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

In this work, we investigate the application of tilted fiber Bragg grating (TFBG) sensors during <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex vivo</i> laser ablation of porcine hepatic tissues. Initially, TFBG’s ability to measure the surrounding refractive index (RI) for different sucrose concentrations and the possibility to measure the RI of the targeted tissue during laser ablation (LA) is analyzed. After, the temperature sensing modality of TFBG is investigated in detail. We have implemented an algorithm for quasi-distributed spatial temperature profile reconstruction along TFBG. The algorithm models the TFBG core mode spectrum as a chain of Bragg gratings (each Bragg grating is modeled <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">via</i> coupled mode theory), where each grating is sensitive to local temperature changes. After, the Gaussian-shape temperature profile along the TFBG is reconstructed using the iterative optimization technique. Temperature measurements have been compared with highly-dense FBG array measurements and with conventional TFBG point temperature measurements based on the core mode tracking techniques (maximum tracking, X-dB Bandwidth, centroid methods). Overall, the proposed reconstruction algorithm is able to provide a quasi-distributed temperature profile along TFBG, which is not possible to obtain using conventional point temperature measurements based on the TFBG’s core mode tracking. The resulted root-mean-square error in comparison to FBG array reference measurements is 7.8±1.7 °C. In general, the results show that the main reliable sensing modality of TFBG during LA is temperature monitoring, which can be significantly improved by the proposed algorithm.

MicroRNAs (miRNAs) serve as critical biomarkers for cancer diagnosis and disease monitoring due to their dysregulated expression in malignancies. However, their low abundance, short length, and high sequence similarity pose significant detection challenges. Here, we present a plasmonic sensing platform that integrates Au@ZIF-8 core-shell nanostructures with a gold-coated tilted fiber Bragg grating surface plasmon resonance sensor. This hybrid architecture enhances plasmonic coupling by generating a three-dimensional localized electromagnetic field. The spatially extended sensing region improves responsiveness to large-scale biomolecular targets. Target recognition utilizes a sandwich probe design combining immobilized ssDNA capture strands with ssDNA-functionalized gold nanoparticles (AuNPs) as signal amplifiers. AuNPs convert minute refractive index (RI) variations induced after miRNA binding into amplified RI perturbations, with the transduction process precisely confined to the cascaded plasmonic coupling sites between the AuNPs and Au@ZIF-8 nanostructures. This selective signal enhancement mechanism improves sensitivity and suppresses nonspecific background signals. We further demonstrate multiplexed detection of two breast cancer-related miRNAs within 30 min, effectively doubling throughput relative to single-target assays. The platform achieves detection limits as low as 5.94 aM (pure water), 13.6 aM (serum), and 7.78 aM (saliva), with recovery rates exceeding 94% in complex biofluids. This ultrasensitive, high-throughput, and noninvasive sensor holds promise for early cancer screening, disease staging, and therapeutic evaluation.

We presented a multifunctional refractive index (RI) bio-sensing platform based on tilted fiber Bragg grating (TFBG) modified with Zinc oxide nanoflower. TFBG mode coupling coefficient or the amplitude of cladding mode will vary sharply with the imaginary part of permittivity of the Zinc oxide nanoflowers. It has been demonstrated that the resonance intensity of the cladding mode decreases obviously with the addition of multifunctional nanoflower. In the experiments two self-assembly methods including silane coupler and polyelectrolytes were introduced to coat ZnO nanoflower on TFBG. By chosing different method and repeating dipping cycle, different thickness of the nanoflower layer could be achieved. Moreover, the refractive index sensitivities of ZnO coated TFBG sensors are respectively 18.4dB/RIU and 22.96dB/RIU, which are significantly improved 5 times compared to conventional bare TFBG. Benefting from strong electron transfer and adsorption as well as stability and high surface to volume ratio of ZnO, this ZnO coated TFBG RI sensor are significant and valuable for high-sensitivity and cost-effective biosensing applications.

We present optical fiber sensors to measurement strain and vibration. The sensors are based on fiber Bragg gratings (FBG). We prepared construction of strain sensors with respect to its implementation on the outer surface of concrete structures and with compensation of potential temperature drifts. These sensors are projected with look forward to maximal elongation and strength which can be applied to the sensor. Each sensor contains two optical fibers with FBGs. One FBG is glued into the sensor in points of fixation which are in the line with mounting holes. This FBG is prestressed to half of measurement range, than the stretching and pressing can be measured simultaneously by one FBG. The second FBG is placed inside the sensor without fixation to measure temperature drifts. The sensor can be used to structure health monitoring. The sensors to measurement vibration are based on tilted fiber Bragg grating (TFBG) with fiber taper. The sensor uses the TFBG as a cladding modes reflector and fiber taper as a bend-sensitive recoupling member. The lower cladding modes (ghost), reflected from TFBG, is recoupled back into the fiber core via tapered fiber section. We focused on optimization of TFBG tilt angle to reach maximum reflection of the ghost and taper parameters. In this article we present complete set-up, optical and mechanical parameters of both types of sensors.

Surface-plasmon resonance (SPR), which results from the strong sensitivity of the plasmon-propagation constant to the permittivity of a nanoscale layer of material on a metal surface, is an exquisitely sensitive technique to detect chemical and biochemical changes occurring between molecules. In this work, we propose and demonstrate an SPR sensor based on a multi-angle tilted fiber Bragg grating (TFBG) coated by a 50 nm-thickness gold film for refractive index (RI) measurement. Experimental results show that wide range of RI, from 1.15 to 1.43, can be detected using the proposed SPR sensor, with sensitivity of 523.41 nm/RIU (refractive index unit). In addition, with designed as a probe, through coating a gold film as a reflection mirror in the cleaved end downstream of the grating, the length of the entire sensor is only 2 cm, making it easy to insert into the media for biochemical measurements.

The temperature dependence of core mode resonance has been thoroughly studied in fiber Bragg gratings (FBGs), but it is not the case for cladding mode resonances in multi-resonance gratings such as tilted FBGs (TFBGs). In this work, the temperature sensitivity of ultraviolet written TFBGs in SMF-28 fibers is assessed, demonstrating in the first, to the best of our knowledge, place that a single gauge factor K T=6.25⋅10-6±0.02⋅10-6 ∘ C -1 can be employed to characterize the response to temperature of the resonances over the full spectrum in the 10°-50°C range. Then, a simulation model is obtained, enabling to predict TFBG spectra in the 10°-50°C range with high accuracy. This requires a calibration of the core index and dispersion of the TFBG measured in air at 25°C, and determination of the glass refractive index thermo-optic coefficient (d n/d T=8.46⋅10-6±0.1⋅10-6 ∘ C -1, common to both core and cladding glasses), leading to a mean error on the wavelength position of resonances between 1 and 3pm. This mean error can be further reduced (less than 1pm) by considering a linear dependence with temperature of d n/d T. Therefore, this model will enable to completely remove the temperature-induced shifts of all resonances in TFBG sensing applications and measure with great accuracy the variables of interest by using the scaled averages of groups of resonances instead of (less accurate) individual shifts.

This work presents a fiber Bragg grating (FBG) wide-range tilt sensor based on a cam structure. Two FBGs are fixed on a specially-designed sensor shell. When one FBG is stretched by the rotating cam driven by tilt, the other FBG moves along a standard semicircle with a constant radius, and compensates for temperature variations of the stretched FBG. By detecting the difference of the central wavelength shifts of two FBGs, variations of the tilt angle can be measured. The detailed mechanical structure design and measurement theory of the sensor is presented, and the sensor prototype is discussed. Testing results of comprehensive performance show that there is a good linear relationship between the tilt angle and the central wavelength of the FBG over three intervals from -90° to 90°. The repeatability and creep resistance of the FBG tilt sensor are good, the measuring accuracy is 1.516°, and the measuring resolution is 0.003°. The measurement range is much larger than comparable existing sensors, and it avoids cross-sensitivity between temperature and strain for FBG. The sensitivity of the sensor can be easily adjusted by changing the outline of the cam, or the length of the optical fiber between two fixed points.

A tilted fiber Bragg grating (TFBG) is sensitive to several physical quantities, which results in cross-sensitivity. The polarization of the incoming light influences the surrounding refractive index (SRI) measurement. In this paper, we analyze the effect of polarization on mode shift and the possibility of minimizing it using the differential shift method. For the second algorithmic method, the average shift of a mode group and the differences in the shifts between the two mode groups are determined. The calculations were based on experimental measurements. An effective reduction in the polarization effect can be achieved using a perpendicular TFBG (PTFBG) grating system. In the PTFBG-compensated system, the maximum and average polarization errors were reduced by more than 10-fold. An additional advantage of the proposed differential methods is the ability to achieve temperature-independent SRI measurements despite the lack of visibility of the core mode for systems with larger TFBG angles.

Mach–Zehnder interferometer cascaded with FBG for simultaneous measurement of RI and temperature

We propose and demonstrate a referencing scheme for dual-wavelength differential detection of tilted fiber Bragg grating sensors that compensates for drift in the laser diode wavelength and dynamic chirp, and results in 40 fm rms stability, and a refractive index change limit of detection of $3 \times 10 ^{-6}$.

The authors describe a detailed investigation on tilted fiber Bragg grating (TFBG) structures with tilted angles exceeding 45deg. In contrast to the backward mode coupling mechanism of Bragg gratings with normal and small tilting structures, the ex-45deg TFBGs facilitate the light coupling to the forward-propagating cladding modes. The authors have also theoretically and experimentally examined the mode coupling transition of TFBGs with small, medium, and large tilt angles. In particular, experiments are conducted to investigate the spectra and far-field distribution, as well as temperature, strain, and refractive-index sensitivities of ex-45deg devices. It has been revealed that these ex-45deg gratings exhibit ultralow thermal sensitivity. As in-fiber devices, they may be superior to conventional Bragg and long-period gratings when the low thermal cross sensitivity is required

Fiber Bragg grating (FBG) sensors play an increasingly important role in structural health monitoring, and tilt sensors are one of the most commonly used among them. Traditional FBG tilt sensors are easily affected by external vibration, and to overcome such a problem, this work proposes a novel FBG tilt sensor with a vibration damping feature. Based on the classical equal strength beam structure, a vibration-isolating spring is used as the vibration-damping connection between the beam and the mass block, while a damping fluid is filled inside the sensor housing as the vibration-damping material. After elaborating on the details of sensor structural design and measurement theory, five sensors were set up for relevant performance tests, and the controlled experiments were carried out for the FBG sensors with and without the vibration isolation springs. Meanwhile, for sensors with vibration isolation springs, the viscosity of filled damping fluid was also varied in the experiments, to better reveal its influence on the sensor performance. The experimental results show that the proposed sensors have a high sensitivity of 112.743 pm/° within a range of ±15° and also have superior creep resistance as well as good temperature compensation capability. From controlled experiments, it is evident that the proposed sensor with a vibration damping structure offers better vibration stability and monitoring accuracy and can be effectively adapted to safety monitoring tasks in different engineering applications by simply adjusting the viscosity of filled damping fluid.

Optical device based on a tilted fibre Bragg grating (TFBG) to perform simultaneous measurement of refractive index (RI) and temperature is proposed. Using two different demodulating techniques, namely monitoring the core mode wavelength and the area of the transmission spectrum, it is possible to measure, separately, temperature and RI, respectively. The proposed method uses a grating, which does not require a reduced diameter, and a single measurement equipment. Measurements with resolution up to 5.7 x 10 -4 and 0.5 °C were achieved, for RI and temperature, respectively. Theoretically, the resolution of RI measurements can go as bellow as 2 x 10 -5 .

Evaluation of thermosetting resin curing using a tilted fiber Bragg grating

A biosensor based on an etched Fiber Bragg Grating (EFBG) for thrombin detection is reported. The sensing system is based on a Fiber Bragg Grating (FBG) with a Bragg wavelength of 1550 nm, wet-etched in hydrofluoric acid (HF) for ~27 min, to achieve sensitivity to a refractive index (RI) of 17.4 nm/RIU (refractive index unit). Subsequently, in order to perform a selective detection of thrombin, the EFBG has been functionalized with silane-coupling agent 3-(aminopropyl)triethoxysilane (APTES) and a cross-linker, glutaraldehyde, for the immobilization of thrombin-binding aptamer. The biosensor has been validated for thrombin detection in concentrations ranging from 10 nM to 80 nM. The proposed sensor presents advantages with respect to other sensor configurations, based on plasmonic resonant tilted FBG or Long Period Grating (LPG), for thrombin detection. Firstly, fabricating an EFBG only requires chemical etching. Moreover, the functionalization method used in this study (silanization) allows the avoidance of complicated and expensive fabrications, such as thin film sputtering or chemical vapor deposition. Due to their characteristics, EFBG sensors are easier to multiplex and can be used in vivo. This opens new possibilities for the detection of thrombin in clinical settings.