Re: A list of textbook miscon: spatial coherence

[William Beaty <billb@eskimo.com>]

On Thu, 12 Feb 1998, Leigh Palmer wrote:

> > Hi Bob!  What part of my long diatribe seems to be discussing temporal
> > coherence?  While writing it, I was visualizing a set of atoms which emit
> > perfectly monochromatic light ("temporally coherent"), so temporal
> > coherence issues can be ignored.  Either some of my thoughts are confused,
> > or I didn't succeed in stating them clearly.
>
> I think you are using an unorthodox definition of coherence.

This could very well be.  I know the definition.  It's the implications,
the "concept net" where I lack expertise. 

For example, suppose I diverge a "LOSER" beam and bounce the light off an
unmoving diffuser 2cm in diameter.  :)  Does the spatial coherence of the
reflected light become much smaller as compared to the original beam?  It
certainly is no longer a point source.  It has huge number of intersecting
"antenna lobes" seen as "speckle".  Yet in theory it can still serve as a
reference beam for holography, as long as the same diffused laser source
and positioning is used to reconstruct the hologram.  Is diffused laser
light still spatially coherent, or is it "nearly incoherent?"  What if I
bounce the diffuse light off of another diffuser, so even the speckle
vanishes?  Is it spatially incoherent yet?

If I measure the spatial coherence of my diffuse laser light at a distance
of 1cm from the diffuser and at a distance of 1km from the diffuser, will
I find that spatial coherence has increased for the light wavefronts at
1km distance? 

I don't have answers to these questions.  I want to refine my "spatial
coherence" concept.  Depending on the answers, my original #16 entry might
require modification or removal from the list:

 16) Spatial coherence of laser light is not caused by atoms' in-phase
     emission. It is caused by the great distance that the wavefronts 
     travel as they reflect within the laser cavity.


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