It is fortunate that John Gastineau caught the error in my laser post. In
responding to William Beaty's neglect (IMHO) of population inversion,
stimulated emission and temporal coherence in his laser description and
giving all credit to the mirrors and spatial coherence, I sinned in the
opposite direction:
I left the easily inferred impression that stimulated emission (via pop
inv.) is almost alone responsible for temporal coherence (narrow spectral
linewidth) while the mirror geometry only adds wavefront configuration and
spatial coherence. This is wrong; the division of labor is not all that
sharp.
The balanced "truth" is that the spectral width obtainable from a
Fabry-Perot interferometer or F.P. interference filter is easily made much
narrower than the line width of the typical laser transition line. (Laser
resonator mirror configurations are simply variations on the Fabry Perot
theme, discussed in intermediate Optics texts.) As John correctly pointed
out, many F.P modes fit under the line profile of the laser transition
emission. The F.P. modes have the "last word" in selecting one or more
narrow slices of spectrum.
But passing the output of an ordinary "monochromatic" light source through a
Fabry Perot filter will not give you laser light coherence. Without
population inversion, there is a net absorption of this light as it passes
between the mirrors, and you get added only that fraction of the newly input
light which fits the F.P modes in frequency and direction. The combination
of population inversion and stimulated emission AMPLIFIES waves of the mode
frequencies and directions, specified by the mirror Fabry Perot geometry.
This amplification comes from energy that was NOT pumped in at the F.P. mode
requirements.
Thanks again, John. Further corrections/amplifications/clarifications are
solicited.
It is good to reason together and get things "straight".