Laser rays, beams and divergence

//21 December 2014

I guess no one says “laser rays” in English, but in Bulgarian the two words for ray (infinitely narrow) and beam (as a shaft or bundle) of light are frequently interchanged. This purely linguistic difference could actually teach us a bit how divergence works.

One of the most well-known and fascinating properties of laser light is that it’s collimated. The beam stays narrow as it propagates in space, unlike the light emitted from the Sun, the lamps at home or any other typical source of light. A truly unique and very useful quality that I will write about in detail some other time.

Now, there is no such thing as perfect collimation and sooner or later every laser beam starts to diverge due to diffraction of light. It is very important to know the divergence of a laser and what is needed for certain applications. This is because, obviously, you cannot have minimal divergence without sacrificing something else. And here comes the ray/beam difference.

We have a collimated light beam that passes through an aperture with width D. Imagine that’s where the beam leaves the resonator. When it’s free to go, it would slowly start spreading out (divergence) and we could estimate the angle alfa (half the angle of the spread) as seen above. In this case k is a positive constant with value near 1, so we could ignore it. So let’s look at D. If it was indeed a laser ray, that implies that D is zero. But as D approaches zero, alfa approaches infinity which means we get a huuuuge divergence and lasers are supposed to have small(er) divergence angles. That’s why it’s called a laser beam with finite width and the bigger D, the smaller the angle, the better the collimation. But we get a big laser spot. So it’s either a narrowly focused beam with big divergence, or a wide beam with small divergence angle.

Also note that alfa is proportional to the wavelength lambda. Then, if we have a constant width, the laser with the shorter wavelength will have weaker divergence. That’s why there’s an interest in blue lasers: with them more information could be recorded.

More in-depth info awaits in my future post on Gaussian beams.

This post was originally published at Photons hit Electrons, a collaborative geek blog mostly ran by Deyan Levski.

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