Liquid crystal based polarized low coherence interferometer for optical demodulation in sensors

The resolution of the measurement detection and sensitivity of a polarized low coherence interferometer (PLCI) can be pre-engineered by optimizing the key parameters of the birefringent wedge, which is rarely reported. In this work, we introduce a liquid crystal (LC) wedge in the PLCI and use it to demodulate Fabry-Perot (FP) cavity length. The birefringence property of the nematic LC is used to convert the optical path difference (OPD) of the sensor into a spatial distribution. This results in the production of localized interference fringe patterns. The formation of PLCI fringes and the related shift of the interferogram with a variation in the displacement of the FP displacement sensor is explained with reference to the OPD matching between an LC wedge and the FP cavity. The displacement value is demodulated from the obtained fringe pattern by tracking the centroid position of the fringe envelope and also considering the birefringence dispersion. An additional simulation study shows that the spatial position of the interferogram signal coupled with the dispersion coefficient is almost identical to the experimental data. The demodulated results from both the simulation and experimental investigations are found to be consistent with each other and closely agree with the actual cavity length. Further, the possibility to enhance the sensing resolution is examined by modulating the interferogram fringes using an electric field. Compared to birefringent crystals, the LC wedge presented here is found to be advantageous for high precision and tunability of the measurement range, which is useful for robust fiber optic sensing applications.