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.