How much would a laser spread out in space?

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Discussion Overview

The discussion centers on the spread of a laser beam in space, particularly in the context of future experiments involving lasers between satellites. Participants explore the factors influencing laser divergence and seek ballpark estimates for this phenomenon.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant inquires about references for laser spread in space and mentions upcoming satellite experiments.
  • Another participant states that the spread depends on the precision of the apparatus, suggesting that in an ideal scenario, a laser would not spread out.
  • A different participant proposes that if a laser were perfect, it would diverge as a Gaussian beam, providing a formula for far-field divergence, while noting that real lasers are not perfect and will diverge more than the ideal case.
  • Another viewpoint introduces the concept of diffraction effects when a beam passes through a circular aperture, referencing the Rayleigh criterion to determine beam width at a distance.
  • One participant suggests researching lasers used for ranging the Moon, hinting at practical applications and historical context.

Areas of Agreement / Disagreement

Participants express varying views on the ideal behavior of lasers and the factors affecting their divergence. There is no consensus on specific numerical values or definitive outcomes regarding laser spread.

Contextual Notes

Limitations include assumptions about the ideality of lasers, dependence on specific parameters like wavelength and waist radius, and the lack of concrete numerical examples in the discussion.

Albertgauss
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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.
 
samblohm said:
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 ??
 

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