Re: [Phys-L] feedback control for zero output

[brian whatcott <betwys1@sbcglobal.net>]

On 8/7/2013 3:55 PM, Carl Mungan wrote:
> 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.
>
> Any help would be greatly appreciated. -Carl
Unless the quality of the light to be nulled is very dynamic, you will 
presumably be
 specifying leading parameters of the light beam which you are to null.
 In this case, a filter centered on parameters of  the light beam being 
nulled,
say its frequency, repetition rate etc., will enhance leakage amplitude 
of the light
 being nulled at the OFF port  and suppress off frequency noise 
components (??)

Brian Whatcott    Altus OK