Re: A list of textbook miscon: spatial coherence

[SCIAMANDA@edinboro.edu]

Bill Beaty wrote:
> . . .
> To simplify my laser, I assume that all atoms emit very long wavetrains. I
> assume that the wavetrains are so long that any phenomena associated with
> finite wavetrain lengths and broadened line widths can be ignored.  If the
> . . .
> William J. Beaty                                  SCIENCE HOBBYIST website

Hi, Bill!
In trying to simplify the laser for your K-12 audience, I think you are 
grievously sinning against Einstein's 11th commandment (paraphrased):
"Thou shalt make things as simple as possible, but no simpler!"
You are also short-changing the laser and its inventors.  You have made 
assumptions which obviate the need for a laser.

The triumph of the laser was not so much to provide a mechanism for 
enhancing spatial coherence as much as harnessing poulation inversion 
and stimulated emission (both absent in your model!) so as to provide 
a new mechanism for enhancing TEMPORAL coherence.

At radio and microwave frequencies we can make macroscopic, temporally 
coherent sources by driving electrons into synchronized oscillation 
at a single frequency.  Given this temporal coherence, wavefront 
configuration and spatial coherence can be controlled by well known 
antenna/waveguide/mirror techniques.

These electronic techniques for producing temporally coherent radiation 
(a "pure" frequency) by brute force are not practical at IR and higher 
frequencies.  Wavelengths are too small and frequencies are too fast.  
In these spectral regions we must abandon the forced 
oscillation of electrons and harness intra atomic, quantum mechanisms of 
emission. This was the triumph of the maser/laser, through population 
inversion and stimulated emission.  Adding the resonant cavity (mirrors) 
was really no big deal; it was not the invention of any new technique. 
(At these wavelengths, even the frequency selectivity of the resonant 
cavity is not the main contributor to temporal coherence (linewidth).)
Laser amplifiers - without mirrors - are in use and have even been 
reported to occur as natural phenomena in space.

In short, given the extreme temporal coherence which you assume to 
already exist with ordinary sources, no fancy wierdness is needed to 
achieve (or explain) spatial coherence.  If we had such temporally 
coherent sources (in the visible, etc) there would have been no need for 
Light Amplification by the Stimulated Emission of Radiation. 

-Bob

Bob Sciamanda           sciamanda@edinboro.edu
Dept of Physics         trebor@velocity.net
Edinboro Univ of PA     http://www.edinboro.edu/~sciamanda/home.html