Lasers: What is Coherence?

A bad textbook diagram, and a widespread misconception

William Beaty 2004

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

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

What does "coherent" mean?

JUST STARTED, VERY UNDER CONSTRUCTION

As a kid I was always confused by explanations of laser coherence. I'd been told that coherence something to do with the sinusoidal shape of photons. Light is transverse waves, so we should imagine that photons look like little wiggling snakes. Whenever all the "snakes" pack together side by side with their wiggles aligned, that's "coherence." The atoms in a laser are all emitting their light in locked phase, and the end result is supposedly special kind of in-phase light where the little sine-waves stack together like egg cartons.

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.

The explanation of coherence found in most introductory textbooks is pure garbage. Light is not a transverse wave. Or more specifically, it's not a transverse wave in the "Ether." A photon isn't shaped like a wiggling snake or like a wave on a string. The "stacked wiggles" explanation falls apart.

And most important of all... I learned that in-phase emission does not create "in-phase light." Instead, in-phase emission creates amplified, brighter light. When atoms emit in phase with incoming waves, the emitted wave adds to the incoming light, making it brighter.

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

...UNDER CONSTRUCTION

Fig. 1 The bad diagram. Light does not behave like this.

If figure 1 is wrong, then what is 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. That's what perfectly coherent light looks like. Coherent light comes from very small light sources. Coherent light is also called "sphere waves" or "plane waves."

[tiny dot sends out a bullseye shape of red waves]
[tiny dot sends out a sunburst of red rays]

A. B.

Fig. 2 A tiny light source emits waves and/or particles

A single small light source sends out electromagnetic waves in all directions as shown above. Of course this diagram is two-dimensional, while the real situation is 3D. Imagine a lightwave to be spherical, like layers of an onion, but where the onion is expanding at the speed of light, and where the tiny light source is adding more layers in the middle. OR... we could imagine that the tiny light source is sending out a stream of particles in all directions. The paths of these particles are the "rays" of light.

OK, if spatially coherent light looks like an expanding bullseye, then what does IN-coherent light look like? It looks like bunches of bullseyes, or it looks like bunches of light rays where the rays cross each other. If we send our coherent light through a frosted screen, it becomes incoherent.

Fig. 3 A frosted screen makes light incoherent.