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

>Is a perfect laser beam that doesn’t get wider possible?

No, at every interface or slit light “bends a corner”; we call this diffraction. A perfectly focused light is limited by the diffraction it experiences, which we call the diffraction limit.

Why light diffracts can only be explained by wave theory, and you can observe it by splashing water waves through a slit; it’s simply how waves “do”, and yes it can be a pain when building optical systems.

This same limit applies to all optical systems like microscopes, fiber optics, telescopes, etc., not just lasers. The fundamental thing is: we need to guide light, so we use optical elements, and slits and interfaces will always cause diffraction.

However, lasers continue to have a uniquely low divergence, which is why we use them nevertheless.

We use several numbers to characterize how “good” the laser beam is, but they mostly derive from the M^(2) number; if it’s 1, then you have a beam so good that if not for the diffraction at the slit, it would not diverge and remain a “straight beam” forever (i.e diffraction limited beam). If M^(2) is larger than 1, then it’s not ideal, and usually it isn’t, we can have perfectly optimal systems with larger values for M2.