piclist 2001\02\04\122155a >
Thread: Again a laser discussion
face BY : Bob Blick email (remove spam text)

>Um, how do you change the output power produced by one of those laser
>diodes, anyway?  I thought they were all the sort where you essentially
>detected output via the built-in photodiode, and didn't let the current
>climb any higher than that?  Wouldn't that make the actual output wholely
>dependent on the laser diode itself?

Hi Bill,

You can have the monitor photodiode in the circuit as a feedback element,
and varying the load on it with a trimpot sets the output power. You also
have a current limit set to the maximum rated current, so when the
efficiency drops because of high temperature, you don't burn it out from
too much current in a vain attempt to maintain output power. If you adjust
the power real low, say under 1 mW, it looks a little noisy during the
first microsecond because the laser diode is so nonlinear there's no way
the frequency compensation in your circuit can match it. But as long as you
are running continuous wave you can pretty much adjust the output from
almost nothing to full.

You don't have to use the monitor photodiode, just drive the laser diode
with a constant current source, but the output varies with temperature, and
you need to drive it with enough current to reach the lasing threshold,
which can be very close to hurting the diode from too much optical power(if
you are at a lower temperature than you expected). In a pinch you can put a
low value(100 ohms) PTC thermistor in series with the diode and just ignore
the photodiode. But that would be wrong :-)

Cheerful regards,

Bob Blick

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