Seiji Kawamura, National Astronomical Observatory, Japan
TAMA is the Japanese project of building a gravitational wave detector. The detector employs a standard power recycled Fabry-Perot interferometer with an arm length of 300m. It is located on the campus of National Astronomical Observatory in Tokyo. The project started in 1995 with the collaborating efforts of National Astronomical Observatory, The University of Tokyo, Institute for Cosmic Ray Research, Institute for Laser Science, High Energy Accelerator Research Organization, Yukawa Institute of Theoretical Physics, and Osaka University. Miyagi University of Education joined the project later.
In 1997 the facility and the vacuum system were completed. The 10W Nd:YAG laser using the injection locking technique was developed by Sony Corporation. The ring-cavity mode cleaner was installed and checked in performance with the 500mW Nd:YAG laser. In 1998 the one arm cavity consisting of two suspended mirrors was installed and locked using the 700mW Nd:YAG laser with both mirrors controlled in orientation by wave front sensing signals (Ward method). We then installed the other arm cavity, the beam-splitter, and two pick-off mirrors. We are now trying to lock the Fabry-Perot Michelson interferometer. In parallel with this effort, we are connecting the 10W laser to the mode cleaner. In fall 1998 we will combine the 10W laser and mode cleaner with the Fabry-Perot Michelson interferometer. When we increase the sensitivity to a reasonable level in spring 1999, we plan to take data of the detector for one month. We will then install the recycling in the interferometer. We expect to have a full sensitivity of TAMA at the end of 1999.
The mechanism of the excess noise produced in the light transmitted through the mode cleaner at the RF frequency of the transmitted sidebands was well understood, and the noise was well suppressed. This ensures that the excess noise does not preclude the shot noise limited sensitivity of the interferometer.
The one arm cavity was held locking very stably for ten days with only several losses of locking. The wave front sensing signals were well diagonalized for the two mirrors, and used to control the orientation fluctuation of the mirrors.
The absolute length of the 300m arm cavity was measured with the accuracy of 1 micrometer using a new technique, which used the phase modulation sideband transmitted through the arm cavity. The result showed that the motion of the 300m arm cavity was dominated by peaks of 20 micrometers which occurred regularly everyday. It turned out that the peaks were caused by pumping the underground water by a hospital nearby.
We will use a new signal extraction method for the recycled interferometer. In the conventional way it is difficult to get the recycling cavity length signal, because the signal is dominated by the common-mode arm cavity length signal due to its high finesse. It was found that the signal extraction matrix can be diagonalized by optimizing the reflectance of the recycling mirror to the sideband rather than to the carrier. The principle of this method has been already verified in the 3m prototype.
As for the future project after TAMA, Institute for Cosmic Ray Research declared that they have adopted the building of a km-class gravitational wave antenna as the institute's future main project. This greatly increases the chances for a future km-class antenna in Japan.
More information about TAMA can be found at our web site. http://tamago.mtk.nao.ac.jp/