From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 05/11/2017 04:11 PM, laloum.eric@gmail.com wrote:   
   > Dear all,   
   >   
   > It's a practical as well as a theoretical question. There are some   
   > conditions where light signal that we need to detect is below the   
   > detector noise. I was guessing if it is possible in some conditions,   
   > to add a controlled constant background light (which would act as an   
   > offset), so that the total signal gets higher than the detector   
   > threshold detection limit. Linked to this question, is it possible   
   > that a photodetector (including the sensor and the attached   
   > electronics)can't detect, let's say 0.1 mWatt light out of the dark   
   > (because of it's dark current or other kind of noise), but can   
   > resolve between 1 mW and 1.1 mW. Of course I'm aware that background   
   > light would carry an associated photon shot noise, and the trick   
   > would be to add a minimum amount of background just to get above the   
   > detector noise level.   
   >   
   > Thanks,   
   >   
   > Eric   
   >   
      
   Adding more incoherent light just makes the problem worse--the signal to   
   noise ratio decreases steadily as you add more.  Coherent detection, on   
   the other hand, can get you sensitivity equivalent to an ideal photon   
   counting system (the shot noise of the signal beam dominates   
   everything).  Of course you have to be detecting laser light from a   
   small (i.e. unresolved) region of space for that to work well.   
      
   I've built quite a few coherent detection instruments--in the right   
   application they're like magic.   
      
   Cheers   
      
   Phil Hobbs   
      
   --   
   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC   
   Optics, Electro-optics, Photonics, Analog Electronics   
      
   160 North State Road #203   
   Briarcliff Manor NY 10510   
      
   hobbs at electrooptical dot net   
   http://electrooptical.net   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)