Dual-Taper Double-Clad Fiber Compact Curvature Sensor With External Refractive Index and Strain Insensitivity

We propose and experimentally demonstrate an optical fiber curvature sensor capable of strain- and refractive index (RI)-insensitive curvature measurements using a compact double-clad fiber (DCF)-based Mach-Zehnder interferometer. The DCF-based interferometer was implemented by forming double tapers
$\sim $
12 mm apart on DCF with a standard fiber fusion splicer and is referred to as the DT-DCF. From the interference spectrum of the fabricated DT-DCF, which is generated by the phase retardation difference between the inner cladding mode coupled from the core mode at the first taper and the uncoupled core mode, two resonance dips were selected as sensor indicators and designated DA and DB. These indicators, DA and DB, were highly sensitive to bending, showing average curvature sensitivities of −24.62 and −25.33 nm/
$\text{m}^{-1}$
in a curvature range of 2.806–4.695
$\text{m}^{-{1}}$
, respectively. They were also nearly insensitive to longitudinal strain, exhibiting a low strain sensitivity of less than 0.1 pm/
$\mu \varepsilon $
. The strain insensitivity of our sensor head can minimize strain-induced curvature measurement uncertainty. In particular, the indicators showed little dependence on the external RI, which facilitates embedding the DT-DCF in another base material without further packaging. Furthermore, the effect of external RI variation on curvature measurement results was investigated using another DT-DCF.