When a laser beam passes through an optic, a small amount of the beam's energy is absorbed in the bulk and surface material of the optic, inducing temperature gradients within the optic. A temperature dependence in the material's index of refraction means that these temperature gradients directly translate into index gradients, which insures that laser light passing through the optic will suffer an added phase distortion: a phenomenon termed "thermal lensing". With the high power laser beams circulating in LIGO (even moreso, advanced versions of LIGO), this effect becomes significant enough that it must be compensated for.
Here at MIT, we are working on developing a novel new system which would make the affected optics "thermally adaptive". The basic idea is to radiatively heat the exterior of the optic (minimal mechanical interaction with the optics is a must!) in such a way as to "flatten" out the thermal lens induced by LIGO laser beams. In fact, one can also nullify the effects of macroscopic defects in the optic (variations in the optic's radius of curvature, inhomogeneities in the bulk and surface absorptivity, etc) by heating the optic such that the induced thermal lens compensates for these defects. Detailed numerical simulations and a little bit of theory have shown that this idea of thermally adaptive optics should work.
The experiment we are setting up here at MIT will test the true feasibility of thermal compenation. A large test optic sits in a vacuum where we radiatively heat it (with a nichrome ring suspended above the face, or with a scanning laser beam), and then we use a wavefront sensor to measure how a laser beam is distorted upon passage through the optic.
| Theory | Pictures |
|---|---|
| Data | Experimental Sketch |
