# There may be phase variations $\ge \lambda/4$ across the diameter of each mirror in any laser cavity?

I am currently studying the textbook Laser Systems Engineering by Keith Kasunic. Chapter 1.2.2 Spatial coherence says the following:

The concepts of spatial coherence and beam quality arise because there may be phase variations $$\ge \lambda/4$$ across the diameter of each mirror in any laser cavity.

The value of $$\lambda/4$$ seems quite specific, but, at the same time, the author seems to introduce it out of nowhere. Where does this value of $$\ge \lambda/4$$ come from? Is it derived from some physical property related to laser cavities?

In high quality optical systems, $$\lambda/4$$ is often considered the maximum allowed peak to peak variation of the optical path difference allowed in the beam that forms an image. Many also use a maxiumum allowed rms (root mean square) variation of 0.07 waves.
Since ideally a laser cavity would be diffraction limited, so that it produces a diffraction limited beam, then it seems reasonable to put a $$\lambda/4$$ limit on allowed phase variations (or optical path difference) on optics inside the cavity. In practice, laser manufacturers would typically specify much tighter specifications on mirrors: $$\lambda/20$$ would be common. They are trying to get as high a beam quality and power as possible out of their laser. Since I'm not a laser expert, I can't comment on how much phase variations affect the extraction of power from within the laser cavity.