Salinity Sensing Characteristics Based on Optical Microfiber Coupler Interferometer

In this paper, we report a novel and compact sensor based on an optic microfiber coupler interferometer (OMCI) for seawater salinity application. The OMCI device is fabricated by connecting Faraday rotating mirrors to the two out-ports of the microfiber coupler, respectively. The sensor signal processing is based on a wavelength demodulation technique. We theoretically analyze the sensing characteristics with different device structure parameters. Besides, the results show that the date reading error decreases with the thinner waist region and longer arm difference. Through the experiment, the reflection spectra red-shifted as the sea water salinity increased; the highest response sensitivity of the OMCI salinity sensor reached 303.7 pm/parts per thousand for a range of 16.6-23.8 parts per thousand, and the resolution was less than 0.03 parts per thousand. This study provides a new technical solution for the development of practical optical fiber seawater salinity sensors.