Development of cryogenic mirrors for interferometric gravitational wave detectors

引力波的探测证明了引力波本身的存在，使天文观测致密恒星等引力波源的动力学、分布和距离成为可能。它也使检验万有引力理论成为可能。因此，探测到引力波将为物理学和天文学带来划时代的知识。在本研究项目中，我们进行了低温反射镜的研制和利用该技术降低热噪声的研究，目的是研制更灵敏的引力波探测低温激光干涉仪。当反射镜的温度从300K降低到30K时，其热噪声的幅值在第一次近似中下降到三分之一。实际上，由于低温下振动的机械质量因子的增加，可以预期提高一个数量级。由于镜面是由细纤维悬吊在超高真空中，因此，除了通过纤维进行热传导外，没有其他方法可以冷却镜面。然而，我们发现晶体蓝宝石纤维可以有效地代替传统的金属或熔融石英纤维传递热量。我们还发现，即使在低温下，用蓝宝石纤维悬挂的传统方式的蓝宝石棒也能满足激光干涉探测器的要求。此外，我们还定量地评估了低温反射镜通过反射镜大小的孔径暴露在室温真空壁上的污染速度。这些结果显示了实现低温激光干涉仪的可能性。为了实际系统的应用，我们开发了一种低温致动器系统，并将其应用于激光干涉仪系统，成功实现了锁定操作。这是世界上首次演示低温干涉仪。这些结果满足了研究计划的原始目标，除了证明了使用干涉仪可以降低热噪声本身。它们被应用于1200米基线低温激光干涉引力波探测器，这是一个后续的研究项目，计划从下一个财政年度2002年开始。少

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