XPost: sci.optics.fiber   
   From: siegman@stanford.edu   
      
   In article ,   
    Joseph Gwinn  wrote:   
      
   >   
   > Why would uniform gain yield stable modes, while uniform loss would   
   > yield unstable modes?   
   >   
      
   Short answer:  Higher-order modes have lower filling factors in the core   
   (i.e., they're 'larger', with more of their energy outside the core and   
   in the cladding) than do lower-order modes.  The core is lossy.   
   Therefore, higher-order modes have _lower_ losses than higher-order   
   modes, and therefore survive longer (i.e., propagate further down the   
   waveguide) than do lower order modes.   
      
   Any kind of random scattering along the waveguide scatters energy among   
   the modes; or any kind of deviation from the lowest-order mode in the   
   excitation at an input to the waveguide puts some energy into the higher   
   order modes -- and these modes last longer and eventually dominate.   
      
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