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

本财年，我们在《物理评论快报》上发表了一篇论文，首次展示了频率依赖的压缩真空源在先进引力波探测器中用于宽带量子降噪。这一结果在APS和OSA杂志上得到了强调。目前，世界上主要的引力波探测器都采用了这一技术。在达到这一里程碑之后，我们提高了实验装置的可靠性。特别是，我们实现了波前传感系统，以实现滤光腔与绿光的自动对准。另外，为了减少实验中的光学损耗，我们准备了一种新的OPO和一种新的法拉第光隔离器的实现。最后，我们开发了一种新的滤光腔锁定控制系统。这种新的系统是基于使用用于相干控制的边带来将滤光腔锁定在其工作点上。首先，我们在Phys Rev D上发表了一篇描述该方法的论文。然后实现了该方法，并成功地实现了对滤光腔的锁定。这种方法允许使用相干控制边带，即完全叠加到压缩真空光束的光束来锁定腔。这是以前从未做过的。

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.