Refractive index detection with self-mixing interferometry for biosensing applications

This paper presents a novel method for detecting a change in the refractive index of samples. One of its major applications is sensing molecular interaction in biological samples. In our study a self-mixing interferometer (SMI) was chosen as the instrument for measuring the refractive index in free -space. A GaN blue laser diode was used as a light-emitting source. Compared with traditional interferometric configurations, self-mixing interferometry combined with the laser diode package has the advantage of a compact setup and high sensitivity. Long-term stability issue was first concerned in our research. The results showed that in 15 minutes the movement of the fringe pattern formed by the self-interfered laser beam is 13.6 nm. The measurement of the refractive index was performed by adding a heating element to the external cavity of the SMI. The refractive index of the air in the external cavity was varied by the atmospheric temperature. The change in the refractive index of the air was calculated using both a modified Edlén equation and the recorded self-interfered signals. The results showed that the change in the refractive index observed from the shift in the fringe pattern is compatible with that calculated with the modified Edlén equation, or about 1*10-6/°C with optical path length of 5 cm. Theoretically, the smallest movement of the fringe pattern that can be detected with our measurement setup is 1.6 nm, corresponding to a 10-8 change in the refractive index in the external cavity.