System for studying Parametric INstability and its active Attenuation

The project SPINA is funded by ANR, it is for research on parametric instability (PI), an optomechanical phenomenon which can occur in high-power optical cavities, and ways to actively control it.

Optomechanical Parametric Instability

Cavity optomechanics is the discipline which studies the mutual interaction between light and mechanical motion inside an optical resonator (cavity). This interaction is mediated by the radiation-pressure force resulting from the reflection of the light on the mirrors (mechanical resonators). PI consists in the amplification of the mirror vibrations (mechanical modes), initiated by thermal excitation, due to radiation pressure applied by the beating of two optical cavity-modes. Although classical, this parametric effect is often described in terms of quantum physics: the conversion of one photon of the main laser mode ħω₀ into one phonon ħω_m and one photon at lower energy ħω₁, where ω₁ = ω₀ – ω_m. Two conditions need to be met for the instability to occur: all the three modes involved must be resonant in the cavity (for the optical modes) and in the mirror (for the mechanical mode) and there must be a significant spatial overlap between the beat note and the mechanical mode profiles.

This phenomenon was first predicted by Braginsky, Strigin and Vyatchanin in 2001¹. In that work PI was identified as a limit for the increase of the power inside the Fabry-Perot cavities of gravitaional wave (GW) detectors. PI was eventually observed for the first time in 2014 in the LIGO detector² : a mechanical mode around 15 kHz became unstable and caused saturation of the control systems that maintain correct and stable position of the mirrors. This prevented the detector to function for an intracavity power of 50kW, which is much lower than the nominal power (800 kW).

¹Physics Letters A 287, 331–338. (2001)

²Phys. Rev. Lett. 114, 16 (2015).