Ultra-sensitive seawater temperature sensor using an FBG-cascaded microfiber MZI operating at dispersion turning point

• An ultra-sensitive temperature sensor is demonstrated by using an FBG and a microfiber MZI. • The FBG and MZI respectively work as a rough and a precise measurement tool. • The MZI working at DTP obtained an ultra-high sensitivity of 38 nm/°C. • The temperature measurement resolution of the MZI working at DTP is 0.0003 °C. • The measurement range is enlarged from 2 °C to 36 °C by using FBG. We proposed an ultra-sensitive seawater temperature sensor with extended measurement range by using an optical fiber Bragg grating (FBG) cascaded with a microfiber Mach-Zehnder interferometer (MZI) working at dispersion turning point (DTP). The FBG works as a rough temperature measurement tool, while the ultra-sensitive polydimethylsiloxane (PDMS)-packaged microfiber MZI plays the role of a precise measurement tool. The cooperation of these two measurement tools can realize temperature measurement with ultrahigh sensitivity and extended measurement range by using a single compact sensing structure. The proposed structure can solve the trade-off between ultrahigh sensitivity and large range of the microfiber MZI resulting from the periodic optical spectrum. A temperature sensitivity of about 38 nm/°C in the seawater temperature measurement range of 2.28–38.38 °C is obtained by the FBG-cascaded microfiber MZI which works at DTP. The corresponding temperature measurement resolution is estimated to be 0.0005 °C when the resolution of the interrogation device is 0.01 nm, which is better than that of the commercial electronic seawater temperature sensor. The temperature measurement range of the MZI can be enlarged from around 2 °C to about 36 °C with the aid of an FBG. Besides ultrahigh sensitivity and extended measurement range, the seawater temperature sensor has advantages of compact structure, easy fabrication, economical cost, low loss, making it promising in ocean monitoring.