Temperature and strain self-compensated refractometer based on turning point of Mach–Zehnder interference in three-core fiber

Abstract

We demonstrate an in-line Mach–Zehnder interference with a dispersion turning point (DTP) by sandwiching a section of three-core fiber (TCF) between two short no-core fibers (NCFs). The interference occurs mainly between the center core mode and a cladding mode of the TCF, which are excited and recoupled by the two NCFs. Two interference dips flanking the DTP are sensitive to ambient RI but shift oppositely. Moreover, the interval between these two dips responds linearly to ambient RI as well and has a higher sensitivity ( − 67.56 nm/RIU) within the RI range from 1.3518 to 1.4003. Meanwhile, the third dip near the DTP shows relatively low sensitivity to RI but is highly sensitive to temperature. By using the different responses of the third dip and the interval between flanking dips, this device is proposed to apply as a RI sensor with the compensation scheme of temperature. Finally, all three interference dips show very low sensitivity to axial strain, and the disturbance of strain can be further depressed by using the interval as the indicator. • We demonstrate an in-line fiber refractometer based on a Mach–Zehnder interference with a dispersion turning point, which is achieved by cascading a segment of three-core fiber with two short sections of no-core fibers. The proposed refractometer is featured with a higher sensitivity and the capability to eliminate the cross impact from temperature and axial strain without the assistance of an additional sensing unit. • Due to the special dispersion property, the wavelength interval of two interference dips flanking the dispersion turning point is proposed to be the indicator of the refractometer, since these two dips respond oppositely to the variation of ambient refractive index and their wavelength interval shows a higher linear sensitivity to ambient refractive index. Moreover, the interrogation of the wavelength interval helps to effectively depress the impact of axial strain, since both two dips shift to the same direction at a similar pace with varying axial strain. • By combining the third dip located at the dispersion turning point with the wavelength interval, we propose a temperature self-compensation solution for the fabricated device, in which all the indicators come from the interference itself. It used to be realized by using a second sensing unit, such as a fiber grating, another interference, and so on.