Innovative Quantum Noise reduction strategies for GW detectors

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In FY2022 several steps were taken in order to improve the stability of the filter cavity. The use of a bichromatic control for the filter cavity length, alignment and incident beam pointing degree of freedoms was finalized after being studied since FY2021. This was a crucial step in understanding the relationship between infrared lock and green lock. A paper on Physical Review D was published on it. With this control, a factor of 6 improvement on the filter cavity detuning was achieved, bringing this parameter within the requirement for implementation in KAGRA. In addition, the scheme proposed in the paper was officially adopted also by the Italian-French gravitational wave detector named Virgo. Another step done to improve the stability of the filter cavity was also achieved which was the experimental demonstration of the lock of the filter cavity via the coherent control sidebands scheme proposed theoretically by our group in 2019. The results of this demonstration have also been published in Physical Review D.In addition, several steps have been performed to reduce the impact of optical losses. In particular, the new low losses Faraday isolator has been characterized and the new low loss Optical Parametric Oscillator has been realized and characterized. The new low losses component will allow for a better characterization of scattering losses which will in turn lead to better understanding the role of polishing specs in the filter cavity system.