Coating thermal noise measurement with a multimode resonator

光学涂层是几乎所有围绕着我们的技术的关键部件，从眼镜到相机。极高性能的涂层用于光学原子钟和引力波探测器，这是有史以来最灵敏的时钟和测距仪。光学涂层在光子学的工业应用中也是必不可少的，特别是对于激光二极管器件的小型化和提高激光损伤阈值。光学涂层由不同折射率的交替材料层组成，厚度只有几微米。它们的性能取决于涂层内部散射和吸收的光量，以及由原子布朗运动引起的热噪声。光学薄膜可以由多种材料制成，例如氧化钽、二氧化硅和非晶硅。然而，涂层的最终性能取决于这些材料的固有性能和制造工艺。因此，有必要进行可靠的实验来测试用于精密仪器的新型薄膜。我们建议建立一个国际领先的设备来直接测量新型光学薄膜的性能。这一建议源于最近的两项发现。首先，麻省理工学院的LIGO小组发现，使用多模光学谐振器可以在一周内测量涂层样品。其次，英国、美国和德国的学术界团体开发了一种新的具有极高性能的涂层，可用于精密测量。该中心是高质量涂层商业化生产的关键一步，因为我们需要实验探索涂层生产的整个参数空间，如沉积速度、掺杂材料和热处理温度。该实验的关键思想是在光学谐振器中嵌入涂层样品，并使用三个共振光束测量其性能。这种设置将使除涂层热噪声外的所有位移噪声对这些光束都是通用的。拟议中的设备的主要优势是，它可以在电信激光波长下每周测试一个涂层样品，并有可能成为世界上第一个在该参数空间内使用极高性能涂层的设备。该中心将能够为未来的光学原子钟、下一代引力波探测器、基础物理实验和商业应用直接测量涂层样品。

Optical coatings are key components within almost all technology that surrounds us from our glasses to cameras. Extreme-performance coatings are used in optical atomic clocks and gravitaitonal-wave detectors which are the most sensitive clocks and distance meters ever built. Optical coatings are also essential for industrial applications in photonics, particularly for miniaturisation of laser diode devices and for increasing the laser damage threshold.Optical coatings consist of alternating layers of materials with different refractive indices and are only a few micrometers thick. Their performance is determined by the amount of light scattered and absorbed inside the coating and by their thermal noises caused by the Brownian motion of the atoms. Optical coatings can be manufactured out of a large variety of materials, such as tantalum oxide, silica, and amorphous silicon. However, the ultimate properties of the coatings depend both on the intrinsic properties of these materials and on the manufacturing process. Therefore, it is essential to have a robust experiment to test novel coatings for precision instruments.We propose to build an internationally-leading facility to directly measure the properties of novel optical coatings. This proposal emanates from two recent findings. First, the MIT LIGO group found that coating samples can be measured in one week using a multimode optical resonator. Second, groups in academia in the UK, USA, and Germany developed a new class of promising extreme-performance coatings for applications in precision measurements. The proposed centre is a crucial step in commercial manufacturing of high-quality coatings since we need to experimentally explore the whole parameter space of coating production, such as deposition rate, doping materials, and annealing temperature.The key idea of the proposed experiment is to embed a coating sample in the optical resonator and measure its properties using three co-resonating beams. This setup will make all displacement noises common to these beams, except for the coating thermal noises. The main advantage of the proposed facility is that it can test one coating sample per week at the telecom laser wavelength and has the potential to be the first in the world working with extreme-performance coatings in this parameter space. The centre will be able to directly measure coating samples for future optical atomic clocks, next generation of gravitational-wave detectors, fundamental physics experiments and for the commercial applications.