Optical torques in suspended Fabry–Perot interferometers

High-power Fabry–Perot cavities with suspended mirrors are employed in gravitational wave detection. The mirrors in such interferometers are subject to forces and torques that are exerted by light pressure. This Letter derives a closed-form expression for the static torsional stiffness due to this effect. The torsional stiffness is shown to scale as the fourth power of the beam spot size. In proposed designs for next-generation interferometric gravitational wave detectors the torsional stiffness is shown to be large enough to dominate the dynamics of the mirror suspensions.