From: pcdhSpamMeSenseless@electrooptical.net   
      
   A client of mine is having a very strange problem with what Newport   
   laughingly describes as a nonpolarizing beamsplitter cube.   
      
   Their Michelson interferometer works with one lens in each arm, works   
   with no lenses, but does not work with only one lens.  The fringes just   
   go away.   
      
   The lens is used as a cat's eye reflector, i.e. it has a mirror at   
   focus, and no amount  of tweaking will get the fringes back.   
      
   Ah, I hear you say, it's spatial incoherence--the beam is multimode, so   
   when you spatially invert it, it won't interfere with itself anymore.   
   Invert twice, you're back to full interference.  And that idea is   
   supported by an interesting fact: inverting the beam twice in _one_ arm   
   also works:   
      
           P=1/f  f        2f   P=1/f     2f   
   beam     ()      x           ()             x|mirror   
      
   Pretty diagnostic of a spatial coherence problem, right?   
      
   Thing is, the input beam comes from a single-mode fibre plus collimator,   
   so it isn't spatially multimode at all.   
      
   The laser is actually a SLD used for coherence tomography, so the   
   fringes have about a 10-um range of visibility about zero path   
   difference, so it must be some temporal or polarization problem.   
      
   Everything I've thought of winds up being too small an effect.  Path   
   differences are quadratic in angle, so they're too small, and everything   
   else seems to be small-squared as well.   
      
   The one exception is the beamsplitter hypotenuse, because there the beam   
   is at 45 degrees' incidence, so weirdness can enter in the first order.   
     But what weirdness?   
      
   They're 3000 miles away, so I can't just dive in and poke around with   
   the thing, and I never use NPBSes for anything, so I don't have any in   
   the drawer.   
      
   Before I buy one and measure it, do any of you folks have any good   
   ideas, especially any based on relevant experience?   
      
   Thanks   
      
   Phil "Call me Puzzled" 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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