title: Feedback control of thermal lensing in a high optical power cavity.
abstract: This paper reports automatic compensation of strong thermal lensing in a suspended 80 m optical cavity with sapphire test mass mirrors. Variation of the transmitted beam spot size is used to obtain an error signal to control the heating power applied to the cylindrical surface of an intracavity compensation plate. The negative thermal lens created in the compensation plate compensates the positive thermal lens in the sapphire test mass, which was caused by the absorption of the high intracavity optical power. The results show that feedback control is feasible to compensate the strong thermal lensing expected to occur in advanced laser interferometric gravitational wave detectors. Compensation allows the cavity resonance to be maintained at the fundamental mode, but the long thermal time constant for thermal lensing control in fused silica could cause difficulties with the control of parametric instabilities.
authors.affiliation: School of Physics, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia. fanyh@physics.uwa.edu.au
authors.forename: Y
authors.initials: Y
authors.lastname: Fan
authors.forename: C
authors.initials: C
authors.lastname: Zhao
authors.forename: J
authors.initials: J
authors.lastname: Degallaix
authors.forename: L
authors.initials: L
authors.lastname: Ju
authors.forename: D G
authors.initials: DG
authors.lastname: Blair
authors.forename: B J J
authors.initials: BJ
authors.lastname: Slagmolen
authors.forename: D J
authors.initials: DJ
authors.lastname: Hosken
authors.forename: A F
authors.initials: AF
authors.lastname: Brooks
authors.forename: P J
authors.initials: PJ
authors.lastname: Veitch
authors.forename: J
authors.initials: J
authors.lastname: Munch