Re: How best to detect laser?

From: Bill Sloman (bill.sloman_at_ieee.org)
Date: 07/14/04 

Date: Wed, 14 Jul 2004 11:10:07 +0200

"colin" <no.spam.for.me@ntlworld.com> wrote in message
news:8tQIc.674$w_6.658@newsfe1-win.ntli.net...
> if your modulating the laser then u probably not gona be bothered too much
> with stray light just filter through the frequencie u want.
>
> if its a single high frequency then an inductor as the load for the
detector
> can greatly increase the signal, otherwise the capacitance can rob all the
> signal. i found a dual gate mosfet seems to be the best choice of amp.
>
> an avalanche photodiode detector probably the ultimate, a bit expensive
tho,
> they can detect single photons, or just multiply by 150+, and theyr still
> dam fast.
>
> with such and a small lense ive managed to detect reflected 50mhz signal
> from an object several meters away and still get a usable signal to noise
> ratio.
>
> Colin =^.^=

Modulating your light source is an excellent way to distinguish it from
stray light, but enough stray light can always saturate your detector and
lower levels can still generate enough random noise to make it quite
difficult to detect your modulated source - narrow-band interference filters
can prevent most of the stray light from getting to the detector in the
first place.

Dual gate MOSFET can be attractive as front end amplifiers if you are
modulating at high frequencies, but when I last looked at them (which was a
long time ago) the 1/f noise corner tended to be close to 1MHz, and the DC
stability was rotten.

For most purposes a good FET-input op amp is better, and much easier to
use - the Burr-Brown (now Texas Instruments) OPA655 comes to mind.

If you are working with modulation frequencies of the order of 50MHz, an
inductor can be an attractive load, but you have to make sure that your
inductor's self-resonant frequency is higher than you modulation frequency -
the interwinding capacitance for a single-layer wound inductor is usually of
the order of 1pF, equivalent to a parallel impedance of only 3k at 50MHz.

Alavanche photo-diodes can detect single photons if operated in the Geiger
mode, and can provide a very prompt output, but the recovery time is
relatively long, which restricts them to looking at very low light levels.

Photomultiplier tubes are a lot more flexible, and can offer steady state
gains of up to about ten million, where the avalanche photo-diode is limited
to about 150 when not operating in the Geiger mode.

-------
Bill Sloman, Nijmegen

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