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Re: A list of textbook misconceptions



On Sun, 15 Feb 1998, Leigh Palmer wrote:

But if IN-coherent light is sent into an amplifying medium, won't the
result be incoherent light which is brighter? A laser crystal
amplifies what it is given. It is transparent, so it doesn't alter
the wavefronts or change the coherence length.

No. The coherence time of the light will increase due to stimulated
emission. The amplification of this medium is highly nonlinear, the
gain being maximum near the center of the lasing frequency. Nearby
wavelengths will be amplified less.

Oops!!! I'm still working with a simplified model, as I said before. I'm
trying to, as far as is possible, strip all temporal coherence issues out
of my mental model of a laser, the better to understand spatial coherence.
By "coherence length" in the above, I mean the length measured
perpendicular to propagation direction.

Obviously the narrow absorbtion/emission frequency of a laser medium will
have significant effects on temporal coherence.

My question should have been written thus:

But if SPATIALLY incoherent light is sent into an amplifying medium,
won't the result be SPATIALLY incoherent light which is brighter? AS
FAR AS THE GEOMETRY OF PROPAGATING WAVEFORNTS IS CONCERNED, a laser
crystal amplifies what it is given. It's transparent, so it doesn't
alter the wavefronts or change the SPATIAL coherence length.


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