Development of frequency dependent squeezing technique for sensitivity improvement of gravitational wave detector KAGRA

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This fiscal year we published a paper on Physical Review Letters on the first demonstration of frequency-dependent squeezed vacuum source for broadband quantum noise reduction in advanced gravitational-wave detectors. This result was highlighted in the magazines of APS and of OSA. All the main gravitational wave detectors in the world have now adopted this technology.After this milestone was achieved, we improved the reliability of the experimental apparatus. In particular, we implemented a wave front sensing system in order to automatically align the filter cavity to the green beam. In addition, we prepared the implementation of a new OPO and a new Faraday isolator in order to reduce the optical losses of the experiment.Finally, we developed a new control system for the locking of the filter cavity. This new system is based on the use of the sidebands used for the coherent control to lock the filter cavity at its operating point. First, we published a paper on Phys Rev D describing the method. Then we implemented the method and successfully we locked the filter cavity using it. This method allow locking the cavity using the coherent control sidebands i.e. a beam perfectly superposed to the squeezed vacuum beam. This was never done before.