A refractometer based on a micro-slot in a fiber Bragg grating formed by chemically assisted femtosecond laser processing

Femtosecond laser writing of fiber Bragg gratings using the phase mask technique: a geometrical optics analysis based on the Bravais refractive index

Material modification is produced inside silica-based optical fibers of different diameters using tightly focused near-infrared (central wavelength at 800 nm) femtosecond laser pulses and the phase mask technique which is often employed for laser inscription of fiber Bragg gratings. 1st-, 2nd-, and 3rd-order phase masks designed for the operation at 800 nm are used in the experiments. The inscription is performed at different distances from the fiber's front surface by translating the focusing cylindrical lens along the laser beam propagation direction. The results show that the material modification produced by means of the 2nd- and 3rd-order phase mask can be positioned at any predetermined distance from the fiber's front surface. In contrast, when the 1st-order mask is used for laser writing, the maximum distance from the fiber's front surface at which material modification can be produced is limited and determined by three main parameters: the diffraction angle of the phase mask, the refractive index of the fiber and the diameter of the fiber.

Femtosecond-laser-inscribed Bragg grating in hollow-core fiber for highly sensitive optofluidic sensing

An optofluidic sensor based on a Bragg grating in hollow-core fiber (HCF) is experimentally demonstrated. The grating is inscribed into the HCF by femtosecond laser illumination through a phase mask. Periodic index modulation is introduced into the silica material surrounding the hollow core, causing cladding mode resonance, and multiple reflection peaks are observed in the grating spectrum. These reflection peaks later shift to longer wavelengths when high-index liquid is infiltrated into the HCF. The new reflection peak results from the backward coupling of the liquid core mode of the waveguide, the mode field of which overlaps with the grating modulation surrounding the liquid core. The resonant wavelength of the liquid-core fiber grating increases with the index value of the infiltrating liquid, and optofluidic refractive index sensing is realized with the device. The highest refractive index sensitivity, 1117 nm/RIU, is obtained experimentally in the index range of 1.476-1.54. The infiltrated hollow-core fiber Bragg grating also exhibits high temperature sensitivity due to the high thermal-optic coefficient of the liquid, and a sensitivity of -301 pm/°C is achieved in the temperature range of 25°C to 60°C.

Plug-and-play fiber-optic sensors based on engineered cells for neurochemical monitoring at high specificity in freely moving animals

In vivo detection of neurochemicals, including neurotransmitters and neuromodulators, is critical for both understanding brain mechanisms and diagnosing brain diseases. However, few sensors are competent in monitoring neurochemical dynamics in vivo at high specificity. Here, we propose the fiber-optic probes based on engineered cells (FOPECs) for plug-and-play, real-time detection of neurochemicals in freely moving animals. Taking advantages of life-evolved neurochemical receptors as key components, the chemical specificity of FOPECs is unprecedented. We demonstrate the applications of FOPECs in real-time monitoring of neurochemical dynamics under various physiology and pathology conditions. With no requirement of viral infection in advance and no dependence on animal species, FOPECs can be widely adopted in vertebrates, such as mice, rats, rabbits, and chickens. Moreover, FOPECs can be used to monitor drug metabolisms in vivo. We demonstrated the neurochemical monitoring in blood circulation systems in vivo. We expect that FOPECs will benefit not only neuroscience study but also drug discovery.

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 \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

Optical Fiber Sensors by Direct Laser Processing: A Review

The consolidation of laser micro/nano processing technologies has led to a continuous increase in the complexity of optical fiber sensors. This new avenue offers novel possibilities for advanced sensing in a wide set of application sectors and, especially in the industrial and medical fields. In this review, the most important transducing structures carried out by laser processing in optical fiber are shown. The work covers different types of fiber Bragg gratings with an emphasis in the direct-write technique and their most interesting inscription configurations. Along with gratings, cladding waveguide structures in optical fibers have reached notable importance in the development of new optical fiber transducers. That is why a detailed study is made of the different laser inscription configurations that can be adopted, as well as their current applications. Microcavities manufactured in optical fibers can be used as both optical transducer and hybrid structure to reach advanced soft-matter optical sensing approaches based on optofluidic concepts. These in-fiber cavities manufactured by femtosecond laser irradiation followed by chemical etching are promising tools for biophotonic devices. Finally, the enhanced Rayleigh backscattering fibers by femtosecond laser dots inscription are also discussed, as a consequence of the new sensing possibilities they enable.

Polarization dependent cladding modes coupling and spectral analyses of excessively tilted fiber grating

We report on the detailed analyses of mode coupling from fiber core to cladding in excessively tilted fiber gratings (ETFGs). Cladding modes responsible for the typical dual peak pairs in the transmission spectrum of ETFGs are identified with phase matching condition, which suggests two set of dual peak pairs generated from coupling to cladding modes with even and odd azimuthal order. The polarization dependence of those dual peak pairs are also investigated by calculating the coupling coefficients of cladding modes for two orthogonal polarizations. With the calculated coupling coefficients, the measured polarization dependent spectra can be reproduced numerically.

Femtosecond laser writing of near-surface waveguides for refractive-index sensing

Femtosecond laser writing of optical waveguides and components in glasses has been a remarkably growing research field during the last two decades. However, such laser- inscribed optical components were mostly written within the volume of the glass due to the unavoidable ablation that arises when the focal spot is approaching the glass surface. This has generally limited the interaction of light with the surrounding medium thus preventing sensing functionality. In this paper, we present the inscription of surface and near-surface silver based waveguides in a silver containing glass with no need for additional processing as it is the case for standard type I waveguides. In addition, an ultra-sensitive refractive index sensor in a 1 cm glass chip is obtained based on near-surface waveguides interacting with liquid droplets acting as top-layer on the glass surface. Remarkably, the device exhibits a novel double-wing feature that sharpens the response and enhances its sensitivity. Our results highlight the advantages of silver based waveguides paving the way towards further surface based sensors in fibers.

Chemical-assisted femtosecond laser writing of lab-in-fibers

The lab-on-chip (LOC) platform has presented a powerful opportunity to improve functionalization, parallelization, and miniaturization on planar or multilevel geometries that has not been possible with fiber optic technology. A migration of such LOC devices into the optical fiber platform would therefore open the revolutionary prospect of creating novel lab-in-fiber (LIF) systems on the basis of an efficient optical transport highway for multifunctional sensing. For the LIF, the core optical waveguide inherently offers a facile means to interconnect numerous types of sensing elements along the optical fiber, presenting a radical opportunity for optimizing the packaging and densification of diverse components in convenient geometries beyond that available with conventional LOCs. In this paper, three-dimensional patterning inside the optical fiber by femtosecond laser writing, together with selective chemical etching, is presented as a powerful tool to form refractive index structures such as optical waveguides and gratings as well as to open buried microfluidic channels and optical resonators inside the flexible and robust glass fiber. In this approach, optically smooth surfaces (~12 nm rms) are introduced for the first time inside the fiber cladding that precisely conform to planar nanograting structures when formed by aberration-free focusing with an oil-immersion lens across the cylindrical fiber wall. This process has enabled optofluidic components to be precisely embedded within the fiber to be probed by either the single-mode fiber core waveguide or the laser-formed optical circuits. We establish cladding waveguides, X-couplers, fiber Bragg gratings, microholes, mirrors, optofluidic resonators, and microfluidic reservoirs that define the building blocks for facile interconnection of inline core-waveguide devices with cladding optofluidics. With these components, more advanced, integrated, and multiplexed fiber microsystems are presented demonstrating fluorescence detection, Fabry-Perot interferometric refractometry, and simultaneous sensing of refractive index, temperature, and bending strain. The flexible writing technique and multiplexed sensors described here open powerful prospects to migrate the benefits of LOCs into a more flexible and miniature LIF platform for highly functional and distributed sensing capabilities. The waveguide backbone of the LIF inherently provides an efficient exchange of information, combining sensing data that are attractive in telecom networks, smart catheters for medical procedures, compact sensors for security and defense, shape sensors, and low-cost health care products.

Laser-assisted lateral optical fiber processing for selective infiltration

We propose a new technique to perform precise selective infiltration of an air hole in the photonic crystal fiber (PCF). To carry out the infiltration process, the end face of the PCF is covered by a mask, which is fabricated by femtosecond laser inscription from the lateral direction. This proposed method overcomes the conventional limitation of maximum mask thickness. An analytical model is further proposed and demonstrated accurate determinations of the fabricated channel diameter in the mask.

Etching rate enhancement by shaped femtosecond pulse train electron dynamics control for microchannels fabrication in fused silica glass

The dependence of the etching rate on the ultrafast pulse shaping is observed when microchannels are fabricated in fused silica glass using the method of femtosecond laser irradiation followed by chemical etching. In comparison with the conventional femtosecond pulses, the temporally shaped pulse trains can greatly enhance the etching rate under the same processing conditions. The enhancement is mainly attributed to the localized transient electron dynamics control by shaping the ultrafast pulse, resulting in higher photon absorption efficiency and uniform photomodification zone. Furthermore, processing parameters, including pulse delay and pulse energy distribution ratio, have also been investigated to optimize microchannels fabrication.

Refractive index and temperature sensitivity characteristics of a micro-slot fiber Bragg grating

Fabrication and characterization of a UV inscribed fiber Bragg grating (FBG) with a micro-slot liquid core is presented. Femtosecond (fs) laser patterning/chemical etching technique was employed to engrave a micro-slot with dimensions of 5.74 μm(h)×125 μm(w)×1388.72 μm(l) across the whole grating. The device has been evaluated for refractive index (RI) and temperature sensitivities and exhibited distinctive thermal response and RI sensitivity beyond the detection limit of reported fiber gratings. This structure has not just been RI sensitive, but also maintained the robustness comparing with the bare core FBGs and long-period gratings with the partial cladding etched off.

Rapid fabrication of microhole array structured optical fibers

A microhole array in a common single-mode fiber is fabricated by selective chemical etching of femtosecond-laser-induced fiber Bragg grating (FBG), which has a laser-modified region extending from the fiber core to the cladding-air boundary due to laser self-focusing. The shape and size of the orderly microhole on the fiber surface are controlled via changing conditions of FBG fabrication and chemical etching. A simultaneous sensing for surrounding refractive index and temperature is demonstrated by this microhole array FBG through measurement of the transmission power change and Bragg resonant wavelength shift.

Note: Optical fiber milled by focused ion beam and its application for Fabry-Pérot refractive index sensor

We introduce a highly compact fiber-optic Fabry-Pérot refractive index sensor integrated with a fluid channel that is fabricated directly near the tip of a 32 μm in diameter single-mode fiber taper. The focused ion beam technique is used to efficiently mill the microcavity from the fiber side and finely polish the end facets of the cavity with a high spatial resolution. It is found that a fringe visibility of over 15 dB can be achieved and that the sensor has a sensitivity of ~1731 nm/RIU (refractive index units) and a detection limit of ~5.78 × 10(-6) RIU. This miniature integrated all-in-fiber optofludic sensor may find use in minimal-invasive biomedical applications.

Refractometer based on fiber Bragg grating Fabry-Pérot cavity embedded with a narrow microchannel

We report on inscription of microchannels of different widths in optical fiber using femtosecond (fs) laser inscription assisted chemical etching and the narrowest channel has been created with a width down to only 1.2μm. Microchannels with 5μm and 35μm widths were fabricated together with Fabry-Pérot (FP) cavities formed by UV laser written fiber Bragg gratings (FBGs), creating high function and linear response refractometers. The device with a 5μm microchannel has exhibited a refractive index (RI) detection range up to 1.7, significantly higher than all fiber grating RI sensors. In addition, the microchannel FBG FP structures have been theoretically simulated showing excellent agreement with experimental measured characteristics.

Passive mode-locked lasing by injecting a carbon nanotube-solution in the core of an optical fiber

In this paper, we propose a saturable absorber (SA) device consisting on an in-fiber micro-slot inscribed by femtosecond laser micro fabrication, filled by a dispersion of Carbon Nanotubes (CNT). Due to the flexibility of the fabrication method, efficient and simple integration of the mode-locking device directly into the optical fiber is achieved. Furthermore, the fabrication process offers a high level of control over the dimensions and location of the micro-slots. We apply this fabrication flexibility to extend the interaction length between the CNT and the propagating optical field along the optical fiber, hence enhancing the nonlinearity of the device. Furthermore, the method allows the fabrication of devices that operate by either a direct field interaction (when the central peak of the propagating optical mode passes through the nonlinear media) or an evanescent field interaction (only a fraction of the optical mode interacts with the CNT). In this paper, several devices with different interaction lengths and interaction regimes are investigated. Self-starting passively modelocked laser operation with an enhanced nonlinear interaction is observed using CNT-based SAs in both interaction regimes. This method constitutes a simple and suitable approach to integrate the CNT into the optical system as well as enhancing the optical nonlinearity of CNT-based photonic devices.

Hybrid fiber grating cavity for multi-parametric sensing

We propose an all-fiber hybrid cavity involving two unbalanced uniform fiber Bragg gratings (FBGs) written at both sides of a tilted FBG (TFBG) to form an all-fiber interferometer. This configuration provides a wavelength gated reflection signal with interference fringes depending on the cavity features modulated by spectral dips associated to the wavelength dependent optical losses due to cladding mode coupling occurring along the TFBG. Such a robust structure preserves the advantages of uniform FBGs in terms of interrogation methods and allows the possibility of simultaneous physical and chemical sensing.

Refractive index sensor based on a microhole in single-mode fiber created by the use of femtosecond laser micromachining

A compact in-fiber refractive index (RI) sensor is presented that is based on a microhole created in a conventional single-mode fiber by the use of femtosecond laser micromachining. The transmission properties of such a device with a microhole of different diameters have been investigated in the wavelength region of 1500-1600 nm and in the RI range of 1.30-1.45. It is found that the relationship between the transmission and the RI is critically dependent on the size of the microhole in the fiber core region. The highest resolution obtained is 6.70x10(-5), in the RI range of 1.37-1.42, when the microhole diameter is approximately 8 microm, close to the fiber core size. The in-fiber RI sensor developed in this work is easy to fabricate and can be used to implement temperature-independent measurements.

Optical fibre-based detection of DNA hybridization

A dual-peak LPFG (long-period fibre grating), inscribed in an optical fibre, has been employed to sense DNA hybridization in real time, over a 1 h period. One strand of the DNA was immobilized on the fibre, while the other was free in solution. After hybridization, the fibre was stripped and repeated detection of hybridization was achieved, so demonstrating reusability of the device. Neither strand of DNA was fluorescently or otherwise labelled. The present paper will provide an overview of our early-stage experimental data and methodology, examine the potential of fibre gratings for use as biosensors to monitor both nucleic acid and other biomolecular interactions and then give a summary of the theory and fabrication of fibre gratings from a biological standpoint. Finally, the potential of improving signal strength and possible future directions of fibre grating biosensors will be addressed.

Not-lithographic fabrication of micro-structured fiber Bragg gratings evanescent wave sensors

This work is devoted to present and to demonstrate a novel approach for the fabrication of micro-structured fiber Bragg gratings (MSFBGs) to be employed as technological platform for advanced optochemical sensors. Basically, the MSFBG consists in a localized SRI sensitization of the grating by deep cladding stripping. The introduction of a perturbation or defect along the grating leads to the formation of a defect state inside the FBG spectral response that is tunable through the surrounding medium refractive index. While its spectral features for sensing and communication applications have been widely described and commented elsewhere, here a simple fabrication procedure is presented as suitable technological assessment enabling cost effective and simple MSFBG production. It relies on a two steps technique based on arc-discharge procedure as fiber pre-treatment and mask-less wet chemical etching to locally sensitize the FBG to external refractive index. The new, simple and low-cost approach overcomes some technological drawbacks related to previous fabrication techniques adopting patterned masking procedures during the etching process. This work demonstrates the effectiveness of the proposed method reporting a detailed description of single and two defects MSFBG fabrication.

In-fiber microchannel device filled with a carbon nanotube dispersion for passive mode-lock lasing

Fueled by their high third-order nonlinearity and nonlinear saturable absorption, carbon nanotubes (CNT) are expected to become an integral part of next-generation photonic devices such as all-optical switches and passive mode-locked lasers. However, in order to fulfill this expectation it is necessary to identify a suitable platform that allows the efficient use of the optical properties of CNT. In this paper, we propose and implement a novel device consisting of an optofluidic device filled with a dispersion of CNT. By fabricating a microchannel through the core of a conventional fiber and filling it with a homogeneous solution of CNTs on Dimethylformamide (DMF), a compact, all-fiber saturable absorber is realized. The fabrication of the micro-fluidic channel is a two-step process that involves femtosecond laser micro-fabrication and chemical etching of the laser-modified regions. All-fiber high-energy, passive mode-locked lasing is demonstrated with an output power of 13.5 dBm. The key characteristics of the device are compactness and robustness against optical, mechanical and thermal damage.

In-fiber polymer-glass hybrid waveguide Bragg grating

A 1.2 microm (height) x 125 microm (depth) x 500 microm (length) microslot along a fiber Bragg grating was engraved across the optical fiber by femtosecond laser patterning and chemical etching. By filling epoxy in the slot and subsequent UV curing, a hybrid waveguide grating structure with a polymer core and glass cladding was fabricated. The obtained device is highly thermally responsive with linear coefficient of 211 pm/ degrees C.