Response Of Fiber Bragg Gratings Research Articles

Fiber Bragg Grating Based Thermometry.

In recent years there has been considerable interest in developing photonic temperature sensors such as the Fiber Bragg gratings (FBG) as an alternative to resistance thermometry. In this study we examine the thermal response of FBGs over the temperature range of 233 K to 393 K. We demonstrate, in hermetically sealed dry Argon environment, FBG devices show a quadratic dependence on temperature with expanded uncertainties (k=2) of ≈500 mK. Our measurements indicate that the combined measurement uncertainty is dominated by uncertainty in determining peak center fitting and thermal ageing of polyimide coated fibers.

Hygroscopic polymer microcavity fiber Fizeau interferometer incorporating a fiber Bragg grating for simultaneously sensing humidity and temperature

This work proposes a hygroscopic polymer microcavity fiber Fizeau interferometer (PMFFI) incorporating a fiber Bragg grating (FBG) for the simultaneous measurement of relative humidity (RH) and temperature (T). The PMFFI was fabricated by attaching the hygroscopic polymer to a single-mode fiber endface to form a low-finesse Fabry–Perot resonant microcavity. This work is the first to investigate polymer of the Norland Optical Adhesive (NOA) series with particular porous structures that responds well to the moisture and can thus be used in the fiber-optic sensing of relative humidity. Additionally, the NOA materials have a high thermal expansion coefficient and so can be used in the highly sensitive measurement of temperature. The adsorption/desorption of water molecules and variations in temperature both change the optical path of the microcavity, shifting the fringes in the interference spectra. Incorporating a general FBG that is nonreactive to RH but sensitive to T judges the variation in T. The combined sensor supports the simultaneous measurement of both parameters RH and T by gauging of the individual spectral responses of PMFFI and FBG, respectively. Experimental results demonstrate that the RH and T can be simultaneously measured with high sensitivity and accuracy using the proposed sensing configuration.

A Needlelike Probe for Temperature Monitoring During Laser Ablation Based on Fiber Bragg Grating: Manufacturing and Characterization

Temperature distribution monitoring in tissue undergoing laser ablation (LA) could be beneficial for improving treatment outcomes. Among several thermometric techniques employed in LA, fiber Bragg grating (FBG) sensors show valuable characteristics, although their sensitivity to strain entails measurement error for patient respiratory movements. Our work describes a solution to overcome this issue by housing an FBG in a surgical needle. The metrological properties of the probes were assessed in terms of thermal sensitivity (0.027 nm °C−1 versus 0.010 nm °C−1 for epoxy liquid encapsulated probe and thermal paste one, respectively) and response time (about 100 ms) and compared with properties of nonencapsulated FBG (sensitivity of 0.010 nm °C−1, response time of 43 ms). The error due to the strain caused by liver movements, simulating a typical respiratory pattern, was assessed: the strain induces a probes output error less than 0.5 °C, which is negligible when compared to the response of nonencapsulated FBG (2.5 °C). The metallic needle entails a measurement error, called artifact, due to direct absorption of the laser radiation. The analysis of the artifact was performed by employing the probes for temperature monitoring on liver undergoing LA. Experiments were performed at two laser powers (i.e., 2 W and 4 W) and at nine distances between the probes and the laser applicator. The artifact decreases with the distance and increases with the power: it exceeds 10 °C at 4 W, when the encapsulated probes are placed at 3.6 mm and 0 deg from the applicator, and it is lower than 1 °C for distance higher than 5 mm and angle higher than 30 deg.

Instrumentation of integrally stiffened composite panel with fiber Bragg grating sensors for vibration measurements

We evaluate the performance of fiber Bragg grating (FBG) sensors for the measurement of dynamic strains in complex composite structures. The particular structure used in this study is an integrally stiffened composite panel for which the stiffeners and skin are fabricated in a single layup and cure process. Surface-mounted FBG sensors are bonded to the panels after curing, whereas embedded FBG sensors are successfully incorporated during the fabrication process. A finite element model was also constructed of the stiffened panel. The panels were subjected to repeated impacts and the post-impact vibration response of the panel was measured through the FBG sensor responses. Little change to the global response of the panel was observed after the repeated impacts, through the dynamic response of the surface-mounted FBGs. Pulsed phase thermography and micro-computer-tomography imaging of the panel confirmed that the damage was localized near the impact locations, producing negligible changes to the global response of the panel. All of the embedded FBG sensors survived the fabrication and multiple impacts; however, as these were embedded close to the neutral axis of the panel, they were not very sensitive to the vibration modes. Excitation of the panel near the first natural frequency did produce a measurable response in the FBG sensors, confirming their functionality.

Response of fiber Bragg gratings bonded on a glass/epoxy laminate subjected to static loadings

Fiber Bragg gratings (FBG) may be used to monitor strain over the surface of a structure as an alternative technology to conventional strain gauges. However, FBG bonding techniques have still not been established to yield satisfactory surface measurements. Here, two adhesives were investigated, one with low viscosity and the other with high viscosity for bonding FBGs on glass/epoxy sandwich skins. First, instrumented elementary specimens were tested under tension. FBG strain results were analyzed together with digital image correlation (DIC) measurements. The influence of the bonding layer on the measured strain and on the integrity of the sensor was investigated by considering different regions of interest. Next, an instrumented structural sandwich beam was tested under four-point bending. FBG rosettes were compared to conventional strain gauge rosettes. The high viscosity adhesive demonstrated behaviors that affected FBG accuracy. Brittleness of the bonding layer and poor interface adhesion were observed using DIC and X-ray tomography. By contrast, the low viscosity adhesive demonstrated satisfactory results. The FBG strain measurements appeared to be consistent with those of DIC. The accuracy is also adequate as the FBGs and the conventional strain gauges had similar results in three directions, under tension and under compression.

Single and Multiphase Flow Characterization by Means of an Optical Fiber Bragg Grating Grid

This study introduces a new approach to characterize single and multiphase flow of water and air/water blends, respectively, by means of the utilization of optical fiber Bragg gratings (FBGs) arranged in a grid pattern. Here, the FBGs act as transducers between the force applied on the optical fiber surface by the liquid or air/liquid flow and the strain-induced Bragg wavelength shift. Since the force is proportional to the square of the velocity, associated to the kinetic energy, it is possible to establish a relationship between the Bragg wavelength shift and flow speed for single-phase flow monitoring. When multiphase flows are taken in consideration, a sudden Bragg wavelength shift represents an abrupt change in the force applied onto the fiber, which means a transition between liquid and air. It is hard to localize turbulences in single phase flow or establish the bubble position for multiphase flow from the response of a single FBG. Therefore, the sensors in this study have been arranged forming an 8 × 8 grid, with a total of 16 different FBGs multiplexed in wavelength. FBG grid enables the detection of turbulences or air bubbles within the pipe by means of an adequate aggregation and processing of the response of the FBGs at each crossing point, with a total of 64 crossings (12 crossings are out of the cylindrical shape pipe). Different flow speeds and void conditions with distinct void fractions and flow rates have been studied. The optical fiber sensors performance agreed with that of a wire-mesh system, which is conventionally used as a reference high performance measurement tool for multiphase flow. Results showed the great potential of this technique that reduces in more than a half the costs, complexity and size of actual devices used for the same purpose.

Adaptive ultrasonic sensor using a fiber ring laser with tandem fiber Bragg gratings.

We propose and demonstrate an intensity-demodulated fiber-optic ultrasonic sensor system that can be self-adaptive to large quasi-static background strain perturbations. The sensor system is based on a fiber ring laser (FRL) whose laser cavity includes a pair of fiber Bragg gratings (FBGs). Self-adaptive ultrasonic detection is achieved by a tandem design where the two FBGs are engineered to have differential spectral responses to ultrasonic waves and are installed side-by-side at the same location on a structure. As a result, ultrasonic waves lead to relative spectral shifts of the FBGs and modulations to the cold-cavity loss of the FRL. Ultrasonic waves can then be detected directly from the laser intensity variations in response to the cold-cavity loss modulation. The sensor system is insensitive to quasi-static background strains because they lead to identical responses of the tandem FBGs. Based on the principle, a FRL sensor system was demonstrated and tested for adaptive ultrasonic detection when large static strains as well as dynamic sinusoidal vibrations were applied to the sensor.

Union self-compensated packaging of FBG strain sensor

To solve the cross-sensitivity to temperature of fiber Bragg grating (FBG) strain sensor, an effective method is firstly proposed for the discrimination of strain and temperature by using a single FBG adopting a special compensated structure, which is based on the thermal stress. The relationship of strain and temperature responses of FBG is analyzed theoretically. The experimental results are in agreement with the obtained theoretical analyses. This sensor structure has excellent characteristics, such as simple structure, easy manufacture and higher strain sensitivity.

Characterization of fatigue damage in adhesively bonded lap joints through dynamic, full-spectral interrogation of fiber Bragg grating sensors: 2. Simulations

In this paper, we simulate the response of fiber Bragg grating (FBG) sensors embedded in the adhesive layer of a composite lap that is subjected to harmonic excitation. To simulate accumulated fatigue damage at the adhesive layer, two forms of numerical nonlinearities are introduced into the model: (1) progressive plastic deformation of the adhesive and (2) changing the boundary of an interfacial defect at the adhesive layer across the overlap shear area. The simulation results are compared with previous measurements of the dynamic, full-spectral response of such FBG sensors for condition monitoring of the lap joint. Short-time Fourier transforms (STFT) of the locally extracted axial strain time histories reveal a transition to nonlinear behavior of the composite lap joint by means of intermittent frequencies that were observed in the experimental measurements and are not associated with the external excitation. The simulation results verify that the nonlinear changes in measured dynamic FBG responses are due to the progression of damage in the lap joint.

Investigation of the Thermal Sensitive Spectral Response of Chalcogenide Fiber Bragg Grating

This contribution presents numerical studies of the nonlinear fiber Bragg grating (FBG). The proposed model corresponds well to chalcogenide FBG as a considerable tool for optical switching. The spectral response of this device is discussed theoretically. Simulations based on the nonlinear coupled mode theory are used for investigating the relationships between the spectral transmission of nonlinear chalcogenide FBG and the grating parameters. The influence of the temperature dependence of FBG is numerically investigated. Numerical results show that the ambient temperature has an influence on the spectral response of FBG. The results are indications for applications in optical switches for all-optical communication networks and this can be important for wavelength division multiplexing optical networks.

Experimental modal analysis and dynamic strain fiber Bragg gratings for structural health monitoring of composite antenna sub-reflector

The objective of this work is to develop a new structural health monitoring system for composite aerospace structures based on dynamic response strain measurements and experimental modal analysis techniques. Fiber Bragg grating (FBG) optical sensors were used for monitoring the dynamic response of the composite structure. The structural dynamic behavior has been numerically simulated and experimentally verified by means of vibration testing. The hypothesis of all vibration tests was that actual damage in composites reduces their stiffness and produces an eigenfrequency shifting to lower values in the same sense as mass increase produces. Thus, damage was simulated by slightly varying locally the mass of the structure at different zones. The correlation between the simulated damage and the loss of stiffness was analytically defined. Experimental modal analysis based on the strain responses was conducted and the extracted strain mode shapes were the input for the damage detection expert system. A feed-forward back propagation neural network was the core of the damage detection system. The features-input to the neural network consisted of the strain mode shapes, extracted from the experimental modal analysis. Dedicated training and validation activities were carried out based on the experimental results. The system showed high reliability, confirmed by the ability of the neural network to recognize the size and the position of damage on the structure. The experiments were performed on a real structure i.e. a lightweight antenna sub-reflector, manufactured and tested at EADS CASA ESPACIO. An integrated FBG sensor network, based on the advantage of multiplexing, was mounted on the structure with optimum topology. Numerical simulation was used as a support tool at all the steps of the work. Potential applications for the proposed system are during ground qualification extensive tests of space structures and during the mission as modal analysis tool on board, being able via the FBG responses to identify a potential failure.

Analytical investigation of response of birefringent fiber Bragg grating sensors in distributed monitoring system based on optical frequency domain reflectometry

When Fiber Bragg gratings (FBGs) are used as strain sensors, both longitudinal and lateral strain can be applied uniformly or non-uniformly over the length of the FBGs. In order for the demodulation of such FBG signal, this paper investigates the response of birefringent FBGs which are monitored by distributed measurement system based on optical frequency domain reflectometry. A numerical model of the distributed measurement system is built based on piece-wise uniform approach, which considers polarization states of propagating lights. The numerical model simulates analytical response of birefringent FBGs especially when birefringence induces power fluctuations in the distributed spectra, which can be noise or new opportunity for sensitive monitoring of birefringence. Simulation results show the relationships between the power fluctuations and the polarization states of the propagating lights. Consequently, appropriate methods of polarization control for sensitive distributed birefringent FBG monitoring are discussed.

Highly Reflective Fiber Bragg Gratings Inscribed in Ce/Tm Co-Doped ZBLAN Fibers

We report highly reflective fiber Bragg gratings (FBGs) photoinduced in the core of Ce/Tm co-doped ZBLAN fibers using a 248-nm excimer laser. We compare the characteristics of the FBGs inscribed in ZBLAN fiber to those produced in silica fiber. We also characterize the strain and temperature responses of ZBLAN FBGs and find that the wavelength can be strain- and temperature-tuned at rates that are 45% and 15% higher, respectively, compared to silica FBGs.

On the Effects of the Lateral Strains on the Fiber Bragg Grating Response

In this paper, a combined experimental-numerical based work was undertaken to investigate the Bragg wavelength shift response of an embedded FBG sensor when subjected to different conditions of multi-axial loading (deformation). The following cases are examined: (a) when an isotropic host material with no constrains on planes normal to the embedded sensor's axis is biaxially loaded, (b) when the same isotropic host material is subjected to hydrostatic pressure and (c) when the hydrostatically loaded host material is an anisotropic one, as in the case of a composite material, where the optical fiber is embedded along the reinforcing fibers. The comparison of the experimental results and the finite element simulations shows that, when the axial strain on the FBG sensor is the dominant component, the standard wavelength-shift strain relation can be used even if large lateral strains apply on the sensor. However when this is not the case, large errors may be introduced in the conversion of the wavelength to axial strains on the fiber. This situation arises when the FBG is placed parallel to high modulus reinforcing fibers of a polymer composite.

Intrinsic Temperature Sensitivity of Fiber Bragg Gratings in PMMA-Based Optical Fibers

Thermal response of fiber Bragg gratings (FBGs) in polymethyl methacrylate-based optical fibers doped with trans-4-stilbenemethanol is studied at 0% relative humidity (RH) from -20°C to 60°C. The Bragg wavelength is found to negatively shift upon heating in a nonlinear manner. The higher the temperature, the larger the magnitude of wavelength shift is. The absolute value of calculated temperature sensitivity also increased with temperature. The sensitivity at room temperature is about -1 pm/°C, which is one order of magnitude lower than that measured at constant RH for polymer FBGs and that of silica FBGs. Such low temperature sensitivity challenges the common notion of high temperature sensitivity of polymer FBGs. In addition, it is found that the dependence of Bragg wavelength of the polymer FBGs on RH is nonlinear. Below 30% RH, the Bragg wavelength shift upon RH increase is minimal compared to that above 30% RH.

Life cycle monitoring and advanced quality assurance of L-shaped composite corner part using embedded fiber-optic sensor

L-shaped composite parts are structural key elements in complex-shaped aerospace composite structures, and life cycle cross-sectional strain change is their key. This study developed a fiber-optic-based life cycle monitoring system for L-shaped corner parts. A fiber Bragg grating (FBG) sensor was embedded in the through-thickness center of a corner part and the cross-sectional strain change was continuously evaluated by using a birefringence effect of the FBG. The embedded sensor could capture key strain changes throughout the structural life cycle, confirming that “non-axisymmetric strain” of the FBG sensor is a good indicator for spring-in distortion in manufacturing and through-thickness tensile failure in operation. The developed system was further utilized for advanced quality assurance of assembled L-shaped parts, and a new strength prediction method was proposed based on the FBG response. The monitoring system developed can provide a deep insight about the current internal state of L-shaped parts, and the information obtained is highly valuable for improving the design/manufacturing and for diagnosing the structural health.

Full-Spectral Interrogation of Fiber Bragg Grating Sensors Exposed to Steady-State Vibration

In this paper we measure for the first time the full-spectral response of a fiber Bragg grating (FBG) sensor subjected to vibration. We consider two cases: with and without an initial spectral distortion due to non-uniform strain along the length of the FBG. Previous work has measured only the dynamic response at a single wavelength which is valid when no spectral distortion is present. FBG sensors are mounted near the notch tip on a double edge notch specimen that is also subjected to harmonic vibration. We measure the full-spectral response of the FBG at 100 kHz applying an interrogator recently developed by the authors. The measurements of the FBG response with an initial spectral distortion clearly show the transient response and are verified through simulation. Finally, we demonstrate that the use of the high-speed, full-spectral interrogator permits the separation of the spectral distortion and the harmonic vibration from the FBG response signal through classical filtering and can therefore be applied to measure non-uniform strain fields in noisy environments.

Analysis of ultrasonic frequency response of surface attached fiber Bragg grating

In recent years, fiber Bragg grating (FBG), for the well-known advantages over other fiber optic sensors, has attracted more attention in ultrasonic inspection for structure health monitoring (SHM). Spectrum shift of FBG to ultrasonic wave is caused by the refractive index profile changing along the FBG, which can be attributed to nonuniform perturbation caused by strain-optic and geometric effects of ultrasonic wave. Response of FBG to the above two effects was analyzed firstly by the V-I transmission matrix model, showing high computing efficiency. Based on this model, spectra response of FBG under changing ultrasonic frequencies was simulated and discussed. In experiment, the system was able to detect a wideband ultrasonic wave ranging from 15 to 1380 kHz. These results would provide a guideline for an FBG-based acoustic detection system design in a specific ultrasonic frequency.

Dual functional performance of fiber Bragg gratings coated with metals using flash evaporation technique

Metallic and other type of coatings on fiber Bragg grating (FBG) sensors alter their sensitivity with thermal and mechanical stress while protecting the fragile optical fiber in harsh sensing surroundings. The behavior of the coated materials is unique in their response to thermal and mechanical stress depending on the thickness and the mode of coating. The thermal stress during the coating affects the temperature sensitivity of FBG sensors. We have explored the thermal response of FBGs coated with Al and Pb to an average thickness of 80nm using flash evaporation technique where the FBG sensor is mounted in a region at room temperature in an evacuated chamber having a pressure of 10−6Torr which will minimize any thermal stress during the coating process. The coating thickness is chosen in the nanometer region with the aim to study thermal behavior of nanocoatings and their effect on FBG sensitivity. The sensitivity of FBGs is evaluated from the wavelengths recorded using an optical sensing interrogator sm 130 (Micron Optics) from room temperature to 300°C both during heating and cooling. It is observed that the sensitivity of the metal coated fibers is better than the reference FBG with no coating for the entire range of temperature. For a coating thickness of 80nm, Al coated FBG is more sensitive than the one coated with Pb up to 170°C and it reverses at higher temperatures. This point is identified as a reversible phase transition in Pb monolayers as the 2-dimensional aspects of the metal layers are dominant in the nanocoatings of Pb. On cooling, the phase transition reverses and the FBGs return to the original state and for repeated cycles of heating and cooling the same pattern is observed. Thus the FBG functions as a sensor of the phase transitions of the coatings also.

Diametric load sensor using a fiber Bragg grating and its differential group delay analysis

Different from the basic sensing theory of the fiber Bragg grating (FBG) sensors based on spectral analysis, a new method exploiting the polarization properties of FBG for diametric load sensing is proposed. In this paper, the evolution of the differential group delay (DGD) response of FBG with respect to the diametric load is completely studied. The centroid information of the DGD evolution of FBG is subsequently proposed for diametric load sensing. Experiments were carried out to verify the reasonability and feasibility of the sensing method. Good agreements between experimental results and numerical simulations had been obtained.