Lasers: WTF is Coherent Light?
EM field strengths graphed along a straight line.
*NOT* a plot of flux lines.
The expanding onion: flux lines surrounding a tiny light source.
No wiggling snakes in the field structure.
If we could see light and radio waves, might we find any little
sinewave-snakes anywhere? Nope. Take a look at the second video above
It's an animation of flux lines surrounding a very tiny light
source. The EM waves expand like layers of an onion.
The flux lines break loose from the source, close upon themselves to form
loops, then fly off into space. The field strengths of course would form
sine waves, if we graphed them as in the first video. But the flux itself
sideways all the time. There is no Aether "medium" which wiggles
side-to-side. No little snakes flying through space.
And photons? ...the photons are either dimensionless particles or
quantized wave-energy; they're either like infinitely small bullets flying
in straight lines, or they're like enormous expanding EM pond-ripples from
a thrown pebble. Photons aren't shaped like twisty snakes, they're
nothing like a transverse wave on a string.
In other words, the entire crazy "stacked wiggles" explanation of Coherent
Light falls apart.
And even more important than all of the above... I realized that the
in-phase emissions in lasers don't
even create any "in-phase light" in the first place! [It's important
enough to say twice: coherent light isn't created by in-phase stimulated
emission. That's a big one.] In-phase emissions are important of course.
But they only cause
light amplification. They create amplified, brighter light.
Whenever atoms in a laser are emitting EM waves in phase with incoming EM
waves, the emitted waves add to the incoming light, making it brighter.
Two plus two equals four. But amplification doesn't create any "in phase
light." If two plus two is four, Four is purely a number, and it isn't
concealing any two-plus-two, instead it could be one plus three or nine
minus five. I mean, when two waves add together to create an amplified
wave, the original waves are gone. The larger wave doesn't forever travel
along as two smaller "inphase waves" like all those intro laser
THE BAD DIAGRAM
The laser's in-phase emission also is the basis for transparency of
For example, whenever atoms in a glass window absorb light waves, they
those waves in phase, so the original wave is preserved and the material
acts transparent. In-phase emission prevents the light from
scattering as it interacts with the atoms in the glass. So yes, the atoms
in the laser-rod or laser gas tube emit some light in phase... making the
material transparent, so it preserves whatever coherence that the incoming
light might already have have. Those "in phase" textbook laser diagrams
are actually, heh, explaining transparency, and they never bothered
to tell us how the light became coherent in the first place.
you may need to un-learn it before you can understand coherence.
Coherent light does not behave anything like this.
If fig. 1 is wrong, then what's right? If we could actually see
individual light waves, what would coherent light look like? Fortunately
the explanation is quite simple. Take a look at figure 2A below. That's
what perfectly coherent light would look like if we could see the waves.
Coherent light is simple: it's light which comes from a very small light
source. Spatially coherent light has another name: "sphere waves" or
"plane waves." Or even simpler: "pinhole light" or "pointsource light."
A single small light source sends out electromagnetic waves in all
directions as shown above. Of course these diagrams are only
two-dimensional, while the real situation is 3D. We should imagine a
lightwave to be spherical, like layers of an onion, but where the onion is
expanding at the speed of light, with new layers constantly added in the
OR... we could imagine that the tiny light source is sending out a stream
of particles flying off in all directions. The paths of these particles
are the "rays" of light. Since they all fly outwards from a single point,
none of the rays cross each other. And if this light is passed through a
converging lens, it's focused to a point.
So coherent light is just "pointsource light?" Paraphrasing Feynman: Now
I Understand Evvrrreeeeeeethiiing! Finally it all makes perfect sense:
starlight is ULTIMATELY coherent, that's why Stellar Interferometry works:
starlight has coherence length in thousands of KM, starlight is far more
coherent than any human-made laser light. And the most distant stars are
like ideal point sources. Then I suddenly remember Dennis Gabor,
inventing holography before lasers existed. To create his pseudo-lasers
he just took light from an ordinary mercury-arc lamp and passed it through
a pinhole. Mercury's emission line made it nearly monochromatic, and the
pinhole gave it the spatial coherence.
Pinhole pinhole, ever hear of an
optics device called a
They're used to 'clean up' laser light and make it
much more spatially coherent. A Spatial Filter is just a very small
pinhole with a lens upstream: any "incoherent" parts of the beam will
never make it through the tiny aperture. It restores the
point-sourcey-ness to the imperfect laser.
And finally I know why lasers are so wonderful: lasers are pinhole
light sources which are ...actually bright! It's easy to make some
light, just use a normal light source and an optically small pinhole (a
halfwave diameter.) But an
aperture this small will block nearly all the light from any conventional
source. To experiment with this, get a slide projector and make a slide
with a pinhole: an Al foil layer perforated by a needle. Add a narrowband
green filter, and that's your Gabor-approved 1940s laser source.
Make some holograms? Heh, long exposure-time though.
unattributed diagram found in online archives.
In the distant past, monochromatic coherent sources were also
microwatt light sources, no getting around it. Creating coherent light
meant throwing away almost all of the power. Sending many milliwatts of
light through a wavelength-diameter pinhole was basically impossible.
So, all the bizarre and wonderful capabilities of lasers were unreachable.
But lasers easily solved the problem by, right at the start, creating some
spherewave "pinhole light," as if their entire light output came from a
virtual pinhole; a pinhole less than 500nM across. Aha, those
resonator mirrors, the ones used in lasers? This means that the virtual
in an actual laser is just a non-virtual pinhole-image sitting between the
mirrors. (See wikipedia diagrams for optical cavities,
http://en.wikipedia.org/wiki/Optical_cavity) And all of those
Semiconductor Lasers with parallel mirrors: they just employ an "infinite
tunnel" to put their pointsource at virtual-infinity distance, where it
behaves just like the light from a distant star. During its trip down
the infinite tunnel, all the non-planewave light wanders out the side of
the tunnel. Only planewave light can persist in the tunnel and get
Laser coherence is created by the mirror-tunnel. Or in proper
terms, created by the laser's cavity, and not by any sidways packing of
tiny wiggle-shaped "photons."
And all this means that we have a simple, gut-level intuitive picture
of laser coherence. What is it? It's light produced by an infinite
mirror-tunnel with amplification. Sort of like those disco mirror
infinity things. On each reflection the light passes through the laser
medium and gets slightly brighter. Viewed from the end, every deeper
segment of the "tunnel" appears slightly brighter ...and the far end of
the tunnel looks like an infinitely bright, infinitely tiny star. Stare
into the depths of the Amplifying Disco Mirror, and the "star" is small
and bright enough to punch a hole right through your retina. And it
doesn't even have to be very bright to do this! A hundred-watt
incandescent light bulb doesn't slice up your retina, but a quarter-watt
laser can burn a tattoo permanently into the back of your eye.
"Coherent" can also mean "sharp when focused," since focused Coherent
light will all converge to an infinitely small point. (Yeah yeah
diffraction limit. We're talking simple idealized geometrical optics
OK, if spatially coherent light looks like an expanding bullseye, then
what does INCOHERENT light look like? In the above diagram 2A,
incoherence instead would look like bunches of overlapped bullseyes.
Lots of interference patterns, and probably with the nodes moving around
randomly. Or it would look like bunches of light rays, but where where
the rays come from several separate points in fig. 2b, and the rays all
cross each other throughout the light beam.
With our gut-level intuitive understanding of laser Coherence, we can now construct a basic list of coherent light sources
Sources in increasing coherence
NO JPEG YET
REAL SOLAR DEATH-RAYAnd now I have the answer to a big question that plagued me in childhood. No doubt all nasty little science-boys like me had thought of this one. Why can't I make a death ray light-source? I could just focus sunlight with my big plastic fresnel lens, then somehow collimate it into a half-mm beam. The 0.50mm burning spot would appear anywhere along the parallel beam miles long. Write CHAIRFACE on the freakin' moon! But if you think about this, it turns out to be impossible. Adding extra lenses to your solar furnace just creates a projector, where your parallel solar deathray spreads out and becomes a wide image of the sun. No thin beam is possible unless you include a tenth-micron pinhole in the optical path, and that turns the power into microwatts. The solution to the problem is simple: JUST REPLACE THE SUN WITH A 10KM WHITE DWARF STAR HA HAAAA! Keep the sun's brightness the same, but shrink the sun until it appears in the sky like a tiny star, like an intense pinpoint. Now just use your big lens to gather a square meter of sunlight, focus it down to 1mm, then collimate it with a 1mm water-cooled short-focus quartz lens stolen from an ultraviolet microscope. Yes, the whole device is still a projector, but if you project the image of a pointsource into the distance, the result is an intense collimated beam. Other than a bit of diffraction it should work great: a few hundred watts in a parallel CW beam 1mm wide. Slice-a offs you fingas! Winston Kock, one of the early laser people at Bell Labs, said that laser light is "sharper light" which can be used as a cutting tool. Exactly, exactly! Winston Kock actually gets it.
But the central concept actually is that coherence or "pinhole light"
is the whole reason for the "sharp light" that does the laser cutting.
If our sun was 10KM wide, or reduced to 10^5 times smaller in visual
angle, then its light would be spatially coherent like lasers, or like an
welding arc, and glancing upwards during the day might slice grooves
across our retinas. The lens of your eye will focus the white-dwarf
sunlight to a pinpoint rather than to a dim and safe little solar disk on
your retina. Only because sunlight is
non-parallel, because our sun is an extended source, the 1.5 KWatt/m^2
sunlight doesn't act like dangerous laser light. Hmmm, hold on a sec. If
sunlight is about 1500 watts per square meter, and your eye's pupil is
about 1mm, then your pupil intercepts 1500W/.001^2 = 1.5mW. DOH! WRONG!
OK, staring at white-dwarf sunlight would actually be just like staring
into a cheap laser pointer. Those things don't become dangerous to human
eyes until up around 5mW. AHA, but using binoculars would be lethally
dangerous to your eyesight: 5000X smaller exit aperture, creating an eight
watt parallel beam 1mm in diameter. Binoculars become like icepicks aimed
at your eyeballs. Coherent light can be nasty.