Refractometer With Etched Chirped Fiber Bragg Grating Fabry–Perot Interferometer in Multicore Fiber

We report a sensitive and mechanically robust fiber-optic refractometer with temperature compensation based on a Fabry–Perot interferometer formed by two chirped fiber Bragg gratings on a seven-core multicore fiber. A small part of the fiber cladding at the sensing region is etched to expose the outer cores to the environment. While the optical modes supported by the outer cores are affected by both temperature and refractive index (RI) changes of the surrounding liquid, the optical mode in the central core of the fiber is affected by the temperature changes only. The wavelength shift due to the RI change is obtained by subtracting the wavelength shift of the central core from that of one of the outer cores so that the temperature effect is removed. The demodulation can also remove the errors caused by the wavelength drift of laser. Due to the narrow spectral features of the sensor, the sensor shows a low RI detection limit of ${2.4} \times {10}^{-{6}}$ at a background RI of 1.316 at a wavelength of ~1550 nm. Because only a small part of the cladding is removed, the sensor maintains excellent mechanical strength and stability.