Parametric instabilities are nonlinear optomechnical phenomena that can appear in high-power optical cavities. They involve an interaction between a mechanical vibration of a mirror, the main laser light in the cavity, and a higher-order optical mode.
Light circulating in the funcamental cavity mode can scatter into a higher-order mode. The beating between these two optical modes produces a radiation-pressure force that can drive a mechanical mode of the mirror, which was initially thermally exctied. The resulting motion then increases the scattering, which further increases the radiation pressure force. This feedback loop can grow exponentially and destabilize the system.
This effect limits how much laser power can be safely stored in the fundamental mode of the cavity. In gravitational-wave detectors, the onset of parametric instabilities can lead to a loss of interferometer control, lowering detector sensitivity. Hence, mitigation strategies will have to be implemented in order to control this phenomenon.
Our team is developing an original active mitigation technique based on radiation pressure. As part of T. Harder’s PhD work, we developed a first system capable of scanning the surface of a mirror with an auxiliary laser beam. We are now developing a high-sensitivity Michelson interferometer to detect and characterize thermally excited mechanical modes of a mirror. We have already achieved a displacement sensitivity up to 1.6×10−16 m/√Hz and will soon observe parametric instabilities in a table-top optical cavity.
This project connects laser physics, precision measurement, optomechanics, and gravitational-wave detection, and it provides hands-on experience with advanced optical experiments and feedback control systems. If you are interested on working in this project, please contact: margherita.turconi@oca.eu
Team working in this project: Stéphanie Grabielle (Post-Doctoral researcher), Walid Chaibi (CNRS, Chargé de recherche) and Margherita Turconi (project leader, Associate Professor UCA.
Picture of the experimental setup in Artemis
Publications:
Harder, “Mitigation of parametric instabilities based on radiation pressure in gravitational wave detectors.”, PhD (2022).
Harder et al., “High speed, high power 2D beam steering for mitigation of optomechanical parametric instability in gravitational wave detectors”, Opt. Exp. 31, 2, 1486-1500 (2023).
