Fiber bragg gratings in polymer optical fibers

This paper discusses the design and development of newer packaging schemes for Fiber Bragg Gratings (FBGs) for sensing water pressures inside ocean. The FBGs used in this work have been fabricated using the phase mask technique, in a photosensitive germano silicate optical fiber using a 248nm UV light. The wavelength shifts (¿¿ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> ) in the reflected light from the FBG, resulting from the strain/pressure imparted on the FBGs have been recorded using a high speed Micron-optics FBG interrogation system. Further, an experimental set up has been fabricated to test the efficiency and characteristics of different sensor packages developed. The results obtained indicate that the packages are of good sensitivity, rugged and can be customized depending on the requirement of the application.

Effect of additive concentration on crystalline surface of ZnO nanostructures morphology for enhanced humidity sensing

A high-resolution and temperature-insensitive interrogation scheme for fiber Bragg grating (FBG) based multicore fiber curvature sensor using microwave photonics filtering techniques is proposed and verified by experimental demonstration. The principle of operation exploited for curvature interrogation is based on monitoring the notch frequency shift of a two-tap microwave photonics filter (MPF), which is formed by combining two reflected sample signals from the two FBGs inscribed in multicore fiber (MCF) and a dispersion device. The proposed interrogation scheme with the sensitivity of 92 MHz/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is achieved, whilst it is naturally insensitive to any temperature variations.

A novel and simple sensor interrogation scheme for fibre Bragg grating (FBG)-based sensing systems is demonstrated. In this scheme, a chirped FBG-based Sagnac loop is used as a wavelength-dependent receiver, and a stable and linear readout response is realised. It is a significant advantage of this scheme that the sensitivity and the measurement wavelength range can be easily adjusted by controlling the chirp of the FBG or using an optical delay line in the Sagnac loop.

This paper presents a phase encoding method for fiber Bragg grating (FBG) code division multiplexing (CDM) sensing systems, based on coherent detection technology and Hadamard sequences. A four-channel CDM sensing system has been constructed based on this method, enabling synchronous monitoring of four ultrasonic vibration signals within the frequency range of 50 kHz to 500 kHz. The experimental results demonstrate that the time-domain signal to noise ratio (SNR) of the demodulated signal typically exceeds 30 dB. The crosstalk suppression that meets the engineering standard (15 dB) reaches over 98%, indicating that the sensing system has good monitoring stability. Furthermore, within the 500 kHz test frequency range, the CDM sensing system can simultaneously detect up to 32 channels and has the potential for integration with other multiplexing techniques, such as wavelength division multiplexing (WDM), to further increase the multiplexing number of FBGs in sensing system. Finally, the multiplexing experiment of special waveform signals was conducted to verify the system's applicability in practical scenarios.

Pipe internal pressure measurement is of utmost importance in the oil &amp;amp; gas industry to monitor the extraction process, and thus to prevent hydrate-plugs formation which may occur in specific temperature and pressure conditions. Traditional solutions usually rely on pressure sensors in direct contact with the fluid to monitor, therefore requiring one hole per sensor, but they also weaken the pipe structure, which may prematurely lead to significant leaks. Attempts to develop non-intrusive pressure sensors relying, for instance, on acoustic waves detection or even strain measurements (the pipe wall acting, in some way, like the membrane of a traditional intrusive sensor), are up to now not fully satisfying, mainly due to poor temperature cross-sensitivity compensation. Thus, 1 °C temperature compensation error typically leads for Fiber Bragg Grating (FBG) transducers to pressure measurement biases greater than 26% at 100 bar (e.g.: Ø 4” NPS Sch. 160 steel pipe). Consequently, if such non-intrusive, but biased, solutions could possibly have been considered to monitor, for instance, a Nuclear Power Plant (NPP) primary coolant circuit, it was with the risk of dramatic consequences since the fluid can reach temperatures up to 320 °C. On the other hand, the solution detailed here truly achieves to cancel the temperature cross-sensitivity, and potentially any additional effect on pressure measurement, provided that each effect has the same influence on all transducers. It first relies on a better understanding of the pipe behavior under hydrostatic pressure, supported by a dedicated model developed on purpose, which demonstrates that the internal pressure and the surface temperature variations of a closed pipe can be recovered with at least two direction-sensitive transducers, the temperature dependence of the pressure measurement being simply removed by a straightforward compensation process. This paper explains the underlying principle, thanks to a formal model established with only few hypotheses, but extended to more complex field conditions. It ends with a lab-test validation involving FBG transducers attached to a pressure circuit submitted to temperature variations greater than several tens of °C, and concludes about the advantages and limitations of this novel approach for non-intrusive sensing, and its potential extensions to other measurement techniques.

A Grey-Wolf optimization (GWO) algorithm served for fiber Bragg grating (FBG) strain sensor in structure detection is developed in this paper. A strain profile demodulation method of FBG reflection spectrum based on GWO algorithm is proposed, which uses an improved transfer matrix method for calculation. The GWO algorithm simulates the leadership and hunting mechanism of gray wolves in nature. For leadership, it has four types of gray wolves, such as α, β, δ, and ω wolves. Additionally, three main steps of hunting, searching, surrounding and attacking prey have also been simulated. On this basis, the reconstructed non-uniform strain profile by GWO algorithm was verified by a comprehensive numerical simulation. And the simulation results verify the accuracy and effectiveness of the GWO method. The calculation example shows that the target strain profile is in good agreement with the reconstructed strain profile.

The coupled-mode equations for fiber Bragg grating (FBG) and long period fiber grating (LPFG) undergoing linear and quadratic temperature change were given. The effects of temperature gradient and quadratic temperature change on the reflectivity spectrum of fiber Braggs grating and the transmission spectrum of long period fiber grating were investigated using the numerical simulation, and the dependence relationships of the central wavelength shift, the full-width-at-half-maximum, and the peak intensity upon temperature gradient were also obtained. These relationships may be used to design a novel fiber optical sensor which can simultaneously measure the temperature and temperature gradient.

This work describes a new technique for fibre Bragg grating (FBG)sensor interrogation and multiplexing. The technique combines a scanningFabry-Perot (SFP) bandpass filter used to wavelength-multiplex multiplegratings in a single fibre, and an unbalanced Mach-Zehnder fibreinterferometer made with a 3×3 coupler to detect strain-inducedwavelength shifts. A passive technique for interferometer drift compensationusing non-sensing FBGs is included in the system. A prototype complete systeminterrogates four gratings in a single fibre at a Nyquist sampling rate up to10 kHz, with a noise floor measured near 4 nϵ Hz-1/2 above0.1 Hz. The inclusion of the interferometer drift compensation technique isshown to make quasi-static measurements feasible.

INVITED] New advances in polymer fiber Bragg gratings

Narrow bandwidth Bragg gratings imprinted in polymer optical fibers for different spectral windows

Polymer optical fibers (POFs) have significant advantages for many sensing applications,including high elastic strain limits, high fracture toughness, high flexibility in bending, highsensitivity to strain and potential negative thermo-optic coefficients. The recent emergenceof single-mode POFs has enabled high precision, large deformation optical fiber sensors.This article describes recent advances in both multi-mode and single-mode POFbased strain and temperature sensors. The mechanical and optical propertiesof POFs relevant to strain and temperature applications are first summarized.POFs considered include multi-mode POFs, solid core single-mode POFs andmicrostructured single-mode POFs. Practical methods for applying POF sensors, includingconnecting and embedding sensors in structural materials, are also described. Recentdemonstrations of multi-mode POF sensors in structural applications based on newinterrogation methods, including backscattering and time-of-flight measurements, areoutlined. The phase–displacement relation of a single-mode POF undergoing largedeformation is presented to build a fundamental understanding of the response ofsingle-mode POF sensors. Finally, this article highlights recent single-mode POFbased sensors based on polymer fiber Bragg gratings and microstructured POFs.

Moisture is an essential parameter in the behaviour of capillary-porous construction materials such as timber and concrete. It affects liquid and gas transport phenomena, chemical and biological degradation processes, mechanical properties and, in the case of concrete, hydration. From a scientific point of view, moisture monitoring is essential in order to improve the understanding of material behaviour. Moisture measurements may increase the predictive accuracy of established material behaviour models. In practice, the increased knowledge of material behaviour improves structural maintenance planning. The main objective of this work is to propose a measurement method for non-destructive, in-depth moisture monitoring of construction materials. In this context, an intrinsic point and an averaging fibre optic relative humidity sensor have been developed and tested. The sensors are based on optical fibres that are coated with a hygroscopically swelling transducer polymer. When wet, the swelling of the coating strains the fibre (analogy with the hair hygrometer). The induced strain is assessed with conventional fibre optic strain sensing techniques such as fibre Bragg gratings and Michelson interferometry. Theoretical and experimental studies lead to a detailed understanding of the influence of humidity and temperature on the steady and transient state sensor behaviour. The sensors have an accurate, linear, reversible and reproducible response to relative humidity between 5 and 95 %RH and between 13 and 60 °C, at least. The sensor response time is in the order of 20 minutes. However, when packaged, the sensor responds slower. The temperature cross-sensitivity of the fibre Bragg grating sensor may be compensated with an additional non-hygroscopic grating, while the Michelson interferometric sensor provides an auto-compensation of temperature effects. Tests in mortar and timber samples demonstrate that the sensors preserve their sensing ability when embedded. These tests have also clarified the multiplexing potential of fibre Bragg gratings for forming a multi-point RH sensor. Multi-point sensors are particularly useful for profile measurements.

The time-domain analysis has been widely used in microwave photonic filtering based interrogation for fiber Bragg grating (FBG) array. In order to solve the problem that the interrogation resolution is limited by RF detection bandwidth in traditional time-domain analysis methods, a novel time-domain analysis with refinement capability has been developed. The proposed time-domain analysis algorithm was theoretically deduced and the feasibility of the proposed method was validated by simulation. Compared to traditional time-domain analysis methods, the proposed time-domain refinement algorithm can greatly improve the interrogation resolution and the demodulation speed at the same system parameters.

Fiber-optic shape sensing enables real-time monitoring of structural deformation across a wide range of applications. For large-scale structures, Brillouin-based distributed sensing, typically implemented through Brillouin Optical Time Domain Analysis (BOTDA), offers an extended range for quasi-static measurements, albeit its limited spatial resolution degrades reconstruction accuracy. This study addresses this fundamental limitation through the introduction of a novel error compensation algorithm, particularly suited for a Brillouin-based shape sensing system, yet agnostic with respect to the sensing technology. The method leverages both the initial and final points of the sensing path, performing both forward and backward reconstructions and fusing the two trajectories by testing several polynomial and exponential weighting strategies. The algorithm is experimentally validated on a 28.91 m four-core shape sensing fiber cable (length = L), interrogated through BOTDA operating at 50 cm spatial resolution, and reconstructed through the Frenet-Serret frame formulation. Calibration procedures include radial-offset tuning and segment alignment via a hotspot reference. A non-trivial S-shaped geometry is adopted as a case study, specifically addressing curvature discontinuities arising from mixed straight and curved segments. Reconstruction accuracy is quantified through a Euclidean-distance-based Figure of Merit (FOMs). The cubic weighting strategy demonstrates improvements exceeding 86% in all FOMs compared to classical methods without compensation. Specifically, it achieves an RMSE of 0.145 m (0.50% of L), a MAE of 0.109 m (0.38% of L), and a maximum error of 0.341 m (1.18% of L). Remarkably, these percentage errors are of the same order of magnitude as those reported in the literature for Fiber Bragg Grating (FBG) and Optical Frequency Domain Reflectometry (OFDR) systems, indicating that the proposed compensation strategy enables BOTDA-based shape sensing to achieve comparable reconstruction accuracy despite its lower spatial resolution.