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Frequency dependent squeezing is the main upgrade for achieving broadbandquantum noise reduction in upcoming observation runs of gravitational wavedetectors. The proper frequency dependence of the squeezed quadrature isobtained by reflecting squeezed vacuum from a Fabry-Perot filter cavity detunedby half of its linewidth. However, since the squeezed vacuum contains noclassical amplitude, co-propagating auxiliary control beams are required toachieve the filter cavity's length, alignment, and incident beam pointingstability. In our frequency dependent squeezing experiment at the NationalAstronomical Observatory of Japan, we used a control beam at a harmonic ofsqueezed vacuum wavelength and found visible detuning variation related to thesuspended mirrors angular drift. These variations can degrade interferometerquantum noise reduction. We investigated various mechanisms that can cause thefilter cavity detuning variation. The detuning drift is found to be mitigatedsufficiently by fixing the incident beam pointing and applying filter cavityautomatic alignment. It was also found that there is an optimal position of thebeam on the filter cavity mirror that helps to reduce the detuningfluctuations. Here we report a stabilized filter cavity detuning variation ofless than 10$\,$Hz compared to the 113$\,$Hz cavity linewidth. Compared topreviously published results [Phys. Rev. Lett. 124, 171101 (2020)], suchdetuning stability would be sufficient to make filter cavity detuning driftinduced gravitational wave detector detection range fluctuation reduce from$11\%$ to within $2\%$.

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