LIGO Document T1600153-v1

Reference Cryogenic System for Coating Noise Measurements

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T - Technical notes
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The Advanced Laser Interferometer Gravitational Wave Detectors (aLIGO) are some of the most sophisticated and sensitive sensors for length fluctuations ever made. The data from this sensor is riddled with an array of noises. One source of noise in these sensors is Brownian noise in the coating of the test masses, which arises from coupling between macroscopic degrees of freedom and thermal energy in the mirrors. These can be reduced by using cryogenically cooled test masses. A workbench designed to measure the coating thermal noise in sample mirrors at cryogenic temperatures exists, consisting of two nominally identical optical cavities with independent lasers locked to them. Using this setup for a differential measurement of resonance frequency fluctuations in the cavities, one can directly measure the coating thermal noise that dominates these cavities by design. However, with the incorporation of better, smaller cavities and less noisy lasers, the resonant frequencies of the two cavities can be too far apart (due to small manufacturing tolerances in the cavity lengths) and the beat frequency to fast to measure. The measurement is much simpler if each of these two setups is compared with a third reference setup, as a differential measurements with lower noise than the expected coating noise would help us isolate the coating impact. This reference bench was setup after careful laser and cavity characterization, mode calculations, and incorporating feedback techniques. A visibility of 85% and a stable lock was achieved in the cavity and the output was guided back into the existing work bench to aid the characterization of coating noise.
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SURF Project 2016 (Subham Vidyant)

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