LIGO Seminar

https://caltech.zoom.us/j/87546916051?pwd=aTByZU5WamhUKzlkRVY2bW05bytDdz09

Speaker: Namisha Chabbra 

Title: Coating Thermal Noise - Reaching New Limits in Precision Laser Stabilisation 

Laser light has been a useful tool in allowing us to detect such small path length changes. In fact, lasers are a common choice for many ultra-sensitive measurements such as space and ground-based metrology, spectroscopy, and fundamental physics tests. For gravitational wave detection, the interferometric readout can be used to convert laser light's relatively short and stable wavelength into a very precise probe of strain.  While lasers offer high fractional frequency stability, the required level of precision to measure gravitational waves, especially in space, demands that lasers be actively stabilized to avoid 'measuring stick' errors. Factors such as acoustic coupling from the environment, thermal fluctuations, the sensitivity of the detector's electronics and mechanical imperfections in optical instrumentation can all contribute to ‘noise' that drowns out the anticipated gravitational wave signal [2].  Brownian noise, at present, is the dominating limitation in precision laser stabilization. Brownian noise is a fundamental physical phenomenon and is the thermal noise that comes from thermal fluctuations in a given material, such as bulk glass spacers, mirror substrates and mirror coatings, which are all present in optical cavities.   While current strategies to mitigate thermal noise have demonstrated promising improvements, some are challenged by the demanding environment of space, where size, weight and power requirements are carefully considered.  We propose an optimized method for laser readout that offers a multiplying factor of improvement on current approaches to reduce thermal noise. As these Brownian thermal fluctuations are random, the problem can be looked at statistically – over a certain area and time, these fluctuations average to zero. We propose sampling the cavity with a fundamental mode and several higher-order modes, where the resulting readout signals are combined in digital signal processing to synthesize a flattened beam profile on the cavity mirrors. By widening the effective spatial sampling within the cavity, the Brownian motion over the surface of the mirrors can be better averaged, lowering the effect of thermal noise by up to a factor of 1.6 with just three modes [3]. 

[1]   LIGO Scientific Collaboration and Virgo Collaboration, "Observation of Gravitational Waves from a Binary Black Hole Merger," Phys. Rev. Lett., vol. 116, no. 6, 2016.  
[2]   A. K. Ghatak, "Optical Electronics," Cambridge University Press, 1991. 
[3]   A. Wade, "Mirror Coating Thermal Noise Mitigation Using Multi-Spatial Mode Cavity Readout," arXiv, 2022.