Coating thermal noise measurement with a multimode resonator

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.