Common-path interferometers for the characterization of waveplates and liquid crystals

We present a common-path interferometer for measuring the retardance and azimuth of optical waveplates and liquid crystals, inherently immune to instabilities typical of other kind of interferometers and without the need for additional pre-characterized waveplates. The parameters of interest are calculated from the fringe pattern displacement produced when rotating the waveplate. We demonstrate this concept with two devices: a Young’s double-slit interferometer with orthogonally oriented linear polarizers at each slit, and a Wollaston prism that splits the incoming beam into two orthogonally polarized components. To gain some insight in the advantages of the method, we have numerically analyzed the e ect of using non-ideal polarizers in the measurement process, demonstrating that using polarizers with very low extinction coe cient is crucial for the accuracy of the determination in the retardance. Additionally, for a more in-depth characterization, we have considered the case of ideal polarizers, but waveplates presenting di erent grades of diattenuation. Also, we have built both interferometers considering and removing the e ect caused by tilted or wedged elements. These experimental realizations of the two common-path interferometers allowed us to analyze for several commercial waveplates, obtaining t errors below 0.1º and repeatabilities usually below 1º for the retardance. The azimuth uncertainties remain close to 0.02º and 0.3º for the t and repeatabilities errors, respectively. We have also characterized a liquid crystal variable retarder using the Wollaston prism interferometer as it has proven to be the most accurate and versatile method. All experimental results obtained with our approach show good agreement with those measured using Mueller polarimetry.