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Hi Bill!
Perhaps there is our problem : temporal coherence cannot be ignored, it is
paramount! The radiation of the atoms from an ordinary "monochromatic"
source consists of truncated sine waves of finite length. The Fourier
transform of a truncated sine wave is a sinc function in frequency space,
with a non-zero line width which is inversely proportional to the duration
of the truncated sine segment in time.
In these ordinary sources, spontaneous emission dominates and there is no
correlation among the start and stop phases of the truncated sine wave
emissions of the various atoms.
It is my understanding that the laser
process is dominated by stimulated emission, in which an incident wave is
"cloned" by synchronizing an already excited molecule to emit before its
time, and in phase with the incident (stimulating) wave.
Spontaneous
emission is random in time (phase), there is no reference to which it might
be synchronized. The distinction is really a QED matter; I speak of the
effects in Maxwellian terms.
Other considerations which may be at issue:
In order for stimulated emission to dominate, one needs a population
inversion.
The mirrors provide the positive feedback which turns an
amplifier into an oscillator. The spatial wavefront pattern at right angles
to the propagation direction (spatial coherence) is a function of the
"waveguide modes" imposed by the mirror shapes.
Getting far away from a source will improve its use in a Young double slit
experiment (because you are asking only a tiny fraction of its immense
wavefront to be spatially coherent), but it will not improve its use in a
Michaelson interferometer - a measure of temporal coherence - related to the
duration of the separate (and randomly connected) wave trains [coherence
measured along the direction of propagation]. These phase discontinuities
do not "wash out" just by removing the source to a large distance away,
which is what I read you as saying.