From: noreply@invalid.invalid   
      
   I have taken FTIR spectra of a single frequency infrared laser (a quantum   
   cascade laser) and see in the spectra peaks at 1/2, 3/2 and 2 times the   
   laser frequency that have amplitudes of 0.1% to 0.5% of the laser peak.  The   
   collimated laser beam travels about 2 meters from the laser to a piece of   
   lens tissue near the entrance of the FTIR.  The FTIR takes spectra of some   
   of this scattered light since the collimated beam is well off axis of the   
   FTIR.  Has anyone seen similar behavior and hopefully established a good   
   explanation?   
      
   I can see how saturation or clipping could generate a peak at harmonics of   
   the laser frequency, but I don't see how subharmonics could be generated.   
      
   The laser frequency is 1225 cm^-1, so the subharmonic appears to be at 612.5   
   cm^-1.  When I get back to these measurements, I will drop in a ZnSe or CaF2   
   window to check that there truly isn't a narrow bandwidth beam at 612.5   
   cm^-1.  The only thing I can think of is a weak feedback effect from the   
   FTIR to the laser, but the solid angle on the return is ~5E-6 and the   
   coupling into the FTIR is probably below 1%.   
      
   Thanks,   
   Bret Cannon   
      
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