Re: laser light

[William Beaty <billb@ESKIMO.COM>]

On Fri, 17 Sep 1999, Stefan Jeglinski wrote:

> > Kock is a personal hero of mine just because he refuses to say that laser
> > light is "in phase light" or "coherent light".
>
> Why would he refuse to use terminology that is well-defined and in
> fact a useful and even crucial way of distinguishing laser light from
> incoherent sources?

Because if we describe Laser light as "in phase light", then we spread a
misconception.  The typical high-school explanation for how lasers work is
wrong, and laser light does not act like a bunch of wiggling snakes all
meshed together "in phase."  See
http://www.amasci.com/miscon/miscon4.html#phase


> And why would that make him your hero? Just
> because he believes he is bucking the trend?

No, because he's one of the only people I've ever encountered who is aware
of this widespread "wiggling meshed snakes" misconception.  He "gets"
lasers, even at the very simple level.


> Fill me in here. You are
> fond of accusing others of "demonization of the opponent" (recent
> quote) when you believe they are being close-minded or too

Huh?  Really?  I thought I accused a single person of "demonization" one
time on PHYS-L (today) because he was specifically demonizing me in
accusing me of beliving in conspiracy theories.  If he says that I believe
in conspiracy theories, then he is dishonestly putting words in my mouth,
and I will certainly complain in the strongest possible terms when anyone
does this.

OK, now I see that you are accusing me of being fond of accusing others of
demonizing the opponent.  That is a very serious charge in my opinion.  I
was not aware that I had ever accused anyone of "demonization" in the
past.  Please present your evidence that I do this (show that I am
"fond"), and I will listen. I hope you have some.  If you do not, then I
think it is not *I* who indulges in "demonization"  on this list

Yes, I certainly do accuse others of closemindedness when I see behavior
which clearly is closed-minded in my opinion.  I hope that they will hear
my words, see the same closed-minded behavior, and cut it out.  Perhaps
this is the "demonization" you see me doing?  I had thought it was
something more equivalent to "hey, your condition looks bad, maybe you
should see a doctor about it before it gets worse, or before you become
infectious."


> establishmentarian, and yet you here appear to be foisting
> misinformation that in a very subtle way demonizes those of us who
> would in fact use very practical terminology from traditional schools
> of thought.

No, I am pointing out an actual error.  When two waves are added together
as a laser beam is created, the result is BRIGHTER light, not a special
form of "in phase" light.  The phase-locked emission which occurs during
stimulated emission leads to amplification, not to a special kind of "in
phase" light.  If incoherent light is fed into a pre-pumped amplifier rod
(no resonator mirrors), then *brighter* incoherent light, and not
"in-phase" light, will be emitted from the other end of the rod.  The
spatial character of laser light is caused by the resonator, not by the
"in phase"  emission of waves as pumped atoms dump their stored energy in
microscopic processes.


> Jeez Bill, sometimes it's as if you got some chip on your
> shoulder about those of us who got stupid PhDs or other "traditional"
> degrees and sometimes have something to offer in the way of technical
> knowledge.

Maybe.  I certainly have intentionally pursued the non-academic path,
after (I hope) clearly seeing the pluses and minuses in engineering versus
academia.  I regret that I have had so little opportunity to teach
students.  I hope I don't have much buried jealousy regarding this issue.
Mostly I just feel sadness.

> > Instead he says that laser
> > light is "sharper" than the light from other sources (which is exactly
> > correct.

> And so unbelievably vague that to use the term "exactly correct" is a
> real disservice to everyone on this list.

I think I can explain the reason why I make this assertion.  It is
contained in http://www.amasci.com/miscon/miscon4.html#phase


> > Point-source light can be focused nearly to a geometrical point,
> > while the light from non-laser sources cannot.)

> This is really sloppy at best. Laser light is not point-source, if
> that is what you're implying. Virtually by definition, laser light
> has a finite waist size.

Of course.  By this reasoning, lasers are not "point source" emitters, but
then neither is most anything else.  (Infinitely distant stars the
exception, if "point source" means "infinite transverse coherence
length"?)  However, to a first approximation, light that's purely
plane-wave or purely sphere-wave is the very definition of "point-source
light."  Send any light through a spatial filter, and you "sharpen" it and
"point-source-ify" it, even if you can never approach the condition of
perfect zero-diameter beam waist.


> And, even an ideal point source cannot be imaged to a geometrical
> point (when you say "focused" you are likely to be talking about what
> is seen in the image plane of a lens, but I am not sure - sloppy
> terminology will kill you here).

Yes, "focused" by a simple lens.  When we focus the light from a non-laser
source, we see a little image of the filament, or of an LED, or of an
electrical arc, etc.  When we focus the light from a laser, (say a
single-mode laser for simplicity) we see something that resembles a
geometrical point, at least in the first approximation "high-school
level" mental model of a laser.

> Check Goodman (or any decent text on
> Fourier Optics). Convolution with a finite impulse response (true of
> all actual lenses) prevents a geometrical point image.

Of course.  A real "point source" emitter would have to have infinite
aperature size.  (Maybe a solid state laser made in the form of a
hemisphere, and resting upon a plane mirror would qualify as an emitter of
true "point source" light? )

> Laser light leads to other effects because of its [typically]
> gaussian spatial nature. I admit to not knowing who your man Kock is,
> but I dare say that most if not all texts on Fourier Optics might as
> well be burned if the unique properties of coherence were replaced
> with the rubbish of "sharp."

"Sharp" is a mental model.  It is not intended to replace any advanced
models.  In my opinion, the very first thing a kid should learn about
laser light is that it is very pure in frequency (temporally coherent) and
that it is point-sourcy or "sharp" (small apparent source diameter,
spatially coherent.)  First show what "coherent" means, then later show
just how "coherent" the light really is.

I only understand lasers at the undergrad level. However, this doesn't
mean I cannot juggle non-math visual models of laser operation, or that I
cannot recognize a widespread "textbook misconception" when I see one.

> Get real. I'll keep Goodman because it
> will allow me to get something done. You may dance with Kock and have
> fun with those who don't use the word "sharp."

This is as it should be.  In K-12, and perhaps at the undergrad level, the
"sharp" concept (which same as the pure planewave/spherewave concept)
explains a lot. (Like, how can we explain spatial filters to my
grandmother?)


> By the way, with enough apparatus, I believe one can make virtually
> point-source sunlight that is highly collimated and also rather
> coherent as well.

Cool!  Is this with non-imaging concentrators?  I suspect that a
funnel-shaped optical fiber could concentrate significant sunlight into a
small aperature (like a single-mode fiber), and then if followed by a
lens, this would form a nice parallel beam.  Basically just a spatial
filter, no?  I wonder if the intensity of such a "parallel sun ray"  could
ever be stronger than the intensity of unmodified sunlight?  Wouldn't that
violate Thermo laws, and result in an entropy reversal or something?  (The
intensity of the beam's apparant source would be brighter than the surface
of the sun.)


((((((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))))
William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  http://www.amasci.com
EE/programmer/sci-exhibits          science projects, tesla, weird science
Seattle, WA   206-781-3320          freenrg-L taoshum-L vortex-L webhead-L