Development of cryogenic mirrors for interferometric gravitational wave detectors

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The detection of gravitational wave proves the existence of the wave itself and makes possible the astronomical observation of the dynamics, distributions and distances of gravitational sources such as compact stars. It also makes possible to test the theories of gravitation. Accordingly, the detection of the wave would bring epoch making knowledge to both physics and astronomy. In this research program, we conducted a research on the development of a cryogenic mirror and on the reduction of thermal noise using this technique for the aim of development of a more sensitive cryogenic laser interferometer for gravitational wave detection. If the temperature of the mirror reduces from 300K to 30K, the amplitude of its thermal noise decreases down to one third in the first approximation. In reality, one order improvement can be expected due to the increase of mechanical quality factor of vibration at cryogenic temperature. Since the mirror is suspended by thin fibers in ultra-high vacuum, t … More here is no other way to cool the mirror than the heat conduction through the fibers. However, we found that crystal sapphire fiber can effectively convey the heat in place of conventional metal or fused silica fibers. We also found that a sapphire rod suspended by conventional style with sapphire fibers satisfies the requirement of a laser interferometric detector even at cryogenic temperature. Moreover, we have quantitatively evaluated the contamination speed of the cryogenic mirror that was exposed to room temperature vacuum wall through an aperture of a size of the mirror. These results shows the possibility of the realization of a cryogenic laser interferometer. For the application of a practical system, we have developed a cryogenic actuator system and applied this to a laser interferometer system and succeeded the locked operation. This operation is the first time of the demonstration of a cryogenic interferometer in the world. These results satisfies the original objective of the research program except the demonstration of the reduction of thermal noise itself using the interferometer. They are applied to 1200m baseline cryogenic laser interferometric gravitational wave detector in a succeeding research program planed to start from the next financial year, 2002. Less