*Communication* **Salinity Sensing Characteristics Based on Optical Microfiber Coupler Interferometer**

**Lingjun Zhou 1,2, Yang Yu 2,3,\*, Huimin Huang 1, Yuyu Tao 1, Kui Wen 2, Guofeng Li 1, Junbo Yang 2 and Zhenrong Zhang 1**


Received: 31 August 2020; Accepted: 18 September 2020; Published: 21 September 2020

**Abstract:** 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/‰ for a range of 16.6–23.8‰, and the resolution was less than 0.03‰. This study provides a new technical solution for the development of practical optical fiber seawater salinity sensors.

**Keywords:** optical microfiber coupler; interferometer; salinity sensing; optic fiber sensor
