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Hydrogen, a high-density and clean energy, have been widely used in various critical applications. However, the safety risk caused by hydrogen leakage during storage and transportation is still a non-negligible issue. Therefore, it is necessary to offer hydrogen sensors with fast response and high repeatability, and it will be perfect for achieving in situ monitoring over the lifecycle of hydrogen production and utilization. Here we proposed a compact optical fiber sensor with a short section of tilted Bragg fiber grating (TFBG) inscribed in the fiber core and coating the fiber surface with a 40 nm thickness of palladium and tungsten trioxide (Pd/WO3) combined film. The TFBG excites tens of narrow cladding resonances, part of which possess refractive indexes matching that of the Pd/WO3 coating and provides a high sensitivity to the surrounding hydrogen concentration change. The sensor offers improved sensing characteristics, including a fast response time (less than 10 seconds), high repeatability (over tens of measurements), an excellent linear response (higher than 99.6%) over the 0 to 3% concentration range, with a detection limit of approximately 2417 ppm.

The co-dope hollow TiO2 sphere is achieved via sacrificial template and thermal treatment methods, which remarkably enhances performances of Li–S battery due to the chemical absorption and physical containment to soluble lithium polysulfides.

Pyroelectric electric fields-modulated SERS substrate based on Ag nanowires–BiFeO3/carbon nanofibers enables vibration-free nanogenerators and miniaturizes E-SERS measurements.

One of the key elements for early diagnosis of acute and severe diseases is the development of ultra-sensitive detection methods, which are required to analyze the pathological state of the human body from trace amounts of substances in blood or urine. Surface plasmon resonance (SPR) optical fiber biosensors, particularly those based on tilted fiber Bragg gratings (TFBG), have emerged in recent years as a novel solution for in-situ biomedical detection. TFBGs can sensitively detect the physical and chemical interactions between biomolecules in real-time by sensing minute changes in refractive index, even in vivo. The TFBG-based SPR method can achieve rapid and accurate analysis of biological samples by demodulating the wavelength, intensity, phase and polarization state of the optical spectrum. In addition, TFBG-based SPR fiber optic sensors are capable of simultaneously detecting the absolute or relative values of multiple parameters. This effectively eliminates the interference from the ambient environment and ensures the stability and reliability of the sensor.

The apparent increase in magnetic field measurement has led to a growing demand for new sensing technologies. However, a long-lasting challenge unaddressed is how to achieve ultrahigh sensitive magnetic field measurement in both amplitude and orientation. Here, we demonstrate a compact-in-size and simple-to-implement fiber-optic magnetometer for highly sensitive vector magnetic field measurement. The sensor is based on magnetic nanoparticles coated tilted fiber Bragg grating inscribed in a commercial single mode fiber. The sensing mechanism stems from the backward coupled cladding mode resonances with narrow bandwidth (Q factor > 104), which is highly sensitive to slight surface refractive index perturbation. The switching of the magnetic field direction on the surface of fiber induces redistribution of coated magnetic nanoparticles with spatial anisotropy density and scattering by formation of magnetic chains hereafter the effective index is changed, which in turn modulates the cladding mode resonance with wavelength shift and amplitude attenuation. Through calculating the peak-to-peak intensity of target cladding modes of TFBG, the external magnetic field was precisely measured with an intensity sensitivity of 0.39 dB/mT and a direction sensitivity of 0.038 dB/rad. The inherent core mode can be used as a reference to get rid of temperature crosstalk and light source disturbance. The proposed sensor is compact in size and easy to be used in hard-to-reach place, showing good potentials for industrial applications.

Plasmonic hot carriers have the advantage of focusing, amplifying, and manipulating optical signals via electron oscillations which offers a feasible pathway to influence catalytic reactions. However, the contribution of nonmetallic hot carriers and thermal effects on the overall reactions are still unclear, and developing methods to enhance the efficiency of the catalysis is critical. Herein, we proposed a new strategy for flexibly modulating the hot electrons using a nonmetallic plasmonic heterostructure (named W18O49-nanowires/reduced-graphene-oxides) for isopropanol dehydration where the reaction rate was 180-fold greater than the corresponding thermocatalytic pathway. The key detail to this strategy lies in the synergetic utilization of ultraviolet light and visible-near-infrared light to enhance the hot electron generation and promote electron transfer for C-O bond cleavage during isopropanol dehydration reaction. This, in turn, results in a reduced reaction activation barrier down to 0.37 eV (compared to 1.0 eV of thermocatalysis) and a significantly improved conversion efficiency of 100% propylene from isopropanol. This work provides an additional strategy to modulate hot carrier of plasmonic semiconductors and helps guide the design of better catalytic materials and chemistries.

A method to enable surface plasmon resonance (SPR) sensors to discriminate between bulk and surface-localized refractive index changes is demonstrated with modified gold-coated tilted fiber Bragg grating SPR sensors (TFBG-SPR). Without this capability, all high-resolution SPR sensors should be using reference channels and strict temperature control to prevent the contamination of the desired detection of surface-localized chemical or binding events by drift of the refractive index of the medium, in which the experiment is carried out. The very fine comb of high-quality-factor resonances of a TFBG-SPR device coupled to the large differential sensitivity of some of the resonances to various perturbations is used to measure unambiguously the refractive index changes within a surface layer thinner than 25 nm from those of the bulk surrounding. The enabling modification of the conventional TFBG-SPR is a reduction of the gold coating from its optimum value near 50-30 nm: at this lower thickness, a surface plasmon wave can still be excited by a limited number of cladding mode resonances, but at the same time, the metal is thin enough to allow modes away from the SPR to tunnel across the metal and probe the bulk RI value. Measurements and simulations of the deposition of a self-assembled monolayer of 1-dodecanethiol in ethanol show that the bulk refractive index changes as small as 0.0004 can be distinguished from the formation of a 1 nm thick coating on the surface of the fiber.

We proposed and demonstrated a method to improve the birefringence of tilted fiber Bragg gratings (TFBGs) via high-refractive-index indium tin oxide (ITO) coatings. Firstly, we find the mode splitting phenomenon in an ITO film-coated TFBG. Numerical results show that the ITO coating can modulate the wavelength separations between two adjacent p- and s-polarized resonances. A cyclic dependency between the resonance separation and the thickness of the ITO film has been discovered. With proper film thickness selection, two groups of well-separated orthogonally polarized resonances can be achieved. Then we investigated the twist angle sensing capability of such a sensor with an optimal film thickness of 270 nm. The experimental results show that the intensity of the split resonances present reverse responses to applied torsion, which can be used for vector twist measurement. By monitoring the peak-to-peak differential intensity of these two orthogonally polarized resonances, the proposed sensor shows a twist sensitivity of ∼0.27 dB/deg and a linear response of 99.8% for the torsion angle range from -45° to 45°, together with the capability of orientation recognization. We believe that this configuration with improved birefringence opens research directions for vector physical parameter measurements such as vibration, bending, and pressure.

Photonics plays a critical role in the modern information society and human's daily life. Photonic technology is increasingly used in instrumentation and measurement for applications in oil and gas industry, electric power system, space missions, transportation, civil structure, military and defense, ocean exploration, biomedical detection, environmental monitoring and other areas. The field is related to many of the fastest evolving engineering professions in a challenging time but with increasing opportunities. With the rapid development of Internet of things (IoT), artificial intelligence (AI), 5G communication, and other emerging technology, the demand for photonic technology has been explosively increased in recent years.

Lithium-sulfur (Li-S) batteries have long been expected to be promising high-energy-density secondary batteries because of their high theoretical specific capacity and element abundances. Yet, their poor cyclability and low rate-capacity strongly limited their practical application. Herein, a nitrogen and sulfur dual doped hollow TiO

In situ and continuous monitoring of thermal effects is essential for understanding photo-induced catalytic processes at catalyst’s surfaces. However, existing techniques are largely unable to capture the rapidly changing temperatures occurring in sub-μm layers at liquid-solid interfaces exposed to light. To address this, a sensing system based on a gold-coated conventional single-mode optical fiber with a tilted fiber Bragg grating inscribed in the fiber core is proposed and demonstrated. The spectral transmission from these devices is made up of a dense comb of narrowband resonances that can differentiate between localized temperatures rapid changes at the catalyst’s surface and those of the environment. By using the gold coating of the fiber as an electrode in an electrochemical reactor and exposing it to light, thermal effects in photo-induced catalysis at the interface can be decoded with a temperature resolution of 0.1 °C and a temporal resolution of 0.1 sec, without perturbing the catalytic operation that is measured simultaneously. As a demonstration, stable and reproducible correlations between the light-to-heat conversion and catalytic activities over time were measured for two different catalysis processes (linear and nonlinear). These kinds of sensing applications are ideally suited to the fundamental qualities of optical fiber sensors, such as their compact size, flexible shape, and remote measurement capability, thereby opening the way for various thermal monitoring in hard-to-reach spaces and rapid catalytic reaction processes.

Operando tracking the chemical dynamics/states of a battery and its components is critical to their second life for boosting their sustainability. Herein, we demonstrate the feasibility and diversity of the tilted fiber Bragg grating (TFBG) optical fiber sensors to operando access the chemistry and states of electrolytes. We show how a single TFBG sensor can simultaneously measure the temperature and refractive index evolution inside the batteries, which pertains to the chemical evolution of the electrolyte. We also, for the first time, monitor the turbidity of electrolytes via the particulate-induced optical scattering and absorption. These unravelled electrolyte characteristics by TFBG help determine the electrochemical reaction pathways, being strongly correlated to the capacity loss. This type of sensing offers intriguing opportunities to building a “lab-on-fiber” platform for deeper chemistry and practical applications, provided we can delicately converge electrochemistry, photonics, and data science.

Microwave photonic interrogation of a high-speed and high-resolution refractive index (RI) sensor based on a tilted fiber Bragg grating with surface plasmon resonance (TFBG-SPR) is proposed and experimentally demonstrated. Instead of demodulating the wavelength shift or intensity change of a TFBG-SPR spectrum in the optical domain, we convert the TFBG-SPR spectrum to the time domain based on spectral shaping (SS) and wavelength-to-time (WTT) mapping and use a digital signal processor (DSP) to extract the RI information at a high speed and high resolution. In the experiment, when an Au-coated TFBG is immersed in a solution, the TFBG-SPR spectrum will produce a dip. When the RI changes, the location of the dip in the TFBG-SPR spectrum will shift, which is a function of the RI. By passing a broadband frequency-chirped optical pulse generated by a frequency swept laser source to the TFBG-SPR and detecting the optical pulse at the output of the TFBG-SPR at a photodetector (PD), due to SS-WTT mapping, a temporal waveform with its shape identical to the optical spectrum is produced. A DSP is then used to extract the SPR envelope information from the temporal waveform. By monitoring the changes of the SPR envelope, the RI information is interrogated with a high resolution of 1.123 × 10−6 RIU at a high speed over 20 kHz.

High-voltage transmission towers in coastal areas can be damaged by destructive hurricanes and strong seasonal winds. With real-time condition monitoring systems, severe and critical damage can be detected in the early stage, and appropriate solution taken. In this paper, we investigate the possibilities for real-time monitoring of wind-induced vibration of transmission towers, using an all-optical fiber sensing system. The optical sensing probe is made by splicing coaxially a thin-core fiber, with tilt fiber gratings inscribed into its core, to a lead-in single mode fiber. We experimentally demonstrated that our sensor can provide linear response higher than 97% over a range of 0.1-6.5 m/s 2 . Finally, in collaboration with a power grid company, we demonstrated the feasibility of the proposed all-optical fiber sensing system during field trials.

Oxygen sensor is an important technique in various applications including industrial process control, medical equipment, biological fabrication, etc. The reported optical fiber-based configurations so far, using gas-sensitive coating do not meet the stringent performance targets, such as fast response time and low limit of detection (LOD). Tin-based halide perovskites are sensitive to oxygen with potential use for sensor applications. Here, the halide perovskite-based oxygen optical fiber sensor by combining phenylethylammonium tin iodide (PEA

Real-time and high-sensitivity monitoring of heavy metal ions in solution is of great significance in environmental monitoring. The traditional electrochemical methods suffer from electrochemical noise and environmental interferences for heavy metal trace detection. To address these issues, we propose a novel electrochemical surface plasmon resonance (EC-SPR) optical fiber sensor. The sensor comprises a tilted fiber Bragg grating imprinted in a commercial single-mode fiber and coated with a nanoscale gold film for high-efficiency SPR excitation. The gold-coated optical fiber serves as a working electrode, performing the dual functions of anodic stripping voltammetry and SPR optical sensing. We demonstrate detection of Pb2+ as an example of a typical heavy metal ion. We show a stable and reproducible correlation between the real-time ion deposition–stripping cycles and the optical transmission of the optical fiber, with a limit of detection of 10−10 M and a dynamic range of nearly five orders of magnitude. Moreover, by taking derivative of the SPR amplitude change, we can clearly identify the peak stripping potential of the detected ions, and therefore, realize specific ion identification. The method proposed is inherently immune to temperature cross-talk because the core mode is temperature dependent but insensitive to the surrounding media. The proposed EC-SPR fiber-optic sensor has the advantages of compact size, flexible shape, and remote operation capability, thereby, opening the way for other opportunities for electrochemical monitoring in various hard-to-reach locations and remote environments.

We propose and demonstrate an ultra-sensitive plasmonic fiber-optic hot-wire anemometer. This instrument comprises a highly tilted fiber Bragg grating-assisted surface plasmon resonance (SPR) sensor, which is coated with carbon nanotubes as the photothermal conversion element. The carbon nanotubes on the sensor surface efficiently convert light from the heating laser, of which wavelength matched to the SPR window, into heat, thereby setting the sensor in an elevated temperature state. Air flow draws away surface heat, thus inducing both a strong SPR wavelength shift and power modulation. Using this anemometer, we experimentally achieve a dynamic range of 0.05 to 0.65 m/s for wind speed measurement. Furthermore, we demonstrate real-time monitoring of wind speed by measuring the intensity of the heating laser. This sensor is simple and robust in structure. It has a wide range of potential applications in both scientific research and industrial production.

We present an optical fiber biosensor that employs both long-range and localized surface plasmon resonances (SPRs) to sensitively detect very small changes in the resonances due to the surrounding refractive index. A tilted optical fiber Bragg grating inscribed in standard single-mode fiber couples light into the fiber's cladding, where it induces micrometer-scale SPRs in a 50-nm-thick gold coating. The limit of detection of the targeted protein, thrombin, is enhanced by resonance between the long-range SPRs and localized SPRs induced in 13-nm-diameter gold nanoparticles bonded via aptamers to the protein molecules. The sensor demonstrates stable and reproducible response to thrombin concentration down to a limit of detection of 1 n·M, which is three orders of magnitude more sensitive than previous optical fiber protein sensors. Meanwhile, it does not require accurate temperature control because of the elimination of the temperature cross-sensitivity inherent in TFBG devices (core mode). The high sensitivity, simplicity, compact size, and remote monitoring capability of the sensor open many new opportunities for environmental, biological, and medical sensing.

A reflective fiber-optic refractometer based on a broadband cladding-to-core recoupling for a wide range of refractive index measurement is proposed and experimentally demonstrated. The sensing probe contains a tilted fiber Bragg grating (TFBG) together with an in-fiber “cladding” mirror upstream. The “cladding” mirror is made by a 0.3-μm-thick gold film with a transparent aperture to permit all the core-guided light go through but reflect the light in the cladding. The downstream TFBG couples the forward propagating core mode to backward propagating cladding modes and such cladding modes are reflected to the TFBG again by the in-fiber “cladding” mirror and recoupled to the fiber core through the same TFBG. With this special design, all the transmitted cladding modes now can be effectively monitored in reflection. Compared with the reported “in-fiber” and “fiber-to-fiber” coupling TFBG refractometer, our sensor demonstrates a much broader surrounding refractive index operating range with a very linear sensitivity of 528 nm/RIU. A tunable laser is used to improve the measured accuracy and a sensitivity of 7973 dB/RIU was obtained, with a limit of detection of 10−6 RIU. The sensor works with very controllable cross-sensitivities because the core mode is temperature dependent but insensitive to surrounding refractive index.

We have proposed and experimentally demonstrated a plasmonic tilted fiber Bragg grating based biosensor for in-situ detection of the small biomolecule S-adenosyl-L-homocysteine (AdoHcy). The sensor can also monitor the molecule's interaction with the protein Set7 in real time. The biosensor is made by coating a 50-nm-thick gold film over an 18° tilted fiber Bragg grating. A spectrally dense comb of backward-propagating cladding resonance modes with Q-factor as high as 104 is excited due to the tilted grating. The spectral overlap between the excited cladding modes and the broader absorption feature of the surface plasmon makes this device a unique tool for high-accuracy measurement of small shifts of plasmon resonance wavelength. Surface biomolecular functionalization of the gold film enables us to not only detect the corresponding biomolecule but also study its interaction with other biomolecules. Our experimental results demonstrate that our biosensor can detect AdoHcy at concentration as low as 1 nM. Furthermore, we have successfully real-time monitored interaction between the protein Set7 and AdoHcy at different concentrations. Our measurements are consistent with the biological theory that the association rate of a pair of biomolecules depends on their concentrations.

Aptamer biosensor for label-free thrombin detection based on a tapered fiber-optic interferometer has been proposed and demonstrated. The biosensor probe can be easily fabricated by tapering a commercial double-cladding fiber down to 5 μm in diameter and it provides a high refractive index sensitivity of 1660 nm/RIU. The changes are determined by tracing the wavelength shift of the dip in the transmission spectrum with variable surrounding refractive index. With the help of the surface functionalization of thrombin-binding aptamers, the proposed fiber-optic biosensor proves its capability in label-free detection of thrombin with a lowest detectable concentration of 0.1 μm, together with high specificity in various concentrations of pure and mixed thrombin solutions.

The mode control enabled by specialized gratings allows simultaneous sensing of multiple parameters in a single fiber strand—a potential game-changer.

Hydrogen sensors are of great importance to detect leakage in time because the hydrogen-air mixture is highly flammable. Based on optical fiber-based configurations reported so far, using palladium coating only does not meet stringent performance targets, such as fast response time and limited deactivation caused by poisoning. Here, a palladium-gold alloy-coated optical fiber hydrogen sensor, i.e., highly tilted fiber Bragg grating (TFBG), was proposed by using its narrow bandwidth cladding modes whose effective refractive index (ERI) extends to 1.0 where the gas measurement is possible, which led to faster specific hydrogen measurement response time (a shorter stabilization time during the association and dissociation phases less than 20 s and 30 s, respectively) and improved deactivation resistance (higher than 99% per test cycle). Meanwhile, the temperature cross-sensitivity can be eliminated via referencing the “target” spectral combs to the core mode. We are sure that this promising configuration extends research directions for rapid, repeatable and high deactivation-resistance in hydrogen gas detection.

We proposed and experimentally demonstrated a metal coated tilted fiber grating (TFG) for calmodulin detection. The sensor was designed as a reflective probe type by depositing a gold mirror in the down-stream of the TFG. A 50-nmthick gold nanocoating was radio-frequency magnetron sputtered over the fiber surface of an 18° TFG, and the transient receptor potential channels were bond onto the metal surface for calmodulin detection. The experimental results demonstrated that our biosensor can detect calmodulin with concentration as low as 1 μM. This result was better than that obtained using the isothermal titration calorimetry method. In addition, with the use of a custom-designed microfluid system, the volume of sample solution required by our biosensor was only 20 μL. Our proposed sensor is simple to fabricate and easy to implement, and it can be used for rapid, label-free, and microliter-volume biomolecule detection

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.

We have proposed and experimentally demonstrated a plasmonic tilted fiber Bragg grating (TFBG) biosensor for labelfree detection of small biomolecule S-adenosyl-L-homocysteine (AdoHcy) in different concentrations. The biosensor is designed as a reflective probe type with a 50-nm-thick gold film coating over the fiber surface of an 18° TFBG, together with a surface functionalization with protein lysine methyltransferase Set7 working as bio-detector. The sensor is fabricated without any destruction of fiber structural integrity, thereby ensuring the stability and reproducibility. The experimental results demonstrated that our biosensor can detect biomolecule AdoHcy with concentration as low as 1 nM. We have also measured AdoHcy in different concentrations, including 0.01, 0.1 and 10 nM, and the results indicated that as the AdoHcy concentration increases, the association ratio between AdoHcy and Set7 arises correspondingly, agreeing with the theory.

Optical microfiber is a class of specialty fibers, which is featured with wavelength scale diameters. With such small dimensions, the optical microfiber offers large fractions of evanescent fields and high surface field intensities, making it highly sensitive to disturbances in the surrounding medium. Thus, the optical microfiber is an ideal building block for high-performance photonics sensing devices. In this chapter, recent progress in optical microfiber-based sensors is reviewed. It starts with a brief introduction of the fundamental optical properties of optical microfibers and the well-developed fabrication techniques. Then, a brief summarization of the well-established microfiber-based refractive index sensing schemes is given, including working principles and sensing performances. Following this section, the latest progress on new effects and strategies for sensing enhancement is reviewed. In the last, the conclusions and an outlook are presented.

Surface plasmon resonance (SPR) optical fiber sensors can be used as cost-effective and relatively simple-to-implement alternatives to well established bulky prism configurations for high sensitivity in-situ biomedical measurements. The miniaturized size and remote operation ability offer a multitude of opportunities for single-point sensing in hard-to-reach spaces, even possibly in vivo. The biosensor configuration presented here utilizes a nano-scale metal-coated tilted fiber Bragg grating (TFBG) imprinted in a commercial single mode fiber core with no structural modifications. A unique feature of our TFBG sensor is the use of a single layer graphene coating over the gold-coated fiber surface, functionalized with a selective DNA aptamer for highly sensitive detection of target molecules. We have demonstrated the capture of dopamine molecules by the DNA aptamer, resulting in aptamer well-defined conformational changes in response to dopamine surface affinities. This process amplifies the surface refractive index modulation over the fiber surface to enable precise dopamine concentration measurement in real time via monitoring of the surface plasmon resonance signals. The sensor shows a linear response for dopamine concentration in the range from 10−13 M to 10−8 M with a lower limit of detection of 10−13 M. This limiting concentration is lower than the concentration fluctuations of dopamine in the human brain. The sensor works with minimal cross-sensitivities because of the core mode calibration inherent in the TFBG. Integration of the TFBG with a hypodermic needle should allow similar measurements in vivo, presenting an appealing solution for rapid, low power consumption and highly sensitive detection of analytes at low concentrations in medicine as well as in chemical and environmental monitoring.

Recently, although great attention has been paid to the design and exploitation of new classes of near-infrared (NIR) light-induced materials, the photothermal dynamics of these materials have not been fully explored. However, understanding the photothermal dynamics of NIR-light-responsive composites is of fundamental importance from the viewpoint of smart material design and processing at the nanoscale, and for the understanding of a number of related phenomena. Herein, an alternative approach to observe the dynamics of the photothermal process is developed, which relies on probing the local refractive index change in the nanocomposite matrix with a silica microfiber interferometer. In this approach, the light-induced morphological change of the polymer is captured by the microfiber because of the strong evanescent-field interaction, and is translated into a significant wavelength shift in the interferometric fringe. Therefore, probing the matrix to study the local photothermal dynamics is possible. The optical microfiber records various phase-transformation stages of the photothermal nanocomposites induced by different optothermal mechanisms, especially revealing the reconstruction process of Ag@reduced graphene oxide (Ag@G) nanosheets during the initial stage of the photothermal process. The feasibility of using optical fibers for studying the inner mechanism of material phase change is presented herein and it provides a new approach for fundamental investigations into smart material development at the nanoscales.

The high price and volatile supply source of oil as well as the desire to move towards greener sources of energy have created a growing interest in the use of alternative fuels, such as hydrogen. It is vital to have an inexpensive device that can quickly, sensitively, reliably and safely monitor hydrogen concentrations in case there is a leak. In this paper, we report measurements of the complex permittivity of 7 nm thick palladium thin films exposed to concentrations of 0 to 3% (by volume) hydrogen in nitrogen at wavelengths near 1310 nm. Measurements were carried out with a tilted fiber Bragg grating first coated with 30 nm of gold and then palladium at several thicknesses between 3 and 43 nm. The tilt angle of the grating was fixed at 23 degrees in order to allow the excitation of surface plasmon resonances on the gold surface by high order cladding modes in air at wavelengths near 1310 nm. Simulations of the grating response based on measurable experimental parameters indicate that for palladium thicknesses between 3 and 10 nm, the measured changes in the grating transmission become independent of thickness. Therefore, for a 7 nm thick film, any change in transmission can be traced directly to changes in permittivity. The relative change of the permittivity of the palladium layer was found to scale linearly with hydrogen concentration with a scaling factor of −0.15/%H2 for hydrogen concentrations between 0 and 1.7%. The limit of detection of the grating configuration used was determined to be 380 ppm at three times the standard deviation for measurements averaged over 80 seconds. Using standard fiber optic instrumentation and a single mode fiber, the signal to noise ratio was over 100.

An optofluidic system based on photothermal spectroscopy is proposed, which combines molecular photothermal effect with Nb2CTx MXene-tilted fiber Bragg grating (TFBG) for the detection of organophosphorus pesticides (OPs) with temperature compensated. Under the irradiation of excitation light, the photothermal effect of OPs produces a detectable change in the refractive index of the sample, and the concentration of chlorpyrifos can be quantified using TFBG. The Nb2CTx MXene coated TFBG allow more molecules to be absorbed on the surface of TFBG, which enhances the interaction between light and matter, and improves the sensitivity of detection. The temperature compensation is performed by referring to the core mode of TFBG, thereby eliminating the influence of ambient temperature on the photothermal detection. The experimental results show that the sensitivity reaches 1.8 pm/ppm with a limit of detection (LOD) of 0.35 ppm, and the obtained temperature compensation coefficient is 4.84 ppm/°C. This photothermal biosensor has the advantages of low LOD, temperature compensation and real-time online monitoring, making it a good candidate in medicine, chemistry and environmental monitoring.

We propose and experimentally demonstrate a compact fiber-optic vibration sensor in which a tilted Bragg grating is inscribed in the core of a depressed cladding fiber. The sensor provides resolution up to $2\times 10^{-2} \text {m/s}^{2}$ and a linear acceleration response between 0.2 and 6.0 m/s2, together with a flat frequency response between 1 and 120 Hz. The sensor takes advantage of a cost-effective interrogation scheme by monitoring the total reflection power of the fiber sensor. We use the calibrated output to eliminate the potential light source and fiber loss-induced power fluctuations. The proposed sensing system has been used to measure the vibration of a mechanical damper in electrical transmission lines and the results agree well with those from a commercial sensor.

Fiber Bragg gratings (FBGs) are inherently sensitive to temperature, axial strain, and pressure, which can be easily measured by a shift of the Bragg wavelength in their reflected/transmitted power spectrum. FBG sensors acquire many more additional sensing modalities and applications when the polarization of the interrogating light is controlled. For the polarization to have an effect, the cylindrical symmetry of the fiber must be broken, either by the structure of the fiber itself, by that of the FBG, or by the perturbation to be measured. Polarization control allows for sensing parameters that are spatially oriented, such as lateral force, bending or twist, and also for measurements of the properties of anisotropic media. Most importantly, polarization control enables high quality all-fiber surface plasmon resonance (SPR) FBG sensors and localized SPR-assisted sensing. This tutorial will cover the theory of polarized measurements in fiber gratings, their experimental implementation, and review a selection of the most important applications.

We propose and experimentally demonstrate a novel electric field intensity sensor by using a tilted fiber Bragg grating (TFBG) interacting with liquid crystal. The TFBG is surrounded by the liquid crystal whose effective refractive index changes over the range from 1.53 to 1.70 in response to the strength of external electric fields. The guiding properties of cladding modes excited in the TFBG are, therefore, sensitive to the external electric field. By monitoring the amplitude of a selected cladding mode resonance, we measured electric field strength over a range of 1.0–4.8 kV/cm. The sensor demonstrated better than 99% linearity with a sensitivity of 0.287 dB/kV/cm. Temperature cross-sensitivity can be effectively reduced by referenced to the core mode of the proposed TFBG sensor.

A reflective fiber-optic biosensor based on microfiber Bragg grating (mFBG) for DNA hybridization detection using layer-by-layer self-assembly technology is demonstrated. By searching the wavelength separation of two well-defined resonances of mFBG in reflection, temperature-compensated refractive index measurement has been achieved. With the help of poly(ethylenimine) (PEI), poly(acrylic acid) (PAA), and single-stranded DNA (ssDNA) for the preparation of a polyelectrolyte multilayer film, more target DNA chains are absorbed by the (PEI/PAA)4(PEI/DNA)1 multilayer films immobilized on the surface of mFBG. The hybridization processes of the target ssDNA with the concentration of 1 μM have been monitored in real-time together with high specificity of the noncomplementary DNA target.