Beating fabry-perot cavity resonant eigen-frequencies to detect gravitational waves

The Virgo interferometer, located near Pisa, Italy, is taking data to look for gravitational waves. The Virgo interferometer is able to "hear" the final spirals before coalescence of a neutron-star binary at ∼9.5 Mpc. A gravitational wave, a space-time distortion, of amplitude h(t) induces a relative variation h(t)/2 of the frequency of an optical resonant cavity with suspended mirrors. For an optimal polarization of the gravitational wave, the variations of the frequencies of two orthogonal Fabry-Perot cavities have opposite signs. A laser compares the resonant frequencies of the cavities disposed in a Michelson interferometer with a resolution ∼10-21. The laser frequency is stabilized to make this eigenfrequency comparison, with an in-loop phase noise of -155 dBc/sqrt(Hz) between 10 Hz and 10 kHz. I explain the oise budget of our interferometer, and discuss upgrades to improve the resolution. © 2010 American institute of Physics.