Authored By Hui Chen
The answer is no, POP does not support M2 value as an input parameter to define the initial beam. M2 is a parameter that’s often used by laser engineers to describe the quality of a laser beam. A Gaussian laser beam is described by a beam waist size, a wavelength, and a location in object space. The Gaussian beam is an idealization that can be approached but never attained in practice. For aberrated, mixed-mode beams, an extension to the fundamental Gaussian beam model has been developed by Siegman [1]. The method uses a term called the beam quality factor, usually denoted by M2. The factor M2 can be thought of as "times diffraction limited" number. An ideal Gaussian beam has an M2 value of unity, but real lasers will always have an M2 value greater than unity. The M2 factor determines the size of the real, aberrated Gaussian beam by scaling the size and divergence of the embedded Gaussian mode by M.
You can think of M2 as a fudge factor that tells you how much the real beam has deviated from an ideal Gaussian beam, and how much faster the beam diverges compared to an ideal Gaussian beam. However, M2 is only an output characteristic of the beam and not a parameter that can be used to uniquely specify the electric field distribution of a laser beam. For example, we have displayed three beams below, all with the same waist size and same computed M2 = 5, but their beam profiles are drastically different. It’s impossible for POP to know which beam to use just based on the given M2 value. This is why POP can compute M2 value for a given aberrated beam, but it cannot take M2 value as input to define the initial beam. In order for POP to propagate a beam, it needs to know the exact electric field distribution.
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[1] A. E. Siegman, Lasers, University Science Books (1986), R. Herloski, S. Marshall, and R. Antos, "Gaussian beam ray-equivalent modeling and optical design"