From: rongibbs@talktalk.net   
      
   "Phil Hobbs"  wrote in message   
   news:4F2AC955.4EB1A9F2@electrooptical.net...   
   >>   
   >> Why not use a spatial filter?   
   >   
   > I'm not a fan of spatial filters in general, because they don't work   
   > very well for low spatial frequency stuff like moderate astigmatism and   
   > coma, unless you use an unresolved pinhole.  Even then they're not that   
   > great, and you still lose a lot of light, plus there's the complexity,   
   > the tweakiness, and the poor pointing stability over temperature, due to   
   > the motion of the pinhole and lenses.   
   >   
   > One exception is if you use a short length of single-mode fibre,   
   > stripped and embedded in an absorber such as soft black epoxy.  That   
   > really works--excellent beam quality and unmatched pointing stability,   
   > assuming you can keep the collimating lens in place accurately enough,   
   > and that the epoxy is thin.   
   >   
   > Cheers   
   >   
   > Phil Hobbs   
      
   I never understood why spatial filters became so universally popular in   
   laser systems. The name suggests something clever and sophisticated   
   happening in fourier space. The reality is just as Phil describes. Even with   
   a single-mode fibre (English spelling), power stability can be a problem, if   
   launch conditions change with time. There is no magic cure for imperfect   
   laser modes and optics. There, I've said it.   
      
   Ron   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)