TVo, Georgia

Last night we powered up to 20 W before losing lock. Today TVo and I had a look at the ETM HWSs to see if we can see any thermal lensing caused by absorption. There is some absorption visible on ETMY HWS.

The first screenshot shows the PSL power [W], ETMX HWS spherical power [diopters], and ETMY HWS spherical power [diopters]. We can estimate the absorption of the optic from the circulating power, the measured thermal lens on the HWS, and the substrate and surface deformations (or lensing per unit power). A back-of-the-envelope calculation puts a first estimate of total absorption at 0.21 ppm. The absorption on ETMX is not visible above the noise.

The HWS beam passes through the substrate of the ETM, reflects off the HR surface, and passes through the substrate again. It samples the substrate lens twice and the surface lens once. So the thermal lens measured on the HWS is

L_tot [D] = (2 A_sub [D/W] + A_surf [D/W] ) * P_circ [W] * alpha

Where L_tot is the total lens in diopters, A_sub and A_surf are the substrate and surface deformations respectively in diopters per W, P_circ is the circulating power in the arms, and alpha is the optic absorption.

Going from 2 W to 15 W we see ~15 uD of thermal lens on the Hartmann. The circulating power at 15 W is ~95 kW (assuming 45 PR gain, 280 arm gain, and 50% reflectivity of the beamsplitter). The substrate and surface deformations as assumed to be 3.86e-4 D/W and -2.93e-5 D/W respectively (numbers acquired from the TCS simulation tool, not sure if they need updating with new test mass coatings).

This gives us an absorption of 0.21 ppm, which seems reasonable.

Final screenshot shows the HWS contour plots between the cold and hot ETM states.