Re: Laser light

[William Beaty <billb@eskimo.com>]

On Thu, 5 Dec 1996, David Winsemius wrote:

> billb@eskimo.com wrote:
> > LASER LIGHT IS NOT "IN PHASE" LIGHT 
> > When light waves of various phases are combined, they inextricably
> > add together. When the light within a laser causes atoms to emit light
> > in phase with the stimulating beam, the result is not "in phase"
> > light, the result is a single, more intense, amplified wave of light.
> > Single waves are always in phase with themselves, but it's misleading
> > to imply that a single wave is something called an "in phase" wave.
> > Laser light could more accurately be called "single wave" light, or
> > "pointsource" light. The physics term for this is "spatially coherent"
> > light. Light bulbs, flames, the sun, etc., emit "extended source"
> > light. Starlight and the light from arc welders is "pointsource" light 
> > and is quite similar to laser light; both of these give light which is
> > highly spatially coherent. 

Hi David!  I'd be happy to hear arguments and improve my stuff [or take
the chance of getting ego boost from finding I'm right after all! ;)  ]

> Laser light is different because it is emitted from an excited 
> population of electrons which are "in phase" because the electrons 
> which were in a metastable state have been decayed in sync with the 
> advancing wave train.

Well, yes and no.  The key is "advancing wave train."  The photons emitted
from the excited electron population are phase-locked with the stimulating
photons.  However, the resulting light beam will have high spatial
coherence only if the *stimulating* light had high spatial coherence to
begin with.  The stimulated emission process is not the source of
coherence.  If ten photons having random phase relationship hit ten atoms
and cause stimulated emission, the resulting twenty photons constitute an
incoherent beam which is twice as bright.  So where does the high spatial
coherence of lasers come from?  Not from the atoms, since they can only
amplify what they are given.

My main point is that the high spatial coherence of laser light comes from
the fact that the light has travelled an immense distance via the
mirror-bounces, and any separate wavefronts which were initially
incoherent have had a chance to superpose and create a single wave where
various points across the wavefront now have the same phase.  You're
aware that starlight has extremely high spatial coherence, with coherence
length measured in hundreds of KM?  Similar effect. 

> Since free photons are wave-particles (not 
> waves) I believe you are propagating the incorrect Huygens model by 
> referring to the light beam as though it is this homogeneous. 

Partly true.  This is the difficulty that comes from explaning QM concepts
to 6th grade kids.  However, the concept that "light=waves" goes deep:
even single-photon light creates interference patterns, implying that even
a single photon does homogeneously fill space (or in virtual-particle
terms, that even a single photon is associated with staggering numbers of
virtual photons, and they fill space.)  As a concequence I believe that
the usual idea of light being a hail of particles can be misleading. 
While flying through space, light acts like a wave which homogeneously
fills space.  When interacting with matter, it acts like point particles. 

> The fact 
> that it is emitted from a point source is also not what distinguishes 
> lased light from incoherent light. If you finely collimate an 
> incoherent light source you do not make it coherent, even though you 
> make it a point source.

I have to disagree.  If I pass the light from a frosted light bulb through
a spatial filter (a lens and pinhole), I greatly increase its spatial
coherence.  The smaller the pinhole, the greater the coherence.  Light
from a perfect point source (or infinitely small pinhole) has perfect
spatial coherence.  Gabor used spatial filtering to convert sodium-vapor
light into "pseudo-laser" light.  The line-emission of the gas provided
the temporal coherence, while the spatial filter provided the spatial
coherence. 

> Lased light is phase coherent even when it is 
> not from a point source.

But if I bounce a laser beam off white surface, the spatial coherence of
the light coming from the surface is terrible.  It may still be
phase-locked, but that doesn't mean it has high spatial coherence. 

Maybe we have terminology difficulties?  By "spatial coherence" I mean the
coherence length measured transverse to the propagation direction (as
opposed to "temporal coherence", the coherence length measured in the same
direction as the propagation.)  Starlight is an example of light with high
spatial coherence and low temporal coherence.  Light from a gas discharge
tube is an example of high temporal coherence and low spatial coherence. 
Laser light is both, it's like single-frequency starlight.  And if I shine
a laser at white paper, I'd call the reflected waves "phase-locked yet 
incoherent". 

Can anyone on phys-L poke holes in my reasoning?  I'm convinced that
stimulated emission causes brighter light, not "in phase" light, and the
light-amplifying properties of lasing media is not the source of the
coherence of laser light. 

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