What does the M² specification for a laser mean?

M² is a measurement of the quality of the beam propagation. It is a ratio of the actual beam propagation over the diffraction limit. It can be used to identify how the beam will change as it travels when compared to a Gaussian beam. The closer the value is to 1, the better the performance of the laser. An M² value less than or equal to 1.2 is generally considered good performance. The value is useful in determining maximum focused spot sizes and the effects on beam delivery systems.