The Virgo Experiment: Characteristics The scheme of the Virgo interferometer is shown in fig. 1. We define l1 , l2 the lenghts of the Michelson interferometer arms (distances from the beam splitter to the input mirrors of the FP cavities), L1, L2 the lenghts of the cavities and lr the distances between the beam splitter and the recycling mirror. Fig 1: The Virgo Interferometer The principal characteristics are: Laser: it is a Nd:Yag emitting at λ = 1064 nm. Its output power in the TEM00 mode is about 20 W. The stability requirement are: "P $ 3*10 / Hz #5 & for f = 10Hz - power: <% P & 3*10#7 / Hz ' for 100 < f < 1000Hz "v $10 / Hz & #4 for f = 10Hz - frequency: <% ! v &10#6 / Hz ' for 100 < f < 1000Hz This level of frequency stability requires a gain of about 107 in the control loop. ! Input bench: it is under vacuum and suspended from a short superattenuator. It is equipped with a rigid triangular referente cavità for the frequency pre- stabilization, the input and output mirrors of the mode cleaner and the main beam expansion and alignment optronics. Mode cleaner: it is a triangular cavità 144 m long. Two mirrors (input and output) are mounted on the input bench. The third mirror is suspended from a short tower, connected to the input bench tower by a dedicated pipe. Power recycling: the recycling cavity is made by the recycling mirror and the interferometer itself. The cavità length is defined as lr = l0+(l1+l2)/2. The foreseen recycling factor is about 50: a power of about 1 kW is expected on the beam splitter. Fabry-Perot cavities: the FP will be 3 km long and will have a finesse F = 50. The interferometer output i sto be kept on the dark fringe for the carrier, while the sidebands are to be transmitted to carry the signal. Detector system: it is composed of a detection bench, suspended from a short tower and a optical table (outside the vacuum vessel). The bench supports the output mode cleaner (a small monolithic triangular cavità, designed to increase the contrast by filtering the spurious mode) and the optics for splitting and collimatine the beam on the photodiodes. The table supports the photodiodes (InGaAs, quantum efficiency η = 0.85) for the locking and the signal detection.
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