Showing papers by "Peter R. Herman published in 2021"

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

Abstract: 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 $${n}_{{{{{{\rm{H}}}}}}}$$ = 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

Conical phase front and aberration beam shaping for manipulating femtosecond laser chemical etching

Abstract: The countering and enhancing influence of surface aberration in combination with conical phase front beam shaping was examined over shallow to deep focusing inside of fused silica to facilitate the geometric control of chemical etching tracks formed by femtosecond laser inscription. The tuning of the collective shaping effects is shown to compensate for a wide focusing range to favorably manipulate the chemical etching rate, the resolution, the surface roughness, and the cross-section profile of the resulting micro-channels. Using only a single laser scanning track, micro-channels have been tailored from symmetrical to highly asymmetric cross-section of up to 1:13 aspect ratio, permitting distortion-free processing to depths of 1.5 mm. The combined shaping effects thus expand the utility of laser chemical etching for micromachining of transparent substrates.

From filaments to light-sheets: tailoring the spectrum of fiber Bragg gratings with femtosecond lasers

Abstract: 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.

2D filament grating array: enabling an efficient, high-resolution lens-less all-fiber spectrometer

Abstract: 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.

Femtosecond laser welding of silica glass fiber for robust Bragg grating sensing in high temperature environment

Abstract: 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.

In-fiber Polarization Control Using Nematic Liquid Crystal in Nano-Capillary Bragg Grating Array

Abstract: 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.

Optical Properties of Nanogratings Inscribed with Conical Phase Fronts

Abstract: Femtosecond laser direct writing in fused silica can lead to different modification types, one of them being the so-called nanogratings [1] - [3] . This modification type consists of self-assembled, grating-like, nanoscopic structures, that exhibit form-birefringence and a higher etching rate than pristine material. These characteristics have been exploited in various applications, including microfluidic devices [4] , data storage [5] , waveplates [6] and quantum gates [7] , to name just a few. The nanograting properties can be tuned using the laser parameters during inscription, such as number of overlapping pulse exposures, focussing condition and repetition rate. Most commonly, nanogratings are inscribed using traditional Gaussian beams. Further manipulation of the nanograting properties may arise from tuning of the inscription beam shape, which has not been systematically explored to date.

Conical Beams for Directing Chemical Etching along Deeply-Focussed Femtosecond Laser Modification Tracks

Abstract: Femtosecond laser modification of transparent materials offers selective chemical etching, facilitating three-dimensional (3D) micro-structures to open into flexible shapes as configured by multi-line scanning [1] . While surface aberration and other beam shaping methods have been widely studied in processing of transparent glasses [2] , [3] , the influence of such beam shaping on chemical etching has not been addressed. This paper examines the microstructure resulting in fused silica from beams with concave and convex conical phase front [4] . The conical beam shaping is shown to improve etching rates and surface morphology, compensate for deep focussing aberration and provide an attractive tailoring of the microchannel cross-sectional profile.

Off-Axis Filament Based Fiber Bragg Gratings for Azimuthally Resolved Displacement Sensing

Abstract: 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] .

High-Speed Writing of Volume Gratings Inside of Transparent Materials

Abstract: The laser inscription of the volume gratings in transparent materials by inducing localized refractive index modification [1] is promising for generating internal colouring effects in flexible designs for wide ranging applications. Nano-explosion of open cavity voids such as provided by elongated laser filaments in glasses [2] are further appealing for enabling high diffraction efficiency due to the large refractive index contrast. While Bessel beams [3] have been widely employed to induce high aspect ratio laser modifications, our group has studied the interplay between Kerr and plasma focusing and surface aberration in forming long filaments in glasses [4] . Spatial light modulators (SLM) present additional opportunities in beam shaping and splitting [5] to enable high resolution multi-positioned interaction volumes of 3D periodic nanostructure. The extension of such techniques to transparent polymers have not be widely explored. Moreover, the point-by-point fabrication technique is slow, time consuming, and prone to positioning errors in 3D space. This paper presents new prospects for high-speed structuring of 3D photonic gratings in polymers.