How can laser diffusion be reduced?

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:

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 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.