From: tjroberts137@sbcglobal.net   
      
   On 8/20/19 2:59 PM, Douglas Eagleson wrote:   
   > The laser has an outcome of a coherent beam.   
      
   Hmmm. There are inherent limits to the coherence of a laser beam.   
      
   > I believe these are termed the outcome of an oscillating system.   
   > [...]   
      
   I've never seen the term "oscillating system" used in this way. Lasers   
   do not oscillate, they operate via stimulated emission between quantum   
   energy levels.   
      
   > On consideration of the subject my question becomes the basic   
   > relation of coherent emission.   
      
   A laser beam has several parameters that characterize it, determined by   
   the properties of the laser. The most notable is its coherence length --   
   the maximum path-length difference in a Michelson interferometer for   
   which good fringes can be obtained. A typical undergraduate lab He-Ne   
   laser has a coherence length of 10-20 cm; in our optics lab when we lock   
   our lasers to an optical cavity [#] their beams have coherence lengths ~   
   5 meters; much longer coherence lengths can be achieved (working on   
   it...). Longer coherence length corresponds to narrower linewidth, and   
   in our application narrow linewidth is important.   
      
        [#] Pound-Drever-Hall locking of a semiconductor laser, 1560nm.   
      
   Coherence length and linewidth are longitudinal measures. There are also   
   transverse limits to the coherence of a laser beam (e.g. beam   
   divergence, emittance, and brightness), but these are not important in   
   many applications.   
      
   > Can I simply say that a coherent beam is polar and the outcome of a   
   > polar system? Then the question becomes how to use a polar axis in   
   > the oscillative origin of coherent beams.   
      
   I have no idea what you are asking; I have never seen "polar" or   
   "oscillative" used in that manner (and lasers do not oscillate).   
      
   Tom Roberts   
      
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