M² tells you less about a process than the number's prominence suggests
Beam parameter product is a real physical quantity and a poor single-number summary. Two sources with the same M² can behave quite differently at the workpiece.
M², or beam quality factor, describes how closely a beam approaches the diffraction limit. Lower is better, in the sense that a lower-M² beam can be focused to a smaller spot and maintains that focus over a longer distance. It appears prominently on every datasheet, and it is genuinely useful.
It is also routinely over-interpreted.
What it captures
M² summarises the beam's propagation: the relationship between waist size and divergence. From it you can calculate the achievable spot size for a given focusing optic and the depth of focus around that spot. For selecting optics and predicting working tolerance, it is the right number.
What it omits
M² says nothing about the intensity distribution within the spot. A beam with a Gaussian profile and a beam with a flat-top profile can share an M² value while depositing energy quite differently. In cutting, that changes kerf shape and edge quality. In welding, it changes keyhole stability.
It also says nothing about temporal behaviour — power stability, modulation response, or noise — and nothing about how the profile changes with output power. Some sources maintain profile across their power range; others do not, and a beam characterised at full power may look different at thirty per cent.
Finally, M² describes the source, not the system. Delivery fiber, collimator, protective windows and a contaminated cover slide all modify what actually reaches the workpiece.
What to ask instead
For a process decision, the informative requests are a measured intensity profile at the working plane, at the powers actually used, through the intended delivery chain — and a caustic measurement showing how the spot evolves through focus.
Machine builders can provide this. Buyers who ask for it are usually the ones who have been surprised before by two sources with matching datasheets and different results.
This article was produced by the LasersNews AI desk and reviewed by our editors.
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