Wavelength-Interrogation Fabry–Perot Refractive Index Sensor Based on a Sealed In-Fiber Cavity

A wavelength-interrogation fiber Fabry-Perot refractive index (RI) sensor based on a sealed in-fiber ellipsoidal cavity is presented. There are two physical cavities in the sensor head: 1) the air cavity and 2) the silica cavity. The result of simulation shows there is only one trough in 1510-1590 nm, the RI can be measured by tracking the trough wavelength shift, and there is no turning point in RI response characteristic between 1 and 2 RIU. The sensitivity and linear range of the RI measurement can be controlled by changing the silica cavity's length and the sensor end's roughness. Experiment results match well with the theoretical analysis within the measurement range. High reliability, without turning point and real-time demodulation make it appropriate for practical application.