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A broadband vibration sensor using reflected excessively tilted fiber grating (ExTFG) with clamped beam was proposed. The two ends of reflected ExTFG were fixed, and the fiber grating part was suspended, thus forming a clamped beam structure. The resonance frequency and vibration mode of the proposed sensor were calculated and analyzed by numerical calculation and software simulation. Axial strain, acceleration response and vibration frequency detection experiments were designed and conducted. Experimental results showed that the axial strain sensitivity of TE/TM mode was −2.57 pm/ $\mu \varepsilon $ and −1.6 pm/ $\mu \varepsilon $ , respectively. The first-order resonant frequency of the sensor was 1900Hz with a flat region ranging from 100 Hz to 800 Hz, and the maximum acceleration sensitivity of TE/TM mode in the flat region was 38.79 mV/g and 31.43 mV/g at 800Hz, respectively. In addition, it could detect the vibration frequency range of 100–10000 Hz, and provide a highest signal-to-noise ratios (SNR) of 55.1 dB at 1900 Hz. The proposed sensor possessed a wide response frequency range, and was able to realize dual-parameter detection for acceleration and vibration frequency, which would make its good application prospects in the mechanical vibration detection for large-scale projects.

In the present work, we introduced a highly sensitive vibration sensor, which is based on the dispersion turning point (DTP) microfiber Mach-Zehnder interferometer. The axial strain and vibration sensing characteristics of the microfiber Mach-Zehnder interferometer were investigated. First, we theoretically analyzed the spectrum evolution characteristics of the microfiber Mach-Zehnder interferometer caused by axial strain. Second, the microfiber with different diameters was fabricated using the electrode discharge and fused taper method, and the axial strain experiments were conducted; the maximum sensitivity of the DTP microfiber with a diameter of ∼2.2 µm reached -45.55 pm/µɛ at ∼1550 nm. Finally, based on the axial strain principle of the microfiber, we designed a highly sensitive vibration sensor using a DTP microfiber integrated into a rectangular through-hole cantilever beam. The 30-3500 Hz vibration signal monitoring could be realized, the maximum signal-to-noise ratio (SNR) was ∼75 dB at 52 Hz, and the acceleration sensitivity reached as high as 0.764 V/g at 45Hz. These results suggested the high performance of the microfiber in axial strain and micro-vibration sensing fields.

A simple U-shaped plastic optical fiber evanescent-wave sensor was fabricated for temperature-independent highly sensitive detection of humidity. The sensing region of the sensor was subjected to five cycles of heating–cooling to improve temperature independence. The effects of the polyimide (PI) coating thickness, number of graphene oxide (GO) coating layers, and alternate PI–GO coating sequence were investigated to optimize sensitivity. The fabricated sensor exhibited high-temperature independence and good sensitivity of 0.17 × 10 − 2 (% relative h u m i d i t y ) − 1 in the temperature range of 10°C to 70°C.

An orientation-dependent optic-fiber accelerometer based on the excessively tilted fiber grating (ExTFG) inscribed in SM28 fiber is demonstrated, which is based on the optical power demodulation scheme. Without any complicated processing, the cladding mode resonances of the bare ExTFG show high sensitivity to slight perturbation of bending. Due to its excellent azimuth-related bending properties, such a bare ExTFG fixed on a simple cantilever beam has exhibited strong orientation-dependent vibration properties. The experimental results show that a TE mode of the sensor can provide a maximum acceleration sensitivity of 74.14 mV/g at 72 Hz and maximum orientation sensitivity of 9.1 mV/deg while, for a TM mode, a maximum acceleration sensitivity of 57.85 mV/g at 72 Hz and maximum orientation sensitivity of 7.4 mV/deg could be achieved. These unique properties enable the sensor to act as a vector accelerometer for applications in many vibration measurement fields.

We study the method to improve the acceleration sensitivity of excessively tilted fiber grating (ExTFG) vibration sensors. The results showed that the acceleration sensitivity of the ExTFG would increase with the decrease of fiber cladding.

We report the bending strain and low frequency vibration characteristics of few-layers graphene-coated excessively tilted fiber grating (ExTFG), which enhanced the mechanical characteristics at the expense of sensitivity. The experimental results show that, in the curvature range of 0~0.5 m-1, bending strain sensitivities of transverse electric (TE) and transverse magnetic (TM) modes based on wavelength variation are -2.78 nm/m-1 and -1.49 nm/m-1, respectively, and those based on intensity variation are -0.11 dB/m-1 and -0.24 dB/m-1, respectively. For the low frequency vibration, the maximum acceleration sensitivities of TE and TM modes at 3dB points are S TE =46.64 mv/g and S TM =73.84 mv/g, respectively, which also the larger than those of other wavelength positions. The proposed graphene-coated ExTFG may endow potential application prospects in optical fiber strain, torsion and vibration sensors.

An optic-fiber vibration sensor based on the reflected 81° tilted fiber grating (81° TFG) integrated with a symmetrical flexible hinge is proposed and experimentally demonstrated in this paper. The vibration sensor is composed of a symmetrical flexible hinge and a reflected 81° TFG, the ends of which are simply fixed on the upper surface of the mass. The theoretical model of the proposed vibration sensor is analyzed, by which the important parameters related to the resonant frequency of the sensor are simulated and discussed; then, the vibration sensing experiments are conducted. Experiment results show that TE/TM mode of the 81° TFG can provide the maximal acceleration sensitivity of 338.28 and 299.94 mV/g at 400 Hz in the flat area of the amplitude-frequency response (50–400 Hz), which is increased by 9.95 and 11.5 times as compared with the optical fiber cantilever beam structure, respectively. Further, the signal-to-noise ratio in the flat area (50–400 Hz) is about ∼ 66.275 d B under the acceleration of 2 g, which is increased by ∼ 20 d B . Furthermore, it can be used for detecting mechanical vibration of medium-high frequency ranging from 50 to 3500 Hz. The proposed 81° TFG vibration sensor has the characteristics of small volume, simple package, high acceleration sensitivity, and wide vibration signal response range, which will ensure it has broad application prospects in the field of mechanical vibration.

We report an immunosensing platform with a large detection range using an excessively tilted fiber grating (ExTFG) coated with graphene oxide (GO). ExTFG was inscribed in standard single-mode fiber; GO film was coated on the fiber surface through hydrogen bond. The effectiveness and uniformity of GO deposited on the ExTFG surface were investigated by field emission scanning electron microscopy and energy spectrum method. Bovine serum albumin (BSA) monoclonal antibodies (MAbs) were used as biometric units to link the GO film through a covalent bond for the specific detection of BSA, so as to evaluate the performances of the proposed biosensor. The whole dynamic immobilization process of BSA MAbs and BSA detection were observed by the spectral evolution of the sensor. Experimental results show that the fabricated GO-coated ExTFG biosensor has a large detection range from 1.5 nM–75 nM and fast response for BSA antigen; the limit of detection is ∼0.88 nM by using an optical spectrum analyzer with a resolution of 0.03 nm, and the dissociation constant KD and the affinity constant KA are calculated to be ∼6.66×10−9 M and ∼1.5×108 M−1, respectively. The proposed GO-coated ExTFG immunosensing platform could lay a foundation for the specific detection of other biomolecules.

We investigated the spectral and refractive index (RI) sensing characteristics of the excessively tilted fiber grating (Ex-TFG) with different cladding diameters. The Ex-TFG is inscribed in standard single-mode fiber, and the cladding reduces from 125 μm to around 15 μm by the chemical etching method. Experimental results show that the number of cladding modes decreases, and the spacing of adjacent resonance peaks becomes larger and larger with the reduction of the cladding diameter in the observed wavelength range of 1250-1650 nm. The average RI sensitivity in the index region of 1.33-1.38, the one near 1.33, and the one at around 1.38 of the etched Ex-TFG with a diameter of 15 μm is ∼6.3, ∼5.3, and ∼6.67 fold compared to those of the no-etched Ex-TFG, respectively. Also, the RI sensing performances of the etched Ex-TFG with a diameter smaller than 30 μm are better than those of the Ex-TFG inscribed in SM1500 (4.2 μm/80 μm) fiber in the index region of 1.33. The proposed micronano Ex-TFG has higher RI sensitivity and a more compact structure in biosensing applications, compared to the standard Ex-TFGs and Ex-TFGs inscribed in SM1500 fiber.

We present a refractive index (RI) distribution sensor based on an etched fiber Bragg grating (FBG). The FBG is etched by hydrofluoric acid solution until the residual diameter of its core is about 5.73 μm, and then it is installed in a micro-tube where the RI of the liquid medium decreases linearly along its axis. The reflection spectrum of the etched FBG changes from a single resonant peak into many micro-resonant peaks under the inhomogeneous liquid medium. The difference of the wavelengths between the maximum micro-resonant peak and the minimum one changes linearly with the RI gradient, and the average change of the wavelengths of them changes linearly with the average change of the RI as well. Experimental results show that the sensitivity of RI gradient and the average sensitivity of RI for this etched FBG sensor is about 336.2 nm mm/RIU and 49.44 nm/RIU, respectively, when the surrounding RI ranges from 1.330 to 1.3586. Associated with the sensitivity of the RI gradient and the average sensitivity of the RI, the linear RI distribution of inhomogeneous liquid medium surrounding the etched FBG can be obtained.

Although the numerous advantages of polymer optical fiber (POF) sensors have been applied in different fields, the measurement consistency and sensitivity of POF evanescent wave (EW) sensors are still affected by its thermal stability and water absorption. Therefore, we perform a study to demonstrate the mechanism of the effect of heat treatments on physical and optical properties of POF EW sensors. We investigate the surface morphology, composition, refractive index, geometry, and weight of the fiber-sensing region subjected to water and vacuum heat treatments. We examine the spectral transmission and transmitted light intensity of POF sensors. We present a theoretical investigation of the effect of heat treatments on the sensitivity of POF EW sensors. The performance of the prepared sensor is evaluated using glucose and Chlorella pyrenoidosa analytes. We discovered that the spectral transmission and transmitted light intensity of the fibers shows little effect of vacuum heat treatments. In particular, the sensors, which subject to vacuum heat treatment at 110 °C for 3 h, exhibit temperature-independent measuring consistency and high sensitivity in glucose solutions in the temperature range 15–60 °C and also show high sensitivity in Chlorella pyrenoidosa solutions.

In this paper, we present a high-sensitivity polymer fiber-optic evanescent wave (FOEW) sensor with a three-layer structure that includes bottom, inter-, and surface layers in the sensing region. The bottom layer and inter-layer are POFs composed of standard cladding and the core of the plastic optical fiber, and the surface layer is made of dilute Canada balsam in xylene doped with GeO2. We examine the morphology of the doped GeO2, the refractive index and composition of the surface layer and the surface luminous properties of the sensing region. We investigate the effects of the content and morphology of the GeO2 particles on the sensitivity of the FOEW sensors by using glucose solutions. In addition, we examine the response of sensors incubated with staphylococcal protein A plus mouse IgG isotype to goat anti-mouse IgG solutions. Results indicate very good sensitivity of the three-layer FOEW sensor, which showed a 3.91-fold improvement in the detection of the target antibody relative to a conventional sensor with a core-cladding structure, and the novel sensor showed a lower limit of detection of 0.2ng/l and a response time around 320s. The application of this high-sensitivity FOEW sensor can be extended to biodefense, disease diagnosis, biomedical and biochemical analysis.

In this work, the theoretical model of FBG when it is affected by double external variables (temperature and transverse pressure) and triple external variables (temperature, axial strain and transverse pressure) is deduced, respectively. The simulated results show that the properties of cross sensitivity would change in a certain regulation with the external effects and fiber parameters. At the same time, some valuable suggestions about how to improve the measurement accuracy are put forward as well.

A fiber-optic differential absorption sensor was developed to accurately monitor biomass growth in a photobioreactor. The prepared sensor consists of two probes: the sensor and the reference. The sensor probe was employed to monitor the biomass and changes in the liquid-phase concentration in a culture. To separate the liquids from photosynthetic bacteria CQK 01 and measure the liquid-phase concentration, a proposed polyimide-silica hybrid membrane was coated on the sensing region of the reference probe. A linear relationship was observed between the sensor output signal and the biomass from the lag phase to the decline phase.

We demonstrate a fiber refractive index (RI) sensor based on an excessively tilted fiber grating (ExTFG) immobilized by large-size plasmonic gold nanoshells (GNSs). The GNSs are covalently linked on ExTFG surface. Experimental results demonstrate that both the intensity of the transverse magnetic (TM) and transverse electric (TE) modes of ExTFG are significantly modulated by the localized surface plasmon resonance (LSPR) of GNSs due to the wide-range absorption band. The wavelength RI sensitivities of the TM and TE modes in the low RI range of 1.333–1.379 are improved by ∼25% and ∼14% after GNSs immobilization, respectively, and the intensity RI sensitivities are ∼599%/RIU and ∼486%/RIU, respectively.

In this study, a simple wet etching technique is developed by employing aqueous solutions of acetic acid and ultrasonic irradiation for the fabrication of a high-quality plastic optical fiber (POF) sensor. The effects of acetic acid concentration and temperature and ultrasonic power on the etching rate and surface morphology of the etched POFs are investigated. The transmission spectrum and sensitivity of the etched POF sensors are evaluated using glucose solutions. We discovered that the POF sensors, which are fabricated using an aqueous solution of acetic acid with a concentration of 80 vol. % under an ultrasonic power of 130 W and temperature of 25°C, exhibit good light transmission and a high sensitivity of 9.10 [(RIU)(g/L)]−1 in the glucose solutions.

In this work, we present a luminous-exothermic hollow optical element (LEHOE) that performs spectral beam splitting in the visible spectral range for the enhancement of biofilm growth and activity. The LEHOE is composed of a four-layer structure with a fiber core (air), cladding (SiOsub2/sub), coating I (LaBsub6/subfilm), and coating II (SiOsub2/sub-Agarose-Medium film). To clarify the physical, optical and photothermal conversion properties of the LEHOE, we determined the surface morphology and composition of the coating materials, and examined the luminous intensity and heating rate at the LEHOE surface. The biofilm activity on the biocompatible LEHOE is far greater than that of commercial fibers, and the biofilm weight on the LEHOE is 4.5 × that of the uncoated hollow optical element.

A microstructured fiber Bragg grating (MSFBG) was created to accurately and simultaneously monitor the cell growth of photosynthetic bacteria (PSB) Rhodopseudomonas palustris CQK 01 and the temperature in a photobioreactor. The proposed sensor was made from an FBG unit that was separated into three regions, an unperturbed region, and two etched regions with smooth surfaces. The unperturbed grating region was employed to monitor the temperature. To eliminate the effects of the liquid concentration and temperature on the biomass, a polyimide-silica hybrid membrane was created and coated on an etched grating region to separate the liquids from the PSB; that is, this thinned region was developed to analyze the liquid concentration and temperature. Another etched grating region with a smaller diameter was used to determine the response to the temperature, biomass, and liquid concentration. In addition, two models were also presented to demonstrate accurate simultaneous measurement of the biomass and temperature. We discovered that the MSFBG sensor can rapidly and accurately determine the difference in the Bragg wavelength shifts caused by changes in the temperature, biomass, and liquid-phase concentration. The measured biomass is highly correlated with the real cell growth, with a correlation of 0.9438; the hydrogen production rate and temperature difference from metabolic heat production reached 1.97 mmol/L/h and 2.8 °C, respectively, in the PSB culture.

We present the mechanism of refractive index (RI) distribution measurement based on an etched fiber Bragg grating (FBG) with a residual diameter of about 5.73 μm. The etched FBG is installed in a micro-tube where the RI of the liquid medium decreases linearly alone its axis. Experimental results show that the sensitivity of RI gradient and the average sensitivity of RI of this sensor is 336.2 nm·mm/RIU and 49.44 nm/RIU, respectively, when the RI ranging from 1.330 to 1.3586. The RI distribution of the inhomogeneous liquid medium can be obtained by the RI gradient sensitivity and the average RI sensitivity.

We demonstrate a structured fiber Bragg grating (FBG) sensor for the measurement of the refractive-index (RI) gradient of the inhomogeneous liquid medium. The double-layer fiber waveguide models and the transfer matrix method are utilized to calculate the reflection spectrum of structured FBG under inhomogeneous liquid medium, the refractive-index (RI) distribution of which is linear and range from 1.330 to 1.360, showing that the reflection spectrum of the structured FBG would split into several tiny resonant peaks, and the D-value of the wavelengths of the tow resonant peaks at the tow sides of reflection spectrum was approximately proportional to the RI gradient. In the experiment, a FBG was etched to a diameter of about 5.73μm for the RI gradient measurement of the diffusion process of the sucrose solution in the micro channel, the experimental results conformed the rationality of the simulated results, indicating the possibility of the RI gradient measurement by using the structured FBG sensors.

Based on the optical waveguide theory and the FBG mode coupling theory, a rigorous analysis of the reflectivity of the fiber Bragg grating (FBG) with different diameters were presented. In conditions of cladding etched and core etched, the power attenuation properties of the FBG based on evanescent wave were analyzed. The normalized reflectivity of the FBG, which was the function of the surrounding refractive index (SRI) and the diameter of the FBG, was derived. The theoretical results exhibited that the normalized reflectivity become smaller as the increase of the SRI, and the deeper the FBG was etched, the smaller the normalized reflectivity was. Thus, we can make the conclusion that both the increase of the SRI (small than the cladding refractive index) and the etched depth can reduce the intensity of light propagating in the fiber and enhance the interaction between the evanescent wave and the surrounding medium, leading the sensitivity of the sensor to increase. In addition, theoretical simulation showed good consistency with the experimental characterization. The proposed study of the normalized reflectivity properties of FBG based on evanescent wave is significant in improving the sensitivity of FBG.

Based on the fiber waveguide models, a modified transfer matrix method was utilized to calculate the reflection spectrum of the thinned fiber Bragg grating (ThFBG) under the uneven surrounding refractive index (SRI) environment. Tow SRI ranges, including the high SRI region (from 1.42 to the fiber cladding index) and the low ones (from 1.33 to about 1.36), were considered. Numerical results showed that the responsive characteristics of the reflectance spectrum of the ThFBG were closely related to the properties of the SRI distribution, first, the original reflection spectrum of the ThFBG would split into many tinny resonant peaks and the reflectance spectrums are asymmetric since the uneven SRI distributions, second, the number of the resonant peaks, the decline of the amplitude, and the degree of the asymmetric of the reflectance spectrums would increase as the increase in the SRI gradient and the D-value of the SRI between the tow ends of the ThFBG. The same numerical approach could be used to analyze the responsive characteristics of the ThFBG under the uneven medium environment where the SRI distribution was any other functions.

The characteristics of Bragg wavelength movement in the core-etched process of fiber Bragg grating (FBG) are theoretically calculated and analyzed under several different conditions. The numerical analysis shows that, the Bragg wavelength decreases non-linearly and more and more faster as the decrease in core diameter, furthermore, the decreasing speed and decreasing degree in Bragg wavelength are higher for the core-etched FBG with a lower surrounding refractive index (SRI) than that with a higher SRI. In addition, the variation of Bragg wavelengths are almost identical in the core-etched FBG of different grating period when other conditions being the same. These theoretical results would provide valuable guidance to the research on the fabrication and applications of core-etched FBG refractive index sensors by hydrogen fluoride (HF) solution etching.

The applications of Fiber Bragg grating (FBG) sensors have been concerned considerably for many years. However, it is usually difficult to obtain an accurate result by single FBG in practical applications, since the phenomenon of cross sensitivity caused by the multi-external fields always exists. In the past, most of researchers presented their studies only on the cross sensitivity of two parameters including temperature and axial strain, temperature and transverse pressure, and so on. Actually, FBG sensors might be affected by temperature field, axial strain field and transverse pressure field simultaneously in some applications, but no previous analysis has been presented on the cross sensitivity caused by these three parameters. This work extends the previous works on the cross sensitivity of two parameters to that of three parameters in a FBG sensor. The sensing model of FBG in three external fields is deduced and analyzed theoretically from the perspective of its physical mechanism, through which the expression of the cross sensitivity function is obtained. Meanwhile, examples of different external physical factors and fiber parameters are considered for illustration purposes, and the cross sensitivity term is discussed by the numerical simulation. The simulated results show that the properties of cross sensitivity would change in a certain regulation with the changes of external effects and fiber parameters. Compared to the two-parameter analyzing method, this work makes a more comprehensive analysis to the research object. Some valuable suggestions about how to reduce the systematic error and improve the measurement accuracy are put forward as well.

We report a highly sensitive, high Q-factor, label free and selective glucose sensor by using excessively tilted fiber grating (Ex-TFG) inscribed in the thin-cladding optical fiber (TCOF). Glucose oxidase (GOD) was covalently immobilized on optical fiber surface and the effectiveness of GOD immobilization was investigated by the fluorescence microscopy and highly accurate spectral interrogation method. In contrast to the long period grating (LPG) and optical fiber (OF) surface Plasmon resonance (SPR) based glucose sensors, the Ex-TFG configuration has merits of nearly independent cross sensitivity of the environmental temperature, simple fabrication method (no noble metal deposition or cladding etching) and high detection accuracy (or Q-factor). Our experimental results have shown that Ex-TFG in TCOF based sensor has a reliable and fast detection for the glucose concentration as low as 0.1~2.5mg/ml and a high sensitivity of ~1.514 nm·(mg/ml)⁻¹, which the detection accuracy is ~0.2857 nm⁻¹ at pH 5.2, and the limit of detection (LOD) is 0.013~0.02 mg/ml at the pH range of 5.2~7.4 by using an optical spectrum analyzer with a resolution of 0.02 nm.

According to the Lambert-Bee law, we can see that the photic absorption coefficient is related to the matter's concentration, the distance of the light through the absorption medium and the transmitted light intensity. The paper just according to the physical phenomena and the theory make the reflex energy relate to the concentration testing of the electrolyte, at the same time the electrolyte's concentration is related to the capacity of lead-acid battery on a corresponding function relation, so we can know the capacity state of the lead-acid battery according to the measurement on the electrolyte's concentration. According to the experiment and research the author deeply discussed how the temperature change affects the capacity of lead-acid battery and the concentration's changing relation, according to the analyses of the thermo-optic effect, we made a new reflecting fiber sensor based on the comparative temperature testing theory and absorption which can eliminate the temperature effect on the tested signal namely the output signal just related to the concentration, so really reflects the change of the capacity of the lead-acid battery when it is in the charge and discharge process. The results of the experiment and theory analyses show that this method is easy to realize the online testing of the capacity of lead-acid battery. This sensor has many merits such as precise measurement, sensitive reaction, long-life use etc. It can be widely used in the electric capacity testing of the automobile lead-acid battery, the electric capacity testing of the industry lead-acid battery, liquor's concentration testing and salinity testing of the sea and have a bright future.

Refractive index measurement by optical fiber sensors has proved to be effective in the research of biochemical and biomedical applications. The theoretical principles and technology underlying several microstructured fiber Bragg grating refractive index sensors developed over the past decade are classified and briefly summarized, and their future developments are considered.