Polarization-sensitive optical coherence tomography for birefringence measurement of calcite nonlinear uniaxial crystal

Abstract

We report on the new application, to the best of our knowledge, of a time-domain optical coherence tomography (TD-OCT) device used to measure the ordinary n o and extraordinary n e indices of calcite birefringence crystal at room temperature. A 1.25±0.05mm thick slab of calcite crystal is cut, polished, and used as a sample in the OCT arm. While the calcite slab is axially scanned, the raw carrier ordinary signals that came from its front and rear facets are received and denoised with a set of digital filters. The extraordinary signals are generated by the change of beam polarization using a 90°-rotating polarizer plate. It is found that the wavelet transform is capable of reaching the highest signal-to-noise ratio (SNR) of about 24.50 and 23.91 for denoising the ordinary and extraordinary signals, respectively. Quantitative measurement of n o and n e is carried out by extracting a desired envelope from the denoised signals using standard methods. Average values of 1.660 and 1.444 are obtained for n o and n e , respectively, using the wavelet-denoised signals. The weights of the results are finally searched with ones obtained from two sets of dispersion equations. We found a very good agreement between the wavelet-denoised OCT- and dispersion equation-based values with a very low relative differences of 0.04% and 2.8% for n o and n e , respectively, when the Ghosh equation is used and averaged ones of 1.3% and 4.2% for n o and n e , respectively, when the Zhao et al. equation is applied.