Fiber-optic temperature profiling for thermal protection system heat shields

For the improvement of monitoring accuracy, a vibration monitoring for aircraft wing model using a fiber Bragg grating (FBG) array packaged by vacuum-assisted resin transfer molding (VARTM) is proposed. The working principle of the vibration monitoring using FBG array has been explained, which can theoretically support the idea of this paper. VARTM has been explained in detail, which is suitable for not only the single FBG sensor but also the FBG array within a relatively large area. The calibration experiment has been performed using the FBG sensor packaged by VARTM. The strain sensitivity of the VARTM package is 1.35 pm/μϵ and the linearity is 0.9999. The vibration monitoring experiment has been carried out using FBG array packaged by VARTM. The measured rate of strain changes across the aluminum test board used to simulate the aircraft wing is 0.69 μϵ/mm and the linearity is 0.9931. The damping ratio is 0.16, which could be further used for system performance evaluation. Experimental results demonstrate that the vibration monitoring using FBG sensors packaged by VARTM can be efficiently used for the structural health monitoring. Given the validation and great performance, this method is quite promising for in-flight monitoring and holds great reference value in other similar engineering structures.

A fully polymer-based flexible arc-shaped three-dimensional (3D) fiber Bragg grating (FBG) array bionic tactile sensor is presented, in which both the FBG array and the encapsulation layer are made of polymers, and its application in multidirectional force identification of mechanical finger is verified. This innovative design utilizes the unique flexibility, biocompatibility, and mechanical properties of polymers to achieve excellent integration and performance consistency. The FBG array, with three FBGs inscribed in a single-mode polymer fiber using femtosecond laser micromachining, featuring progressively increasing lengths to compensate for the reflectivity degradation typically observed in conventional FBGs. The 3D arc-shaped arrangement of the FBG array endows the sensor with the ability to achieve 3D force perception, including normal stress, left shear stress, and right shear stress. By accurately measuring the wavelength shifts of the FBGs under different force stimuli, the sensor can accurately distinguish multi-directional forces. Experimental results show that the sensor has high force sensitivities to the left shear force, normal force, and right shear force with sensitivities of 4.973nm/N, 4.363nm/N, and 4.746nm/N, and measurement resolutions of 0.02N, 0.023N, and 0.021N, respectively, within a verified linear working range of 0 N to 0.8 N. Its sensitivity is as high as 17 times that of the traditional quartz optical fiber FBG sensor. our sensor can accurately measure multi-dimensional forces with very good sensitivity and significantly reduce the crosstalk caused by humidity, demonstrating very good performance. Its all-polymer design exhibits high flexibility and environmental robustness—completely eliminating crosstalk caused by humidity changes. It is expected to have potential applications in advanced robotics and intelligent tactile sensing systems.

The monitoring of temperature during operation of lithium ion batteries is crucial for a stable and safe performance of a battery system. However, a major challenge is to obtain exact internal temperature progression upon charge and discharge processes. Though commonly used resistance temperature sensors can give considerably accurate temperature values, they are limited to the application outside of the cell due to the distortion of their signal through induction effects of the applied cell currents, if installed inside the pouch cell and therefore, they are not suitable for in situ measurements. This poses serious problems especially for big cell stacks, as core temperatures for those cells differ greatly from the outer parts and particularly the surface of the cell encasement. Fiber Bragg Grating (FBG) sensors on the other hand, provide data that are unaffected by induction effects as their operating principle is solely based on the reflection of light of certain wavelengths within a glass fiber. However, a specific difficulty is the distinction between temperature and pressure effects, as the reflected signal is sensitive to both. Often times additional tubing around the sensor is needed to eliminate pressure effects or an additional sensor is placed in close proximity, inside or outside, to the FBG-sensor as a reference. The former, however, can affect the cell stack physically acting as a defect spot with unwanted side effects.In our work, we show that a careful calibration of the FBG-sensor can be accomplished, so that both effects can be separated and the FBG-sensor can then be used to gather information not only about temperature but also about volume changes in an operating cell. Therefore, a temperature calibration took place, with the cell being held at a constant SoC. Subsequently, a calibration of the SoC - dependent pressure-induced mechanical deformation was performed at very low C-rate to minimize thermal effects. The so gathered calibration parameters can then be applied to all performed cycling tests, regardless of the C - rate. A calibration routine and mathematical solution to separate the influences of temperature and pressure to receive exact in situ temperature values for a 10 Ah lithium ion battery cell is presented. The calibration method is applied to cycling and rate tests at various C-rates.The results show, that wavelength shifts upon charging and discharging processes are mainly resulting from volume changes of the active cell components, meaning that the FBG – sensor has a very high sensitivity towards pressure changes. With the before mentioned calibration method the impact of mechanical deformation on the FBG - sensor can be subtracted from the raw data and allow for a direct temperature reading. The temperature data correlates with data gathered from commonly used sensors placed on the outside of the pouch cell. Tests with varying discharge rates confirm these results and show, that the method produces precise temperature data even at different C-rates, even though only slow C-rates were applied during calibration.In detailed comparison, the FBG - sensors show a faster response towards temperature changes compared to conventional external temperature sensors, which can be related to the insulating effect of the pouch foil. In case of high discharge rates, we were able to show that the measured temperatures inside of the pouch cell are higher in comparison to the outside ones. Furthermore, SoC - induced changes in pressure measured by two FBG-sensors, which were located at different positions on the cell stack, are almost identical. This indicates that pressure build-up within a pouch cell occurs evenly through the cell stack.

Over the past years, Thulium (Tm) -doped fiber lasers in the 2μm region have gained a lot of interest due to many potential applications in materials processing and biophotonics. Based on the broad gain regions spanning from 1800nm to 2100nm, they offer the perfect basis to implement broadly tunable and user-friendly light sources like they are increasingly demanded in spectroscopic applications. Recently, a novel tuning mechanism based on a fiber Bragg grating (FBG) array as versatile spectral filter has been reported. This concept combines unique spectral freedom for customized tuning ranges and ultrabroad bandwidths with a fiber-integrated setup in order to maintain the advantages of the waveguide geometry. In this work, we demonstrate such a dispersion tuned and pulsed fiber laser in the Tm domain around 1950nm using a modulator and a discrete FBG array to control the emission wavelength. In order to comply with the demands of potential applications in biophotonics, for the first time, this tuning concept is realized in a polarization maintaining (PM) configuration ensuring linearly polarized output. While a simple FBG array is employed containing five gratings inscribed in PM fiber, we also outline the prospect to implement FBGs fabricated in a standard single mode fiber. The emission characteristics of the system are investigated showing pulse durations down to 11ns and a good spectral signal contrast. In order to highlight the prospect for tunable high-power operation, we have also implemented an amplification stage scaling the average power to more than 25W.

An efficient fiber Bragg grating (FBG) sensor integration system, which utilizes a small number of standard Telecom opto-electronics components, is presented in this paper. Wavelength-swept optical pulses were generated by driving a standard telecom DFB laser diode by high amplitude (>3 A) and short duration (300 ns) current pulses. A total laser diode's wavelength sweep in excess of 10 nm was achieved, which allows for simultaneous interrogation of several FBGs within similar wavelengths. The application of short duration wavelength-swept optical pulses also allows for time division multiplexing of small FBG arrays containing FBGs with overlapping characteristic wavelengths. When a short, wavelength-swept optical pulse is launched into the fiber containing single or multiple FBGs along its length, fractions of the launched pulse are back-reflected towards the detector. Since the launched pulse wavelength changes over the time, the reflections from the FBGs occur in the time moments that depend on the FBGs' characteristic wavelengths. Measurement of the time delay among back-reflected optical pulses is then used to determine the FBGs characteristic wavelengths. The experimental system demonstrated FBG wavelength readout resolution, which exceeded 3 pm at 1 KHz sampling rate, maximum sampling rate of over 40 KHz, and capability to readout/demultiplex over 30 FBGs located down a single optical fiber.

In this study, a down-scaled wind turbine blade was designed and fabricated using glass and carbon fiber materials for the skin and stiffener, respectively. In the course of its fabrication, an array of FBG (fiber Bragg grating) sensors was embedded in the composite laminates. The embedded FBG sensor array was used to measure the residual strain in the stiffener before and after the curing process, and after fabrication of the blade, the FBG array was used to monitor the structural conditions, including structural dynamic behavior during the structural testing of the blade. The results of the tests showed that the FBG sensor array effectively measured the residual strain distribution of the blade stiffener before and after the curing process. And the measured natural frequencies and mode shapes by the FBG array matched the results obtained from the FE analysis and conventional accelerometers.

We demonstrated a MHz speed wavelength-swept fiber laser based on the active mode locking (AML) technique and applied to interrogation system of an array of fiber Bragg grating (FBG) sensors. MHz speed wavelength sweeping of wavelength-swept fiber laser can be obtained by programmable frequency modulation of the semiconductor optical amplifier (SOA) without any wavelength tunable filter. Both static and dynamic strain measurement of FBG sensors were successfully characterized with high linearity of an R-square value of 0.9999 at sweeping speed of 50 kHz.

A three element, 15.3 cm, fiber Bragg grating array (FBGA) operating at 1550 nm wavelength is fabricated using a single mode photosensitive fiber. The FBGA is initially simulated using in-house developed software based on the Transfer Matrix Method, then fabricated using a double frequency Argon laser and a phase mask technique, and interrogated using Optical Frequency Domain Reflectometry. A single fiber Bragg grating (FBG) is accurately strain calibrated using a Fabry-Perot interferometer and piezoelectric actuation. The piezoelectric is linearly ramped, and the shifts in the Bragg wavelength along with the fringe count from the Fabry-Perot interferometer are recorded. The fringe count is then used to determine the strain on the FBG and compared to changes in the Bragg wavelength in-order to calculate the strain gage factor. This result is used to calibrate the FBGA for strain measurements. The FBGA is then bonded to a cantilever beam with three electric strain gages attached next to each FBG in the array. The axial strain results obtained from the electric strain gages and FBGA are compared for various displacements of the cantilever beam. The Fabry-Perot interferometer and piezoelectric calibration method is a non-destructive process that eliminates the need to bond the FBG to an external support during the calibration process, and can also be used to calibrate electric strain gages.

We have studied health-monitoring systems using embedded optical fiber sensors for damage detection in FRP (Fiber Reinforced Plastics) composite laminates. For sensing of strain or temperature changes in these composite materials, we developed the FBG (Fiber Bragg Grating) sensors using the polyimide-coated small-diameter optical fiber with cladding diameter of 40 micrometers , which can be embedded in FRP composite laminates without inducing any structural defects. The FBG was subjected to high-temperature heat treatment at 300 degree(s)C in order to suppress the optical characteristics change of the FBG over a long period of time. Even after such high temperature treatment, the FBG retained sufficient mechanical strength and reflectivity. We embedded the FBG into FRP composite laminates and measured the optical characteristics against temperature and tensile strain. These experimental results show this type of the embedded FBG is very promising for practical health-monitoring systems. In addition, we have investigated various multipoint measuring systems and propose the time division multiplexing system using these FBG sensors.

Fiber Bragg Grating (FBG) sensor is a new type of optical sensor that is light in quality, small in volume, easy to achieve distributed measurement, and easy to embed. When FBG sensor technology is introduced to the damage detection of the CFRP shaft, the shortcomings of conventional damage detection technologies, including ultrasonic testing and radiographic testing, can be effectively overcome. Considering the working load, operating environment and life requirements of the CFRP shaft , combined with the relevant national and industry standards for composite material testing, Prepare three CFRP shaft with embedded fiber gratings (non-destructive CFRP shaft, single hole CFRP shaft , double hole CFRP shaft), the hole diameter is 1mm, and the layering of CFRP shaft is ([0 / 90 / 45 / 0 / 0 / 45 / -45 / 0 / -45 / 0 / 90 / 90 / 0 / -45 / 0 / -45 / 45 / 0 / 0 / 45 / 90 / 0 ]). FBG is embedded between 18 and 19 layers, pasted on the surface of CFRP shaft with 10 FBG sensors (named FBG1, FBG2... FBG10). The strain information of the CFRP shaft is obtained through fiber grating to obtain the deformation of the CFRP shaft under a three-point bending load. Each CFRP shaft is loaded 12 times (A1, A2, A3, B1, B2, B3, C1, C2, C3, D1, D2, D3). Through the BP neural network method, the mapping relationship between strain distribution and damage model was established. In the model, the FBG sensor measures the strain of the CFRP shaft and forms the input of the neural network. Combined with the damage situation of the composite material, the learning samples of the network formed. After learning and training, the neural network can damage recognition. To better verify the accuracy of the neural network to distinguish and identify the CFRP shaft non-destructive, single-hole, and double-hole damage patterns. The finite element analysis was used to simulate the damage of three CFRP shafts under the same experimental conditions, and extract the eigenvalues at the same position to form the input of the network, combined with the damage situation of the composite material, constitutes a learning sample of the network. The experiment shows that the CFRP shaft experiment based on BP neural network verifies the reliability of the CFRP shaft damage recognition method. The finite element analysis is used to extract the strain information of CFRP shaft measuring points with different damage modes. The damage position information and strain information were input into the BP neural network. The experimental results are in good agreement with the simulation results, which verifies the accuracy of the BP neural network to distinguish the CFRP shaft non-destructive, single-hole, and doublehole damage patterns.

This paper proposes multipoint optical fiber sensing system based on fiber Bragg grating (FBG) sensor array. The low reflectivity FBG sensors having the same Bragg wavelength are connected cascaded in the same fiber line as the sensing elements of optical fiber sensing system. This can increase the monitoring points and reduce manufacturing costs of FBG sensor. To verify the effectiveness, the system simulator based on equivalent circuit model by microwave technology was developed. Simulation result shows that though multi-reflections can be caused by FBG sensors with the same Bragg wavelength connected cascaded in the same line, the effect may become weak as reflectivity of FBG sensors are reduced to 0.02.

This paper presents the experimental work on distributed temperature sensing making use of Fiber Bragg grating (FBG) array sensor for possible applications in the monitoring of temperature profile in high temperature boilers. A special sensor has been designed for this purpose which consists of four FBGs (of wavelengths λB1 = 1547.28 nm, λB2 = 1555.72 nm, λB3 = 1550.84 nm, λB4 = 1545.92 nm) written in hydrogen loaded fiber in line with a spacing of 15 cm between them. All the FBGs are encapsulated inside a stainless steel tube for avoiding micro cracks using rigid probe technique. The spatial distribution of temperature profile inside a prototype boiler has been measured experimentally both in horizontal and vertical directions employing the above sensor and the results are presented.

We report on the use of a frequency-rnodulated continuous wave (FMCW) technique for multiplexing fiber Bragg grating (FBG) sensors. This technique is based on the modulation of light intensity from a broadband source by a linear swept-frequency RF carrier. Signals from the FBG sensors located at different positions in an array are separated in frequency-domainand demodulated using a tunable optical filter. The potential and limitation of the technique are discussed. A 3-sensor FMCWmultiplexed FBG array of parallel topology and a 6-sensor hybrid FMCWIWDM system were experimentally demonstratedwith -30 dB crosstalk between sensors and 2 tc resolution in terms ofroot-mean-square strain value.Keywords: optical fiber sensors, fiber Bragg grating sensors, frequency-modulated continuous wave, strain measurements. 1. Introduction Fiber Bragg gratings (FBGs) have been identified as an enabling technology for a variety of applications from aerospace, marine, to civil engineering structural monitoring, from medical, nuclear power industry, to electrical power industry[1]. In many applications such as condition monitoring of civil structures such as highways, bridges and buildings. FBG sensorsneed to be multiplexed in order to achieve quasi-distributed measurement and to reduce cost per sensing point to compete withconventional electrical or optical sensors.The most popular technique for multiplexing FBG sensors is the wavelength division multiplexing (WDM) technique[2,3]. The maximum sensor number that can be multiplexed using the WDM technique is determined by the ratio ofthe sourcespectral width over the spacing between the Bragg wavelengths of the FBGs. For applications requiring a larger multiplexinggain, a combination ofthe WDM and time-division multiplexing (TDM) techniques [3] may be used.A TDM system employs a pulsed or gated CW source to illuminate gratings in sequence as the pulse travels along thefiber, the reflected signals from the gratings are separated in time domain [4]. The physical separation between the gratings isdetermined by the input pulse width. A fast time-gated photodetector can be used to select the signal from an individual grating

One challenging need for inspection capabilities is in adhesively bonded joints between composite components, a common location of premature failure in aerospace structures. In this work we demonstrate that dynamic, full spectral scanning of FBG sensors embedded in the adhesive bond can identify changes in bond quality through the measurement of non-linear dynamics of the joint. Eighteen lap joint specimens were fabricated with varying manufacturing quality. Ten samples also included fiber Bragg grating (FBG) sensors embedded in the adhesive bond for real-time inspection during a simulated flight condition of these single-lap joints. Prior to testing, pulse phase thermography imaging of the pristine specimens revealed defects such as air bubbles, adhesive thickness variations, and weak bonding surface between the laminate and adhesive. The lap joint specimens were then subjected to fatigue loading, with regular interrogation of the FBG sensors at selected load cycle intervals. The FBG data was collected during vibration loading of the lap joint to represent an in-flight environment. Changes in the lap joint dynamic response, including the transition to non-linear responses, were measured from both the full-spectral and peak wavelength FBG data. These changes were correlated to initial manufacturing defects and the progression of fatigue-induced damage independently measured with pulse phase imaging and visual inspections of the failure surfaces.

Optical fiber Bragg grating (FBG) sensors have significant potential for use as embedded devices to monitor the structural integrity of engineering materials. The main drawback of the FBG strain sensor is its cross-sensitivity to temperature. This paper reports a simple scheme for multiplexing a FBG and an extrinsic Fabry-Perot interferometric (EFPI) sensor to enable the decoupling of strain from temperature. The EFPI sensor was constructed using a precision bore quartz capillary tube which housed two cleaved optical fibers. The gap between the fiber end- faces served as the Fabry-Perot cavity. Since the coefficients of thermal expansion between the optical fiber and the capillary tube were similar, the EFPI sensor has a very low sensitivity towards temperature. Therefore, when both sensors are placed close together, the EFPI sensor can act as the strain sensor, and temperature can be determined from the FBG wavelength shift after taking out the strain effect. The signal processing for the EFPI sensor was based on a channelled spectrum method using a CCD spectrometer. The same CCD spectrometer was also used to determine the wavelength shift of the FBG. The cross-talk between the EFPI and FBG sensors was evaluated. The feasibility of conducting simultaneous strain and temperature measurements was demonstrated.