Measurement of angular antispring effect in optical cavity by radiation pressure

We present a measurement of an angular antispring effect caused by radiation pressure in an optical cavity with a mirror of 20 mg suspended by a silica fiber of 10 μm in diameter. The antispring effect occurred since the torque on the suspended mirror is increased with the higher radiation pressure force, pushing the system towards instability. We measured shifts of the rotational resonant frequencies of the suspended mirror from 2.0 Hz to 1.0 Hz with the increased circulating power. It is verified that the result agrees with the theoretical curve to show the antispring effect. The result proves that it will be possible to make a reliable control system model of the radiation pressure effect for the second generation of the gravitational wave detectors.