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From: Phil Hobbs
Subject: Re: laser diode operating range
Date: Tue, 19 Nov 2002 12:01:17 -0500
Organization: IBM T. J. Watson Research Center
NNTP-Posting-Date: 19 Nov 2002 17:01:19 GMT
X-Mailer: Mozilla 4.61 [en] (OS/2; U)
Chuck Simmons wrote:
> 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.
The two main failure mechanisms in diode lasers are:
1. Blowing a facet right off the die due to ESD (the gigantic laser
pulse due to a 100A, 1 ns discharge causes the material to fail).
Dielectric breakdown at the surface can also happen under more normal
pulsed operating conditions. Since this is a highly nonlinear effect,
it happens fairly suddenly as the optical power increases.
2. Propagating dislocations down into the active region, producing
recombination centres that prevent the carrier density from becoming
large enough for efficient laser action. This can happen quickly or
slowly, and is primarily a thermal and hot-carrier effect. It will also
happen when the device is reverse-biased significantly.
The 2:1 ratio (sometimes more like 5:1, especially in longer-wavelength
devices) is the ratio of the bias currents at damage vs threshold.
Laser threshold is usually defined in much the same way as FET gate
threshold: extrapolate the straight-line part of the power vs bias
current to the current where it crosses zero optical power.
> 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.
Not so. There are lots of times when having high spatial coherence is
vital. Lasers can be focused down to spots millions of times brighter
than you can get with any thermal source whatever. With a thermal
source, it is impossible to get a surface radiance
(watts/meter**2/steradian) higher than that of the source itself, which
in turn is limited by the maximum attainable temperature (3400K for
tungsten, perhaps 6000-8000K for a flashtube). Using a higher-powered
bulb merely increases the area you can illuminate at a given
brightness. There are also many applications for rapid modulation (e.g.
optical communication) in which the rapid temporal response and high
spatial coherence of a diode laser is vital.
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