Dual-wavelength intensity-modulated Fabry–Perot refractive index sensor driven by temperature fluctuation

A refractive index sensor based on an in-line Fabry-Perot interferometer is proposed and experimentally demonstrated. Two lasers are combined and injected into the sensor head. The power responses of two wavelengths are measured by a dual-channel optical power meter simultaneously. The two reflected power signals distribute along an ellipse. The refractive index of the liquid is calculated from the half length of the longer axes of the fitted ellipse. The refractive index sensing system is demonstrated to measure the refractive index of the salt solutions with different concentrations. The demodulated results matched well with the refractive index measured by the Abbe refractometer, and a resolution of 0.0017 was obtained. Since the temperature is eliminated during the ellipse fitting, the measuring result is insensitive to the temperature fluctuation. The proposed refractive index sensing sensor has outstanding advantages, such as low demodulation cost, simple fabrication, easy cleaning, and good mechanical strength, and will be of importance in biological detection, chemical analysis, and water pollution monitoring.