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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]

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]

• 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]

• 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]

• 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]

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]

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]

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]

A new design for a hybrid fiber‐planar refractive index sensor based on the surface plasmon resonance effect is presented. The key element of the design is a thin layer of gold covering a tilted fiber Bragg grating. In the study, a specially developed microfluidic chip is used to perform measurements directly in the flow of the investigated liquid. Rigidly mounting the sensor to the base of the fluid chip is proposed. The presented experimental data demonstrate the high stability of the sensor readings and the low noise level during measurements in a liquid flow. The proposed construction is a promising decision for application in compact immunoassay systems based on microfluidic technologies. A new design of a combined fiber‐planar plasmon sensor is presented. The sensor based on tilted fiber Bragg grating is integrated into a microfluidic chip channel, and rigidly mounted to the chip base along the entire length. High stability of the sensor readings and the low noise level during measurements are demonstrated. This design is perspective for different lab‐on‐chip applications. [ABSTRACT FROM AUTHOR]