Hi all,  I just had a call from someone who wants to do FM spectroscopy.   
   (I had to put him off for a bit since I'd never heard of it.)   
   http://link.springer.com/article/10.1007%2FBF00688820#page-1   
   But I think I see it.   
      
   I modulate a laser (somehow) and put sidebands on it.   
   The absoprtion signal then gets modulated at the side band frequency.   
   I mix this down and detect it... So I need a fast detector.   
      
   I wonder if I can do it at relatively low modulation frequencies?   
   (Like 100kHz for the slow PD's I have.)   
      
   Sorry thinking out loud to sci-optics,   
   george H.   
      
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