Re: A list of textbook misconceptions

[William Beaty <billb@eskimo.com>]

On Wed, 11 Feb 1998, Leigh Palmer wrote:

> > > 13) Laser coherence is caused by atoms' in-phase emission
>
> Why is this a misconception?

Hi Leigh!

It is my understanding that in-phase emission only explains why a laser
medium is transparent, and why it has gain.  A pumped amplifier medium
would be more than transparent, it would preserve wavefronts but increase
the propagating wave's amplitude (by adding in more Huygens' Wavelets?) 
;)

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

(By "coherent" I mean "having large spatial coherence length.)

So, if laser medium amplifies light regardless of its coherence, why 
do lasers produce coherent light?

If I understand it correctly, the coherence comes about from the great
distance travelled by the light wavefronts.

Picture it like this: ruby crystal hit by strobe light.  Ruby crystal
flouresces incoherently.  Some of the waves hit the cavity mirrors and are
directed back into the crystal.  This incoherent light is amplified as it
travels, but its 3D waves also superpose in space as they go.  Random
phases and directions gradually cancel, leaving an average phase and
direction.  Amplification keeps the wave from dying away.  As the wave
repeatedly reflects from the cavity mirrors, it travels a great distance,
and eventually a single phase in a single direction remains.

It's like light from the sun vs. light from a star.  Sunlight is
incoherent,  starlight has coherence lengths of meters or km or 100s of
km.   (temporal coherence is MOSTLY a separate issue, at least when
explaining lasers in K-12)

Thought experiment: if we had a pumped laser rod which was 300,000 km
long, and we stimulated one end with a flashlight, coherent light would
leave the other end a few seconds later.  But if we replaced the laser rod
with a glass rod, the same thing would happen, only the light from the far
end would be somewhat dimmer. 

Or if we simply viewed a flashlight bulb from a distance of hundreds of
thousands of km, the light we received would probably be far more
spatially coherent than laser light.

Spatial coherence is linked to the size of an extended source.  A perfect
point source must be spatially coherent.

A laser essentially takes a flashlight bulb, moves it many light-seconds
distant, then preserves the light by amplification so it does not
grow uselessly dim.  Viola', the extended source is transformed into a
nearly-ideal point source.

Anyone who REALLY understands lasers can jump in at any time and correct
my misconceptions!

Textbook after textbook attempts to illustrate the coherence-creating
power of lasers by drawing little sine wave wiggles which mesh together
like stacked egg crates.  I say they are wrong.  The wiggle-line diagrams
only explain why laser media have gain and why they are transparent.  If
emission was not in-phase, then the laser medium would scatter and/or
absorb light.  But an amplifier is not a coherent lightsource.

To illustrate the source of coherence, draw a pair of parallel plane
mirrors, and draw the infinite virtual tunnel within them.  See the
infinitesimal point at the end of the virtual tunnel?  That is the source
of the laser light.  It's like an artificial star, shining at an
artificial stellar distance, all inside a 1-ft HeNe tube. 

I hope I'm wrong in all of the above, because when I arrived at my
understanding of laser coherence, I did not feel like a student who has
defeated a naiive conception.  I felt like a person who has been lied to
for decades.  Textbooks presented a very different picture of laser
coherence. (This is K-12 textbooks. I don't recall if lasers even appeared
in my undergrad physics texts.  I certainly hope that college-level
optics texts explain laser spatial coherence correctly!) 

Here's one very rare author who did not spread the misconception: Winston
Koch.  He said something to the effect that laser light is a "sharp tool." 
He pointed out that it is different than ordinary light not because of its
intensity, but because it can be focussed to an exceedingly tiny spot.



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