Two-frequency paired polarization interferometer for Faraday rotation angle detection

A highly sensitive two-frequency paired linear polarized interferometer (TPPI) for measuring the Faraday rotation angle and Verdet constant of the Bi₁₂SiO₂₀ (BSO) crystal in real time was set up by an amplitude-sensitive detection method. TPPI features a common-path heterodyne interferometer in conjunction with a highly correlated paired linear polarized laser beam. Then, the antisymmetry of polarized heterodyne signals is produced and Faraday rotation angle detection by a balanced detector scheme is satisfied automatically. As a result, shot-noise-limited detection of Faraday rotation angle is possible. In addition, the Faraday rotation angle detection is also insensitive to the scattering and absorption caused by the specimen because of the common-path propagation of the paired linear polarized laser beam. Experimentally, the sensitivities of Faraday rotation angle and Verdet constant measurements of the BSO crystal under the arrangement with a single pass of the laser beam in TPPI are 4:93 × 10^-5 rad/mm and 2:6 × 10^-7 rad/(mT.mm), respectively. This suggests that the Faraday rotation angle detection sensitivity has the potential to be on the order of 10^-8 rad/mm if a Fabry-Perot cavity with a finesse of F = 120 is used in TPPI.