Low Crosstalk Deep Ocean Salinity Monitoring at 2 μm Wavelength Band

Accurate ocean salinity detection and monitoring are crucial for protecting the ocean’s ecological environment, with low crosstalk sensing heads playing a significant role in this process. To achieve low crosstalk while maintaining high sensor sensitivity, devices operating in the longer wavelength band could be a viable candidate. In this article, we demonstrate an all-optical fiber marine salinity sensor designed for deep ocean environments, formed by adiabatic-tapered few-mode fiber (FMF) with a $40~\mu $ m waist diameter to balance sensitivity and mechanical stability, operating at the $2~\mu $ m wavelength band. In the experiment with a refractive index ranging from 1.306 (a salinity of ~0‰) to 1.314 (a salinity of ~50‰), the sensor shows a refractive index sensitivity of 1200.7 nm/RIU, which corresponds to a salinity sensitivity of 0.192 nm/‰. Additionally, the sensor shows low sensitivity to ambient temperature and pressure, and the performance impact caused by their variations at different ocean depths is negligible in practical applications. The proposed all-optical fiber sensor offers a reliable and practical solution for marine salinity sensing across varying ocean depths. The fiber sensor can be further developed to enable the mixture of communication and sensing.