Looking more at lasers, specifically for a 532 nm diode-pumped solid state laser, I’ve found two companies, Coherent and Klastech, that claim to solve the “green noise” problem with these. I didn’t know there was a problem! But apparently when various laser cavity modes interact with atomic electron state modes, the result can be a noisy mess. Klastech’s solution is based on a Fox-Smith interferometer, and from what I can tell Coherent’s solution depends on using a semiconductor for the 532 nm lasing material so that the relaxation time is faster than the cavity travel time for the photons, thus avoiding any of the unwanted interference that might happen in a harmonic crystal.
Klastech also claims that their design leads to “inherently single frequency” operation. They claim <1MHz linewidth, which is ~2E-9 of the frequency (~5.6E14).
One other note, I found another series of useful entries from RP Photonics’s Encyclopedia of Laser Physics and Technology, more on the M^2 beam factor, one of many quantitative measures of laser beam quality. RP explains it as how close a beam is to being diffraction-limited. The closer a beam is to being diffraction-limited, the easier/more accurately it can be focused down, and similarly the lower the beam divergence. Beam divergence is how fast the beam expands “far from the beam waist,” or in other words far from the focal point. Makes some sense, but how do you take account of, say, the focal length of the lens that’s being used to create the beam waist in the first place? Here’s an article by Siegman with more insight on the subject of beam quality: Defining, measuring, and optimizing laser beam quality.