Study of the measurement of solution concentration by surface plasmon resonance heterodyne interferometer

In this study a non-contact method for accurately measuring small concentration of solutions by surface plasmon resonance heterodyne interferometer is proposed. Firstly, a linearly polarized heterodyne light source is transmitted hrough a test box filled with pure water. The transmitted light is incident on the base of a hemi-spherical prism of a surface plasmon resonance apparatus. Then the reflected light passes through an analyzer and generates an interference signal on a photo-detector. Secondly, when the incident angle is chosen at resonant angle, a significant phase difference between the s- and p-polarized components occurs. This phase difference is a function of the incident angle at the base of the hemi-spherical prism. Finally, when the test box is filled with a test solution, the incident angle at the base of the hemi-spherical prism is changed. This causes a variation in the phase difference that can be detected by the heterodyne interferometry. Therefore, the concentration of the tested solution can be accurately determined with special derived equations. The validity of this method was demonstrated experimentally. The advantages of the propose method include a simple apparatus, rapid measurement, high stability, and high resolution. Due to the introduction of a common-path structure, the interference signal is not affected by surrounding fluctuations and can be captured easily.