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

Abstract

Braginsky, Gorodetsky, and Vyatchanin have shown that thermo-refractive 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 ∼ 5 in the low-frequency part of the GEO600 band, compared to the previous estimates. We also find, by contrast with previous calculations, that thermo-refractive fluctuations result in white noise between 600 Hz and 39 MHz, at a level of 8.5 · 10 −24 Hz −1/2 . Finally, we describe a new type of thermal noise, which we call the thermo-chemical 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 thermo-chemical noise for GEO600.

Thermorefractive noise of finite-sized cylindrical test masses

“…Thus, regions III and IV show no physical relevance for silicon. In contrast, for fused silica at room temperature region III shows physical relevance again as it starts at a frequency around 1 kHz [14]. the substrate.…”

“…(3), (12), (14), (16), and (18) we finally arrive at an expression for the displacement noise for a read out homogeneous along the z-direction.…”

“…On the other hand the TR effect at the antinodes is amplified. In 2009 Benthem and Levin [14] were the first to bring this idea into the field of gravitational wave detection. As they did only show a few calculation steps we give a detailed derivation of the effect of standing waves on TR noise in this section.…”

“…In that work he validated the result of the model of the infinite disc. With the same model of the infinite disc the TR noise level of the GEO600 beam * svyatchanin@physics.msu.ru † Present address: Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa, Chiba 277-8582, Japan splitter was determined in 2009 [14]. There the effect of a standing wave emerging in the substrate was considered.…”

Reducing thermal noise in future gravitational wave detectors by employing Khalili etalons

“…Fundamental thermodynamic fluctuations of temperature lead to thermo-elastic noise through the material's thermal expansion. Similarly thermo-refractive noise [10,11] is caused by temperature fluctuations leading to fluctuations of the refractive index and therefore fluctuations of the optical path length inside the material. These results were obtained for the model of an infinite test mass, i.e.…”