So, a laser works by bouncing photons back and forth between two mirrors until they straighten each other out and exit a small hole, like this:

laser formation

The problem is that no matter how much the photons straighten out and how small the hole is, it will never be perfect, and there will at least some diffusion.

My question is this: Once the laser beam has exited the device, how can the diffusion be reduced?

I haven't found anything online about how this is done, but I assume that it can be done since LIGO exists and I'm pretty sure they wouldn't just make the most concentrated laser they can and hope for the best.

Ideally, the process wouldn't cause the laser to be focused to a point, as this would still create diffusion, even if there is less of it; but beggars can't be choosers.


There is no straightening! Lasers rely on the induced emission of photons, which means that every new photon is identical to the one that caused its emission. That is they are identical in their frequency, direction, mode structure, polarization, etc.

There is indeed homogeneity due to various imperfections: the low quality of the resonator, Doppler effect in gas lasers, different level widths in atoms due to perturbations of crystal lattice, etc. This homogeneity varies depending on the type of the laser: lasing semiconductor diodes are among the worse, gas lasers (like Helium-Neon) are among the best, $H_2$ maser serves as the frequency standard.

  • $\begingroup$ So, what you are saying is that the diffusion can't be fixed? $\endgroup$
    – The Eye
    Apr 6 '20 at 9:45
  • $\begingroup$ I am saying that there is no diffusion - at least not in the sense that you described it, which is more applicable to the conventional light sources. In the same time, some of the lasers/masers have extremely high quality of emission, e.g. en.wikipedia.org/wiki/Hydrogen_maser $\endgroup$ Apr 6 '20 at 9:51
  • $\begingroup$ My question has little to do with how a laser is produced (be it conventional or quantum), and more to do with the reduction of an "error" that results from real world imperfections. Even with the hydrogen maser you have referenced, I expect there will be some noticeable difference between the radius of the of the beam when it first leaves the maser and the radius of the beam at some extreme distance, even in a vacuum. $\endgroup$
    – The Eye
    Apr 6 '20 at 10:01
  • $\begingroup$ However, this is precisely the point: laser/maser beam has very low divergence in comparison with the conventional light sources. The issue that you are talking about is simply not there. They projected a laser beam on the Moon and it made a spot of a few hundred meters. $\endgroup$ Apr 6 '20 at 11:18
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    $\begingroup$ I've deleted a number of inappropriate comments and/or responses to them. $\endgroup$
    – David Z
    Apr 7 '20 at 9:45

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