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
right. 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. Of course if we graphed the field strengths on a voltage
axis, they would form sine waves, as in the first video. But the flux
itself points entirely sideways all the time, like closed rings with no
sine wiggles. And there isn't an "Aether medium" which wiggles
side-to-side. No little snakes flying through space.
And photons? ...the photons are either dimensionless particles, or they're
quantized wave-energy. They're either like infinitely small bullets
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"
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, the resulting 4 is purely a number,
and it isn't
concealing any 2 + 2. Instead it could be one plus three, or nine
minus five. I mean, when two in-phase waves add together to create an
wave, the original waves are gone. The larger wave doesn't forever travel
along as two smaller "inphase waves" like all those intro laser
explanations depict. Instead, all those diagrams should show that smaller
waves add together to create single, larger waves. Amplification, not
THE BAD DIAGRAM
The laser's in-phase emission appears in other places: it's the basis
for transparency of materials. For example, whenever atoms in a glass
window absorb light waves, they re-emit those waves in phase, so the
original wave is preserved and the material acts transparent. In-phase
emission prevents the light from scattering when it interacts with the
atoms in the glass. So yes, the atoms in the laser-rod or laser gas tube
emit light in phase... making the laser material transparent, and this
preserves whatever coherence that the incoming light might already have
had. 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 can visualize a
lightwave to be spherical. The waves are like layers of a spherical
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 perfect point.
Coherent light is just some:
So coherent light is just "pointsource light?" Paraphrasing Feynman: Now
I Understand Evvvvvrrreeethiiing! Finally it all makes perfect sense:
starlight is ULTIMATELY coherent, that's why Stellar Interferometry works:
starlight has coherence-lengths 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. I remember AA Michelson discovering that
Betlegeuse is far less coherent than other stars. Ha, far less like a
microscopic pointsource! 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 converging lens upstream: any "incoherent" parts of the
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 always been easy to make some
coherent 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, a bit
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
confocal/concentric resonator mirrors, the ones used in lasers? This
means that the "virtual pinhole" in an actual laser is just a non-virtual,
very real pinhole-image sitting in the space 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
mirror 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
sides of the tunnel. Only planewave light can persist in the tunnel and
So ...laser coherence is created by the mirror-tunnel. Not by stimulated
emission or 'stacked sinewaves." Or in proper
terms, coherence is created by the laser's Fabry-Perot resonator cavity,
and not by any sideways
packing of long narrow string-like "photons."
And all this means that we have a simple, gut-level intuitive picture
of laser coherence. What is it? It's just light produced by an infinite
mirror-tunnel with amplification. Sort of like those disco-era
mirror-infinity toys from Spencer Gifts.
But the depths of their tunnel wouldn't be dark.
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. If
into the depths of the Amplifying Disco Infinity Mirror, the "star" is
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 dynamically
swerving around. 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
As a little kid, did you believe that the light from clear incandescent
bulbs was more magical than the frosted ones? And the light of garage
welders was even more magical still? If so, you were intuitively
experiencing optical coherence. Your little brain was wanting to mess
around with laser sources, rather than overcast daylight.
A perfect ideal pointsource gives perfectly coherent light, while a wide
source gives the least coherent light. Turn the idea backwards: if we
start out with perfectly coherent laser light, but then we send it through
a frosted screen, the light remains just as monochromatic, but it becomes
incoherent. Hey, I noticed that we can actually buy an incoherent-izer,
opto device for our optical bench. They're just a rotating frosted screen
with a little motor (since an unmoving frosted screen still leaves a small
bit of micro-scale coherence or "laser speckle."
NO JPEG YET
REAL SOLAR DEATH-RAYAnd now I have the answer to a big question that plagued me in childhood. No doubt all the nasty little science-boys like me had thought of this one. Why can't I make a death ray light-source? I could just get my big plastic fresnel lens and focus sunlight, and 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 we think about this now, it turns out to be impossible. Adding extra lenses to our solar furnace just creates a projector, where our parallel solar deathray spreads out and becomes a wide image of the sun. The darned sun isn't a pointsource. No thin beam is possible unless we 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 extremely intense pinpoint. Now just use the 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 we 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!
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.
Lasers aren't particularly bright. Hundred watt light bulbs? 5,000 watt
spotlights for school play?? Or daytime sunlight?
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 0.3deg solar
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 really dangerous to
eyes until up around 5mW. AHA, but using binoculars would be bad, very
bad: 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.