# How much would a laser spread out in space?

Gold Member

## Main Question or Discussion Point

Hi all,

Does anyone know of any references on how much a laser would spread out in space? I'm not sure if this is the right place for this, but I know there are various experiments planned in the future for space that rely on lasers going back and forth between satellites. Just looking for ball park numbers.

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That all depends on how precise and accurate the apparatus is. Assuming it is perfect, which it isn't, it never would. I don't really have numbers for you though.

D H
Staff Emeritus
Assuming it is perfect, which it isn't, it never would.
Assuming it is perfect, it would diverge as a Gaussian beam. The far-field divergence is given by
$$\theta \simeq \frac{\lambda}{\pi w_0}$$where λ is the wavelength and w0 is the waist radius. Even a laser isn't perfect, so the divergence is always more than that for a Gaussian beam.

One way of looking at it would be through the diffraction effects from passing the beam through a circular aperture, using the Rayleigh criterion.

Once you find out where the first minima occurs you can find how wide that beam is at any given distance by multiplying the angle by the distance to the target, assuming small angles. You can have a play around with the numbers with this app.
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/cirapp.html

Research the lasers used to 'range' the Moon via the retro-reflectors at Apollo sites and on Lunokhod (sp) rover ??