Optical Fiber Bragg Grating Research Articles

Micrometeoroid/Orbital Debris (MMOD) Impact Detection and Location Using Fiber Optic Bragg Grating Sensing Technology

Spacecraft in low earth orbit (LEO) experience a variety of hazards including exposure to micrometeoroids and orbital debris (MMOD). Average impact speeds for orbital debris on spacecraft in LEO are 9 to 10km/s, and 20km/s for micrometeoroids. Due to their high speeds, MMOD can cause considerable impact damage to sensitive spacecraft surfaces such as windows, structural elements, electronic boxes, solar arrays, radiators, thermal protection system (TPS) materials covering crew/cargo return vehicles, as well as crew modules. Prolonged exposure to the on-orbit MMOD environment can potentially compromise the TPS covering return vehicles such as the future crewed vehicles expected to visit and remain for a half-year or longer at the International Space Station (ISS). However, determination of MMOD impact damage to the TPS on crew/cargo return vehicles, or damage to other orbiting spacecraft currently requires visual inspection. For human-rated spacecraft such as the ISS and, previously, the Space Shuttle Orbiter, this has required crew time as well as vehicle assets to identify damage due to MMOD strikes. For unmanned spacecraft, there are no human assets present to conduct detailed surveys to identify potential damage. While the practice of visual inspection may successfully indicate the location of a debris strike, it does not currently allow precise determination of exactly when the debris strike occurred. It is only possible to determine that a debris strike has occurred between two successive inspection events unless damage to other components can allow inference that a debris strike occurred at a specified time. Initial development testing of a novel MMOD sensing concept amenable for potential implementation in a structural health monitoring system is discussed. Using fiber Bragg grating (FBG) sensing technology, a test article representative of a spacecraft MMOD shield layer or spacecraft structure was subjected to hypervelocity impact testing. The FBG array successfully detected that an impact had occurred, when it occurred and where it occurred on the structure. Detection of both the impact wave and the residual strain in the structure can be used to infer the location of the impact. The potential application of this technology to spacecraft is discussed.

Improved KLT Algorithm for High-Precision Wavelength Tracking of Optical Fiber Bragg Grating Sensors

Fiber Bragg Gratings (FBGs) are among the most popular optical fiber sensors. FBGs are well suited for direct detection of temperature and strain and can be functionalized for pressure, humidity, and refractive index sensing. Commercial setups for FBG interrogation are based on white-light sources and spectrometer detectors, which are capable of decoding the spectrum of an FBG array. Low-cost spectrometers record the spectrum on a coarse wavelength grid (typically 78–156 pm), whereas wavelength shifts of 1 pm or lower are required by most of the applications. Several algorithms have been presented for detection of small wavelength shift, even with coarse wavelength sampling; most notably, the Karhunen-Loeve Transform (KLT) was demonstrated. In this paper, an improved algorithm based on KLT is proposed, which is capable of further expanding the performances. Simulations show that, reproducing a commercial spectrometer with 156 pm grid, the algorithm estimates wavelength shift with accuracy well below 1 pm. In typical signal-to-noise ratio (SNR) conditions, the root mean square error is 22–220 fm, while the accuracy is 0.22 pm, despite the coarse sampling. Results have been also validated through experimental characterization. The proposed method allows achieving exceptional accuracy in wavelength tracking, beating the picometer level resolution proposed in most commercial and research software, and, due to fast operation (>5 kHz), is compatible also with structural health monitoring and acoustics.

Analysis of fiber slip regulation and solution in array gratings fabrication

A kind of novel fabrication system of array optical fiber gratings is proposed and multi-wavelength array optical fiber gratings are obtained on 8-core ribbon optical fiber through this structure with only one phase mask. By means of image processing, the dynamic images are collected during the array gratings fabrication and the relative pixel shifts are monitored, and then fiber slip regulation are analyzed and obtained. In order to resist the influence of fiber slip, the specially-designed fixture is designed, and the material and shape of the clamping surface are improved many times. The experiment results show that the macrobending surface of the clamping surface can not only fix the ribbon optical fiber, but also keep the coating in good condition. We successfully obtained array optical fiber Bragg gratings with 0.2 nm bandwidth, wavelength interval less than 0.5 nm, wavelength deviation less than 0.05 nm and reflectivity more than 80% on ribbon optical fiber by the above system.

Irradiation Campaign in the EOLE Critical Facility of Fiber Optic Bragg Gratings Dedicated to the Online Temperature Measurement in Zero Power Research Reactors

The control of temperature during operation of zero power research reactors participates to the overall control of experimentation conditions and reveals itself of a major importance when measuring neutronic parameters such as multiplication factors. Within the framework of the refurbishment of the MASURCA facility, the development of a new temperature measurement system based on the optical fiber Bragg grating (FBG) technology is under consideration. In a first step, a series of FBGs is irradiated in the EOLE critical facility with the aim to select the most appropriate. Online temperature measurements are performed during a set of irradiations that should allow reaching a fast neutron fluence of nearly $5\cdot 10^{14}$ n.cm $^{-2}$ and a total gamma dose lower than 4 kGy. The results obtained, especially the Bragg wavelength shifts during the irradiation campaign, are discussed in this paper and compared to data from standard PT100 temperature sensors to highlight possible radiation effects on sensor performances. Work to be conducted during the second step of the project, aiming to a feasibility demonstration using a MASURCA assembly, is also presented.

UV-transparent fluoropolymer fiber coating for the inscription of chirped Bragg gratings arrays

A fluoropolymer optical fiber coating based on the thermoplastic copolymer of chlorotrifluoroethylene and vinylidene fluoride is presented. Such coatings can be used as a UV-transparent material for writing single Bragg gratings or arrays of chirped fiber Bragg gratings directly through the fiber coating with the use of excimer laser radiation at 248nm. As an optimum radiation density that does not lead to significant degradation of the fluoropolymer coating, an exposure time not exceeding 200s with a 10Hz laser pulses repetition rate at 70mJ/cm2 was identified. With such inscription parameters it was possible to inscribe arrays of fiber Bragg gratings in hydrogen-loaded birefringent optical fiber with an elliptical stress cladding through a 12µm thick coating, so that stripping of the coating is avoided and good mechanical strength is preserved. The reflection spectrum width of the chirped Bragg gratings was about 3.5nm with a reflectance coefficient of the most effective grating of up to 38%. Such Bragg grating arrays are especially interesting as reflective elements in fiber interferometers.

PMMA Based Optical Fiber Bragg Grating for Measuring Moisture in Transformer Oil

Water contamination can cause serious problems that compromise in transformer's safe operation and reduce its lifetime. Online monitoring of moisture concentration in transformer oil would permit the control of moisture buildup. This letter presents a direct optical measurement of moisture concentration in transformer oil using a poly(methyl methacrylate) (PMMA)-based optical fiber Bragg grating (POFBG). The refractive index and volume of PMMA-based optical fiber vary with the moisture in the surrounding transformer oil, changing the reflecting wavelength of the grating. A sensitivity of POFBG wavelength change to moisture content of 29 pm/ppm is demonstrated in this letter, indicating detectable water content better than 0.05 ppm.

Identification of modal strains using sub-microstrain FBG data and a novel wavelength-shift detection algorithm

The presence of damage in a civil structure alters its stiffness and consequently its modal characteristics. The identification of these changes can provide engineers with useful information about the condition of a structure and constitutes the basic principle of the vibration-based structural health monitoring. While eigenfrequencies and mode shapes are the most commonly monitored modal characteristics, their sensitivity to structural damage may be low relative to their sensitivity to environmental influences. Modal strains or curvatures could offer an attractive alternative but current measurement techniques encounter difficulties in capturing the very small strain (sub-microstrain) levels occurring during ambient, or operational excitation, with sufficient accuracy. This paper investigates the ability to obtain sub-microstrain accuracy with standard fiber-optic Bragg gratings using a novel optical signal processing algorithm that identifies the wavelength shift with high accuracy and precision. The novel technique is validated in an extensive experimental modal analysis test on a steel I-beam which is instrumented with FBG sensors at its top and bottom flange. The raw wavelength FBG data are processed into strain values using both a novel correlation-based processing technique and a conventional peak tracking technique. Subsequently, the strain time series are used for identifying the beam's modal characteristics. Finally, the accuracy of both algorithms in identification of modal characteristics is extensively investigated.

Recording Bragg gratings in hydrogenated birefringent optical fiber with elliptical stress cladding

This paper presents comparative results of the recording of fiber Bragg gratings in birefringent optical fiber with an elliptical stress coating, subjected and not subjected to hydrogenation, as well as with an increased concentration of germanium dioxide. It is shown that both techniques for increasing the photorefraction of a fiber lightguide are effective for solving the problem of recording Bragg gratings. Time dependences are obtained for the shifts of the Bragg resonances of fiber gratings recorded in hydrogenated and unhydrogenated optical fibers.

On the use of strain sensor technologies for strain modal analysis: Case studies in aeronautical applications.

This paper discusses the use of optical fiber Bragg grating (FBG) and piezo strain sensors for structural dynamic measurements. For certain industrial applications, there is an interest to use strain sensors rather than in combination with accelerometers for experimental modal analysis. Classical electrical strain gauges can be used hereto, but other types of strain sensors are an interesting alternative with some very specific advantages. This work gives an overview of two types of dynamic strain sensors, applied to two industrial applications (a helicopter main rotor blade and an F-16 aircraft), FBG sensors and dynamic piezo strain sensors, discussing their use and benefits. Moreover, the concept of strain modal analysis is introduced and it is shown how it can be beneficial to apply strain measurements to experimental modal analysis. Finally, experimental results for the two applications are shown, with an experimental modal analysis carried out on the helicopter main rotor blade using FBG sensors and a similar experiment is done with the aircraft but using piezo strain sensors instead.

Perimeter System Based on a Combination of a Mach-Zehnder Interferometer and the Bragg Gratings

Fiber optic interferometers and Bragg gratings belong to the group of very precise and sensitive devicesthat allow measuring very small deformation, temperature or vibration changes. The described methodology presentsthe useof a Mach-Zehnder interferometer and Bragg gratings together as a sensor system for detecting and monitoring movement within thedefined perimeter of 2.5×1m. Analyses of the dynamic changes in interferometric patterns were a basis for this method. Also the signal maximum amplitude was measured and compared with the noise background. Perimeter disruptions can be detected by Bragg gratings due to its large deformation sensitivity in transversal or perpendicular directions. The result is then evaluated in the spectral domain. In terms of detected persons it showed very good results. The combination of these sensors was chosen for monitoring both the static and dynamic phenomena. Author's aim is to take advantage of both devices' positive properties. Thus, the system has the abilityto identify people due to frequency analysis in case of interferometers as well as dynamic weighting thanks to Bragg gratings.

A novel method of identifying damage types in carbon fiber-reinforced plastic cross-ply laminates based on acoustic emission detection using a fiber-optic sensor

We applied a highly sensitive phase-shifted fiber-optic Bragg grating (PS-FBG) sensor with a broad bandwidth to acoustic emission (AE) detection in a composite material. Based on AE detection, this research proposed a novel method to identify damage types in carbon fiber-reinforced plastic (CFRP) laminates. Because the PS-FBG sensor measured pure dynamic strain, the detected AE signals had great physical reliability, which made elastic wave theory suitable for analysis of the signal. The analysis clarified the different characteristics of Lamb wave modes in three types of AEs caused by a transverse crack, delamination and fiber breakage. Additionally, the ratios of the amplitudes of the S0 mode to the A0 mode and the peak frequencies quantitatively evaluated the mode characteristics in the AE signals. Simultaneously using the two physical parameters, we identified the three types of damage among the AE signals detected in a three-point bending test. Furthermore, the finite element method-based AE wave propagation simulation agreed well with the identification results. Hence, the proposed identification method with the PS-FBG sensor has great physical reliability for evaluating damage in composite laminates.

Total Temperature Measurement of Fast Air Streams With Fiber-Optic Bragg Grating Sensors

The suitability of using fiber Bragg gratings (FBG) as sensor elements in total temperature probes for turbine compressor applications is shown. A series of five FBG-based probes was built and characterized experimentally in the harsh environment of an open-jet test bench at air stream velocities of up to Mach 0.8. The performances of the FBG-based probes were compared with that of a conventional thermocouple-based total temperature sensor. All fiber-optic probes showed good mechanical robustness and recovery factors in the range of 0.79–0.99. Employing fiber-optic sensor elements allowed a significant reduction of the diameters of the stagnation tubes of the transducers. Because of the smaller probe design and thinner signal transmission cables, less distortion of the air stream is expected with the fiber-optic sensors, when compared with the conventional instrumentation.

English

English

Compaction in Optical Fibres and Fibre Bragg Gratings Under Nuclear Reactor High Neutron and Gamma Fluence

Compaction in Optical Fibres and Fibre Bragg Gratings Under Nuclear Reactor High Neutron and Gamma Fluence

Aviation Fuel Gauging Sensor Utilizing Multiple Diaphragm Sensors Incorporating Polymer Optical Fiber Bragg Gratings

A high-performance fuel gauging sensor is described that uses five diaphragm-based pressure sensors, which are monitored using a linear array of polymer optical fiber Bragg gratings. The sensors were initially characterized using water, revealing a sensitivity of 98 pm/cm for four of the sensors and 86 pm/cm for the fifth. The discrepancy in the sensitivity of the fifth sensor has been explained as being a result of the annealing of the other four sensors. Initial testing in JET A-1 aviation fuel revealed the unsuitability of silicone rubber diaphragms for prolonged usage in fuel. A second set of sensors manufactured with a polyurethane-based diaphragm showed no measurable deterioration over a three month period immersed in fuel. These sensors exhibited a sensitivity of 39 pm/cm, which is less than the silicone rubber devices due to the stiffer nature of the polyurethane material used.

Evaluation of the physical properties of dental resin composites using optical fiber sensing technology

ObjectivesThe characterization of the physical properties of dental resin composites is fraught with difficulties relating to significant intra and inter test parameter variabilities and is relatively time consuming and expensive. The main aim of this study was to evaluate whether optical fiber Bragg grating (FBG) sensing system may become a viable tool to study dental material characteristics. Of particular focus was the potential for the system to demonstrate a multi parameter all-in-one feature. MethodsA miniature FBG was embedded in six different dental resin composites and employed as a sensor to evaluate linear polymerization shrinkage, thermal expansion and water sorption. Six commercially available dental composites with different filler types and volume are evaluated. The tests are repeated with three sets of samples. The curing characteristics and residual strain gradient exhibited by the cured dental composites were also observed and commented. ResultsAmong the studied samples, SDR shows lowest polymerization shrinkage, while Beautifil FO3 shows the highest. The results also show clear distinction between particle filler type and fiber reinforcement based composites in their polymerization shrinkage properties. The agreement of the results with existing literatures show that FBG based system provides accurate results. Polymerization shrinkage rate of the samples are also obtained. Thermal expansion of the composites are measured using the FBG sensing method for the first time and is correlated with resin type, volume, filler type and glass transition temperature. The water sorption characteristics of the dental composite are also successfully measured using the FBG sensing method. The high level of repeatability and the low standard deviations shown in the results indicate good reliability with the use of FBG sensors. SignificanceThis study demonstrates how optical fiber technology can provide simple and reliable methods of measuring the critical physical properties of dental composites. In addition due to the embedding and preservation of the sensor within the samples multiple parameters can be tested for with the same sample. These features are expected to greatly assist material science researchers in dentistry as well as other biomedical fields. Of some interest the phenomenon of stress relaxation of dental composite at higher temperature was observed.

Characterization of Polymer Fiber Bragg Grating With Ultrafast Laser Micromachining

We report that the main photosensitive mechanism of poly(methyl methacrylate)-based optical fiber Bragg grating (POFBG) under ultraviolet laser micromachining is a complex process of both photodegradation and negative thermo-optic effect. We found experimentally the unique characteristics of Bragg resonance splitting and reunion during the laser micromachining process providing the evidence of photodegradation, while the mean refractive index change of POFBG was measured to be negative confirming further photodegradation of polymer fiber. The thermal-induced refractive index change of POFBG was also observed by recording the Bragg wavelength shift. Furthermore, the dynamic thermal response of the micromachined-POFBG was demonstrated under constant humidity, showing a linear and negative response of around −47.1 pm/°C.

Numerical Analysis of Thermal Dependence of the Spectral Response of Polymer Optical Fiber Bragg Gratings

Numerical Analysis of Thermal Dependence of the Spectral Response of Polymer Optical Fiber Bragg Gratings

Measurement of sound pressure and temperature in tissue-mimicking material using an optical fiber Bragg grating sensor.

The experimental investigation of an optical fiber Bragg grating (FBG) sensor for biomedical application is described. The FBG sensor can be used to measure sound pressure and temperature rise simultaneously in biological tissues exposed to ultrasound. The theoretical maximum values that can be measured with the FBG sensor are 73.0MPa and 30°C. In this study, measurement of sound pressure up to 5MPa was performed at an ultrasound frequency of 2MHz. A maximum temperature change of 6°C was measured in a tissue-mimicking material. Values yielded by the FBG sensor agreed with those measured using a thermocouple and a hydrophone. Since this sensor is used to monitor the sound pressure and temperature simultaneously, it can also be used for industrial applications, such as ultrasonic cleaning of semiconductors under controlled temperatures.

The Role of Photo-Striction in Tailoring the Nano-Scale Phase Changes in Amorphous Selenium Thin Films

The photo-structural changes in a 2 μm thick amorphous selenium (a-Se) thin film, thermally evaporated on Si substrate, have been studied by micro-Raman spectrometer with 532 nm laser at different laser power densities (1 to 64 W/mm 2 ) and exposure times (15 – 60 s). In addition, the nanoscale photo-structural changes/photo-thermodynamic phase changes are examined on 90 nm a-Se film coated on SiO 2 cladding of the optical Fiber Bragg Grating (FBG) sensor on prolonged exposure to a low power (1 mW/mm 2 , 10 mW/mm 2 ) 532 nm laser, which is measured by means of photo-striction (light induced strain) in the material.