Pump absorption heats the middle of a laser crystal more than the cooled edges, and since refractive index changes with temperature, and the faces bulge slightly, a crystal such as Nd:YAG acts like a positive lens whose power grows with pump level. It shifts the resonator's stability, mode size and beam quality as power changes. High-power transmissive optics show the same effect from absorbed laser light, which moves the focus during a job.
Beam's fine at 20 percent and blows up at full power, that's thermal lensing in the rod, the cavity was designed for the wrong pump level.
A laser that needs twenty minutes before its beam settles is usually waiting for its thermal lens to reach equilibrium.
Also heard as
- thermal lens
Related terms
Resonator
The arrangement of mirrors that sends light back and forth through the gain medium to build up laser oscillation.
M-squared
The beam quality factor: how many times worse a beam focuses and diverges than an ideal Gaussian of the same wavelength.
Pump
The energy source that excites a gain medium, or the act of supplying it, usually with diodes, flashlamps or current.
Damage threshold
The maximum fluence or power density an optic can take before it is permanently damaged.