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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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${n}_{{{{{{\rm{H}}}}}}}$$\end{document}nH = 1 to 1.67 as the nano-holes were readily wetted and filled with various solvents and oils through an intact fibre cladding., Engineered stop bands to sense an ambient environment can enable many applications. Here, the authors demonstrate well-controlled processes to open high-aspect ratio nanoholes through optical fibre for Bragg gratings in the telecomm spectrum and to enable high-resolution refractive index sensing

Femtosecond laser light has been shaped with a spatial light modulator (SLM) to generate optically thin, aberration-free sheets of uniform intensity for inscription of fiber Bragg gratings (FBGs) inside the core of SMF-28 telecommunication fiber. The combination of flexible beam shaping and sheet-by-sheet writing offers facile means in controlling the coupling to cladding or radiation modes while facilitating spectral tuning flexibility that is not available with interference-based techniques. Spectral responses of uniform first order FBGs fabricated with single-pulse exposures are presented.

Femtosecond laser welding was extended to optical silica fiber (SMF-28) by focusing through fused silica substrates and ferrules to form all-glass weld seams. Laser radiation was focused into the fiber cladding to create a welding zone, which drove molten glass to fill as much as a 3 𝜇𝑚 gap around a contact line to form a crack free pseudo-continuous welding seams along the contact line. The strong weld seams up to 30 𝜇𝑚 wide were generated in fiber-to-plate and fiber-to-ferrule geometries without inducing photochemical or thermal degradation of a fiber Bragg grating (FBG) positioned only 62.5 𝜇m from the weld zone. Welding was optimized by real-time monitoring of the FBG thermo-optical shift during laser scanning. Four-point bending tests confirmed a high mechanical strength while thermal annealing showed stable mechanical and FBG responses up to 1000 ˚C. Femtosecond laser writing and welding thus demonstrated a flexible means for photonics fabrication and packaging of FBGs, enabling reliable, high frequency vibration sensing suited for high temperature and strain environments.

A two-dimensional (2D), point-by-point writing technique of forming filament arrays with femtosecond laser pulses was applied inside of single-mode optical fiber to open new opportunities for 2D photonic bandgap engineering and highresolution spectroscopy. A small grating period of ~300 nm provided first-order diffraction externally to the fiber cladding, with spectral, blazing, and self-focusing properties tailored by varying the 1D and 2D grating design. The spectral properties of the lens-less, all-fiber spectrometer have been tuned with varying grating dimension, chirping rate, and blazing design that can meet wide ranging criteria for design of compact grating spectrometers in narrow to broad spectral ranges of the visible and telecommunication bands.

Nematic liquid crystal was introduced into a nano-hole array grating formed by femtosecond laser filaments in a telecommunication fiber. Capillary alignment has resulted in a strong polarization extinction ratio within a broad Bragg resonance.

Beam shaping of femtosecond lasers was applied in the Kerr-lensing and aberration regime to enable high-aspect-ratio filament tracks to form uniformly through the silica cladding and core waveguide of single-mode fiber (SMF28/450). One- and two-dimensional filament arrays were embedded along the waveguide to form weak to strong photonic stopbands. The filament shape enhanced transverse light scattering into narrow azimuthal radiation zones. Tailoring of chirp and 2D patterns further facilitated high-resolution (~350 pm) spectral focusing onto a CCD camera, defining a compact “Spectrometer-in-fibre” over the visible spectrum. At higher exposure, the filaments opened into narrow nano-channels (200-400 nm diameter) presenting a novel Bragg grating for refractive index sensing of the ambient environment. This lab-in-fiber technology presents a robust, flexible, and ubiquitous communication platform for nano-scale sensing across expansive networks or into tightly confined, sinuous spaces.

A chirped-array of filament tracks, formed in single-mode fiber with femtosecond laser pulses, provided radiative emission and lens less focusing to define an all-fiber spectrometer with 0.3 nm resolution in the visible spectrum.

Femtosecond laser pulses were shaped into uniform intensity sheets for point-by-point writing of micro-volume gratings inside of single-mode optical fiber core, opening flexible means for tailoring spectral properties of fiber Bragg grating devices.

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.

Femtosecond laser irradiation followed by chemical etching (FLICE) was applied to standard telecommunication fiber, embedding an open-filament, grating array through the silica cladding and guiding core cross-section. Second and third order Bragg air-gratings are presented for the first time, providing nano-fluidic channels accessible to the outer cladding surface. Pishifted fiber Bragg grating responses to chemical etching were used to optimize a novel refractive index sensor for fluids.

Fiber Bragg gratings (FBGs) serve in widely varying applications across research and industry as optical sensors, signal multiplexers, and fiber laser mirrors. While interference-based phase mask inscription of FBGs offers advantages in precision and uniformity, point-by-point writing with femtosecond lasers provides a strong localized material response for exceptional flexibility in apodizing, chirping, and spatially varying the grating structure. Moreover, spatial beam shaping opens possibilities for controlling the 3D geometry of the grating elements, for example, by stretching into long and uniform filaments to influence the cladding or radiation mode coupling [1] . To this end, our group has harnessed surface aberration from glass plates to fabricate an all-fiber radiative spectrometer with low-contrast gratings [2] or to drive filament nano-explosions and open nano-capillary FBG sensing holes for accessing the external cladding environment [3] . The present work introduces crossed-filament gratings with off-centered positioning that enables tailoring of the strain-optic response for azimuthally resolved displacement sensing. The off-centering accommodates an otherwise zero photoelastic sensitivity in traditional FBGs. The laser processing is a straight-forward modification of fiber relative to the complex interferometric or microstructured assemblies used in bend sensing [4] . The filament grating promises lower crosstalk and loss relative to FBGs that use tilted gratings and cladding-core recoupling to detect bending. Since the gratings were embedded in common SMF-28 fiber, alignment and splicing is facile relative to bend sensors based on asymmetry such as multicore and eccentric core fiber [4] .