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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: Tue, 19 Nov 2002 03:46:00 GMT
NNTP-Posting-Date: Mon, 18 Nov 2002 19:46:00 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?
The few laser diodes I have specifications for tend to have about a 1 to
2 range between threshold and damage and sometimes a bit less. From what
I have read, the major cause of failure is exceeding the optical power
density between the end reflectors. This power level can be exceeded
several times for short pulses and this fact is used in CD and DVD
writers. There are some "high power" laser diodes that have a wider
useful range probably by use of more robust end reflectors.
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.
Now you've got me thinking about getting a few low cost laser diodes and
trying speckle interferometry at home. I've intended to mess with that
for years but never get around to it.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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