Abstract
Advanced Virgo is a 2nd-generation laser interferometer based in Cascina (Italy) aimed at the detection of gravitational waves (GW) from astrophysical sources. Together with the two USA-based LIGO interferometers they constitute a network which operates in coincidence. The three detectors observed the sky simultaneously during the last part of the second Observing Run (O2) in August 2017, and this led to two paramount discoveries: the first three-detector observation of gravitational waves emitted from the coalescence of a binary black hole system (GW170814), and the first detection ever of gravitational waves emitted from the coalescence of a binary neutron star system (GW170817). Coincident data taking was re-started for the third Observing Run (O3), which started on 1st April 2019 and lasted almost one year. This paper will describe the new techniques implemented for the longitudinal controls with respect to the ones already in use during O2. Then, it will present an extensive description of the full scheme of the angular controls of the interferometer, focusing on the different control strategies that are in place in the different stages of the lock acquisition procedure, which is the complex sequence of operations by which an uncontrolled, “free” laser interferometer is brought to the final working point, which allows the detector to reach the best sensitivity.
Highlights
The detection of gravitational waves is based on the physical principle of the interference pattern of a Michelson interferometer, where different configurations of the length difference between the two arms can lead to an interference pattern ranging from constructive to destructive at the output port of the interferometer
The application of this PRCL to SSFS feed-forward technique is appreciable in Figure 3, where the transfer function and the coherence between the PRCL and the DARM degrees of freedom (DOF) are shown during two separate noise injections, in order to make a precise measurement of the coupling of PRCL to DARM: as it was explained in Section 2, the reduction of the coupling between auxiliary DOFs does have an effect on the noise level of DARM as, for example in this case, the reduction of the PRCL contribution to the frequency noise level assures that the contribution to DARM of the SSFS itself is effectively reduced
From the PRCL to SSFS feed-forward, the Adaptive 50 Hz feed-forward is theoretically aimed at a very definite frequency, which is exactly 50 Hz; looking at Figure 12, and because of the technical qualities described in Section 2.2, it is noticeable that the mains line is completely removed from DARM, and the coherence is greatly reduced in a band which is wider than the simple line itself, with a reduction factor in the range 2 to 8 in the band ranging from around 49 Hz to 51 Hz
Summary
The detection of gravitational waves is based on the physical principle of the interference pattern of a Michelson interferometer, where different configurations of the length difference between the two arms can lead to an interference pattern ranging from constructive (bright fringe) to destructive (dark fringe) at the output port of the interferometer. Fabry-Pérot cavities on resonance, and the relative mirror angular orientation with respect to the main laser beam, a series of control loops is needed, which maintain the correct microscopic working point of each optic both in relative position (longitudinal controls) and relative orientation (angular controls). These loops are engaged during a complex sequence which brings the interferometer from a free uncontrolled state to the final working point; such sequence is called lock acquisition.
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