Dave Mann wrote:
> Patrick White writes:
>> If it hasn't already been suggested, you might try using an LED.
>> They generate a small voltage when light hits them, and I'd bet they
>> have a way faster response to light than the photocell.
> I was under the impression that an LED generates light, not senses
> it :) You may be referring to a photodiode (or phototransistor)?
> The photocell I was using has a pretty quick response time (it's
> basically a really tiny silicon solar cell) and proved adequate for the
> purposes of my test. I wasn't really wanting to put too much effort into
it :)
Hi Dave,
Patrick is absolutely right about LEDs. Not only will a narrow band of
optical emission result when the LED is *electrically* excited (by imposing
a bias across the semiconductor junction), but a junction bias will also
result when the junction is *photoexcited* by light of the same energy as
the bandgap. This has got to be buried somewhere in those EE books you've
got -- if not, it will be in your physics books. :-)
But back to your original detector.... Silicon photodiodes are
extraordinarily fast devices (compared to a focal plane shutter). I've used
them to monitor pulse shapes of nanosecond dye lasers. Ten- or
20-nanosecond pulse widths were no problem at all. The only trick -- if you
can call it that -- is to make sure that the circuit inputs and outputs are
low-impedance. For the detector circuit, something as simple as a battery
(20 or 30 volts) in series with the photodiode works just fine. For the
oscilloscope, make sure that both the scope trigger and the scope signal are
low-impedance inputs. Use a 50-ohm terminator at the scope input. If
you're saturating the diode with that 10 mW laser, just paint a little
white-out (liquid paper) over the lens. If possible, you may want to employ
one photodiode placed at the edge of the focal plane to trigger the scope
and a second in the middle of the focal plane to monitor the pulse width.
The scope trigger could then be high-impedance (for larger trigger signal
and more reliable triggering).
Alternatively, you could stick with higher-impedance circuits all around,
but try a variation on the trick Mark Roberts alluded to yesterday. Fashion
a little optical chopper with a toy motor (or a Dremel tool??) and a
spinning paper disk into which is cut a series of radial slots. Turn on the
toy motor, shine the laser through the moving slits, and voila -- instant
modulated source. Now instead of measuring pulse width of the signal, you
count the number of "humps" in the captured transient signal. The effect of
circuit impedance in this case is to demodulate and phase-shift the
transient with respect to the incident modulated light. You don't care if
the signal gets smeared out a little bit, as long as you can distinguish the
humps.
Gee, this is sounding much more fun than what I'm supposed to be doing right
now. :-) Anyway, hope this helps.
Bill Peifer
Rochester, NY
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