In-Fiber Hybrid Cladding Waveguide by Femtosecond Inscription for Two-Dimensional Vector Bend Sensing

A 2D vector bending fiber sensor is demonstrated based on a femtosecond (fs) laser inscribed hybrid cladding waveguide (CWG) structure, which is mainly composed of two eccentric cladding waveguides distributed oppositely in a single mode fiber (SMF). The structure can enable light in the central core to efficiently couple into different CWGs under respective resonant conditions, thereby providing two separate local dips in response to the two CWGs in the transmission spectrum. When the bending is applied, the two CWGs will suffer asymmetric effective refractive index (RI) variation, resulting in that the wavelength shift for each dip differs significantly. The curvature sensitivity can be enhanced by tracking the wavelength interval between the two dips, reaching up to -14.25 nm/m
^-1
and +14.80 nm/m
^-1
at the 0° and 180° orientations ranging from 0 m
^-1
to 8 m
^-1
, respectively. Featured with comparatively high sensitivity, relatively large curvature range, 2D orientation identification, compact size, and reduced temperature crosstalk, the proposed structure offers the opportunity to be applied from public infrastructure health monitoring to wearable artificial devices.