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

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 degrees angle of incidence was characterised along with a QWP of birefringence of lambda/4 + lambda/324 . The cavity mirror optics of a 10 m prototype polarisation Sagnac speedmeter were measured to have birefringence in the range 1 x 10(-3) to 2 x 10(-5) radians. This level of birefringence, along with the QWP imperfections, can be cancelled 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.

Automated summary

The polarisation Sagnac speedmeter interferometer shows promise as a replacement for the Michelson interferometer in future ground-based gravitational wave detectors. The quality of polarisation optics, including the polarisation beam-splitter and quarter-waveplate, along with consideration of birefringence effects, are crucial for the quantum noise benefits of this speedmeter. The leading limitation for the polarisation Sagnac speedmeter in terms of polarisation effects is the extinction ratio of the PBS, which can be impacted by careful adjustment of the QWP angle.