the issue. Maybe
> there is no real "issue;" I was simply commenting on the fact
> that once one has APC as you call it, it's tempting to use that
> for attenuation instead of neutral-density filters... However,
> only a limited current range produces coherent narrow-band light.
> One thing missing from my design is a BELOW LASING THRESHOLD
> warning indicator.
That's a bit tough really. If your operating point is that marginal and
your APC (this is common industry jargon) is stable on both sides of
threshold, you can't really detect much of anything. For one example of
laser I use, I have to go by the optical unit manufacturers
specification which states I must not exceed 85ma and that I must have
at least 200 microwatts out of the objective lens. The population seems
to meet that with currents between 50ma and 75ma. Generally, I see below
10 microwatts below threshold so the transition is abrupt but hard to
detect. The usual wisdom is to pick a monitor diode current (or voltage
for photovoltaic mode) that you consider to be the edge of the cliff and
put some safety margin on it. On your side is the fact that dropping
below threshold is like going over a cliff. The slope is very steep. A
counter and DAC type APC (integrated product) could be made to detect
below threshold based on slope of the monitor diode current verus laser
current in that the slope can always be seen because of the +- a couple
of LSBs of the counter. The DAC dither will give a larger AC current out
of the monitor diode. I would be nervous operating that close to
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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