Optical losses as a function of beam position on the mirrors in a 285-m suspended Fabry-Perot cavity

Reducing optical losses is crucial for reducing quantum noise in gravitational-wave detectors. In fact, with equal input power, a lower level of round-trip losses in the arm cavities allows more power to be stored. Moreover, losses are the main source of degradation of the squeezed vacuum, which, along with increasing the power, is the most effective strategy to reduce quantum noise. Frequency-dependent squeezing obtained via a filter cavity is currently used to reduce quantum noise over the whole detector bandwidth. Such filter cavities are required to have high finesse in order to produce optimal squeezing angle rotation, and the presence of losses is particularly detrimental for the squeezed beam, as it does multiple round trips within the cavity. Characterizing such losses is crucial to assess the achievable quantum noise reduction. Here we present an in situ measurement of the optical losses, for different positions of the beam on the mirrors of the Virgo filter cavity. We implemented an automatic system to map the losses with respect to the beam position on the mirrors, finding that optical losses depend clearly on the position at which the beam hits the input mirror, varying from 42 to 87 ppm (parts per million), while they are much more uniform when we scan the end mirror (53 to 61 ppm). We repeated the measurements on several days, finding a statistical error less than +/- 4 ppm. The lowest measured losses are not much different from those estimated by individual mirror characterization performed before installation (30.3-39.3 ppm). This means that no major loss mechanism has been neglected in the estimation presented here. The larger discrepancy found for some beam positions is likely to be due to contamination. In addition to a thorough characterization of the losses, the methodology described herein allowed an optimal cavity axis position to be found for which the cavity round-trip losses are among the lowest ever measured. This work can contribute to achieving the very challenging loss goals for the optical cavities of future gravitational-wave detectors, such as the Einstein Telescope and Cosmic Explorer.