Refractive Index Sensor Based on No-Core Fiber With Optoelectronic Oscillator Interrogation

In this letter, a refractive index sensing system is proposed and experimentally demonstrated. Acting as the sensing head, the no-core fiber spliced with two sections of single-mode fiber (SNS) is fabricated to form a fiber ring laser (FRL). The measured salt water changes the self-imaging wavelength of the NCF as well as the central wavelength of the FRL. After that, the light is introduced into the dual-loop optoelectronic oscillator (OEO). With the help of the dispersion compensation module in the OEO loop, the wavelength change is mapped into the frequency shift of the generated microwave signal. Hence, the RI of the measured liquid can be recovered by tracking the frequency change. Both the simulation analysis and the proof of concept experiment are performed. Results show that both the central wavelength of the FRL and the microwave signal increase exponentially with the increased RI, agreeing with the simulated analysis. In the range of 1.33-1.38, the linear fitting results manifest that the sensitivities of central wavelength change vs. RI and microwave signal shift vs. RI are 87.70 nm/RIU and 83.89 MHz/RIU, the corresponding resolutions are $2.28\times 10 ^{-4}$ and $1.19\times 10 ^{-5}$ , respectively. The proposed method shows potential for realizing high sensitivity, fast, and real-time RI measuring.