Femtosecond laser irradiation is applied to a single-mode optical fiber to embed a filament array through the silica cladding and guiding core and form chirped Bragg gratings. Unlike a planar-shaped refractive index modification, the long and uniform filament facilitates efficient optical scattering into azimuthally narrowed radiation modes, external and transverse to the fiber cladding. Chirping of the grating period further provides spectral focusing. The combined spectral and azimuthal focusing permits lens-less recording of bright and high-resolution spectra spanning across most of the visible band with a low-cost charged coupled device camera. The flexible point-by-point writing enables fiber tapping of light with engineered spectral and geometric focusing properties, permitting the design of new compact photonic devices based on the all-fiber spectrometer.

Femtosecond laser filaments are formed by surface aberration to open nano‐hole arrays, passing through the silica cladding and guiding core cross‐section of a standard telecommunication fiber. The hollow filament grating presents a strong capillarity effect to draw nematic liquid crystal (NLC) into homogeneous alignment with the cylindrical walls and presents a high birefringent response in the second‐order Bragg stopband. The NLC provides a strong extinction ratio of up to 20 dB over a ≈5 nm band with only a moderate insertion loss of <1 dB arising between the two polarization states of the shifted stopbands. The Bragg grating follows the thermo‐optic response of the NLC to further offer dynamic tuning and switching of the polarization extinction response without an increase in the insertion loss. The flexible laser writing enables tailoring of the Bragg spectral and polarization responses in the telecommunication C‐band.

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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 $${n}_{{{{{{\rm{H}}}}}}}$$ n H  = 1 to 1.67 as the nano-holes were readily wetted and filled with various solvents and oils through an intact fibre cladding.

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