All-polarisation beamsplitters for interferometer applications

Optical beamsplitters with similar properties for orthogonal, linear polarisation modes are required for realising polarisation-based speedmeter schemes to reduce back-action noise in gravitational-wave interferometers. In this paper, we investigate two beamsplitter coatings obtained from Laseroptik GmbH and Optoman on a best-effort basis that aim for a 50/50 power splitting ratio and equal overall phase shift for two orthogonal, linear polarisation modes interacting with the optic. We show that while Laseroptik GmbH opted for coating stack with 22 alternating layers of Ta2O5 and SiO2, Optoman produced a much thinner coating made of 5 SiO2 and SiOx (0 < x < 2) layers. With these strategies, the Laseroptik coating achieves an equal power reflectivity of 51% at 46 deg angle of incidence, and zero phase shift between both polarisations at 44.25 deg angle of incidence. The Optoman coating achieves power reflectivities of 49% for s-polarisation and 51% for p-polarisation with a differential phase shift around 5 deg largely independent of the angle of incidence.