Room-temperature tests of an optical transducer for resonant gravitational wave detectors

A two-oscillator transducer incorporating a laser-illuminated Fabry–Perot cavity with a finesse of 77,500 and a power dissipation of 1.2 μW was tested at room temperature. The energy of the last resonator with a mass of 1.25 g was measured to be k_BT within 8%, and no back action from the sensor could be detected. The lowest value of the noise measured away from resonance was $1.0\times {10}^{-15}\hspace{0.17em}\text{m}/\sqrt{\text{Hz}}$, and the electronic noise was $3.2\times {10}^{-17}\hspace{0.17em}\text{m}/\sqrt{\text{Hz}}$. That transducer is designed for a 2400-kg gravitational wave antenna operating at cryogenic temperatures. At 4.2 K and for mechanical quality factors of 3 × 10⁶, the measured thermal and electronic noise levels would translate into a sensitivity in h equal to 7.0 × 10⁻¹⁹ and 1.5 × 10⁻¹⁹, respectively.