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Plasmonic tilted fiber Bragg gratings (TFBGs) have emerged as versatile tools for refractometric analyses and biochemical sensing. Their applications have significantly blossomed these last years, from proteins and cellular bioassays to operando monitoring in batteries, to cite just a few. They are widely recognized for their cutting‐edge performance and low limits of detection, arising from their dense multimodal spectral nature featuring tens of narrowband cladding mode resonances. Their comb‐like spectrum is so rich that numerous demodulation techniques have been reported, without benchmark of their relative performance while they possess important distinctions. This review highlights developments in detangling techniques from the pioneering works based on single‐peak analysis up to the most recent approaches involving Fourier analysis, the implementation of machine learning, and cascaded spectral decomposition processes. To fairly compare the different techniques of the literature, we implemented each analysis on original experimental refractometric calibrations, revealing the assets of the most updated methods. This paper therefore reviews these demodulation techniques based on the same datasets, obtained under the same conditions. We show and discuss the results obtained from bioassays and pinpoint the importance of advanced analytical methodologies to maximize the reproducibility, reliability and performance of plasmonic‐based TFBGs biosensors. [ABSTRACT FROM AUTHOR]

The presence of nitrite, a prevalent contaminant in natural environments, presents a significant environmental and human health concern. Hence, it is imperative to develop a sensor with the ability to quantitatively detect nitrite. This study focuses on the design and development of i) probe 1: tilted fiber Bragg gratings (TFBGs) and ii) probe 2: fiber optic tip-based plasmonic sensors utilizing ion-imprinted polymers. The concentration of nitrite was assessed at various levels using both sensing configurations. The outcomes indicated that the TFBGs-based sensor exhibited a sensitivity and limit of detection (LOD) of 0.469 nm/ln(μg/mL) and 0.142 μg/mL in the linear detection range of 0.5–50 μg/mL. The fiber optic tip-based sensor exhibited a sensitivity and LOD of 1.16 nm/ln(μg/mL) and 0.176 μg/mL within the 1–50 μg/mL linear detection range. The obtained sensing results reveal that the sensors presented in this study are able to accurately detect nitrite at various concentrations in a quantitative manner. Moreover, an assessment was conducted to examine the selectivity and reusability of the sensor individually, yielding satisfactory results.

This article proposes an unequivocal method of labeling and numbering the cladding modes propagating in single-mode opticalwaveguides with tilted periodic structures. The unambiguous determination of individual propagating modes in this type of optical fiber is crucial for their use in sensory systems. The selection of the appropriate spectral range and mode determines the sensitivity and measuring range of tilted fiber Bragg grating (TFBG) sensors. The measurement methods proposed by individual research teams using TFBGs as transducers are usually based on the selection of specific modes. Unification of the labeling of modes and their numbering enables comparison of the basic metrological parameters of individual measurement methods and reproduction and verification of the proposed sensors and methods in the laboratories of other scientific and research centers.

Featured Application: The research results could find applications in many areas where measurement of bending radius or distance between elements is required in harsh environments with explosion danger or in flammable gases atmospheres. The use of optical fibers and light as a information medium eliminates the danger of sparks and explosions as a results. In this article we deal with the influence of polarization on determining the bending radius in TFBG (tilted fiber Bragg grating) sensors. The spectrum of TFBG structures changes under the influence of many factors at the same time. In the case of bending radius measurements, additional factors may be the temperature and polarization state of the introduced light. So far, only the cladding mode envelope algorithm has been used to determine the bending radius. An interesting issue seems to be establishing the influence of cross-sensitivity of the spectrum on changes in polarization during bending measurements. In addition to the envelope algorithm, algorithms for spectral length and average deviation from the local mean were examined. As a result of the analysis of experimental data, it was found that the level of polarization's influence on the result may be significant in determining the bending radius. Reducing the influence of polarization can be achieved by using algorithms providing wavelength parameters. Additionally, in order to reduce the influence of polarization, we proposed the use of the PLS (partial least squares regression) algorithm for the processed spectrum. [ABSTRACT FROM AUTHOR]

This article proposes an unequivocal method of labeling and numbering the cladding modes propagating in single-mode opticalwaveguides with tilted periodic structures. The unambiguous determination of individual propagating modes in this type of optical fiber is crucial for their use in sensory systems. The selection of the appropriate spectral range and mode determines the sensitivity and measuring range of tilted fiber Bragg grating (TFBG) sensors. The measurement methods proposed by individual research teams using TFBGs as transducers are usually based on the selection of specific modes. Unification of the labeling of modes and their numbering enables comparison of the basic metrological parameters of individual measurement methods and reproduction and verification of the proposed sensors and methods in the laboratories of other scientific and research centers. [ABSTRACT FROM AUTHOR]

In this paper, we proposed a high-sensitive glucose sensor using the tilted fiber Bragg gratings (TFBGs). The TFBGs with different tilt angle has been fabricated by phase mask scanning method with an argon ion double-frequency 244 nm laser. As tilt angle increases, the bandwidth of the cladding mode excited by TFBGs gradually expands and cladding mode region shifts towards shorter wavelengths. The higher-order cladding modes appeared at the shorter wavelength side demonstrate a larger separation in the spectrum. TFBGs with different tilt angles for glucose concentration measurement have been investigated experimentally. When the concentration of glucose solution increases, the surrounding refractive index changing make the cut-off mode resonance shift to a longer wavelength. In addition, the tilt angle of TFBGs does affect the glucose sensitivity, that the larger tilt angle is, the higher sensitivity could achieve. In this work, a maximum glucose sensitivity of 80.91 pm/(mg/ml) could be obtained for TFBGs with tilt angle of 10°. The proposed TFBGs with superiorities of high glucose sensitivity and wider detection range, makes it potential for glucose detection in the field of chemical and biomedical sensing. • A high-sensitive glucose sensor based on tilted fiber Bragg gratings (TFBGs) is proposed, with detailed spectral analysis. • The cut-off mode resonance shows a red-shift with glucose concentration increasing, proving the potential in glucose detection. • The glucose sensitivity of TFBGs varies with tilt angle, which could achieve 80.91 pm/(mg/ml) with 10° TFBGs. [ABSTRACT FROM AUTHOR]

In this article we deal with the influence of polarization on determining the bending radius in TFBG (tilted fiber Bragg grating) sensors. The spectrum of TFBG structures changes under the influence of many factors at the same time. In the case of bending radius measurements, additional factors may be the temperature and polarization state of the introduced light. So far, only the cladding mode envelope algorithm has been used to determine the bending radius. An interesting issue seems to be establishing the influence of cross-sensitivity of the spectrum on changes in polarization during bending measurements. In addition to the envelope algorithm, algorithms for spectral length and average deviation from the local mean were examined. As a result of the analysis of experimental data, it was found that the level of polarization’s influence on the result may be significant in determining the bending radius. Reducing the influence of polarization can be achieved by using algorithms providing wavelength parameters. Additionally, in order to reduce the influence of polarization, we proposed the use of the PLS (partial least squares regression) algorithm for the processed spectrum.

Surface plasmon (SP) excitation in metal-coated tilted fiber Bragg gratings (TFBGs) has been a focal point for highly sensitive surface biosensing. Previous efforts focused on uniform metal layer deposition around the TFBG cross section and temperature self-compensation with the Bragg mode, requiring both careful control of the core-guided light polarization and interrogation over most of the C + L bands. To circumvent these two important practical limitations, we studied and developed an original platform based on partially coated TFBGs. The partial metal layer enables the generation of dual-comb resonances, encompassing highly sensitive (TM/EH mode families) and highly insensitive (TE/HE mode families) components in unpolarized transmission spectra. The interleaved comb of insensitive modes acts as wavelength and power references within the same spectral region as the SP-active modes. Despite reduced fabrication and measurement complexity, refractometric accuracy is not compromised through statistical averaging over seven individual resonances within a narrowband window of 10 nm. Consequently, measuring spectra over 60 nm is no longer needed to compensate for small temperature or power fluctuations. This sensing platform brings the following important practical assets: (1) a simpler fabrication process, (2) no need for polarization control, (3) limited bandwidth interrogation, and (4) maintained refractometric accuracy, which makes it a true game changer in the ever-growing plasmonic sensing domain.

Gold-coated tilted fiber Bragg gratings (TFBGs) have been highly studied over the last years, mainly for biosensing purposes. They present a comb-like spectrum of narrow-band cladding mode resonances that is often demodulated by tracking the change of a selected peak. In this paper, we report a twentyfold more sensitive demodulation method based on the intersection of the upper and lower envelopes of gold-coated TFBG spectra. This method has been successfully applied in biosensing experiments towards the detection of HER2 (Human Epidermal Growth Factor Receptor-2) proteins, a relevant biomarker for breast cancer. Practical improvements have also been implemented. First, a uniform FBG has been superimposed on the TFBG to reduce the read-out wavelength span to 10 nm instead of 70 nm while keeping the temperature-compensated measurements. Second, a micro-fluidic system has been implemented to smoothly deliver the samples to the sensor. These 3 originalities make this sensing platform even more attractive for use in practical applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study proposes a refractive index sensor based on tilted fiber Bragg gratings (TFBGs) that connect metal-reflective spherical ends. The proposed fabrication method enables it to reflect light to increase the overall energy spectrum. We used the end of the fiber with a spherical shape and the coating reflective metal to compare the reflective energy in TFBGs. The reflective technique was employed to deposit nickel (Ni) and silver (Ag) onto the surface of the spherical end of TFBGs to precisely form the spherical-shaped tilted fiber Bragg gratings (STFBGs). The Ni-covered NiSTFBG sensor had an average sensitivity of 8.028 nm/RIU wavelength, which is a 1.64-fold improveme nt. The result shows the backward reflected wave is reflected by the metal layer has improved performance for the wavelength and transmission loss via the reflected metal spherical-shaped end of tilted fiber Bragg gratings. [ABSTRACT FROM AUTHOR]

• Machine learning approach for automatic estimation of responses using different TFBGs. • Estimation based on the optical spectral characteristics of TFBGs as a function of RI. • Root mean squared errors as small as 0.008 RIU for the trained samples. • The approach can be used to enhance the reproducibility of large set of TFBG sensors. This paper presents the development of an approach for automatic estimation of the regression coefficients as a function of the refractive index (RI) in Tilted Fiber Bragg Gratings (TFBGs) based on their optical spectral characteristics. The methodology is based on machine learning approach, where different samples are analyzed in various test conditions to obtain a dataset for the training of a kNN regression algorithm. In this case, the spectral characteristics related to the wavelength spacing between adjacent cladding mode resonances, number of valleys and amplitude of the normalized spectra are correlated with the linear regression algorithm for RI estimation using the machine learning approach. The analysis are performed as a function of the wavelength shift and optical power, considering two regions on the transmitted spectrum and the envelope of the curve. Furthermore, the number of valleys and the area of the envelope curve are also analyzed as a function of the refractive index, which result in six analyzed features. The Principal Components Analysis (PCA) is used to reduce the number of features to two principal components. The results of the proposed algorithm are obtained with different trained and untrained samples, which indicates a root mean squared error (RMSE) of around 0.003 RIU for the trained samples and 0.008 RIU for the untrained ones. Therefore, the results indicate the usefulness of the proposed approach, which can be used to enhance the reproducibility of large set of TFBG sensors in practical applications. [ABSTRACT FROM AUTHOR]

The coupling of less than 80 µW of in‐plane polarized near‐infrared light in a 6 nm‐thick graphene layer deposited on an optical fiber produces important permittivity changes leading to bistability and self‐starting 50% modulation of over 1 W of continuous wave light in the core. These features arise from resonant coupling of core‐guided light into the cladding by a 12‐degree tilted, 1 cm long fiber Bragg grating via narrowband, polarization‐dependent resonances that allow the selection of cladding modes with electric fields polarized in the plane of the graphene. The pulse repetition rate of the modulation increases from 10 to 269 Hz for input powers ranging from 0.3 to 1.33 W in the core, with no evidence of saturation. Investigations into the origin of these effects through physical modelling and different experimental conditions point to photo‐induced Joule heating in the graphene layer giving rise to temperature increases of the order of 60 °C and corresponding permittivity changes in the graphene and underlying silica fiber. Those changes lead to shifts in the resonance positions which result in the equivalent of saturable absorption for light guided in the core without direct contact with the absorbing graphene layer. [ABSTRACT FROM AUTHOR]

This study focuses on the tilted fiber Bragg gratings. The paper describes the method of their inscription into optical fiber using the Talbot interferometer and KrF excimer laser system. The method allows precise selection of the resonant wavelengths and tilt angles of the created structures in a wide range of values. In course of the study, an original method was proposed for inscription of a superposition comprising two structures being identical in parameters but having the opposite tilt angles of grating strokes relative to the optical fiber cross-section. The described method makes it possible to compensate the decrease in the reflective efficiency of fiber Bragg gratings with an increase in their tilt angle. [ABSTRACT FROM AUTHOR]

In this paper, we studied the basic characteristics of tilted fiber Bragg gratings (TFBGs), inscribed line-by-line. Experimental results showed that if the TFBGs were located within different planes parallel to the fiber axis, the spectra performed differently. For 2°TFBG, if it was located near the central plane, the Bragg resonance was stronger than ghost mode resonance, and the order reversed if it was located near the boundary between core and cladding. As the tilted angle increased, the range of cladding mode resonance increased. When the tilted angle was larger than 12°, the birefringence effect was observed. Based on the birefringence phenomenon, torsion characteristics were experimentally studied; the sensitivity was about 0.025 dB/degree in the linear variation range. The harmonic order of TFBGs also affected the transmission spectrum. Leaky mode resonance was observed in the 8th order TFBG, and torsion (or polarization) influenced the spectrum of the 8th order TFBG. Our research represented the theory of line-by-line inscribed TFBGs and provided an inscription guidance for TFBGs.

Multicore optical fibers are of great interest in the optical sensing field. Their core diversity and spatial distribution enable the development of sensing mechanisms that are not possible in single-core fibers. In this paper, we study the use of the inter-core crosstalk phenomena for the implementation of a surrounding refractive index (SRI) sensor. The selective inscription of a tilted fiber Bragg grating (TFBG) intentionally increases the inter-core crosstalk between the inscribed cores and makes it sensitive to the SRI. With this technique we simplify the measurement of the SRI and improve the identification and tracking of the excited cladding modes, as compared with the analysis of the transmission spectrum of a TFBG in single-core fibers. The proposed device is also sensitive to temperature. Temperature is obtained from the crosstalk wavelength shift with a measured sensitivity of 9.75 pm/°C. The SRI is obtained from the measurement of the crosstalk optical power. For increasing SRIs the cladding modes gradually fade, reducing the crosstalk optical power. We observed that the higher the tilt, the higher the sensor sensitivity. For a 7° TFBG the SRI sensitivity obtained is −74.2 dB/RIU from 1.31 to 1.39 and −250.8 dB/RIU from 1.39 to 1.44. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A sensitivity-enhanced strain sensor constructed by a 7-mm-long thin-core fiber (TCF) splicing with a 10-mm-long tilted fiber Bragg grating (TFBG) was proposed. The modal interferometer formed by the TCF will interact with the spectrum of the TFBG to form an overlapped spectrum, which will extremely increase the axial strain sensitivity of the TFBG. Experimental results showed that the wavelength shifts of the resonant dip in the overlapped spectra for the applied axial strain have a high sensitivity of 0.67 pm/ $\mu \varepsilon $. Meanwhile, the proposed sensor has the temperature self-compensation function for the variation of the external temperature simply by measuring the relative wavelength shifts of the Bragg mode of the TFBG. [ABSTRACT FROM AUTHOR]

• The GSST-TFBG has an extended SRI range, and the resonance amplitudes remain strong and detectable beyond the cut-off point. • The s-mode resonances are practically insensitive whereas the p-mode remains sensitive to the changes of SRI. • Refractive index measurement for transformer oil using GSST-coated TFBG. In this work, a tilted fiber Bragg gratings coated with a 130 nm thick Ge-Sb-Se-Te (GSST) layer, a high refractive index soft glass had been fabricated and demonstrated for the refractometric test of transformer oil. The findings indicated that the introduction of the high refractive index coating had enhanced the polarization property and extended the refractive index detection range of the tilted fiber Bragg gratings beyond the cut-off point of the cladding modes. In the experiment with transformer oils, the Ge-Sb-Se-Te -coated tilted fiber Bragg gratings has shown a good linear sensitivity of 9.952 dB/RIU for the refractive index range of 1.472–1.484 RIU. [ABSTRACT FROM AUTHOR]

A new technique of depositing a poly(vinyl alcohol) (PVA) coating with an airbrush on the cylindrical surface of an optical fiber with real‐time spectral monitoring is presented. The method is applied for creating hygroscopic PVA coating as a part of the tilted‐fiber Bragg‐grating (TFBG)‐assisted fiber hygrometer samples. By monitoring the transmission spectra of the samples placed in a climatic chamber with variable relative humidity, the performance of the resulting coating is evaluated. Experimental data are presented, demonstrating the effectiveness of the described technique for humidity‐sensing purposes. [ABSTRACT FROM AUTHOR]

In this paper we present a novel twist measurement technique based on the influence of changes in the input light polarization state on the tilted Bragg grating. In the proposed sensor with the interrogation method, changes in the amplitudes of high-order cladding modes are measured by wavelength-matched fiber Bragg grating filters working in reflection mode. The key element of the method is the use of parameters of two selected parts of one cladding mode that are in close proximity to each other. Application of appropriate technological parameters to the manufactured photonic structures of the sensor and interrogation filter elements allows to obtain high-resolution measurement with simultaneous immunity to temperature fluctuations. The differential character of the proposed method also provides independence from changes in the optical power of the illuminating light source. [ABSTRACT FROM AUTHOR]

The paper reports on the results of the development of tilted fiber Bragg grating-assisted selective biosensors. The main sensing principle is based on the dependence of an optical evanescent field parameters on the composition of the medium under study. Sensors feature functional coatings of antibodies against protein molecules common in blood composition – fibrinogen and D -dimer. The work explores and clearly demonstrates the selective sensitivity of sensors to specific proteins by the example of simultaneous measurements using a pair of sensors functionalized with different antibodies. [Display omitted] • Fiber sensors with functional coatings provide for specific detection of proteins. • Complex processes in biological solutions affect the refractive index unpredictably. • Differential signal of two similar sensors could be used as a mean of detection. • Specific binding of an antigen to a functional coating can change layers' geometry. [ABSTRACT FROM AUTHOR]

Surface plasmon resonance-based fiber-optic sensors are of increasing interest in modern sensory research, especially for chemical and biomedical applications. Special attention deserves to be given to sensors based on tilted fiber Bragg gratings, due to their unique spectral properties and potentially high sensitivity and resolution. However, the principal task is to determine the plasmon resonance wavelength based on the spectral characteristics of the sensor and, most importantly, to measure changes in environmental parameters with high resolution, while the existing indirect methods are only useable in a narrow spectral range. In this paper, we present a new approach to solving this problem, based on the original method of determining the plasmon resonance spectral position in the automatic mode by precisely calculating the constriction location on the transmission spectrum of the sensor. We also present an experimental comparison of various data processing methods in both a narrow and a wide range of the refractive indexes. Application of our method resulted in achieving a resolution of up to 3 × 10−6 in terms of the refractive index.

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

We report on the study of an intensity-based optical fiber sensor for gasoline quality monitoring. The sensor setup employs two Bragg gratings with different spectral responses to interrogate the optical response of a tilted Bragg grating. The sensor operation is based on the tilted Bragg grating sensitivity to external refractive index changes, which are translated as power variations by the interrogation scheme. Gasoline–ethanol solutions with concentrations ranging from 0% to 60% ethanol were used to demonstrate the sensor performance. The results allowed to estimate that the sensor is able, within its resolution limit, to detect ethanol concentration variations of 1.5% in gasoline–ethanol solutions and discriminate temperature variations of 0.5 °C. The all-optical sensor setup is compact and robust, making it a competitive alternative for the realization of fuel quality analyses in practical applications.

• The method for refractive index measurement using cascaded TFBGs is presented. • Proposed method allows to significant reduction of interferences from input light polarization. • Sensitivity of proposed setup is compared to conventional setup. • Influence of the input light polarization is reduced by 90% compared to single TFBG. This paper presents a possibility to improve the reliability of the measurement of the liquid solution refractive index using tilted fiber Bragg gratings (TFBGs) under various inputs of light polarization angle. The application of a specialized setup of cascaded TFBGs provides a great improvement in the stability of cladding resonances under various light polarization. In the present paper, the analysis of the amplitude of the cut-off cladding mode shows that the dependence of this parameter in the case of the proposed structure is reduced by 96.101% compared with a single TFBG with the same internal tilt angle of 7° and a depth of periodic refractive index perturbations. Stabilization of the spectra under various polarizations of inputted light could be observed in the case of all cladding modes, which could be used to improve the surrounding refractive index (SRI) measurement over a wider range. [ABSTRACT FROM AUTHOR]

Tilted fiber Bragg gratings (TFBGs) have shown to be a suitable tool for exciting surface plasmon waves in a comparable manner as in typical attenuated total reflection Kretschmann configurations. In this paper, we present a comparative analysis of the characteristics of prism and grating configurations for generating surface plasmon resonance (SPR) for 800 and 1550 nm wavelengths. Results indicate that longer wavelength SPR may present advantages in sensing. Recent label-free biochemical sensing results using NIR TFBGs with and without SPR coatings demonstrate limits of detection ranging from the nM to the pM range without the need for thermal stabilization. [ABSTRACT FROM PUBLISHER]

The tilted fiber Bragg grating (TFBG), as a versatile and robust tool for many sensing applications with a particular focus on vapor deposition processes, is reviewed. Recent work employing the TFBG as an optical probe for monitoring the metal-organic (MO)CVD of noble-metal, single-source precursors is discussed extensively. This work also establishes the broad scope that utilizes TFBGs and other optical fiber configurations to interrogate thin film growth and the associated optical properties. While it cannot possibly cover the full scope of applications with respect to the TFBG device, this review highlights the recent advances of TFBG-based sensing for CVD, and progress towards the applicability of TFBGs as evanescent field-based sensors. [ABSTRACT FROM AUTHOR]

The ubiquity and importance of anions in many crucial roles accounts for the current high interest in the design and preparation of effective sensors for these species. Therefore, a tilted fiber Bragg grating sensor was fabricated to investigate individual detection of different anion concentrations in ethyl acetate, namely acetate, fluoride and chloride. The influence of the refractive index on the transmission spectrum of a tilted fiber Bragg grating was determined by developing a new demodulation method. This is based on the calculation of the standard deviation between the cladding modes of the transmission spectrum and its smoothing function. The standard deviation method was used to monitor concentrations of different anions. The sensor resolution obtained for the anion acetate, fluoride and chloride is 79×10−5 mol/dm3, 119×10−5 mol/dm3 and 78×10−5 mol/dm3, respectively, within the concentration range of (39–396)×10−5 mol/dm3. [ABSTRACT FROM AUTHOR]

Radiation-mode coupling is stronger and more efficient in tilted fiber Bragg gratings than in other fiber gratings; it has good advantage in such fields as optical communication and optical sensors. A simplified coupled-mode theory (CMT) approach is proposed for what we believe to be the first time, whose validity is demonstrated by comparing its simulation results with that of the complete CMT equations. With the simplified CMT approach, a theoretical spectral analysis of coupling from core mode to counter-propagating radiation modes in reflective tilted fiber Bragg gratings is presented. The influence of grating length, refractive index modulation amplitude and tilt angle is exhaustively investigated on the transmission spectrum characteristics. The different dependences between s-polarized and p-polarized radiation-mode coupling on grating tilt angle are discussed, and the coupling strength of 45°-tilted gratings shows the greatest polarization dependence with the limitation of backward-propagating radiation-mode coupling. [ABSTRACT FROM AUTHOR]

The polarization properties for tilted fiber Bragg gratings (TFBGs) are investigated theoretically based on coupled-mode theory and Mueller matrix method. The expression of wavelength-related polarization-dependent loss (PDL) for TFBGs with different tilt angles is derived and calculated. Simulation results are compared, and the results indicate that the polarization capability of TFBGs with 45° angle is stronger than other TFBGs with smaller angles. The degree of polarization for unpolarized light passing TFBGs is also simulated to further evaluate the polarization properties for various tilt angles. In addition, the relationship between physical parameters of a TFBG and its polarization capability is discussed. [ABSTRACT FROM AUTHOR]

In this paper, we studied the basic characteristics of tilted fiber Bragg gratings (TFBGs), inscribed line-by-line. Experimental results showed that if the TFBGs were located within different planes parallel to the fiber axis, the spectra performed differently. For 2°TFBG, if it was located near the central plane, the Bragg resonance was stronger than ghost mode resonance, and the order reversed if it was located near the boundary between core and cladding. As the tilted angle increased, the range of cladding mode resonance increased. When the tilted angle was larger than 12°, the birefringence effect was observed. Based on the birefringence phenomenon, torsion characteristics were experimentally studied; the sensitivity was about 0.025 dB/degree in the linear variation range. The harmonic order of TFBGs also affected the transmission spectrum. Leaky mode resonance was observed in the 8th order TFBG, and torsion (or polarization) influenced the spectrum of the 8th order TFBG. Our research represented the theory of line-by-line inscribed TFBGs and provided an inscription guidance for TFBGs. [ABSTRACT FROM AUTHOR]

• Multiplex cross-sensitive have been implemented through the TFBGs manufacturing process. • Available sensors for biochemical and mechanical that can be caused by a controlled tilted angle and exposure dose. • TFBGs sensing spectra with particular emphasis on cladding, ghost, and Bragg mode have been proposed. The controllable cross-sensitivities of tilted fiber Bragg gratings (TFBGs) are strongly related to the control of the parameters during the manufacturing process. The TFBG spectrum characterization, which can demonstrate cross-sensitivities, is controlled by the process parameters. Cross-sensitivities analyses in biochemical surrounding refractive index (SRI) (cladding mode) and mechanical sensing (Bragg and ghost mode) are evaluated through processing parameters, including the rotating tilted angles of phase-mask, laser exposure dose time, and reducing fiber diameters. The results of this study show the cross-sensing ability in both biochemical and mechanical sensing that can be caused by a controlled tilted angle was 10° and exposure dose of five minutes while obtaining the cladding and Bragg modes of TFBGs with broad spectral coverage. [ABSTRACT FROM AUTHOR]

A simple refractive-index fiber sensor based on a tilted Bragg fiber grating (TFBG) interacting with multimode fiber (MMF) is described. The sensor structure is formed by insertion of a small section of MMF between the single-mode fiber and the TFBG. The average reflective power in the cladding modes of the TFBG reflected a different power as the surrounding refractive index changes, while the power of the reflected Bragg mode keeps unchanged. Therefore, the power of the reflected Bragg mode can be used as a reference to compensate any possible fluctuation of the light source power. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1375–1377, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25178 [ABSTRACT FROM AUTHOR]

Abstract: A new type fiber bending sensor based on a tilted fiber Bragg grating (TFBG) interacting with a multimode fiber (MMF) is presented. The sensing head is formed by insertion of a small section of MMF between a single-mode fiber (SMF) and the TFBG. The average reflective power in the cladding modes decreases with the increase of curvature. The measurement range of the curvature from 0 to 2.5m−1 with a measurement sensitivity of −802.4nW/m−1 is achieved. The proposed sensor is also proved as temperature-independent from the experimental investigation. [Copyright &y& Elsevier]

In this letter, we propose a novel method of edge filter linear demodulation using tilted fiber Bragg gratings (TFBGs) in a fiber sensor system. Based on the filter characteristics of TFBGs and the same temperature characteristics as the FBG, the strain sensor demodulation system with dynamic temperature compensation can be achieved. An experimental system is built up to achieve one or multichannels of sensor demodulation. The experimental results were analyzed in detail. Using this method based on intensity, linear demodulation with a bandwidth of 6 nm can be achieved. The advantages of this system are an all-fiber design, quasi-static and dynamic operation, and potential high-speed demodulation. [ABSTRACT FROM PUBLISHER]

This work reports on the experimental investigation of the spectral effects induced by uniform high refractive index nano-coatings deposited on weakly tilted fiber Bragg gratings (TFBGs). To this aim, weakly TFBGs with different tilt angles were fabricated and then coated with thin overlays of syndiotactic polystyrene. Automated dip coating technique was used to obtain ring-shaped coatings with thickness ranging in hundreds of nanometers. The transmission spectra of the obtained devices were collected in order to outline the effects of thickness, tilt angle, and surrounding refractive index. Experimental results reveal interesting differences with respect to the uncoated case, both in terms of spectral envelope and wavelength shifts related to cladding modes coupling. [ABSTRACT FROM PUBLISHER]

A wavelength monitor based on a pair of tilted fiber Bragg gratings is reported. The monitor exhibits a subpicometer wavelength selectivity over a broad wavelength range (>30 nm), and a low insertion loss (<1 dB) for laser light with polarization matching the reference polarization of the device. Simulations show that a wavelength coverage up to 100 nm is possible and that the resolution degradation due to changes in the polarization state can be avoided by proper design of the gratings. [ABSTRACT FROM PUBLISHER]

A novel method for measuring modal power distribution (MPD) in multimode fibers is proposed using built-in tilted fiber Bragg gratings (TFBGs). For each degenerated guided mode group, a portion of the guided power can be coupled to the corresponding radiation mode through a TFBG imprinted into the same fiber, and then measured separately with a spatial filter. The power coupling coefficients between the guided and radiation modes are obtained by solving a set of linearly independent equations acquired under a number of launching conditions. With these coefficients obtained, the MPD under any excitation condition can then be obtained by simply measuring the powers of radiation modes. The proposed method is successfully demonstrated with a few-mode fiber. [ABSTRACT FROM PUBLISHER]

Surface plasmon resonance-based fiber-optic sensors are of increasing interest in modern sensory research, especially for chemical and biomedical applications. Special attention deserves to be given to sensors based on tilted fiber Bragg gratings, due to their unique spectral properties and potentially high sensitivity and resolution. However, the principal task is to determine the plasmon resonance wavelength based on the spectral characteristics of the sensor and, most importantly, to measure changes in environmental parameters with high resolution, while the existing indirect methods are only useable in a narrow spectral range. In this paper, we present a new approach to solving this problem, based on the original method of determining the plasmon resonance spectral position in the automatic mode by precisely calculating the constriction location on the transmission spectrum of the sensor. We also present an experimental comparison of various data processing methods in both a narrow and a wide range of the refractive indexes. Application of our method resulted in achieving a resolution of up to 3 × 10−6 in terms of the refractive index. [ABSTRACT FROM AUTHOR]

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