Optical Fiber Gratings Research Articles

Individual inscription of spectrally multiplexed Bragg gratings in optical multicore fibers using small spot direct UV writing.

We have demonstrated the inscription of Bragg gratings into five individual cores of a seven core fiber using small spot direct UV writing. With this technique, we defined spectrally multiplexed Bragg gratings consecutively in separate cores as well as spectrally multiplexed gratings at the same longitudinal location in different cores. The effect of bending on the optical spectrum was evaluated to allow the differentiation between cross-exposure and cross-talk, and an alignment process to reduce cross-exposure by 13 dB was found.

Optical detection of the percolation threshold of nanoscale silver coatings with optical fiber gratings

The metal-to-dielectric transition of silver films deposited on single-mode optical fibers is monitored by measurements of the transmission spectra of tilted fiber Bragg gratings inscribed in the core of the fiber. In situ, real-time measurements of the spectrum at wavelengths near 1550 nm during the wet etching of a 50 nm thick silver coating show a sudden and temporary decrease of more than 90% in the amplitudes of the core-to-cladding mode coupling resonances when the film thickness reaches 18 nm. Confirmation that this observation corresponds to the percolation threshold is obtained from simultaneous measurements of the silver coating conductivity during etching and by simulations of the grating response. The characteristic spectral signature of the percolation threshold is only obtained for cladding modes polarized azimuthally in the fiber cross section, i.e., parallel to the film surface.

Selective detection of cadmium ions using plasmonic optical fiber gratings functionalized with bacteria.

Environmental monitoring and potable water control are key applications where optical fiber sensing solutions can outperform other technologies. In this work, we report a highly sensitive plasmonic fiber-optic probe that has been developed to determine the concentration of cadmium ions (Cd2+) in solution. This original sensor was fabricated by immobilizing the Acinetobacter sp. around gold-coated tilted fiber Bragg gratings (TFBGs). To this aim, the immobilization conditions of bacteria on the gold-coated optical fiber surface were first experimentally determined. Then, the coated sensors were tested in vitro. The relative intensity of the sensor response experienced a change of 1.1 dB for a Cd2+ concentration increase from 0.1 to 1000 ppb. According to our test procedure, we estimate the experimental limit of detection to be close to 1 ppb. Cadmium ions strongly bind to the sensing surface, so the sensor exhibits a much higher sensitivity to Cd2+ than to other heavy metal ions such as Pb2+, Zn2+ and CrO42- found in contaminated water, which ensures a good selectivity.

Periodic and solitary waves in optical fiber Bragg gratings with dispersive reflectivity

The system of partial differential equations for moving optical solitons in fiber Bragg gratings are studied. The traveling wave reductions are used to look for solutions of the system of equations. The compatibility conditions for the overdetermined system of equations are analyzed and discussed. Periodic and solitary waves in optical fiber gratings are found and illustrated.

Generation of leaky mode resonance by metallic oxide nanocoating in tilted fiber-optic gratings.

This work investigates the excitation of dense comb-like enhanced leaky mode resonance (eLMR) in tilted fiber Bragg grating (TFBG) integrated with indium tin oxide (ITO) nanocoating. The ITO overlay leads to a large reduction in mode loss and a great increase of propagation length for s-polarized leaky modes, which means the leaky modes become guided. The guidance of leaky modes enhances significantly the interaction with the core guided mode, which leads to the generation of strong dense comb-like eLMR. The results show that the ultra-narrow eLMR bands present promising sensing performance with an extended measurement range and provide advantages of high Q measurement over the case of surface plasmon resonance (SPR) and lossy mode resonance (LMR). The similarities and differences between the eLMR and SPR and LMR are also discussed. This study offers new opportunities to develop eLMR-based multifunctional fiber-optic devices with high performance.

Design and characterization of surface relief grating on etched multimode optical fiber for refractive index sensing

We report a simple and compact optical fiber sensor based on surface relief gratings in conventional graded-index multimode optical fiber (GIMMF) for refractive index sensing. Bragg gratings on MMF are less explored compare to its single mode counterpart, even though the MMF platform possesses several advantages such as robustness, enhanced light-matter interaction and easy light coupling. In this work, surface relief gratings are directly written on one side of the cladding etched MMF core surface by using focused ion beam milling (FIB). A detailed theoretical analysis of the structure and sensing principle is analysed. The device performance as a refractive index sensor has been investigated experimentally by immersing the device in different concentration of NaCl solution. Experimental results show a sensitivity of 70 nm/RIU for a refractive index range of 1.333 to 1.345. The device is easy to fabricate with high repeatability and low cost, compact in design and competent to be used in harsh environmental conditions. The surface grating on etched MMF provides a better interaction between the sensing region and the environment which makes it a potential candidate for various chemical and biological sensing applications.

Mining Failure Response Characteristics of Stope Floor: A Case of Renlou Coal Mine

Currently, shallow coal resources are being exhausted gradually, mining depth is continuing to extend downward, and hydrogeological conditions are becoming increasingly complex. Therefore, accurate determination of the failure floor position is necessary to perform multiple‐seam mining. In this study, the 7255 working face of the Renlou coal mine is regarded as the research object. Through a comprehensive measurement of ground penetrating radar detection and fixed‐point grating optical fibers, the law of floor deformation and failure is analyzed dynamically, and the characteristics of the floor rock deformation response are discussed. The results of on‐site monitoring indicate that the mining effect of the working face is greater than that of the tectonic stress. With the advance of the working face, the deformation of the shallow area (0–8 m) first increases gradually, then increases rapidly, and finally increases gradually again; the middle area (8–19 m) experiences three stages, from a gentle increase to temporary stability and then a rapid increase; the deep area (19–29 m) undergoes three stages, from being stable to increasing and then being stable again. After mining, the floor of the working face can be classified into four areas in the vertical direction: complete failure area (0–5 m), poor severe influence area (5–11 m), failure development area (11–19 m), and elastic deformation area (19–29 m). Mining‐induced stresses cause resistance at the interface of different lithologies and weaken the effect of downward propagation. Coal seams and the interface between different lithologies are more prone to deformation. The results can provide a certain reference basis for the advanced exploration scheme of the underlying seam mining under the multiple‐seam mining method, as well as provide a new approach for floor failure depth measurement.

Simulative Study of Wavelength Division Multiplexing Fiber Bragg Grating in Nuclear Reactors Monitoring

The technologies of wavelength division multiplexing (WDM) have been theoretically studied and analyzed for multiplexing fiber Bragg grating (FBG) in a single optical fiber. This method allows a single fiber to carry many of identical FBGs, making this sensor more appropriate in the nuclear reactors. The analysis demonstrates that the multiplexing capacity can be incredibly enhance small data rates and high channel spacing. The interference effect among FBGs multi-reflections channels must be taken into account. This paper simulate WDM based FBG for a channel spacing of 0.1, 0.3, 0.5, 0.8, 1 nm Gaussian apodized FBGs at data rates of 2.5, 10, 40,100,160, 250 Gb/s respectively for nuclear applications. All simulations were performed in Optisystem software.

Palladium-coated plasmonic optical fiber gratings for hydrogen detection.

Surface plasmon resonance excitation with tilted fiber Bragg gratings has been typically studied using gold films to target biochemical sensing applications. However, surface plasmons can be excited on other metal coatings as well. In this work, plasmonic optical fiber grating platforms are developed using palladium films. Since the optical properties of this metal differ from the ones of gold, simulations are carried out to define the optimal thickness. Due to the phase transition of palladium in the presence of hydrogen, intensity changes in the optical transmission of the devices are produced. It is demonstrated that these platforms can be used for hydrogen detection at concentrations way below the lower explosive limit.

Hollow and filled fiber bragg gratings in nano-bore optical fibers**Project supported by the Beijing Natural Science Foundation, China (Grant No. 4192047), the Fundamental Research Funds for the Central Universities, China (Grant Nos. 2018JBM070 and 2018JBM065), and the National Natural Science Foundation of China (Grant No. 61675019).

To combine the technical functions and advantages of solid-core fiber Bragg gratings (FBGs) and hollow-core optical fibers (HCFs), the hollow and filled FBGs in nano-bore optical fibers (NBFs) with nano-bore in the GeO2-doped core are proposed. The fundamental mode field, effective mode index, and confinement loss of NBF with 50 nm–-diameter hollow and filled nano-bore are numerically investigated by the finite element method. The reflected spectra of FBGs in NBFs are obtained by the transmission matrix method. The hollow FBGs in NBFs can be acheived with (∼5% power fraction in the bore and the (∼0.9 reflectivity when bore diameter is less than . The filled FBGs can be realized with (∼1% power fraction and 0.98 reflectivity with different fillings including o-xylene, trichloroethylene, and chloroform for 800-nm bore diameter. The feasibility of the index sensing by our proposed NBF FBG is also analyzed and discussed. The experimental fabrication of hollow and filled FBGs are discussed and can be achieved by current techniques. The aim of this work is to establish a principle prototype for investigating the HCFs and solid-core FBGs-based fiber-optic platforms, which are useful for applications such as the simultaneous chemical and physical sensing at the same position.

Optical Fiber Gratings Immunoassays.

Optical fibers are of growing interest for biosensing, especially for point-of-care and biomedical assays. Their intrinsic properties bestow them sought-after assets for the detection of low concentrations of analytes. Tilted fiber Bragg gratings (TFBGs) photo-inscribed in the core of telecommunication-grade optical fibers are known to be highly-sensitive refractometers. In this work, we present different strategies to use them for label-free immunoassays. Bare, gold-sputtered, gold-electroless-plated (ELP) and hybrid configurations are biofunctionalized with antibodies, aiming at the detection of cancer biomarkers. We discuss the relative performances of the tested configurations and show that each leads to singular key features, which therefore drives their selection as a function of the target application. The most sensitive configuration presents a limit of detection of 10−12 g/mL in laboratory settings and was successfully used ex vivo in freshly resected lung tissues.

Microwave Hilbert Transformer and Its Applications in Real-Time Analog Processing

A microwave Hilbert transformer is introduced as a new component for Real-time Analog Processing (RAP). In contrast to its optical counterpart, that resort to optical fiber gratings, this Hilbert transformer is based on the combination of a branch-line coupler and a loop resonator. The transfer function of the transformer is derived using signal flow graphs, and two figures of merits are introduced to effectively characterize the device: the rotated phase and the transition bandwidth. Moreover, a detailed physical explanation of its physical operation is given, using both a steady-state regime perspective and a transient regime perspective. The microwave RAP Hilbert transformer is demonstrated experimentally, and demonstrated in three applications: edge detection, peak suppression and single sideband modulation.

The Application of a Nanomaterial Optical Fiber Biosensor Assay for Identification of Brucella Nomenspecies.

Bacteria in the genus Brucella are the cause of brucellosis in humans and many domestic and wild animals. A rapid and culture-free detection assay to detect Brucella in clinical samples would be highly valuable. Nanomaterial optical fiber biosensors (NOFS) are capable of recognizing DNA hybridization events or other analyte interactions with high specificity and sensitivity. Therefore, a NOFS assay was developed to detect Brucella DNA from cultures and in tissue samples from infected mice. An ionic self-assembled multilayer (ISAM) film was coupled to a long-period grating optical fiber, and a nucleotide probe complementary to the Brucella IS711 region and modified with biotin was bound to the ISAM by covalent conjugation. When the ISAM/probe duplex was exposed to lysate containing ≥100 killed cells of Brucella, or liver or spleen tissue extracts from Brucella-infected mice, substantial attenuation of light transmission occurred, whereas exposure of the complexed fiber to non-Brucella gram-negative bacteria or control tissue samples resulted in negligible attenuation of light transmission. Oligonucleotide probes specific for B. abortus, B. melitensis, and B. suis could also be used to detect and differentiate these three nomenspecies. In summary, the NOFS biosensor assay detected three nomenspecies of Brucella without the use of polymerase chain reaction within 30 min and could specifically detect low numbers of this bacterium in clinical samples.

Label-free cocaine aptasensor based on a long-period fiber grating.

In this Letter, we combined a promising bioreceptor, a cocaine aptamer MN6, with an ultrasensitive optical platform long-period fiber grating (LPFG) to create a new cocaine biosensor. The cocaine induces a conformational rearrangement of the aptamer which changes the refractive index around the LPFG producing a measurable shift of the transmission spectrum. We were able to track subtle interaction between the receptor and cocaine molecules over a concentration range of 25 to 100μM. The presented biosensor does not require labeling or signal enhancement, resulting in a simple user-friendly device.

Latest Achievements in Polymer Optical Fiber Gratings: Fabrication and Applications

Grating devices in polymer optical fibers (POFs) have attracted huge interest for many potential applications in recent years. This paper presents the state of the art regarding the fabrication of different types of POF gratings, such as uniform, phase-shifted, tilted, chirped, and long period gratings, and explores potential application scenarios, such as biosensing and optical communications.

A review on nanomaterial-modified optical fiber sensors for gases, vapors and ions.

The mesmerizing properties of nanomaterials and the features offered by optical fibers can be combined to result in an attractive new platform for chemical sensing. This review (with 230 refs.) summarizes the progress made in the past five years in the field of fiber-optic sensors: The first group comprises metals and metal oxides and their composites, and the second group comprises graphene, graphene oxides and CNTs, and its composites. By combining these nanocomposites with various optical fiber geometries, numerous sensors have been realized. Following an introduction, first section summarizes fiber-optic configuration for chemical sensing (including Fabry-Perot and Mach-Zehnder interferometry, surface plasmon resonance, and optical fiber gratings of the FBG and LPG type). The second section covers typical nanomaterials used in such sensors, with a first subsection on metals, metal oxides, their composites and nanostructured modifications, and a second subsection on graphenes, graphene oxides, carbon nanotubes, and their derivatives. Section 3 summarizes sensors (i) for various gaseous species (NH3, H2, CH4, H2S, CO2, NO2, O2), (ii) for volatile organic compounds (such as ethanol, methanol, acetone, toluene, and formaldehyde), and (iii) for heavy metal ions (such as Hg2+, Pb2+, Mg2+, Cd2+, Ni2+, and Mn2+). The merits and limitations of these nanomaterials and numerous examples for nanomaterial-based sensors are discussed and presented in the form of tables. A concluding section addresses technological challenges and future trends. Graphical Abstract Schematic presentation of an optical fiber modifiedwith various nanomaterials such as metal oxides (MOXs), metals, carbon-nanotubes (CNTs) and graphene. Such sensors are based on several fiber-optic configurations like Fabry-Perot interferometers (FPI), Mach-Zehnder interferometer (MZI) (includes an in-line MZI), surface plasmon resonance (SPR) (includes coating on cladding and unclad part of an optical fiber) and fiber gratings (FGs) (includes fiber Bragg gratings (FBGs) and long-period gratings (LPGs), these are explored for detection of various gases (NH3, H2, H2S, CH4, O2, CO2), vapors (VOCs), and ions.

Formation and characterization of non uniform reversible optical fiber gratings with single and double sided loading

In this paper the formation and characterization of non uniform single side loaded and double side loaded optical gratings is done. The non uniform long period reversible optical grating is practically induced in communication grade single mode fiber. In reversible optical fiber gratings the fiber is subject to periodical stress, which results in alternated regions under compression and stretching that modulate the refractive index via the photo elastic effect. The spectral responses of single side loaded optical grating is then compared with double side loaded optical grating.

Temperature and stress sensing characteristics of symmetrically fused tapered fiber grating

The stress-sensitive and temperature-insensitive characteristics of the tapered fiber grating can be used effectively to suppress the cross-sensitive problem of temperature and stress. In this paper, a fiber grating with a symmetric double-cone shape is proposed, which is made by using a fused taper technology. The theoretical model of sensing characteristics is established and analyzed by the transfer matrix method. Firstly, the factors affecting the change of radon coefficient are studied, and the relationship between the radon coefficient and the amount of grate length change is obtained, and then the spectral characteristics of the symmetric fused-tapered fiber grating are analyzed to discuss the origin of dense modulation at the short wavelength of the spectrum. The effects of temperature and stress on the reflection spectrum of symmetrically fused-tapered fiber grating are studied, and the relationship between the corresponding center wavelength and spectral bandwidth is obtained. In order to solve the problem of low stress sensitivity of the fiber grating, a scheme is presented that the radius difference of the optical fiber in the sensing cone region is enhanced by using polymer to coat the tapered area. Finally, a fused taper technology is used to prepare the symmetrically molten fiber grating, and verify the correctness of theoretical simulation in experiment, indicating that its stress sensitivity is 0.11391 nm/N. Firstly, the ripple coefficient of the symmetrically fused-tapered fiber grating is linearly related to the amount of change in the length of the grating. Secondly, because the grating cycle is small at the end of the symmetrically melt-pull-cone fiber-optic grating, and the reflectivity is less than 1, the left-hand transmission light and the right-hand reflected light will produce interference, so the spectral short wavelength will present dense modulation phenomenon. Thirdly, the center wavelength shifts to long wavelength region and the reflection bandwidth is broadened as stress is raised, and the center wavelength and reflection bandwidth are both linearly changed with the applied stress. Finally, the center wavelength shifts to long wavelength region as the temperature rises gradually, and the effect on the spectral bandwidth can be ignored. The stress sensitivity of the fiber grating increases hundreds of times by increasing the difference in fiber optic grating radius in the sensing tapered area, and the stress sensitivity can be further improved by increasing the fused taper variation of the grating. The spectral bandwidth of the symmetrical fused tapered fiber grating is only sensitive to stress but not to temperature. The characteristics can be used to realize the double-parameter measurement of temperature and stress.

Optically controlled phase array antenna [Invited

To overcome the beam squint in wide instantaneous frequency, we review a number of system-level optical controlled phase array antennas for beam forming. The optical delay network based on a fiber device in terms of topological structure of an N-bit optical switch, fiber grating, high-dispersion fiber, and vector-sum technology is discussed, respectively. Lastly, an integrated circuit is simply summarized.

Research of the Spectral Characteristics of Apodized Fiber Bragg Gratings

Fiber Bragg Gratings (FBG) are widely used in different areas of the state-of–the -art fiber optics. Every task imposes specified requirements to the FBG spectral characteristics, which are scheduled at the gratings manufacturing stage. Manufacturing and using the Bragg fiber-optic gratings is impossible without measuring their characteristics at every stage of manufacturing the gratings themselves and devices on their basis. To select FBG’s optimal parameter we will compare the parameter SGW with several different the most widely used apodization functions. Upon manufacturing the FBG there applied strict requirements to their parameters. Recording or manufacturing the fiber Bragg gratings might be classified according to the type of the laser being used, radiation wave length, recording techniques, irradiation material and grating type. The article is dedicated to the techniques of computing and measuring the FBG’s principal parameters; it is necessary to define optimal parameters of the characteristics for the grating quality operation.