Experimental investigation of the limitations of polarisation optics for future gravitational wave detectors based on the polarisation Sagnac speedmeter

TL;DR

This study investigates the limitations of polarisation optics, specifically PBS and QWP, in a polarisation Sagnac speedmeter for gravitational wave detection, highlighting birefringence effects and their mitigation.

Contribution

The paper provides detailed measurements of polarisation optics and birefringence effects, identifying the primary limitations for implementing the polarisation Sagnac speedmeter.

Findings

PBS extinction ratio is the main limitation for polarisation purity.

Birefringence in cavity mirrors can be compensated by adjusting the QWP angle.

High-quality polarisation optics are essential for future gravitational wave detectors.

Abstract

The polarisation Sagnac speedmeter interferometer has the potential to replace the Michelson interferometer as the instrumental basis for future generations of ground-based gravitational wave detectors. The quantum noise benefit of this speedmeter is dependent on high-quality polarisation optics, the polarisation beam-splitter (PBS) and quarter-waveplate (QWP) optics that are key to this detector configuration and careful consideration of the effect of birefringence in the arm cavities of the interferometer. A PBS with an extinction ratio of better than 4000 in transmission and 700 in reflection for a $41^{\circ}$ angle of incidence was characterised along with a QWP of birefringence of $\frac{\lambda}{4} + \frac{\lambda}{324}$. The cavity mirror optics of a 10m prototype polarisation Sagnac speedmeter were measured to have birefringence in the range $1\times10^{-3}$ to $2\times10^{-5}$ radians. This level of birefringence, along with the QWP imperfections, can be canceled out by careful adjustment of the QWP angle, to the extent that the extinction ratio of the PBS is the leading limitation for the polarisation Sagnac speedmeter in terms of polarisation effects.