LIGO Document T1600285-v3

Temperature control of silicon mirrors in locked cavities at 123 K

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T - Technical notes
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Thermal noise is one of the multiple areas in the LIGO noise budget that affects its overall sensitivity. The main idea of a cryogenic LIGO is to reduce this noise by working at low temperatures. Investigations into this method at this stage were performed using short optical cavities made of silicon held at 123 K inside a cryostat. These cavities have been locked into resonance with a 1550 nm laser beam, using the PDH technique for frequency stabilization. At this temperature, the thermal expansion coefficient is at a zero-crossing point, and thus any slight temperature fluctuations around that point translate into length fluctuations. A temperature control system has been designed, to monitor the mirror temperatures of the optical cavities setup and thus quantify deviations from resonance.

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