Hi Phil...    
      
   On my current system I get about 5nA from a 7mm2 PD. My current system gain is   
   about 6M times.    
      
   No reflector tape is possible, however I will push the laser to a higher power   
   while reducing the duty cycle to that average power is preserved, the issue is   
   even a low frequency of 1-2MHz rise time becomes a problem . I will probably   
   give a higher    
   negative bias to reduce the capacitance so that the rise time could be gained   
   back. I expect to gain 6db by way of pushing laser power and some other   
   optical means, if I can squeeze the duty cycle to half or more another   
   additional 3db lurking somewhere    
   there, so all in all perhaps I can gain another 10db but I think I need more   
   like 25db improvement to achieve a good signal SNR.    
      
   Anyway, I will spin a board to see if I could solve the problems this way.   
   Last resort is APD.    
      
      
      
   >    
   > > Phil,   
   >    
   > >   
   >    
   > > I own a copy of your book. It is fantastic great job.   
   >    
   > >   
   >    
   > > My current system, I sense an object that is far away using a very   
   >    
   > > weak laser. (For various reasons, it needs to be weak and it is   
   >    
   > > pulsed at a low frequency of 1MHz) My distance of sensing is 400mm.   
   >    
   > > Now for a next version, I need to extend my distance of sensing to   
   >    
   > > 4Meter. I currently use a si-pd and I have done a lot to make this   
   >    
   > > current system work at 400mm, now I need 10x the improvement, based   
   >    
   > > on my experience I don't think I can get this done, hence the   
   >    
   > > Avalanche PD idea.   
   >    
   > >   
   >    
   >    
   >    
   > The thing about APDs is that they multiply the shot noise of the ambient    
   >    
   > light as well as the signal, so they're really only useful when you'd    
   >    
   > otherwise be in the Johnson noise limit.  A gain of M lets you drop the    
   >    
   > feedback resistance by a factor of M**2, so in some situations that's a win.   
   >    
   >    
   >    
   > Advanced front ends are much cheaper, though--usually only a few dollars    
   >    
   > in onesies.   
   >    
   >    
   >    
   > How much photocurrent are you expecting to have, and how big is your    
   >    
   > detector?   
   >    
   >    
   >    
   > Any possibility of putting some retroreflecting tape on the target?    
   >    
   > That's good for a factor of about 3000 (70 dB electrical) compared to a    
   >    
   > Lambertian scatterer.   
   >    
   >    
   >    
   > 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   
      
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