1. M-square factor
This is the most widely used parameter.
| M2 = | πWoΘ λ | (1) |
The capital letters Wo and Θ are beam waist radius and half-width divergence respectively for a general multimode Gaussian beam (I use lower-case wo and θ for the "imbedded" fundamental Gaussian beam [1]).
2. Beam parameter product (BPP) [2]
This parameter is defined as the product of beam width and full beam divergence:
| BPP=2W0*2Θ=4M2λ/π | (2) |
3. Brightness
Brightness is essentially equivalent to radiance [3], i.e., power in a unit cross-sectional area and a unit solid angle:
| B = | P πW02πΘ2 | = | P (M2)2λ2 | (3) |
References:
[1] T.F. Johnston, "Beam propagation (M2) measurement made as easy as it gets: the four-cuts method", Appl. Opt. 37, 4840 (1998).
[2] X. Gao et al., "Beam-shaping technique for improving the beam quality of a high-power laser-diode stack", Opt. Lett. 31, 1654 (2006).
[3] http://www.rp-photonics.com/brightness.html
