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Description
Squeezed vacuum injection is one of the key upgrades for improving the high-frequency sensitivity of KAGRA. However, the inhomogeneous birefringence of the sapphire input test masses (ITMs) can limit the achievable squeezing benefit. In this work, we evaluate how the measured ITM birefringence affects squeezed-light operation in KAGRA.
The ITM birefringence couples the fundamental interferometer mode, S-polarized HG00, into P polarization and higher-order spatial modes. These components should not be treated only as lost fields. After propagating through the interferometer, they acquire different phase rotations through Gouy phase evolution and polarization-dependent beam-splitter reflection phases, and can coherently recouple to the fundamental mode. This recombination mixes anti-squeezed noise into the detected squeezed quadrature.
Using a mixing model that includes HOMs, S/P polarization, and the asymmetry of the two KAGRA arms, we find that the present ITM birefringence can strongly suppress the squeezing gain. For 10 dB input squeezing, the high-frequency sensitivity can become worse than the unsqueezed case in part of the band. We also investigate how the degradation changes when the birefringence strength and injected squeezing level are varied, and discuss the implications for allowable ITM birefringence and future squeezed-light commissioning in KAGRA.