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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: Diode Laser Drive Circuit
Date: Mon, 11 Nov 2002 23:06:33 GMT
NNTP-Posting-Date: Mon, 11 Nov 2002 15:06:33 PST
Alex Cusenza wrote:
> It's been a while since I posted, but I'm always peaking for interesting
> Anyway, can someone please explain to me how a driver board functions for a
> laser diode. I have a simple differential feedback circuit (very typical)
> connected with a potentiometer connected to the laser input. As I vary the
> potentiometer, the laser beam varies from dim to bright. WHY? Then, what
> is the purpose for a more fancy driver circuit, that is, higher frequency
> operation and such?
Usually laser diodes are driven with a transconductance amplifier, a
constant current source. Basically for two reasons. The input current
difference between threshold and burn out current is often as small as 2
to 1 or less. Often stable beam power is required and the power is more
or less proportional to current above threshold.
Many laser diodes have a power monitor diode built into the case It is a
separate die that senses the light coming out of the back of the laser
die. The output of the power monitor diode is compared to a reference
and appropriate feedback through the transconductance amplifier
stabilizes the output optical power. This arrangement is universally
found in CD and DVD player and reader devices.
The monitor diodes come in two main configurations. Common cathode and
anode/cathode common. The common cathode types are usually used in
photovoltaic mode and typical monitor diode output is 100 to 200mV. The
anode/cathode common type usually has a PIN photodiode intended for
reverse bias operation. These are typically used in pulsed power
applications such as CD and DVD writers where write pulses are several
times read level and widths may be 50nsec or less.
Laser diodes used for communications and many special applications must
be stabilized for SNR reasons and to protect the diode from overcurrent
conditions. Laser diodes are very fragile and just a few microseconds of
overcurrent can damage the device.
When operating a laser diode, it is necessary to find the threshold
current. The laser emits light below threshold. To detect threshold, you
must detect when the beam becomes coherent. Aiming the laser at a piece
of paper will allow you to see a speckle pattern when the laser
operating regime starts. This is threshold. It is usually safe to run
the laser at about 1.5 times the threshold current. Many devices are
damaged by currents of less than twice threshold.
That is everything I know about the care and feeding of commonly
available laser diodes.
... 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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