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The advanced LIGO detectors are currently in their final design stage, and the installation phase will start at the end of 2010: they will have about 10 times better sensitivity than initial LIGO, with a sensitive band ranging from 10 Hz to 10 kHz. As compared with previous LIGO detectors, there will be increased complexity in the optical configuration, improved seismic isolation system and significantly higher power circulating in the arm cavities. In the new detectors, the control of the angular orientation of the mirrors will be particularly challenging. The advanced LIGO (aLIGO) mirrors need to have a residual angular motion of the order of 1 nrad RMS in order to achieve high sensitivity. In the high power regime, the torque induced by radiation pressure effects will be comparable with the restoring torque of the mirror suspension, such that we must think of the opto-mechanical response, instead of just the mechanical response. These modifications have to be considered in order to design the control strategy for keeping the mirrors well aligned. Moreover, to meet the sensitivity target the alignment control noise coupled to the gravitational-wave channel must be well below at 10 Hz. We developed a model of the alignment sensing and control scheme of aLIGO which takes into account radiation pressure effects and meets the noise target.
Barsotti et al. (Wed,) studied this question.