Improving the stability of frequency dependent squeezing with bichromatic control of filter cavity length, alignment and incident beam pointing

TL;DR

This paper presents a method to improve the stability of frequency dependent squeezing in gravitational wave detectors by controlling filter cavity parameters, significantly reducing detuning fluctuations and enhancing detection sensitivity.

Contribution

The study demonstrates a bichromatic control technique for filter cavity length, alignment, and incident beam pointing that stabilizes detuning variations below 10 Hz, improving quantum noise reduction.

Findings

Detuning variation was reduced to less than 10 Hz from 113 Hz.

Stability improvements could reduce detector range fluctuation from 11% to 2%.

Automatic alignment and beam pointing fixing are effective in mitigating detuning drift.

Abstract

Frequency dependent squeezing is the main upgrade for achieving broadband quantum noise reduction in upcoming observation runs of gravitational wave detectors. The proper frequency dependence of the squeezed quadrature is obtained by reflecting squeezed vacuum from a Fabry-Perot filter cavity detuned by half of its linewidth. However, since the squeezed vacuum contains no classical amplitude, co-propagating auxiliary control beams are required to achieve the filter cavity's length, alignment, and incident beam pointing stability. In our frequency dependent squeezing experiment at the National Astronomical Observatory of Japan, we used a control beam at a harmonic of squeezed vacuum wavelength and found visible detuning variation related to the suspended mirrors angular drift. These variations can degrade interferometer quantum noise reduction. We investigated various mechanisms that can cause the filter cavity detuning variation. The detuning drift is found to be mitigated sufficiently by fixing the incident beam pointing and applying filter cavity automatic alignment. It was also found that there is an optimal position of the beam on the filter cavity mirror that helps to reduce the detuning fluctuations. Here we report a stabilized filter cavity detuning variation of less than 10$\,$Hz compared to the 113$\,$Hz cavity linewidth. Compared to previously published results [Phys. Rev. Lett. 124, 171101 (2020)], such detuning stability would be sufficient to make filter cavity detuning drift induced gravitational wave detector detection range fluctuation reduce from $11\%$ to within $2\%$.