On Thursday, January 8, 2015 at 11:35:02 AM UTC-5, Phil Hobbs wrote:   
   > On 01/08/2015 11:05 AM, ggherold@gmail.com wrote:   
   > > On Wednesday, January 7, 2015 11:23:10 PM UTC-5, Phil Hobbs wrote:   
   > >> On 1/6/2015 8:54 PM, ggherold@gmail.com wrote:   
   > >>> On Monday, December 29, 2014 9:06:55 AM UTC-5, Phil Hobbs wrote:   
   > >>>> I'll probably have to get a couple to play with.  The   
   > >>>> nonpolarizing thing and the asymmetry make them potentially   
   > >>>> harder to use than PBSes for anything at all fancy--for   
   > >>>> instance if the phase shift between p and s is more than a   
   > >>>> degree or two, the output polarization will be a mess.   
   > >>>>   
   > >>>> Cheers   
   > >>>>   
   > >>>> Phil Hobbs   
   > >>>   
   > >>> Hi Phil, (and I thought SO was dead.) If there is any absorption   
   > >>> you get a phase shift. (But you know that.) We've used the phase   
   > >>> shift in metallic beam splitters.   
   > >>   
   > >> Interesting--what do you use it for?  IME it's usually a nuisance.   
   > >> (You folks have some pretty cool ideas.)   
   > >>   
   > >> Cheers   
   > >>   
   > >> Phil Hobbs   
   >   
   > >   
   > > This was my colleagues idea... we bought a bunch of inconel (I   
   > > think) metallic beam splitters from thor labs, and found the one that   
   > > gives ~90 degree phase shift.  This was stuck into a Michelson   
   > > interferometer and then one looks at both the output beams.  (You   
   > > need a second beam splitter to pick off the beam that goes back to   
   > > the light source.)  Then with two signals in quadrature you can get   
   > > direction information as you move one of the mirrors.  Pretty neat.   
   > > You also get much better noise immunity when counting fringes... it's   
   > > almost all phase noise and no amplitude noise. (If you understand   
   > > what I mean by that.... the quadrature signal looked at x-y on a   
   > > 'scope is a circle and noise (bumping the table) moves around the   
   > > perimeter of the circle, but doesn't change the radius, that was not   
   > > for you Phil, but for those reading along at home.)   
   > >   
   > > George H.   
   >   
   > Cute, and much cheaper than using waveplates, if you can afford the   
   > power loss.  (BTW I never feel like you're talking down to me, don't   
   > worry.  There's all sorts of stuff I don't know, or have forgotten, or   
   > haven't thought about.)   
   >   
   > I usually use a QWP in each arm and a polarizing beam splitter.  That   
   > gets rid of the beam going back to the laser, and the recombined beams   
   > are in orthogonal linear polarizations.  Another PBS or (ideally) a   
   > Wollaston prism at 45 degrees in the output arm produces two   
   > interference signals 180 degrees out of phase.  Subtracting them after   
   > detection gets rid of the residual AM noise (autobalancing helps a lot).   
      
   Oh... that's fun.  I think we have all the pieces/ parts except for the QWP   
   (in the visible.)   
   I do have some 3-D glasses I nicked from the movie theater.   
   I think those have both polarizer and waveplate...   
      
   George H.   
   >   
   > Coincidentally I have a project going that will use a frequency counter   
   > to do Doppler velocity measurements.  It's a great deal simpler than a   
   > big digitizer/FPGA back end (which I wouldn't know how to build anyway),   
   > at the cost of ~ 5 dB of SNR.   
   >   
   > Running a comparator at the zero crossings of the difference signal is a   
   > win for that.  (An AM detector supplies the gate signal for the counter,   
   > so we only count when the SNR is reasonable, i.e. more than about 8.)   
   >   
   > 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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