From: sam@repairfaq.org   
      
   Phil Hobbs  writes:   
      
   > 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?   
      
   Without looking at other options, you say the fiber is SM, but is it   
   SM for the wavelength in question?   
      
   --   
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   > 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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