A new method to improve the sensitivity of nitrate concentration measurement in seawater based on dispersion turning point

Abstract

The nitrate concentration is extremely low in seawater, which needs highly sensitive sensors to measure. In this paper, a new method to improve the sensitivity of nitrate concentration measurement in seawater based on dispersion turning point (DTP) theory is demonstrated using a microfiber mode interferometric sensor. The high sensitivity is achieved by the interference of HE11 mode and HE12 mode. Different parameters that influence DTP have been calculated theoretically, including positions of dips and diameters for obtaining DTP, the relationship between DTP and group effective refractive index difference, and the effects of refractive indexes of solutions on the DTP. Calculated results show that the sensitivity of dip will be ultra-high near the DTP and the ultra-high sensitivity region is very narrow in a certain band or a certain diameter. In the experiment, six sensors with different diameters are fabricated to realize the concentration of sodium nitrate sensing in seawater. By optimizing the parameters of sensor, the maximum sensitivity and detection limit of sodium nitrate in seawater can reach to be 5.98 pm/ppm (refractive index sensitivity is 50396.09 nm/RIU) and 0.17 ppm, respectively. The sensor demonstrated here shows advantages of easy to construct, low cost, high precision and high sensitivity, which provides a new optical detection method for in-situ detection of marine environment or low concentration substances sensing in other liquids.