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The mid-infrared birefringence coefficients, including photoelastic coefficients, phase delay, and fast axis angle, are important indicators for evaluating infrared crystal materials. These coefficients can detect deviations introduced during optical glass processing and stress birefringence generated under external forces, which are mostly used to measure the influence of birefringence deviation of optical window under atmospheric pressure before assembly. This paper introduces a dual photoelastic modulators (PEMs) system for detecting birefringence in the near infrared to midinfrared range. The hardware of this system includes a 3390nm laser, a -45°polarizer, a 0°PEM, a +45°PEM, a 90°polarizer, a photodetector, a lock-in amplifier with filter circuit module, and a host computer, which can achieve modulation and demodulation of the polarization state of light and extraction of weak electric signal. The software interface of this system includes, data acquisition card channel setting interface, electrical signal waveform display interface and birefringence coefficient data processing interface. The polarization state analysis method using Stokes parameters and Mueller matrices, along with data processing techniques, enables automated and precise measurement of mid-infrared birefringence coefficients in crystals. Multiple sets of measurement data yield a phase delay of 9.863541 nm for a 15mm thick Si crystal and a phase delay of 8.971042 nm for a 12mm thick ZnS crystal. The repeatability of the measurement device is 0.020, and the measurement uncertainty is 0.019. This system can be extended for precision measurement of stress birefringence in multi-wavelength infrared crystals. It not only fills the gap in testing equipment for mid-infrared crystal photoelastic coefficients but also provides valuable guidance for the design, evaluation, and application of crystal stress birefringence systems.
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