Accurate polarization characterization of thin film polarizing beamsplitters with cavity ring-down technique

The cavity ring-down (CRD) technique is employed to accurately measure the polarization characteristics of thin film polarizing beamsplitters (PBS's). The measurement principle and related theory are provided. By establishing <i>s-</i> and <i>p-</i>polarized two-channel CRD configurations, the s- and p-polarization transmittance and reflectance values (<i>T_s</i>, <i>T_p</i>, <i>R_s</i>, and <i>R_p</i>) and the polarization extinction ratio (<i>PER</i> = <i>T_p</i>/<i>T_s</i>) of the thin film PBS's are measured with high accuracy. The experimental results with a 45-degree and a Brewster angle PBS demonstrate that the two-channel CRD is capable of measuring the <i>p</i>-polarization transmittance <i>T_p</i> of ∼99.0% and higher with an error of ±0.01∼0.02% or lower, and the <i>s</i>-polarization transmittance <i>T_s</i> < 1 ppm with an uncertainty of < ±0.01 ppm or ∼ ± 1.0%. A <i>PER</i> as high as 10⁶:1 is determined by CRD with an error of only ±1.0∼2.0%. Comparisons between <i>PER</i> measurements using CRD and spectrophotometry show that that CRD offers much higher accuracy for high PER measurements than spectrophotometry. The two-channel CRD is expected to measure the <i>PER</i> of thin film PBSs with an uncertainty of ∼1.0% across the entire range from approximately 100:1 to >10⁹:1.