Crystalline silicon is a strong contender to replace fused silica as the core optics substrate material in third generation cryogenic gravitational wave detectors such as LIGO Voyager. Naturally, before we can commit to such a drastic design change, the enticing low-temperature characteristics of silicon optics need to be thoroughly investigated. While silicon's thermal expansion as a function of temperature is well documented, and its bulk mechanical loss has been explored with encouraging results, the necessary transition towards longer wavelengths requires modifications to the coating topology.
While high reflectivity coatings for 1550 nm can be constructed from Tantala-Silica stacks, these would inherit the Brownian noise that is already expected to limit Advanced LIGO in its sensitivity. Alternative coating solutions are under investigation in particular for their mechanical loss, Young's modulus, and thermo-optic properties, which all appear in coating thermal noise models. Each property can be targeted separately by dedicated experiments, but only a direct observation of the induced characteristic broadband coating noise can build trust in the models, and ultimately the coating solution. In the Caltech CryoLab a mirror-intrinsic displacement noise test bench is under construction that aims to observe the optical path length fluctuations due to coating noise of selected test mirrors. With the capability to go to cryogenic temperatures, it assesses the differential, uncorrelated length noise of two nominally identical cavities in a beat note measurement between individually stabilized lasers. Through a detailed noise budget we will be able to extract the coating noise contributions and compare them to model predictions.
As part of my previous research a similar experiment emerged out of a laser frequency stabilization and heterodyne phase measurement test bed for the Laser Interferometer Space Antenna project at the University of Florida. In my presentation I will connect the dots between the two experiments and discuss their relevance, current status, and ongoing improvements.
We plan to broadcast these talks using TeamSpeak. Use a sub-channel of
LIGO Lab called "LIGO Seminar", which is not password protected.
NOTE: These and all other scheduled LIGO seminars are listed on the LIGO
Laboratory seminar calendar for convenient reference.