Encapsulation Research of Microfiber Mach-Zehnder Interferometer Temperature and Salinity Sensor in Seawater

Encapsulation of microfiber Mach-Zehnder interferometer (MMZI) sensor for temperature and salinity sensing in seawater is developed. It includes a C-shape metal tube with a slit on the bottom and polymer adhesive with large elasticity modulus, by which a more robust sensor with relatively fast response can be obtained. To evaluate the performance of sensor after encapsulation, effects of different polymer adhesives on tension withstanding, temperature and salinity sensitivities are evaluated, and dependence of response time on shape of metal tube is investigated. Results show that sensitivities of the sensor in temperature and salinity sensing can reach to be about -1.77 nm/degrees C and 1.18 nm/ parts per thousand, and response times are about 219.0 ms and 12.5 ms, respectively. By comparing temperatures and salinities measured by sensor with those measured by commercial device, accuracy of the sensor after encapsulation is proved. In addition, cross-sensitivities of strain/pressure and short/long-term stability after encapsulation are also evaluated. Finally, based on the basic structure, further improvements are attempted, such as double encapsulation with stainless steel mesh and complex encapsulation with two kinds of polymer. Encapsulations of MMZI demonstrated here show advantages of low cost, improved robustness, relatively fast response, long-term stability and easy expandability, which will not only afford a critical support for optical sensors operated in the ocean, but also offer some useful references for microfiber sensors used in other sensing applications.