Abstract
The sensitivity of interferometric gravitational wave detectors is optimized, in part, by balanced finesse in the long Fabry–Perot arm cavities. The input test mass mirrors of Advanced Virgo feature parallel faces, which creates an etalon within the substrate, adding variability in the total mirror reflectivity, in order to correct imbalanced finesse due to manufacturing tolerances. Temperature variations in mirror substrate change the optical path length primarily through varying the index of refraction and are tuned to correct for a finesse imbalance of up to 2.8% by a full etalon fringe of 0.257 K. A negative feedback control system was designed to control the mirror temperature by using an electrical resistive heating belt actuator for a heat transfer process modeled as a two-pole plant. A zero controller filter was designed which achieves temperature control within 2.3% of the etalon fringe and recovers to within 10% of the working point within 32 hours after a step input of one etalon fringe. A preliminary unlock condition control designed to compensate when the interferometer unlocks shows that the control remains stable even after a drastic change in the plant due to the absence of the laser heating. Further improvements to the control must also consider the full heat transfer mechanisms by using modern control state space models.
Highlights
Advanced Virgo (AdV) is a recycled Michelson interferometric gravitational wave detector [1].It is a second generation detector, part of a worldwide network, which along with LIGO, allowed for the recent discoveries in gravitational wave research [2,3,4]
These results indicate that an uncontrolled etalon effect will push the finesse asymmetry out of the 1% requirement
This paper presents the current application of the etalon control in AdV
Summary
Advanced Virgo (AdV) is a recycled Michelson interferometric gravitational wave detector [1]. The finesse asymmetry requirement specified a mirror coating tolerance which surpassed manufacturing capabilities at the time of construction These limitations were avoided by including the etalon effect into the AdV arm cavity design, first implemented in the previous generation Virgo+. Punturo [11] describes how the optical path length (OPL) dependent etalon resonance condition is varied by the input mirror substrate temperature and the subsequent effect on the arm cavity finesse. Etalon control was implemented in Virgo on a previous generation of the detector, Virgo+ [18,19] The gain of this controller was tuned to achieve an accuracy of 30 mK, which greatly stabilized the performance of the ITF, in the BNS range. A separate unlock condition control methodology is developed to account for the drastic plant change when the ITF unlocks and the laser heating is removed
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