LIGO Document P0900304-v8

Gravitational wave detector derived error signals for the LIGO thermal compensation system

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Thermal lensing has been one of the sensitivity limiting factors for the LIGO detectors since their inception. Although estimates of such lensing was assumed when LIGO's core optics were specified, in practice a thermal lensing compensation system (TCS) was installed in 2004 in order to improve mode matching to the injected beam and ultimately detector sensitivity. This subsystem’s primary purpose was to induce a corrective thermal lens in the input test mass mirrors or LIGO's 4 km Fabry-Perot arms. A few empirically-motivated means of monitoring the focal parameters of the input couplers were employed for the 2005-2007 science run, “S5.” We discuss results of a numerical model study, a set of signals, “focal discriminants,” that could have been used during S5 to set TCS compensation levels. Most of these signals would not have needed the installation of any new equipment or software. If investigated further, these “focal discriminants” may find utility in pathfinder projects as the next generation LIGO detectors is commissioned.
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Final Version Submitted to Classical and Quantum Gravity (IOP)
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