Re: Screwy laser diode with monitor photodiode


Ethan wrote:
So it sounds like using the monitor diode is not going to work for me.
Does anybody have suggestions for how to get a tightly controlled
output from a laser?

I am using this to measure optical density of a fluid. I shine a laser
into the fluid and measure how much light comes out. I am modulating
the laser at a couple kHz, just to make the detection easier, and to
make it easier to cancel out external light sources.

This is pretty simple and for the most part this works pretty well. The
problem is the light source is drifting with temperature.

I have a few bad ideas, they are all physically cumbersome, such as a
beam splitter and an external monitor diode. Or a heater/thermister to
maintain a constant temperature just above ambient.

Ideas anyone?

Thanks

Ethan

P.S. I am realizing I don't know as much about VCSELs and other lasers
as I thought. Any suggestions on good books, or other references?

It's fairly common to control the temperature of a diode laser.
Usually a TEC (thermoelectric cooler) is used to cool it. A heater
would be easier to set up, but you may be shortening the lifetime; if I
did that I'd run the diode at reduced (half?) power (if your
application can still work with less power). Of course, you'll need a
thermistor or other temperature sensor, plus additional circuitry. PID
or PI controllers are available from Wavelength Electronics,
http://www.wavelengthelectronics.com -- their site seems to be down
right now or I'd hunt down the temp controller part # I used to use at
my previous job.

HTH,

Mark

.

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