Design of two-color phase-modulated birefringence equipment

A two-color phase-modulated birefringence equipment, used for the simultaneous and real-time measurement of birefringence and orientation angle of flow field, was designed by using a two color Argon-ion laser as light source coupled with the phase-modulate technique. Low filter circuits and lock-in amplifiers were used to measure the intensities of the incident light and the fundamental component of emergent light respectively, then the analog signals of light intensities were changed to digital signals, and thus the calculation of birefringence and orientation angle was achieved. The employment of the Bessel function of order 2 was avoided for the reason of a two-color laser was used as light source, so the calculation and the process of experiment became simplified. Experiments proved that this apparatus can accurately measure the birefringence and orientation angle simultaneously, where the maximum measurement error is smaller than 5 percent.