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

该研究项目探索了先进 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。 通过了解并最大限度地减少反射镜材料的机械损耗，可以降低涂层热噪声。 传统的高反射率涂层是多层介电材料，通常是非晶态金属氧化物，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失的主要来源；低折射率材料是熔融石英，它在非晶介电材料中具有异常低的损耗。在该研究项目中，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，从而可以通过高温退火来减少复合涂层的损耗。 此外，砷化铝镓 (AlGaAs) 等结晶涂层的机械损耗比大多数非晶介电涂层低约 10 倍。 AlGaAs 涂层可开发为用于第三代探测器的 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。 该研究计划将探索这些晶体涂层机械损失的根源，包括与大型光学器件涂层的缩放和应用相关的任何问题。 最后，上变频噪声是将低频噪声（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构的频率之和或差处。 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性（相干性的高阶形式）来确定上变频噪声的来源。 涂层热噪声是高级 LIGO 中心频段的主要噪声源，也是探测器整体灵敏度的主要限制。减少涂层热噪声可以直接提高探测器的灵敏度，并使其预期事件发生率成倍增加。 因此，即使噪音的小幅降低也很重要。涂层噪声的显着改善将加速LIGO直接探测引力波并开启引力波天文学时代的到来。 除了引力波研究之外，涂层热噪声现已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，由于高级 LIGO 的本底噪声较低，上变频预计将成为比初始 LIGO 更为突出的噪声源。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中常见的噪声问题，因此开发有效的工具可能远远超出了引力波界的范围。

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.