Filament-arrayed Bragg gratings for azimuthally resolved displacement sensing in single-mode fibers.

Filament arrays were inscribed off-axis in the core of standard single-mode telecommunication fiber, using femtosecond laser pulses. The flexible line-by-line writing formed uniform, parallel filaments, permitting Bragg grating sensing of the photoelastic response from inside of the narrow grating plane. Active monitoring of the Bragg resonance wavelength while driving a lateral fiber tip displacement directly informed on the fiber mechanics when coupled with opto-mechanical modelling. Overlaying of parallel and orthogonal gratings further provided a strongly contrasting azimuthal sensitivity, which paves the way for multi-dimensional displacement sensing with improved precision.

Laser nano-filament explosion for enabling open-grating sensing in optical fibre.

Embedding strong photonic stopbands into traditional optical fibre that can directly access and sense the outside environment is challenging, relying on tedious nano-processing steps that result in fragile thinned fibre. Ultrashort-pulsed laser filaments have recently provided a non-contact means of opening high-aspect ratio nano-holes inside of bulk transparent glasses. This method has been extended here to optical fibre, resulting in high density arrays of laser filamented holes penetrating transversely through the silica cladding and guiding core to provide high refractive index contrast Bragg gratings in the telecommunication band. The point-by-point fabrication was combined with post-chemical etching to engineer strong photonic stopbands directly inside of the compact and flexible fibre. Fibre Bragg gratings with sharply resolved π-shifts are presented for high resolution refractive index sensing from [Formula: see text] = 1 to 1.67 as the nano-holes were readily wetted and filled with various solvents and oils through an intact fibre cladding.