From: boxman@voyager.net   
      
   On 7/21/2013 11:09 AM, Phil Hobbs wrote:   
   > 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   
   >   
   >   
      
   I'm way out of my area of expertise here since it's been many years   
   since I've dealt with interferometry, but one suggestion for where the   
   problem might be if it is the cube causing the issue would be the   
   quality of the assembly.  If the cementing was done incorrectly then you   
   might have problems resulting from that interface.  Don't know if that   
   could explain it, but that's where I've found problems with cube beam   
   splitters in the past in other setups.   
      
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