Theoretical and experimental study of liquid refractive index measurement method based on the double external-cavity-laser feedback effect

Abstract

A liquid refractive index (RI) measurement method based on the double external-cavity laser feedback effect is proposed. The method uses a laser beam to simultaneously enter the top and bottom surfaces of the liquid to be tested, with the light reflected from the two surfaces returning to the laser cavity along the original path, forming a double external-cavity system. When the liquid level rises, the self-mixing interference signal, caused by the optical path from the top and bottom liquid surfaces, superimposes on the tuning intensity of the laser, and the RI of the liquid can be obtained by demodulating the amplitude-frequency feedback tuning curve. This method offers the following innovative performances. Firstly, the structure of the system is simple and the measuring range is large. Secondly, the measurement results can be traced back to one physical quantity, i.e. the RI of air. Thirdly, it has been proved that the laser feedback effect is ubiquitous in all types of lasers, and the change of wavelength does not affect the feedback phenomenon, so that the method is not limited by the type and wavelength of the laser.