What kind of speed do you need on the feedback? If a port goes from intense-on to "nearly off", and then switches back on again, is there a certain time-lag that's acceptable (on the order of microseconds or more), or does it have to be faster than that?
Is an attenuator on the "light" port (or something more sophisticated, like a fluorescent material, which the detector THEN monitors) acceptable?
It sounds like you're essentially 1-bit quantizing the difference between the light port and the dark port. If so, then all ADCs and comparators allow for a bit of jitter on one or both ports, so that bit-flipping isn't always subject to noise. Perhaps you're looking for the term "latched comparator".
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"As a species, we are forever sticking our fingers into the electric socket of the Universe to see what'll happen next. It's a trait that'll either save us or kill us, but by god it's what makes us human beings. I'd rather be in the company of people who look at Mars than people who contemplate humanity's navel -- other worlds are better than fluff." ~~Terry Pratchett
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________________________________
From: Carl Mungan <mungan@usna.edu>
To: phys-l@phys-l.org
Sent: Wednesday, August 7, 2013 12:55 PM
Subject: [Phys-L] feedback control for zero output
I have a device I'm trying to construct in which I want to null all the
light coming out one port and have it all go out another port. However, the
light is very intense, so I don't want to sample the "on" port. I'd rather
have a detector monitoring the "off" port to use as the feedback signal.
But the problem is "off" isn't a very good condition to monitor. (It's very
subject to noise.)
Is there a way to build a feedback control using a null as input? I tried
googling various terms and couldn't find a good starting point to get me
thinking about this stuff productively.