Laser light behaves very differently than light from other sources. Textbooks give two reasons for this:

1. Laser light is monochromatic or very pure in color.
2. Laser light is coherent or "in-phase" light.

JUST STARTED, VERY UNDER CONSTRUCTION

But somehow this explanation just wouldn't stick to my brain. It didn't "fit" with everything else I knew. And worse still, I couldn't use the explanation as a tool. On one hand, the explanation of "monochromatic" was very useful in many situations. Pure color means single frequency, which implies narrow peaks in spectrum graphs and tiny spots on the radio dial. It connects with audio, where a pure tone such as the note from a flute is "monochromatic," while an impure broad-spectrum tone sounds like pink noise. And in holography, whenever the frequency of light moved up and down, I could imagine how this would slide the tiny diffraction patterns around on your film and make holography impossible. Clearly a hologram camera needs a very monochromatic light source.

On the other hand, how do I use those wiggling-snake photons to explain other things? Where does "coherent light" fit into radio antennas, loudspeakers and water waves? And if my laser isn't coherent enough to make holograms, can I draw a picture of the exact nature of the problem? It just didn't connect.

Well, after a few years in the physics business I finally figured it out. I just shoulda known...

That explanation is WRONG.

And most important of all... I learned that in-phase emission does not create "in-phase light." [It's important enough to say twice: coherent light isn't created by in-phase stimulated emittion!] Instead, in-phase emission only causes light amplification. It creates amplified, brighter light. When atoms in a laser are emitting EM waves in phase with incoming EM waves, the emitted wave combine with the incoming light, making it brighter. But amplification doesn't cause the coherence.

In-phase emission is also the basis for transparency of materials. For example, when atoms in a glass window absorb light waves, they re-emit those waves in phase, and the original wave is preserved. In-phase emission keeps the wave from scattering as it interacts with the atoms in the glass. The atoms in the laser rod or gas tube emit light in phase... making the material transparent, so it preserves whatever coherence that the light might already have. Those "in phase" diagrams are actually explaining transparency, and they never tell us how the light became coherent in the first place.

...UNDER CONSTRUCTION

If figure 1 is wrong, then what's right? If we could 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 looks like. Coherent light comes from a very small light source. Coherent light has another name: "sphere waves" or "plane waves."

A.	B.

A single small light source sends out electromagnetic waves in all directions as shown above. Of course this diagram is 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 more layers added in the middle. 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 passed through a converging lens, they focus to a point.

So coherent light is just "pointsource light?" Paraphrasing Feynman: Now I Understand Evvrrreeeeeeethiiing! Finally it all makes perfect sense: starlight is coherent, that's why Stellar Interferometry works: coherence length in hundreds of KM, starlight is far more coherent than any human-produced laser light. And stars are like the ultimate point sources. Then remember Dennis Gabor, inventing holography before lasers existed: to create pseudo-lasers he just took light from a mercury source and passed it through a pinhole. The mercury emission line made it monochromatic, and the pinhole gave it the spatial coherence. Pinhole pinhole, ever hear of a "Spatial Filter?" 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 aperture.

And finally I know why lasers are so wonderful: lasers are pinhole light sources which are actually bright! It's easy to make coherent light, just use an optically small pinhole (a halfwave diameter.) But an aperture this small will block nearly all the light from a conventional source. To experiment with this, get a slide projector and make a slide with a pinhole, a needle-perforated foil layer. Add a narrow green filter, and that's your 1940s laser source. In the past, monochromatic coherent sources used to be microwatt sources, no getting around it. Sending a milliwatt through a wavelength-diameter pinhole was basically impossible, but lasers do it easily by creating spherewave "pinhole light" right at the start, as if it came from a virtual pinhole. Aha, confocal resonator mirrors, the virtual pinhole in a laser is a real image pinhole sitting between the mirrors. Semiconductor lasers with parallel mirrors: they just put the point source at virtual infinite distance, way off down the "infinite mirror tunnel" so it behaves like the light from a distant star.

OK, if spatially coherent light looks like an expanding bullseye, then what does INCOHERENT light look like? In the above diagram 2A it looks like bunches of overlapped bullseyes. Lots of interference patterns, probably with the nodes moving around randomly. Or it looks like bunches of light rays where the rays come from several separate points in fig. 2b, and the rays all cross each other in the light beam.

Sources in decreasing coherence

• Bright cloudy sky (least spatially coherent)
• Fluorescent tube lamp
• Frosted incandescent bulb
• Clear incandescent bulb
• LED
• Laser
• Starlight (coherence leng 1000s KM)

A perfect 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 light, but then we send it through a frosted screen, the light remains monochromatic but it becomes incoherent. Hey, I see that we can actually buy incoherent-izer opto devices. 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."

And now I have the answer to a big question that plagued me in childhood. 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. The 1cm burning spot would appear anywhere along the parallel beam. But if you think about this, it turns out to be impossible. Adding lenses just creates a projector, where your parallel solar deathray becomes a wide image of the sun. No thin intense beam appears. The answer is simple: REPLACE THE SUN WITH A 10KM WHITE DWARF STAR! Keep the brightness the same, but shrink the sun until it appears in the sky like a star, like an intense pinpoint. Now use a 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. It's still a projector, but if you project the image of a pointsource into the distance, the result is a collimated beam. Other than a bit of diffraction, this should work great: a few hundred watts CW in a parallel beam 1mm wide. Slice off 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 sharp tool. Exactly, exactly. But the central concept is that "pinhole light" is the reason for the coherent sharp light that does the cutting. If our sun was 10KM wide, or reduced to 10^5 times smaller in visual angle, its light would be like a laser, and our retinas would be damaged quickly. The lens of your eye will focus the whitedwarf sunlight to a pinpoint rather than to a little solar disk on your retina. Only because our sun is an extended source, the 1.5 KW/m^2 sunlight doesn't act like laser light. If instead it was pointsource, then looking at the sun would be like staring into a 1.5mW laser pointer. Using binoculars would be lethally dangerous: 5000X smaller exit aperture, creating an eight watt parallel beam 1mm in diameter. Coherent light can be nasty.