Compact assembly-free vector bend sensor based on all-in-fiber-core Mach-Zehnder interferometer.

A novel low-cost, compact, assembly-free and sensitive optical fiber curvature sensor is presented. This device consists of an off-axis positive refractive index modified zone (PRIMZ), induced by a direct femtosecond laser, written in a single-mode fiber (SMF) core. The PRIMZ transforms the original SMF section into a few-mode fiber (FMF). As a result, the whole fiber forms an assembly-free "SMF-FMF-SMF" sandwich Mach-Zehnder interferometer. When the device is bent, a direction-dependent spectral shift of the interference pattern is produced. The sensitivity of the sensor is up to 2.53 and 2.24  nm/m-1 for the 0° and 180° orientations in a wide bend range (from 0 to 4  m-1). In addition, the device is immune to the surrounding refractive index and has a low-temperature crosstalk, which makes it very attractive for practical structural monitoring applications.