Using the etalon effect for in-situ balancing of the Advanced Virgo arm   cavities
  Several large-scale interferometric gravitational-wave detectors use resonant arm cavities to enhance the light power in the interferometer arms. These cavities are based on different optical designs: One design uses wedged input mirrors to create additional optical pick-off ports for deriving control signals. The second design employs input mirrors without wedge and thus offers the possibility to use the etalon effect inside the input mirrors for tuning the finesse of the arm cavities. In this article we introduce a concept of maximized flexibility that combines both of these options, by featuring wedges at the input mirrors and using the etalon effect instead in the end mirrors. We present a design for the arm cavities of Advanced Virgo. We have used numerical simulations to derive requirements for the manufacturing accuracy of an end mirror etalon for Advanced Virgo. Furthermore, we give analytical approximations for the achievable tuning range of the etalon in dependence on the reflectance, the curvature and the orientation of the etalon back surface. 
S. Hild, A. Freise, M. Mantovani, S. Chelkowski, J. Degallaix and R.   Schilling