A dimensionless number equal to 1 for a perfect TEM00 Gaussian and larger for real or multimode beams. It is measured by recording beam width at several points through a focus and fitting the propagation curve. Good single-mode lasers reach 1.1 or better; multi-kilowatt cutting lasers run 2 to 30. The related beam parameter product, waist radius times divergence half-angle, equals M-squared times wavelength over pi.
Customer wants M-squared under 1.3 at full power, and at 50 watts the thermal lens pushes us to 1.6.
A single M-squared number hides a lot: an astigmatic beam can have different values in x and y, so always ask for both axes.
Also heard as
- M2
- M²
- beam quality factor
Related terms
TEM00
The fundamental transverse mode of a laser: a single round spot with a Gaussian intensity profile.
Gaussian beam
A beam whose intensity falls off from the center as a bell curve, the shape of an ideal single-mode laser.
Beam waist
The narrowest point of a focused or propagating laser beam, where its wavefront is flat.
Beam profiler
An instrument, usually a camera or scanning slit, that measures a laser beam's intensity distribution and width.
Divergence
How fast a laser beam spreads with distance far from its waist, given as an angle in milliradians.