BASIC STUDY ABOUT A VERTICAL LASER INTERFEROMETER

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Many projects for the first gravitational wave (GW) detection, such as Advanced-LIGO, EGO and LCGT, GW from binary neutron star coalescence, whose event rate is 1/100000 per a year and a galaxy, is mainly targeted. For this reason, they are enthusiastic to enhance their sensitivity to increase the observable galaxy numbers. Recently, theoretical super and hyper-novae explosion simulations seem to require anti-symmetrical (rotational) explosion process, which might produce extremely large amplitude gravitational waves (GW). Accounting for the supernovae event rate in a galaxy of 2/year and its large amplitude GW that can be detected by 100m-scale GW detectors, it also seems to be important to be able to identify positions of GW sources from supernovae in our galaxy as other astronomical observations. For the position identification, three interferometers (IFOs) are at least necessary. In the case that these three IFOs are situated nearby on the Earth, one of the three IFOs is desirable … More to have an arm (a Fabry-Perot cavity arm) in the vertical direction to have no dead angles. Accounting for this background, we started basic study about the construction of a laser interferometer that has a Fabry-Perot cavity in the vertical direction. We have constructed vertical mirror suspension and seismic noise isolation pendulums to construct the vertical Fabry-Perot cavity. This pendulum contained three vertical isolation stages. The first stage had 0.5 Hz mini-GAS filter and the second and the third stages had spring-like isolators (12Hz) using pseudoelasticity property of BOLFUR wires. As a light to be injected to the Fabry-perot cavity, 532nm wavelength laser was used (Innolight Inc. Prometheus-50, 50mW). A Pound-Drever-Hall method was used to obtain an error signal of a Fabry-Perot cavity length. Consequently, the Fabry-Perot cavity whose mirrors are suspended by these vertical pendulum systems, has been successfully controlled to resonate, using either a magnet-coil actuation force between the mirror mass and coils or using a laser frequency control. On the other hand, the traditional horizontal laser interferometer's displacement sensitivity has reached 2x10^-16…3^*10^-17 [m/rHz] from 20Hz to 30Hz, which was the world best displacement noise level, and 3x10^-18 [m/rHz] at 100Hz (LIGO has 1x10^-19 [m/rHz] at 200Hz), that is ever best in Japan. Less