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Description
Precise alignment sensing and control are essential for the stable operation of laser interferometric gravitational-wave detectors. In conventional Wave Front Sensing (WFS) based on the beat between the carrier and phase-modulated (PM) sidebands, signals from the arm cavity axis dominate when the carrier resonates throughout the interferometer, making it difficult to detect signals from other optical axes. To separate multiple alignment degrees of freedom, we propose a WFS method, phase-modulated sideband × phase-modulated sideband WFS (PMPMWFS), which demodulates signals at the difference frequency of two PM sidebands. In this study, we derive the theoretical response of PMPMWFS and demonstrate it experimentally using the KAGRA Power-Recycled X-arm (PRXARM) configuration. Furthermore, we discuss the potential applications of PMPMWFS in gravitational-wave detectors.