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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.

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.

The apparent increase in hormone-induced cancers and disorders of the reproductive tract has led to a growing demand for new technologies capable of detecting endocrine disruptors. However, a long-lasting challenge unaddressed is how to achieve ultrahigh sensitive, continuous, and in situ measurement with a portable device for in-field and remote environmental monitoring. Here we demonstrate a simple-to-implement plasmonic optical fiber biosensing platform to achieve an improved light–matter interaction and advanced surface chemistry for ultrasensitive detection of endocrine disruptors. Our platform is based on a gold-coated highly tilted fiber Bragg grating that excites high-density narrow cladding mode spectral combs that overlap with the broad absorption of the surface plasmon for high accuracy interrogation, hence enabling the ultrasensitive monitoring of refractive index changes at the fiber surface. Through the use of estrogen receptors as the model, we design an estradiol–streptavidin conjugate with the assistance of molecular dynamics, converting the specific recognition of environmental estrogens (EEs) by estrogen receptor into surface-based affinity bioassay for protein. The ultrasensitive platform with conjugate-induced amplification biosensing approach enables the subsequent detection for EEs down to 1.5 × 10−3 ng ml−1 estradiol equivalent concentration level, which is one order lower than the defined maximal E2 level in drinking water set by the Japanese government. The capability to detect EEs down to nanogram per liter level is the lowest limit of detection for any estrogen receptor-based detection reported thus far. Its compact size, flexible shape, and remote operation capability open the way for detecting other endocrine disruptors with ultrahigh sensitivity and in various hard-to-reach spaces, thereby having the potential to revolutionize environment and health monitoring., An optical fiber biosensor displaying superfine plasmonic spectral combs and enhanced by conjugate-induced amplification enables the detection of environmental estrogens down to pg/mL estradiol equivalent concentration level.

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.

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.

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.

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.

[EN] In this work, we present the architecture of a multiplexed refractive index (RI) sensing system based on the interrogation of Rayleigh backscattering. The RI sensors are fabricated by fiber wet-etching of a high-scattering MgO nanoparticle-doped fiber, without the need for a reflector or plasmonic element. Interrogation is performed by means of optical backscatter reflectometry(OBR), which allows a detection with a millimeter-level spatial resolution. Multiplexing consists of a simultaneous scan of multiple fibers, achieved by means of scattering-level multiplexing (SLMux) concept, which uses the backscattered power level in each location as a diversity element. The sensors fabricated have sensitivity in the order of 0.473-0.568 nm/RIU (in one sensing point) and have been simultaneously detected together with a distributed temperature sensing element for multi-parameter measurement. An experimental setup has been prepared to demonstrate the capability of each sensing region to operate without cross-talk, while operating multi-fiber detection., This work was supported in part by the ORAU Programme at Nazarbayev University (LIFESTART and FOSTHER Grants), in part by the Agence Nationale de la Recherche (ANR) Project NanoSlim under Grant ANR-17-17-CE08-0002, in part by the National Natural Science Foundation for Excellent Youth Foundation of China under Grant 61722505, in part by the Key Program of Guangdong Natural Science Foundation under Grant 2018B030311006, and in part by The Spanish Ministry of Economy and Competitiveness under Grant DIMENSION TEC2017 88029-R. The associate editor coordinating the review of this article and approving it for publication was Prof. Marco Petrovich.

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.

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.

A miniature optical fiber ultrasonic sensor based on the Fabry-Perot interferometer (FPI) is proposed and experimentally demonstrated. The cavity of the interferometer is based on a section of hollow core fiber coated with a 353ND diaphragm. A well-define interference pattern is achieved, and the spectral sideband filter technique is used to interrogate the sensor. Because the thin reflective diaphragm is extremely sensitive to sound pressure, the signal-to-noise ratio of the sensor response reaches to 31.22 dB with the pulse ultrasonic excitation of 300 kHz. A clear interface information can be distinguished in physical model imaging, and the longitudinal spatial resolution is 5 mm. This proposed sensor has the advantages of being simple to fabricate, ultracompact in size, cost effective, corrosion resistance, and being highly stable.

An ultrasensitive tapered fiber-optic sensor functionalized with size-selective molecular sieve (Fe 3 O 4 @SiO 2 @meso SiO 2 microspheres) arrays for label-free γ-aminobutyric acid (GABA) detection has been proposed and experimentally demonstrated. The surface morphology of the Fe 3 O 4 @SiO 2 @meso SiO 2 microspheres and optical properties of tapered fiber-optic interferometer endow the as-prepared sensing probe to transfer slight GABA concentration changes into strong refractive index fluctuations with high selectivity (size dominated). Label-free GABA detection with a linear sensitivity of 0.94 nm/log M has been achieved. The proposed fiber-optic sensor provides an improved limit of detection up to 3.51 × 10 −13 M together with size-based selectivity, making it an appealing solution for rapid, low consumption and highly sensitive detection of analytes at low concentrations in medicine, chemical and environmental monitoring.

Ultrasensitive and in situ DNA detection at different pH values, ranging from 4.3 to 8.5, based on a microfiber with a graphene oxide linking layer was proposed and experimentally demonstrated.

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.

It is necessary that accurate and continuous detection of heavy metal ions around our life. Traditional electrochemical methods have security issues and background interference. To solve these problems, a new method based on surface plasmon resonance (SPR) electrochemical optical fiber sensor system has been put forward. Combined with cyclic voltammetry, SPR can be used for heavy metal ions monitoring simultaneous gold coatings modified on the surface of the optical fiber sensor recording the electrochemical signal, the characteristic is that SPR is highly sensitive and excludes electrochemical background interference. The optical fiber coated with gold film works as a sensor and a working electrode. Optical fiber have a lot of advantages, including flexible operation, compact size and variable shape, opens a new way for high sensitive electrochemical detection. In this paper, heavy metal ion $\mathbf{Pb}^{2+}$ detection is carried out with good electrochemical and optical response.

An ultra-sensitive plasmonic fiber-optic photothermal anemometer is proposed and demonstrated. The device consists of a highly-tilted fiber Bragg grating (TFBG) sensor, which is coated with a gold layer exciting surface plasmon resonance (SPR) and then carbon nanotubes as the photothermal conversion element. The carbon nanotubes deposited on the sensor surface efficiently convert light from the heating laser, which is wavelength matched to the SPR signature, into heat. Air flow draws away the surface heat, thus inducing both a strong SPR wavelength shift and a changed of the power modulation. Using this approach, the proposed anemometer accounts for a dynamic range from 0.05 m/s to 0.65 m/s for wind speed measurement. In addition, the real-time monitoring of wind speed has been proved by measuring the intensity of the heating laser source. This device is a valuable candidate for a wide range of potential applications in both scientific research and industrial production, given its simplicity and robustness in structure.

A breast cancer cell detection method by using a tilted fiber Bragg gratings (TFBGs) coated with halloysite nanotubes (HNTs) has been proposed and a limit of detection of 10 cells/mL has been experimental achieved with good reproducibility. Experiments have shown that the capture of MCF-7 cells by peristaltic pump is much higher than that by static cell solution. Compared with normal cells, the HNTs coating is able to capture tumor cells better, because the surface morphology of the HNTs can make cancer cells adhere well. In the detection of tumor cells, the integration of TFBGs with microfluidic channels enables precise measurement control over samples with sub-microliter volumes and does not require accurate temperature control because of the elimination of the temperature cross-sensitivity inherent in TFBG devices. The proposed optical fiber/microfluidic biosensor represents an appealing solution for rapid, low consumption and highly sensitive detection of analytes at low concentrations in medicine monitoring.

The combination of fiber-optic–based platforms for biosensing with nanotechnologies is opening up the chance for the development of in situ, portable, lightweight, versatile, reliable and high-performance optical sensing devices. The route consists of the generation of lossy mode resonances (LMRs) by means of the deposition of nm-thick absorbing metaloxide films on special geometric-modified fibers. This allows measuring precisely and accurately the changes in surface refractive index due to the binding interaction between a biological recognition element and the analyte, with very high sensitivity compared to other optical technology platforms, such as fiber gratings or surface plasmon resonance. The proposed methodology, mixed with the use of specialty fiber structures such as D-shaped fibers, allows improving the light-matter interaction in a strong way. The shift of the LMR has been used to monitor in real-time the biomolecule interactions thanks to a conventional wavelength-interrogation system and an ad-hoc developed microfluidics. A big leap in performance has been attained by detecting femtomolar concentrations in real samples of human serum. The biosensor regeneration has been also studied by using a solution of sodium dodecyl sulphate (SDS), proving the device reusability. Therefore, this technology possibly represents a paradigm shift in the development of a simple, high-specificity and label-free biosensing platform, which can be applied to speed up diagnostic healthcare processes of different diseases toward an early diagnostic and personalized treatment system.

The tilted fiber Bragg grating (TFBG) is a new kind of fiber-optic sensor that possesses all the advantages of well-established Bragg grating technology in addition to being able to excite cladding modes resonantly. This device opens up a multitude of opportunities for single-point sensing in hard-to-reach spaces with very controllable cross-sensitivities, absolute and relative measurements of various parameters, and an extreme sensitivity to materials external to the fiber without requiring the fiber to be etched or tapered. Over the past five years, our research group has been developing multimodal fiber-optic sensors based on TFBG in various shapes and forms, always keeping the device itself simple to fabricate and compatible with low-cost manufacturing. This paper presents a brief review of the principle, fabrication, characterization, and implementation of TFBGs, followed by our progress in TFBG sensors for mechanical and biochemical applications, including one-dimensional TFBG vibroscopes, accelerometers and micro-displacement sensors; two-dimensional TFBG vector vibroscopes and vector rotation sensors; reflective TFBG refractometers with in-fiber and fiber-to-fiber configurations; polarimetric and plasmonic TFBG biochemical sensors for in-situ detection of cell, protein and glucose.

A wideband-tunable Q-switched fiber laser with a high-reflective topological insulator saturable absorber mirror (TISAM) was proposed and experimentally demonstrated. The proposed TISAM is composed of Sb2Te3 and gold films deposited over the fiber tip, and it has the advantage of compact size, easy integration over fiber, good stability, and mass production ability. The modulation depth, saturable intensity, and nonsaturable loss of the TISAM at 1550 nm are experimentally tested to be 6.2%, 143 MW/cm2, and 4.2%, respectively. By using such TISAM, we obtain laser pulses of ~400 ns in pulse width and kHz in repetition rates over the wavelength range from 1530 to 1570 nm. The experimental results indicate that the Sb2Te3 based TISAM have good potentials for Q-switched fiber lasers.

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.

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 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. The sensing probe is well packaged and directly mounted on the transmission tower. The sensor simultaneously provides two wavelength-separated spectral signatures, one originating from the core-guided mode and the other from the cladding modes. The optical power of the cladding modes shows the real-time vibration information, both amplitude and frequency, of the transmission tower. The power of core mode provides in-situ temperature measurement and an inherent self-calibration to remove the fluctuations of light source intensity and optical transmission loss. In collaboration with a power grid company, we demonstrated the feasibility of the proposed all optical fiber sensing system during field trials. A stable and reproducible correlation between the real-time wind speed and vibrational acceleration of the transmission tower was demonstrated. The measurements also showed that the mechanical resonant frequency of the transmission tower remained constant to within 0.01 Hz, independent of the wind speed. Any change in the resonant frequency would thus signal damage to the tower’s structure, thereby giving early warning of a dangerous situation.

In situ and continuous monitoring of electrochemical activity is key to understanding and evaluating the operation mechanism and efficiency of energy storage devices. However, this task remains challenging. For example, the present methods are not capable of providing the real-time information about the state of charge (SOC) of the energy storage devices while in operation. To address this, a novel approach based on an electrochemical surface plasmon resonance (SPR) optical fiber sensor is proposed here. This approach offers the capability of in situ comprehensive monitoring of the electrochemical activity (the electrode potential and the SOC) of supercapacitors (used as an example). The sensor adopted is a tilted fiber Bragg grating imprinted in a commercial single-mode fiber and coated with a nanoscale gold film for high-efficiency SPR excitation. Unlike conventional “bulk” detection methods for electrode activity, our approach targets the “localized” (sub-μm-scale) charge state of the ions adjacent to the electrode interface of supercapacitors by monitoring the properties of the SPR wave on the fiber sensor surface located adjacent to the electrode. A stable and reproducible correlation between the real-time charge–discharge cycles of the supercapacitors and the optical transmission of the optical fiber has been found. Moreover, the method proposed is inherently immune to temperature cross-talk because of the presence of environmentally insensitive reference features in the optical transmission spectrum of the devices. Finally, this particular application is ideally suited to the fundamental qualities of optical fiber sensors, such as their compact size, flexible shape, and remote operation capability, thereby opening the way for other opportunities for electrochemical monitoring in various hard-to-reach spaces and remote environments., Energy storage: Watching supercapacitors at work An optic fiber system developed by researchers in China and Canada can peer inside supercapacitors and batteries to observe their state of charge. Renewable energy sources are naturally inconsistent, and so require new energy storage technologies. Supercapacitors offer rapid charging and long-term storage, but it is important to be able to monitor their working state. To tackle this issue, a team including Tuan Guo and Wenjie Mai at Jinan University adapted a standard telecommunications optic fiber to act as a grating, with a very thin gold coating that supports electron oscillations called surface plasmons. When the researchers installed their fiber in a supercapacitor they found that the optical properties of the surface plasmons changed depending on the supercapacitor’s state of charge, offering a unique, low-cost method for real-time monitoring of energy storage devices in operation.

Interfacing bio-recognition elements to optical materials is a longstanding challenge to manufacture sensitive biosensors and inexpensive diagnostic devices. In this work, a graphene oxide (GO) interface has been constructed between silica microfiber and bio-recognition elements to develop an improved γ-aminobutyric acid (GABA) sensing approach. The GO interface, which was located at the site with the strongest evanescent field on the microfiber surface, improved the detection sensitivity by providing a larger platform for more bio-recognition element immobilization, and amplifying surface refractive index change caused by combination between bio-recognition elements and target molecules. Owing to the interface improvement, the microfiber has a three times improved sensitivity of 1.03 nm/log M for GABA detection, and hence a lowest limit of detection of 2.91 × 10−18 M, which is 7 orders of magnitude higher than that without the GO interface. Moreover, the micrometer-sized footprint and non-radioactive nature enable easy implantation in human brains for in vivo applications.

A platinum-coated singlemode-multimode (SM) structure is investigated in this paper as an optical fibre sensor (OFS) to monitor the phase transition of a phase change material (PCM). Paraffin wax has been used as an example to demonstrate the sensor’s performance and operation. Most materials have the same temperature but different thermal energy levels during the phase change process, therefore, sole dependency on temperature measurement may lead to an incorrect estimation of the stored energy in PCM. The output spectrum of the reflected light from the OFS is very sensitive to the bend introduced by the PCM where both liquid and solid states exist during the phase transition. The measurement of strain experienced by the OFS during the phase change of the PCM is utilized for identifying the phase transition of paraffin wax between the solid and liquid states. The experimental results presented in this paper show that the OFS with a shorter multimode fibre section has better performance for monitoring the phase transition of paraffin wax with a measured phase transition temperature range of 41.5 ºC – 57.7 ºC for the SM based OFS with a 5 mm long multimode fibre section.

This paper presents a brief overview of the technologies used to implement surface plasmon resonance (SPR) effects into fiber-optic sensors for chemical and biochemical applications and a survey of results reported over the last ten years. The performance indicators that are relevant for such systems, such as refractometric sensitivity, operating wavelength, and figure of merit (FOM), are discussed and listed in table form. A list of experimental results with reported limits of detection (LOD) for proteins, toxins, viruses, DNA, bacteria, glucose, and various chemicals is also provided for the same time period. Configurations discussed include fiber-optic analogues of the Kretschmann–Raether prism SPR platforms, made from geometry-modified multimode and single-mode optical fibers (unclad, side-polished, tapered, and U-shaped), long period fiber gratings (LPFG), tilted fiber Bragg gratings (TFBG), and specialty fibers (plastic or polymer, microstructured, and photonic crystal fibers). Configurations involving the excitation of surface plasmon polaritons (SPP) on continuous thin metal layers as well as those involving localized SPR (LSPR) phenomena in nanoparticle metal coatings of gold, silver, and other metals at visible and near-infrared wavelengths are described and compared quantitatively. Graphical AbstractArtist rendering of light from a tilted fiber Bragg grating probing the cladding surface where a thin gold film and biofunctional layer are used to detects analytes

Plasmonic immunosensors are usually made of a noble metal (in the form of a film or nanoparticles) on which bioreceptors are grafted to sense analytes based on the antibody/antigen or other affinity mechanism. Optical fiber configurations are a miniaturized counterpart to the bulky Kretschmann prism and allow easy light injection and remote operation. To excite a surface plasmon (SP), the core-guided light is locally outcoupled. Unclad optical fibers were the first configurations reported to this end. Among the different architectures able to bring light in contact with the surrounding medium, a great quantity of research is today being conducted on metal-coated fiber gratings photo-imprinted in the fiber core, as they provide modal features that enable SP generation at any wavelength, especially in the telecommunication window. They are perfectly suited for use with cost-effective high-resolution interrogators, allowing both a high sensitivity and a low limit of detection to be reached in immunosensing. This paper will review recent progress made in this field with different kinds of gratings: uniform, tilted and eccentric short-period gratings as well as long-period fiber gratings. Practical cases will be reported, showing that such sensors can be used in very small volumes of analytes and even possibly applied to in vivo diagnosis.

A highly sensitive fiber-optic accelerometer based on detecting the power output of resonances from the core dip is demonstrated. The sensing probe comprises a compact structure, hereby a short section of specific core (with a significant core dip) fiber stub containing a straight fiber Bragg grating is spliced to another single-mode fiber via a core self-alignment process. The femtosecond laser side-illumination technique was utilized to ensure that the grating inscription region is precisely positioned and compact in size. Two well-defined core resonances were achieved in reflection: one originates from the core dip and the other originates from fiber core. The key point is that only one of these two reflective resonances exhibits a high sensitivity to fiber bend (and vibration), whereas the other is immune to it. For low frequency (2) measurement, the proposed sensor shows a much higher resolution (1.7 × 10−3 m/s2) by simply monitoring the total power output of the high-order core mode reflection. Moreover, the sensor simultaneously provides an inherent power reference to eliminate unwanted power fluctuations from the light source and transmission lines, thus providing a means of evaluating weak seismic wave at low frequency.

Surface Plasmon resonance (SPR) optical fiber sensors can be used as a cost-effective and relatively simple-to-implement alternative to well established bulky prism configurations for in-situ high sensitivity biochemical and electrochemical measurements. The miniaturized size and remote operation ability offer them a multitude of opportunities for single-point sensing in hard-to-reach spaces, even possibly in vivo. Grating-assisted and polarization control are two key properties of fiber-optic SPR sensors to achieve unprecedented sensitivities and limits of detection. 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. Such sensor provides an additional resonant mechanism of high-density narrow cladding mode spectral combs that overlap with the broader absorption of the surface Plasmon for high accuracy interrogation. In this paper, we briefly review our recent developments of plasmonic tilted fiber grating sensors, including the surface affinity studies of biomolecules for real life problems, electrochemical actives of electroactive biofilms for clean energy resources, the vector magnetic field measurement and the ultra-highly sensitive plasmonic sensing in gas.

An optical fiber twist sensor based on surface plasmon resonance (SPR) excitation and able to operate in both gaseous and liquid media is proposed and demonstrated. The use of a gold-coated 37.5° tilted fiber Bragg grating allows one to generate two windows of cladding mode resonances, providing surrounding refractive index sensitivity in two different external media. The relative optical power of the SPR mode in each case exhibits a linear response to the twist angle of the fiber from 0° to 90°, as well as from 90° to 180°, as a consequence of the polarization dependency of these sensors. The results show sensitivities of 1.15%/° and 1.22%/° with the sensor working in air and liquids, respectively, being suitable for deformation measurement and structural health monitoring applications.

A new configuration of Michelson fiber interferometer is proposed and demonstrated for refractive index (RI) and temperature sensing. The proposed configuration consists of a 2ntn2 3 dB coupler and a section of high birefringence (Hi-Bi) fiber. The Hi-Bi fiber is spliced to an output of the coupler, and its combining with the other output forms and the interference arms of the interferometer. For the spectral response of the proposed interferometer, the extinction ratio of an interference pattern corresponding to conventional Michelson fiber interferometer is modulated by a larger period induced by Birefringence in Hi-Bi fiber. This sensor shows a capacity for simultaneous measurement of liquid RI and temperature by detecting the fringe contrast variation and the wavelength shift, respectively.

A method based on a fiber-optic interferometer to study the pH-dependent interaction between graphene oxide and single-stranded DNA chains is carried out with the assistance of a scanning electron microscope and a confocal laser scanning microscope. The various wavelength shifts of the interferometric fringes of the transmission spectrum reveal the different strengths of interaction between graphene oxide and single-stranded DNA in various pH environments. The present work demonstrates the feasibility of optical fibers for studying the interaction between graphene oxide and biomolecules. It provides a potential means to understand the intermolecular interactions. This technique might effectively supplement the existing tools.

Surface plasmon polaritons (SPP) can be excited on metal-coated optical fibres, enabling the accurate monitoring of refractive index changes. Configurations reported so far mainly operate in liquids but not in air because of a mismatch between permittivities of guided light modes and the surrounding medium. Here we demonstrate a plasmonic optical fibre platform that overcomes this limitation. The underpinning of our work is a grating architecture—a gold-coated highly tilted Bragg grating—that excites a spectral comb of narrowband-cladding modes with effective indices near 1.0 and below. Using conventional spectral interrogation, we measure shifts of the SPP-matched resonances in response to static atmospheric pressure changes. A dynamic experiment conducted using a laser lined-up with an SPP-matched resonance demonstrates the ability to detect an acoustic wave with a resolution of 10−8 refractive index unit (RIU). We believe that this configuration opens research directions for highly sensitive plasmonic sensing in gas., Fibre sensors are key to many minimally-invasive detection techniques but, owing to an index mismatch, they are often limited to aqueous environments. Here, Caucheteur et al. develop a high-resolution fibre gas sensor with a tilted in-fibre grating that allows coupling to higher-order plasmon modes.

A sensitive Ag+ sensor based on nucleic acid probes modified silica microfiber interferometry is designed and developed. The probes on microfiber surface plays the part on catching Ag+ as tentacles, while their conformation change from random coils to hairpins. It induces the fiber surface refractive index change, which is captured by the optical fiber and translated into a significant wavelength shift in the interferometric fringe. Such a combination enables an improved concentration sensitivity of 0.22nm/log M and limit of detection of 1.36 × 10-9M, taking the advantage of real-time and in-situ analysis. It shows good selectivity in the present of many other metal ions and offers potential to analysis in real matrix, especially in the environmental samples must be analyzed in a short time. This may provide insights into the preparation of sensing platforms for optical quantification of other small molecular, supplementing the existing tools.

Spectroelectrochemistry has been found to be an efficient technique for revealing extracellular electron transfer (EET) mechanism of electroactive biofilms (EABs). Herein, we propose a novel electrochemical surface plasmon resonance (EC-SPR) optical fiber sensor for monitoring EABs in situ. The sensor uses a tilted fiber Bragg grating (TFBG) imprinted in a commercial single-mode fiber and coated with nanoscale gold film for high-efficiency SPR excitation. The wavelength shift of the surface plasmon resonance (SPR) over the fiber surface clearly identifies the electrochemical activity of the surface localized (adjacent to the electrode interface) bacterial cells in EABs, which differs from the "bulk" detections of the conventional electrochemical measurements. A close relationship between the variations of redox state of the EABs and the changes of the SPR under potentiostatic conditions has been achieved, pointing to a new way to study the EET mechanism of the EABs. Benefiting from its compact size, high sensitivity, and ease of use, together with remote operation ability, the proposed sensor opens up a multitude of opportunities for monitoring EABs in various hard-to-reach environments.

We propose a novel optical microfiber-loaded plasmonic TE-pass polarizer consisting of an optical microfiber placed on top of a silver substrate and demonstrate its performance both numerically by using the finite element method (FEM) and experimentally. The simulation results show that the loss in the fundamental TE mode is relatively low while at the same time the fundamental TM mode suffers from a large metal dissipation loss induced by excitation of the microfiber-loaded surface plasmonic mode. The microfiber was fabricated using the standard microheater brushing–tapering technique. The measured extinction ratio over the range of the C-band wavelengths is greater than 20 dB for the polarizer with a microfiber diameter of 4 µm, which agrees well with the simulation results.

We propose and demonstrate hydrogen peroxide (H2O2) and glucose concentration measurements using a plasmonic optical fiber sensor. The sensor utilizes a tilted fiber Bragg grating (TFBG) written in standard single mode communication fiber. The fiber is over coated with an nm-scale film of silver that supports surface plasmon resonances (SPRs). Such a tilted grating SPR structure provides a high density of narrow spectral resonances (Q-factor about 105) that overlap with the broader absorption band of the surface plasmon waves in the silver film, thereby providing an accurate tool to measure small shifts of the plasmon resonance frequencies. The H2O2 to be detected acts as an oxidant to etch the silver film, which has the effect of gradually decreasing the SPR attenuation. The etching rate of the silver film shows a clear relationship with the H2O2 concentration so that monitoring the progressively increasing attenuation of a selected surface plasmon resonance over a few minutes enables us to measure the H2O2 concentration with a limit of detection of 0.2 μM. Furthermore, the proposed method can be applied to the determination of glucose in human serum for a concentration range from 0 to 12 mM (within the physiological range of 3-8 mM) by monitoring the H2O2 produced by an enzymatic oxidation process. The sensor does not require accurate temperature control because of the inherent temperature insensitivity of TFBG devices referenced to the core mode resonance. A gold mirror coated on the fiber allows the sensor to work in reflection, which will facilitate the integration of the sensor with a hypodermic needle for in vitro measurements. The present study shows that Ag-coated TFBG-SPR can be applied as a promising type of sensing probe for optical detection of H2O2 and glucose detection in human serum.

A technique for temperature-insensitive force measurement via strain-induced Gaussian-strain-chirped fiber Bragg grating (FBG) couple and differential optical power detection is proposed and experimentally demonstrated. For a temperature range from -10degC to 80degC , a linear force measurement with thermal fluctuations less than 2.5% full-scale is achieved by simply using cost-effective p-i-n photodiodes without any temperature compensation.

Based on differential intensity measurement between the cut-off and Plasmon resonances of an Au-coated tilted fiber Bragg grating, refractometer with RI sensitivity of 920 dB/RIU over the range of 1.332 to 1.357 has been achieved.

A technique for temperature-insensitive force measurement using a single fiber Bragg grating (FBG) based on strain-gradient-induced reflection spectrum-bandwidth modulation and optical-power detection is demonstrated. A specially designed bending cantilever beam (BCB) is used to induce axial-strain gradient along the sensing FBG, resulting in a Bragg bandwidth modulation. The broadening of the FBG spectrum bandwidth and reflection optical power linearly change with the applied force, and both of them are insensitive to spatially uniform temperature variations. For a temperature range from 20 degC to 80 degC, a linear response of force measurement up to 20 N with fluctuation less than 0.8% full-scale is achieved without any temperature compensation. The demodulation process is simplified by optical-power detection via a p-i-n photodiode, and the sensing system is potentially cost-effective