Force-insensitive optical cavity.

We describe a rigidly mounted optical cavity that is insensitive to inertial forces acting in any direction and to the compressive force used to constrain it. The design is based on a cubic geometry with four supports placed symmetrically about the optical axis in a tetrahedral configuration. To measure the inertial force sensitivity, a laser is locked to the cavity while it is inverted about three orthogonal axes. The maximum acceleration sensitivity is 2.5×10⁻¹¹/g (where g=9.81 ms⁻²), the lowest passive sensitivity to be reported for an optical cavity.