User talk:Mpolyanskiy

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M squared
I reverted you at M squared. It's not obvious, but for a given laser cavity, a higher-order mode with M2 > 1 is M times larger in diameter everywhere, compared to the lowest-order (Gaussian) mode of that cavity. The beam gets larger both in near field and far field. If you consider two beams with the same diameter in the far field, you would be correct that the spot would be M2 larger in diameter at the waist.

Putting that another way: you are absolutely correct that for a given size input beam, focus size scales as M2. If you're considering the output of a given laser cavity, however, when the laser transitions from the lowest-order mode (M = 1) to a mixture of higher-order modes, the collimated beam size increases by M and the waist size also increases by M, so that the beam parameter product scales as M2.--Srleffler (talk) 03:24, 11 January 2018 (UTC)