Cause of the high-frequency open-loop model discrepancy

Determine the cause of the discrepancy between the modeled and measured open-loop transfer functions of the Advanced LIGO-style ALS-CARM control configurations above 25 kHz in the CIT 40-meter prototype interferometer.

Background

The paper compares modeled and measured open-loop transfer functions for low-bandwidth and high-bandwidth ALS-CARM control in the CIT 40-meter prototype interferometer. Although the high-bandwidth configuration achieves a unity-gain frequency near 17.9 kHz and vector fitting identifies stable loop-pole locations, the model and measurements diverge outside the control bandwidth, particularly above 25 kHz. The authors explicitly state that the reason for this discrepancy is not clear, leaving its physical or instrumental origin unresolved.

References

A discrepancy was observed between the main loop model and the measured data outside the control bandwidth, particularly above 25 kHz. The reason for this discrepancy is not clear, but vector fitting successfully identified the loop pole locations, supporting the fact that they are stable.

— Fast Control for LIGO Interferometer Lock Acquisition  (2608.19449 - Ono et al., 19 Aug 2026) in Figure \ref{fig:OLTF_and_NoiseSpectrum} caption, Section 3 (Fast Lock Acquisition)