Demonstration of the parametric instability suppression through optical feedback

We demonstrate the suppression of parametric instability in an 80-meter-long Fabry-Perot cavity through optical actuation in an electro-optical feedback loop. The suppression of the high-order mode is achieved by injecting a high-order mode with the same frequency and opposite phase. The beat note signal between the fundamental and high-order mode in the transmission of the cavity is filtered, amplified, and applied to an electro-optical modulator to create a sideband at the high-order mode frequency. Misalignment of the injection couples a small part of this sideband to the high-order mode of the cavity. The paper presents a theoretical analysis and an experimental demonstration of parametric instability suppression for an instability that would normally ring up with a parametric gain of approximately 2.5.