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.