The Ligo Detectors Controls

All three LIGO detectors have reached their design sensitivities. A sky-averaged detection range (SNR > 8) of more than 15 Mpc for inspiral binary neutron stars with masses of 1.4 Msol has been achieved with the two 4 km instruments. The fifth LIGO science started in November 2005 and ended September 2007. About 365 days of coincidence data have been collected. The feedback controls system is a major component to make LIGO work and its performance has been crucial to achieve the present sensitivity. INTRODUCTION Interferometric gravitational wave antennas are based on Michelson interferometers whose sensitivity to small differential length changes has been enhanced by adding multiple coupled optical resonators. The use of optical cavities is essential for reaching the required sensitivity, but sets challenges for the control system which must maintain the cavities near resonance. The goal for the strain sensitivity of the Laser Interferometer Gravitational-wave Observatory is 10 rms, integrated over a 100 Hz bandwidth centered at 150 Hz [1,2]. Figure 1: Schematic view of the optical path in LIGO. The light of a frequency stabilized Nd:YAG laser is passed through a triangular mode cleaner cavity before it is launched into a Michelson interferometer. To stabilize the laser frequency a small fraction of the light is sampled, doubly passed through an acousto-optic modulator (AOM) which serves as a frequency shifter, passed through a Pockels cell and sent to a reference cavity. Using a polarizing beamsplitter (PBS) and quarter-wave plate (λ/4) the light reflected from the reference cavity is measured by a photodetector to obtain the error signal, Sref, which in turn is used to adjust the laser frequency. The main laser light is passed through a premodecleaner (not shown) and a series of Pockels cells which impose the phase-modulated rf sidebands used to lock the mode cleaner and the Michelson interferometer. The mode cleaner locking signal, SMC, is measured by a photodetector in reflection of the mode cleaner cavity. The light which passes through the mode cleaner is sent through a Faraday isolator (FI) which also serves the purpose, together with a polarizer (P), to separate out the reflected light signal, Srefl. The main interferometer consists of a beamsplitter (BS), two arm cavities each of them formed by an input test mass (ITM) and an end test mass (ETM), and the power recycling mirror (PRM). Additional locking signals are obtained at the antisymmetric port, Santi, and by sampling a small amount of light from inside the power recycling cavity, Sprc. ___________________________________________ [email protected] TOAB04 Proceedings of ICALEPCS07, Knoxville, Tennessee, USA