M squared

In laser science, the parameter M², also known as the beam propagation ratio or beam quality factor is a measure of laser beam quality. It represents the degree of variation of a beam from an ideal Gaussian beam. It is calculated from the ratio of the beam parameter product (BPP) of the beam to that of a Gaussian beam with the same wavelength. It relates the beam divergence of a laser beam to the minimum focussed spot size that can be achieved. For a single mode TEM₀₀ (Gaussian) laser beam, M² is exactly one. Unlike the beam parameter product, M² is unitless and does not vary with wavelength.

The M² value for a laser beam is widely used in the laser industry as a specification, and its method of measurement is regulated as an ISO standard.