The laser operator running the SPM can spot variances in the weld signature and interpret them as specific system problems as they develop within the laser system itself.

For example, As mirrors get dirty or worn they heat up and become convex. This moves the focal point of the laser down into the substrate and the weld signature on the monitor reflects the shifting focal point

Laser power fluctuation show up in the weld signature... The negative fallouts in signal amplitude occur and correspond to a reduced thermal penetration, which often occurs when laser power is momentarily lost or reduced.