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I don't see how you can use a retroreflector as one end of a laser cavity. They destroy coherence, since the path length is different depending on where each photon hits the reflective element. Don't lasing mediums require fairly coherent photons for stimulated emission to work?

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I think the path length may be constant. Retroreflectors can be used in Michelson interferometers.

https://patents.google.com/patent/US5309217

I stand corrected.

I don't think this will be a problem. A classic gas laser can have variations of more than 1 wavelength in its cavity without any issues. I believe this was one reason for the relatively short coherence length of HeNe lasers, but I could be wrong.

I bet you the lasers in these transmitting devices will not be gas but small solid state units. As this becomes more popular, like surface-mount camera dies, these SMD IR laser diodes will become smaller and smaller.

They are probably using semiconductor units (with lasers "solid-state" usually refers to non-semiconductor solid lasers, such as ruby lasers). My point with HeNe lasers was that having variations in the cavity length is not a problem for lasing.

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