I read laser beams get wider, like a few feet wide by the time they hit the moon, Is that a manufacturing limit, or just something about the physics of laser light? Is a perfect laser beam that doesn’t get wider possible?

Question

I read laser beams get wider, like a few feet wide by the time they hit the moon, Is that a manufacturing limit, or just something about the physics of laser light? Is a perfect laser beam that doesn’t get wider possible?

In: 5280

Answer 1

Good question.

Obviously for focused laser beams, they are going to de-cohere past their focal point. That is a manufactured property.

But in the specific case of an un-focused collimated beam, like one you might shine on the moon, it really is actually about the physics of the laser light. For a given aperture size and emission wavelength, you cannot do better than a certain amount of decoherence no matter what you do or how perfect you make your laser.

However, that is for a given aperture and wavelength. So, if you are sneaky-minded, you might say to yourself, “So what if you had a really big aperture and a really short wavelength?” And in that sense, you could say that a “perfect” laser beam with no decoherence would be possible if you had an infinitely large aperture and an infinitely small wavelength.

In other words, not actually physically possible, but a cute mathematical thought experiment.