RUI: Investigate Thermal and Upconversion Noise for Advanced LIGO and Third Generation Detectors

该研究计划探讨了高级LIGO和其他引力波干涉仪中的两个主要噪声源：上转换噪声和涂层热噪声。 涂层热噪声可以通过理解和最小化反射镜材料中的机械损耗来降低。 传统的高反射率涂层是多层介电材料，通常是非晶金属氧化物，具有交替的高折射率和低折射率。 构成高折射率层的材料是机械损耗的主要来源;低折射率材料是熔融石英，其在非晶介电材料中具有非常低的损耗。在这项研究计划中，PI寻求开发一种稳定的高折射率材料，其热膨胀系数与低折射率材料相匹配，以便通过高温退火减少复合涂层的损耗。 此外，结晶涂层，如砷化铝镓（AlGaAs），已被证明具有比大多数非晶介电涂层低约10倍的机械损耗。 AlGaAs涂层可以开发为第三代探测器计划的1.5微米激光器的镜面材料。 然而，目前的样品仅限于厘米尺度。 这项研究计划将探索这些晶体涂层中机械损失的来源，包括与大型光学涂层的缩放和应用相关的任何问题。 最后，上变频噪声是低频噪声（主要是地震噪声）耦合到干涉仪的检测频带中。 上变频噪声可能难以表征，因为噪声峰值位于耦合机制的频率的和或差处。 该研究计划将开发一种数据分析工具，该工具将使用双相干（相干的高阶形式）来确定上转换噪声的来源。 涂层热噪声是先进LIGO中心频带的主要噪声源，也是限制探测器整体灵敏度的主要因素。减少涂层热噪声导致探测器灵敏度的直接增加和其预期事件率的立方增加。 因此，即使是很小的噪音降低也是很重要的。涂层噪声的显著改善将加速LIGO直接探测引力波并启动引力波天文学时代的那一天。 除了引力波的研究之外，涂层热噪声现在已经成为其他物理领域的重要噪声源，包括精密光学和利用微谐振器的精密实验。 最后，由于Advanced LIGO的本底噪声较低，预计上转换将成为比Initial LIGO更突出的噪声源。调试团队将欢迎能够识别这些噪声峰值及其来源的工具。 由于上转换是精密物理界常见的噪音问题，因此开发一种有效的工具可能会在引力波界之外发挥作用。

This research program explores two of the primary noise sources in Advanced LIGO and other gravitational wave interferometers: upconversion noise and coating thermal noise. Coating thermal noise can be reduced by understanding and minimizing the mechanical loss in the mirror materials. Traditional high reflectivity coatings are multilayers of dielectric material, usually amorphous metal-oxides, with alternating high and low index. The materials that comprise the high-index layers are the primary source of mechanical loss; the low-index material is fused silica, which has an anomalously low loss among amorphous dielectric materials. In this research program, the PI seeks to develop a stabilized, high index material with a coefficient of thermal expansion matched to the low index material, so that the loss of the composite coating can be reduced through high temperature annealing. In addition, crystalline coatings, such as Aluminum Gallium Arsenide (AlGaAs), have been shown to have a mechanical loss about a factor 10 lower than most amorphous dielectric coatings. The AlGaAs coating could be developed as mirror materials for the 1.5 micron lasers planned for third generation detectors. However, current samples have been limited to the centimeter scale. This research program will explore the source of mechanical loss in these crystalline coatings including any issues related to the scaling and application of the coatings for large optics. Finally, upconversion noise is the coupling of low frequency noise, primarily seismic noise, into the detection band of the interferometer. Upconversion noise can be difficult to characterize because the noise peak is located at the sum or difference of the frequencies of the coupled mechanisms. This research program will develop a data analysis tool that will use bicoherence, the higher order form of coherence, to determine the sources of the upconverted noise. Coating thermal noise is the leading noise source in the central frequency band of Advanced LIGO and is a primary limit to overall detector sensitivity. Reducing coating thermal noise leads to a direct increase in the detector sensitivity and a cubed increase in its expected event rate. Thus even a small reduction in the noise is important. A significant improvement in coating noise will hasten the day when LIGO will make a direct detection of gravitational waves and launch the era of gravitational wave astronomy. Beyond the study of gravitational waves, coating thermal noise has now become an important noise source in other areas of physics, including precision optics and in precision experiments that utilize microresonators. Finally, with the lower noise floor of Advanced LIGO, upconversion is expected to be a much more prominent noise source than in Initial LIGO. The commissioning teams will welcome a tool that can identify these noise peaks with their source. And since upconversion is a common noise problem within the precision physics community, the development of an effective tool could be useful well beyond the gravitational wave community.