Birefringence measurements of substrate materials and coatings for Einstein Telescope

Given the planned tenfold improvement in sensitivity of the future Einstein Telescope, the birefringence of optical elements (substrates and coatings) is becoming an important optical parameter to be measured. A sensitive optical polarimeter has been developed at the Department of Physics and Earth Sciences of the University of Ferrara and INFN - Ferrara Section, Italy to measure ellipticities and rotations induced by substrates and coatings on a linearly polarised laser beam with wavelength λ = 1064 nm both in transmission and reflection. The sensitivity in optical path difference, both in transmission and in reflection, is SD≈ 10-12m/√Hz. The technique takes advantage of both heterodyne detection and of time-dependent signals (ellipticity and rotations) induced by two co-rotating half-wave plates. The method is demonstrated on some crystalline silicon samples and high-reflectivity coatings. Some examples are reported.