In-line Mach-Zehnder interferometer and Bragg grating integrated by femtosecond laser for discrimination of temperature and directional torsion.

Abstract

Femtosecond laser micromachining has been considered as a powerful tool for fabricating versatile fiber devices and received increasing attention in recent years. Here, we report on a compact sensor by integrating a bridge-like waveguide inside a single-mode fiber to construct an in-line Mach-Zehnder interferometer and then inscribing a second-order Bragg grating in the core of the same fiber. The interference dip shows good performance in torsion sensing - the maximum torsion sensitivity of 1.5573 nm/(rad/m), the ability to identify the torsion direction, and low perturbation of axial strain. In order to compensate the cross impact of temperature, the fiber Bragg grating dip is employed as the second indicator and combined with the interference dip for discriminating temperature and directional torsion simultaneously. The proposed device also has the merits such as compact size, high thermal stability, and so on.