Tilted Fiber Bragg Grating Research Articles

Simultaneous measurement of refractive index and temperature using a hybrid-grating sensor

A hybrid-grating-based optical fiber sensor for simultaneously measuring the refractive index (RI) and temperature is proposed. The sensor is fabricated by inscribing a long period grating (LPG) and tilted fiber Bragg grating (TFBG) in the same region of an optical fiber in sequence. The hybrid grating takes advantage of both LPG’s high temperature sensitivity and TFBG’s high RI sensitivity, with consideration of the temperature-insensitive characteristic of wavelength difference between the cladding mode wavelength and core mode wavelength of TFBG. Experiments prove the ability of the sensor to measure the RI and temperature simultaneously. The sensitivities are 606.82 nm/RIU and 268.79 pm °C−1, respectively.

Functionalized etched tilted fiber Bragg grating aptasensor for label-free protein detection

An aptasensor based on etched tilted fiber Bragg grating (eTFBG) is developed on a single-mode optical fiber targeting biomolecule detection. TFBGs were chemically etched using hydrofluoric acid (HF) to partially remove the fiber cladding. The sensor response was coarsely interrogated, resulting on a sensitivity increase from 1.25 nm/RIU (refractive index unit) at the beginning of the process, up to 23.38 nm/RIU at the end of the etching, for a RI range from 1.3418 to 1.4419 RIU. The proposed aptasensor showed improved RI sensitivity as compared to the unetched TFBG, without requiring metal depositions on the fiber surface or polarization control during the measurements. The proposed sensor was tested for the detection of thrombin-aptamer interactions based on silane-coupling surface chemistry, with thrombin concentrations ranging from 2.5 to 40 nM. Functionalized eTFBGs provided a competitive platform for biochemical interaction measurements, showing sensitivity values ranging from 2.3 to 3.3 p.m./nM for the particular case of thrombin detection.

Method for Determining the Plasmon Resonance Wavelength in Fiber Sensors Based on Tilted Fiber Bragg Gratings.

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.

Palladium-coated plasmonic optical fiber gratings for hydrogen detection.

Surface plasmon resonance excitation with tilted fiber Bragg gratings has been typically studied using gold films to target biochemical sensing applications. However, surface plasmons can be excited on other metal coatings as well. In this work, plasmonic optical fiber grating platforms are developed using palladium films. Since the optical properties of this metal differ from the ones of gold, simulations are carried out to define the optimal thickness. Due to the phase transition of palladium in the presence of hydrogen, intensity changes in the optical transmission of the devices are produced. It is demonstrated that these platforms can be used for hydrogen detection at concentrations way below the lower explosive limit.

Highly Sensitive SPR Biosensor Based on Graphene Oxide and Staphylococcal Protein A Co-Modified TFBG for Human IgG Detection

A highly sensitive optical fiber surface plasmon resonance (SPR) biosensor based on graphene oxide (GO) and staphylococcal protein A (SPA) co-modified tilted fiber Bragg grating (TFBG) is proposed and demonstrated for the detection of human immunoglobulin G (IgG) for the first time. The gold film on the surface of the sensor was first fixed with GO and then modified with an SPA to improve the sensitivity of the sensor. Large specific surface area and abundant functional groups of GO can adsorb more antibodies. The combination of SPA and the antibody molecule Fc region makes the Fab area with antigen-binding sites extend outward, resulting in highly oriented antibody immobilization on the sensor surface and high antigen–antibody binding efficiency. The experimental results show that the sensitivity as well as the limit of detection of GO-SPA-modified TFBG-SPR biosensor is around 0.096 dB/( $\mu \text{g}$ /mL) and $0.5~\mu \text{g}$ /mL, showing better responses to human IgG solutions with a concentration range of 30– $100~\mu \text{g}$ /mL compared with the TFBG-SPR biosensors modified singly with GO or SPA. The biosensor exhibits the advantages of small size, ease of fabrication, high sensitivity, label-free, and rapid response, and provides a new solution for detecting low concentration of biological solution, presenting great application potential in the biochemistry field.

Non-enzymatic D-glucose plasmonic optical fiber grating biosensor

Saccharide sensors represent a broad research area in the scope of sensing devices and their involvement in the medical diagnosis field is particularly relevant for cancer detection at early stage. In that context, we present a non-enzymatic optical fiber-based sensor that makes use of plasmon-assisted tilted fiber Bragg gratings (TFBGs) functionalized for D-glucose biosensing through polydopamine (PDA)-immobilized concanavalin A (Con A). Our probe allows a live and accurate monitoring of the PDA layer deposition leading improved surface biochemistry. The SPR shift observed was assessed to 3.83 ± 0.05 nm within 20 min for a 2 mg/mL dopamine solution. Tests performed in different D-Glucose solutions have revealed a limit of detection close to 10−7 M with the highest sensitivity in the 10−6 to 10−4 M range. This configuration has the capability to overcome the limitations of current enzyme-based solutions.

Experimental determination of the characteristics of a transmission spectrum of tilted fiber Bragg gratings

Experimental determination of the characteristics of a transmission spectrum of tilted fiber Bragg gratings

All-optically controlled slow and fast lights in graphene-coated tilted fiber Bragg grating

We demonstrate all-optically controlled slow and fast lights in tilted fiber Bragg grating (TFBG) with a graphene film. By employing graphene’s efficient photothermal effect, the global resonances in TFBG are shifted with slopes of 1.1 and 1.01 pm mW−1 for cladding and core modes, respectively. This enables changes of group delay and thus supports tunable fast and slow lights from −156 to 20 ps in cladding modes and up to 400 ps in core modes. Our demonstration with an all-in-fiber scheme possesses advantages, such as low-cost manufacture, simple and compact configuration, and multi-wavelength operation.

Optical Fiber Gratings Immunoassays.

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.

Refractive Index and Temperature Sensing Using Inter-Core Crosstalk in Multicore Fibers

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.

Gasoline Quality Sensor Based on Tilted Fiber Bragg Gratings

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.

Filtering of stimulated Raman scattering in a monolithic fiber laser oscillator using chirped and tilted fiber Bragg gratings

We have demonstrated here, for the first time to the best of our knowledge, the filtering of stimulated Raman scattering (SRS) in a monolithic fiber laser oscillator, using chirped and tilted fiber Bragg gratings (CTFBGs). We designed and inscribed CTFBGs in large-mode-area fibers according to the operating wavelength of the fiber laser oscillator. With a CTFBG inserted before the output coupling FBG in the oscillator, a maximum SRS suppression ratio more than 10 dB was achieved in spectrum with 976 nm and 915 nm laser diode pumps respectively at a given power level. Obvious decreasing of the Stokes power and increasing of the equivalent Raman threshold was observed. By reducing the insertion loss and improving the transmission spectrum of the CTFBG, an improvement in laser efficiency could be achieved. This work provides a novel idea for SRS filtering in a high-power all-fiber oscillator system, which is very useful for the power enhancing of a fiber oscillator.

Fiber laser twist sensor with hybrid few-mode tilt Bragg grating and few-mode long period grating

A twist sensor with hybrid few-mode tilted fiber Bragg grating (FM-TFBG) and few-mode long period grating (FM-LPG) in fiber laser cavity is demonstrated. The FM-LPG is utilized to excite LP11 core mode. The FM-TFBG is used for sensing. The transverse modes at 1 553.9 nm and 1 550.5 nm are LP01 and LP21 core modes, respectively, which are coupled from forward-propagating LP11 core mode. These two excitation wavelengths have opposite variation tendencies, which participate in sensing. The twist sensitivity of 0.16 dB/° from -40° to 40° is achieved. The proposed sensor has potentially used for structure monitoring in many areas.

New Parameters Extracted from Tilted Fiber Bragg Grating Spectra for the Determination of the Refractive Index and Cut-Off Wavelength.

Tilted fiber Bragg grating (TFBG) is a very popular fiber optic element that is used as a sensor for various physical quantities. The calculation of the refractive index of a substance surrounding the TFBG is based on its spectrum demodulation, which consists of determining a certain parameter that is correlated with the sought quantity. The most commonly used parameter is the area created by the maxima and minima of the cladding mode resonances. In this article, we propose a new group of methods, which are based on calculating the parameters related to the spectrum differences between the local average values in the range of occurrence of the cladding modes. The basic parameter used in this group of methods is the mean absolute deviation from the local mean, which is characterized by the best linearity among the considered group of methods. The calculated parameters, in their cumulative form, can also be used to determine the cut-off wavelength, which can also indirectly indicate the refractive index value. The proposed approaches were compared, in terms of measurement resolution, to the most commonly used methods, such as the cladding modes’ envelope area and the spectral contour lengths.

Wide Range Refractive Index Measurement Based on Off-Axis Tilted Fiber Bragg Gratings Fabricated Using Femtosecond Laser

In this paper, we propose an off-axis tilted fiber Bragg grating (OTFBG) in which the grating structure is limited to one side of the fiber core cross section. The tilted grating planes are fabricated using a femtosecond laser with a phase mask. The wavelength range of the OTFBG cladding mode resonance could be twice that of a conventional tilted fiber Bragg grating with the same tilt angle. The resonance intensity of the OTFBG core mode is significantly enhanced. We also fabricate a serial-double-angle OTFBG (SOTFBG) and a parallel-double- angle OTFBG (POTFBG). The wavelength ranges of their cladding mode resonances are effectively extended. The SOTFBG and POTFBG with tilt angles of 5° and 10° possess a wavelength range of cladding mode resonances of approximately 120 nm. They are capable of measuring the surrounding refractive index (SRI) with a refractive index (RI) sensible range from 1.24 to 1.45. A significantly wide range of cladding mode resonances is achieved by an SOTFBG with tilt angles of 10° and 15°, which is as wide as 215 nm. The RI sensible range of this SOTFBG is estimated to be from approximately 1.00 to 1.41, thereby making it suitable for SRI measurements in both gaseous media and aqueous solutions.

Integrated Differential Area Method for Variable Sensitivity Interrogation of Tilted Fiber Bragg Grating Sensors

A novel demodulation method for tilted fiber Bragg grating (TFBG) refractometry is proposed and experimentally demonstrated. This “integrated differential area” method consists of calculating the difference between measured spectral transmission over a selected bandwidth and a reference spectrum and then integrating the absolute value of the result. This has the effect of multiplying the effect of the individual cladding mode resonance shifts relative to the background noise. The method further allows the use of spectrum analyzers with low resolution, including portable ones with spectral resolution of the order of 150-200 pm without loss of sensitivity. In practical measurements, a reflection configuration is demonstrated using a TFBG cascaded by a chirped-fiber Bragg grating with bandwidth of 19 nm. A refractive index sensitivity of 7.19% transmission change per refractive index unit is achieved for refractive indices ranging from 1.32 to 1.38, along with a limit of detection of 0.00026 refractive index units.

Sensitivity-Enhanced Strain Sensor Based on Thin-Core Fiber Modal Interferometer Interacted With Tilted Fiber Bragg Grating

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.

Polarization-Dependent Tuning Property of Graphene Integrated Tilted Fiber Bragg Grating for Sensitivity Optimization: A Numerical Study

In this paper, the general characteristics of the polarization-dependent tuning property and sensitivity optimization of the graphene integrated tilted fiber Bragg grating (TFBG) are investigated theoretically. The tuning mechanism of the graphene on mode characteristics and transmission spectrum is thoroughly explored. The results reveal that the graphene induces a stronger influence on the s-polarized mode than its p-polarized counterpart, but the TFBGs with or without graphene in all the polarized states have nearly the same wavelength sensitivity ( $\sim \text{540}\; \text{nm/RIU}$ in average). In contrast, the resonance strength is highly dependent on the tunable state of the graphene, especially for the s-polarized case. In consequence, the normalized power response is increased in the low refractive index region. More importantly, the s-polarized leaky mode resonance that is ignored for regular TFBGs is significantly enhanced (by increasing field content in the fiber region and decreasing the confinement loss) whereas the guided counterpart is suppressed greatly by the graphene with lower chemical potential. The sensitivity of up to $ \text{6595.8 dB/RIU}$ (9.17 times higher than that of the regular counterpart) is achieved in response to an extremely small analyte perturbation ( $ \text{10}^{-\text{4}}$ close to 1.333) based on the graphene enhanced leaky mode resonance. This paper provides a novel technique for significantly optimizing the sensing performance of the TFBG, which has great potential in biological and biochemical sensing applications.

Ultrastable Combined Planar‐Fiber Plasmon Sensor (Phys. Status Solidi A 3∕2019)

Plasmon Sensors In article number 1800541 by Kirill A. Tomyshev, Oleg V. Butov and co-workers 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 labon-chip applications.

Weak feedback assisted random fiber laser from 45°-tilted fiber Bragg grating.

We have demonstrated the realization of a high-polarization random fiber laser (RFL) output based on the hybrid Raman and Erbium gain with the tailored effect provided by a 45°-tilted fiber Bragg grating (45°-TFBG), revealing an improvement in the polarization extinction ratio (PER) and achieving a PER of ~15.3 dB. The hybrid RFL system incorporating the 45°-TFBG has been systematically characterized. The random lasing wavelength can be fixed under the extremely weak feedback effect of the 45°-TFBG with reflectivity of 0.09%. In addition, numerical simulation has verified that the weak feedback can boost the random lasing emission with fixed wavelength using a power balance model, which is in good accordance with the experiment results.