Thermorefractive and thermochemical noise in the beamsplitter of the GEO600 gravitational-wave interferometer

Abstract

Braginsky, Gorodetsky, and Vyatchanin have shown that thermorefractive fluctuations are an important source of noise in interferometric gravitational-wave detectors. In particular, the thermorefractive noise in the GEO600 beamsplitter is expected to make a substantial contribution to the interferometer's total noise budget. Here, we present a new computation of the GEO600 thermorefractive noise, which takes into account the beam's elliptical profile and, more importantly, the fact that the laser beam induces a standing electromagnetic wave in the beamsplitter. The use of updated parameters results in the overall reduction of the calculated noise amplitude by a factor of $\ensuremath{\sim}5$ in the low-frequency part of the GEO600 band, compared to the previous estimates. We also find, by contrast with previous calculations, that thermorefractive fluctuations result in white noise between 600 Hz and 39 MHz, at a level of $8.5\ifmmode\cdot\else\textperiodcentered\fi{}{10}^{\ensuremath{-}24}\text{ }\text{ }{\mathrm{Hz}}^{\ensuremath{-}1/2}$. Finally, we describe a new type of thermal noise, which we call the thermochemical noise. This is caused by a random motion of optically active chemical impurities or structural defects in the direction along a steep intensity gradient of the standing wave. We discuss the potential relevance of the thermochemical noise for GEO600.