Solution refractive index sensor based on high resolution total-internal-reflection heterodyne interferometry

•The study develops an optical sensor for measuring the solution refractive index.•The sensor is based on total internal reflection effect and heterodyne interferometry.•The study derived the equations of the phase difference versus the solution refractive index.•This method has the merits of both common-path interferometry and heterodyne interferometry.

In this paper, a solution refractive index sensor is proposed based on a high resolution total-internal-reflection (TIR) interferometry. In the proposed sensor, a half-wave plate and a quarter-wave plate that exhibit specific optic-axis azimuths are combined to form a phase shifter. When an isosceles right-angle prism whose base contacts with a tested solution is placed between the phase shifter and an analyzer with suitable transmission-axis azimuth, it shifts and increases the phase difference of the s- and p-polarization states at one TIR. The increased phase difference relates to the solution refractive index; thus it can be easily and accurately measured by evaluating the phase difference. The feasibility was demonstrated by experimental results. This method has the merits of both common-path interferometry and heterodyne interferometry.