Is it true that longer laser cavities generally produce more collimated beams? I read that longer laser cavities generally produce more collimated beams. Is this true? And, if so, then why? The closest source of information that I could find is this Wikipedia article on collimated beam:

Laser diodes emit less-collimated light due to their short cavity, and therefore higher collimation requires a collimating lens.

However, this obviously does not explain why this is the case.
 A: The Wikipedia article is being a little misleading here. The beam from a laser diverges by an amount which depends on its width not length. The divergence is due to diffraction, and the angular spread of the beam is given approximately by
$$
\Delta \theta = \frac{\lambda}{w}
$$
where $\lambda$ is the wavelength and $w$ the width. But if a laser has a long cavity then clearly the angular divergence of the beam could not be very large or it would spread outside the laser cavity. In other words, a long cavity implies, in practice, that the laser mode is wide enough for not much spreading to be happening as it propagates inside the cavity. This implies that the beam waist cannot be all that small, so in practice the longer cavity will be associated with a less diverging beam. However there is no basic physical requirement that says it has to be this way. One could design a long cavity with a very diverging mode; the concentric design is like this.
Of course if you let the beam spread a bit and then put a suitable lens, then after the lens you will get a nice collimated beam no matter how divergent it was coming out of the laser cavity itself.
