Highly Sensitive Salinity Sensor Based on Virtual Vernier Effect of Micro–Nano Fiber Mode Interferometer

In this paper, a highly sensitive salinity sensor based on the virtual Vernier effect of a micro-nano tapered two-mode fiber (TTMF) is proposed, theoretically analyzed, and experimentally verified. Theoretical calculations show that the dispersion turning point (DTP) occurs when the group effective refractive index difference between the fundamental and higher-order modes of the micro-nano TTMF is equal to zero, and thus ultra-high refractive index sensitivity can be achieved to measure seawater salinity. After considering the robustness of the sensor, the cone size near the DTP was chosen and the sensor was designed in a semi-package. Meanwhile, applying the sensing signal demodulation method of the virtual Vernier effect to the TTMF salinity sensor not only exponentially amplifies the sensitivity of salinity measurement, but also conveniently obtains the tunable sensitivity amplification by changing the modulation function, which has the incomparable flexibility of the conventional optical Vernier effect. The experimental results show that the proposed salinity sensor has a salinity sensitivity of 6.138 nm/‰ in the range of 0 ‰ - 39.22 ‰ and a temperature sensitivity of -3.672 nm/°C in the range of 30 °C - 50 °C, which provides an improvement of 13.64 and 14.81 times in salinity and temperature sensitivities, respectively, when compared with a single TTMF. The TTMF packaged sensor demonstrated in this paper has the advantages of low cost, robustness, and high reproducibility, which provides key technical support for optical sensing applied in the ocean, and also provides some useful references for other sensing applications.