Speaker
Description
While squeezing is essential for reducing high-frequency quantum noise in gravitational-wave detectors, inhomogeneous birefringence in sapphire Input Test Masses (ITMs) poses a challenge for KAGRA. ITM birefringence couples the fundamental S-polarized HG00 mode into P polarization and higher-order modes (HOMs). We show that the impact of birefringence cannot be captured as simple optical loss: the coupled components propagate through the interferometer, acquire distinct phase rotations, and coherently recouple to the fundamental mode, producing a mixing effect.
To quantify this mechanism, we developed a mixing model extending the framework of McCuller et al. (2021) to include HOMs, polarization degrees of freedom (S/P), and realistic arm asymmetry. Our analysis shows that differential phase rotations—arising from Gouy phase evolution and beam-splitter reflection phase shifts—cause anti-squeezed (anti-SQZ) noise to be mixed back into the squeezed (SQZ) field. This leads to sensitivity degradation far exceeding pure-loss predictions.
Using KAGRA’s current birefringence map and 10 dB input squeezing, we find that this anti-SQZ contamination can completely erase the expected sensitivity gain and even make the sensitivity worse than the unsqueezed (0 dB) case. We will present the recombination-driven degradation mechanism and discuss upper limits on allowable ITM birefringence for future KAGRA upgrades.