Demonstration for a two-axis interferometric tilt sensor in KAGRA

Highlights: • Interferometric tilt sensor can directly measure the angle of a target. • The interferometric tilt sensor was modularized so to sense two axes of a target. • They were tested with one of the mode-cleaner mirrors in the gravitational wave detector as the actual application. • The module properly sensed the target mirror motion both in time series and in the frequency domain. • Compared with the currently-used optical levers, our tilt sensors have lower noise above 10 Hz. - Abstract: Recently, a folded Mach–Zehnder interferometer with homodyne in- and quadrature-phase detection was proposed as a high-precision, wide-dynamic range tilt sensor. By way of a practical application and to validate actual performance, two-axis tilt sensors were developed and installed for one mirror of the input mode cleaner cavity in KAGRA, the large-scale cryogenic gravitational-wave telescope in Kamioka, Gifu, Japan. Building on previous work, we have demonstrated that the two-axis tilt sensor has properly sensed the tilt angle changes of the mirror motion with high precision and without calibration. Compared with our initial angular sensor, an optical lever, which is calibrated by using the interferometer tilt sensor, we found that both sensors showed actual tilt motions of the mirror at low frequencies, and the two-axis interferometer sensor has a better sensitivity at higher frequencies.