A Propagation Model for the LIGO Squeezing Subsystem

Frequency-dependent squeezing allows LIGO interferometers to surpass shot noise limitations at high frequencies, while avoiding radiation pressure noise penalties at low frequencies, effectively lowering the noise floor. LIGO currently achieves up to -6 dB squeezing in the phase quadrature. Our models reveal 20-25% optical loss and 10-15 mrad phase noise associated with this level of squeezing, which are critical constraints on the injected squeezing at the output. To reach the target of -10 dB squeezing in future detectors as well as ensuring robust -6 dB performance in present setups, it is essential to carefully characterize the system capabilities. This work focuses on developing a propagation model of the squeezer system as it integrates with the interferometer, providing insights into the optical limitations and guiding improvements for next-generation detectors.