From: noreply@invalid.invalid   
      
   "Salmon Egg"  wrote in message   
   news:SalmonEgg-779763.21333229092010@news60.forteinc.com...   
   > In article ,   
   > "Bret Cannon"  wrote:   
   >   
   >> 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   
   >   
   > I am not spending much time on this. What is the possibility that you   
   > have a two-photon excitation of a parametric oscillator?   
   >   
   > Bill   
   >   
   > --   
   > An old man would be better off never having been born.   
      
   This is an interesting suggestion, but first I want some confirmation that   
   their really is 612.5 cm^-1 light.  The intra-cavity laser power is only a   
   few mW but with a beam area of ~10 microns x 2 microns the intensity would   
   be on the order of  10 kW.  However, I think that a 2-photon excitation   
   would excite electrons out of the quantum wells and into the continuum,   
   which means there would be no resonance enhancement of the 2-photon   
   excitation nor would I expect resonances for the 612.5 nor 1837.5 cm^-1   
   lines.  Finally, without careful design, losses in the waveguide at 612.5   
   cm^-1 would likely be very large.   
      
   Thanks for your idea.   
      
   Bret   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)