When laser light passes through an optic, a small amount of power is absorbed due to impurities, resonances, etc.(about 5 parts per million per cm in the bulk, and about 5 parts per million in the coating on the surface). Since the laser beam has a gaussian profile, more heat is deposited in the center than in the outer portions of the optic, thus inducing temperature gradients. The index of refraction is dependent on temperature (about 1*10^-5 per degree K), so temperature gradients translate directly into index gradients. This results in a net change in optical path which varies across the optic (called the "optical path distortion", or OPD, for short), which behaves like an additional lens added to the system (thus the term "thermal lensing"). Click here for an illustration.
While the effect of thermal lensing in the first generation of LIGO
detectors is manageable by grinding additional curvatures into various
optical components, the problem for advanced detectors (with 20 times
more ciruclating power) is serious enough that it must be actively
compensated for (grinding the additional curvature fixes the problem
in the heated steady state, although it makes the various optical cavities
nearly impossible to bring into resonance in the unheated state). Click
here to see the modeled thermal lens induced
in the LIGO II input test mass (which has 4kW of power circulating in the
bulk, with 200kW of power incident on Fabry-Perot cavity face).
