A fiber-optic refractometer for humidity measurements using an in-fiber Mach–Zehnder interferometer

• We report a refractive index and relative humidity sensor based on an in-fiber Mach–Zehnder interferometer.
• The Mach–Zehnder interferometer (MZI) composed by a hollow-core fiber sandwiched between two single-mode fibers with a length of 8 mm.
• We have theoretically and experimentally demonstrated the MZI for RI and RH measurement, the numerical study of the RI measurement shows that sensitivity of the sensor up to 155.6 nm/RIU and 163.1 dB/RIU, furthermore, humility measurement show the sensitivity is 1.29 pm%−1 and 0.00542 dB%−1.

An in-fiber Mach–Zehnder interferometer (MZI) was proposed and experimentally demonstrated for refractive index (RI) and relative humidity measurements. The sensor is composed of a hollow-core fiber (HCF) sandwiched between two single-mode fibers (SMFs) with a length of 8 mm. A well-defined interference pattern was obtained as a result of fiber–core mismatch and core–cladding mode interference. The selected interference dip was extremely sensitive to the ambient RI and provided a stable RI response with sensitivities of up to 155.6 nm/RIU and 163.1 dB/RIU for water-based solutions close to the RI of 1.33, which makes it capable of environment humidity measurements with a sensitivity of 1.29 pm%−1 and 0.00542 dB%−1.