Optical fiber sensor fusion for aerospace systems lifecycle management

The measurement of the internal deformations occurring in real-life composite components is a very challenging task, especially for those components that are rather difficult to access. Optical fiber sensors can overcome such a problem, since they can be embedded in the composite materials and serve as in situ sensors. In this article, embedded optical fiber Bragg grating (FBG) sensors are used to analyze the vibration characteristics of two real-life composite components. The first component is a carbon fiber-reinforced polymer automotive control arm; the second is a glass fiber-reinforced polymer aeronautic hinge arm. The modal parameters of both components were estimated by processing the FBG signals with two interrogation techniques: the maximum detection and fast phase correlation algorithms were employed for the demodulation of the FBG signals; the Peak-Picking and PolyMax techniques were instead used for the parameter estimation. To validate the FBG outcomes, reference measurements were performed by means of a laser Doppler vibrometer. The analysis of the results showed that the FBG sensing capabilities were enhanced when the recently-introduced fast phase correlation algorithm was combined with the state-of-the-art PolyMax estimator curve fitting method. In this case, the FBGs provided the most accurate results, i.e., it was possible to fully characterize the vibration behavior of both composite components. When using more traditional interrogation algorithms (maximum detection) and modal parameter estimation techniques (Peak-Picking), some of the modes were not successfully identified.

This is an ambitious book aimed at introducing the relatively new concepts and possibilities of optical fibre sensors for structural monitoring to the uninitiated who have an engineering or general physics background.Measures draws the reader into the volume with a description of smart structures - the structural monitoring equivalent of artificial nervous systems - before a series of back-to-basics tutorials on optical theory and photonic technology.The emphasis on smart structures early in the book is a worthy attention-grabber since it elevates the subject of structural health monitoring above just another set of techniques for making engineering measurements. The promise is to `revolutionize engineering design philosophy' by creating `intelligence within otherwise inanimate structures'.In the latter two thirds of the book, the author steps through the main issues of structural monitoring using fibre optic sensors. Intensity-based, interferometric, polarimetric and spectral sensors (including the ubiquitous Bragg grating) are compared and contrasted. The hot topic of strain versus temperature discrimination in fibre sensors earns a whole chapter and several useful techniques for overcoming this cross-sensitivity are portrayed.Installation of sensors is also discussed with reference to retro-fit and co-manufacturing (embedding) approaches. Examples of concrete constructions such as bridges (a frequent theme in the book) and fibre-reinforced plastics such as glass-fibre and carbon composite materials are considered.A chapter on `short-gauge' sensors and applications deals in some depth with the Bragg grating as a strain sensor. The methods of multiplexing and interrogating these devices are explored with many examples from both Measures' own research and the work of other groups worldwide. The Beddington Trail bridge trial in Calgary, one of the first such installations of Bragg gratings, followed by the more ambitious Confederation Bridge, also in Canada, provide concrete examples of the technology's application.The material is marred somewhat by the inferior reproduction of some of the photographs, especially those showing field installations of the optical sensors.Other applications are not neglected. A description of trials aboard a Norwegian Naval vessel with composite hull monitored by Bragg gratings is also given.Interferometric sensors in similar applications trials are also covered in chapters on short and long gauge length devices.Distributed strain and temperature sensing techniques using Fourier transform, low coherence and stimulated backscattering are covered in the penultimate chapter, which draws together distributed measurementat a small physical scale in the form of intra-Bragg grating strain profile measurements (on the scale of millimetres)and measurements over kilometres using stimulated Brillouin scattering.In this reviewer's opinion the book dwells on strain monitoring in civil engineering structures at the expense of a broader scope, which could have included, for example, the detection of impacts or the acoustic emissions from crack propagation and other forms of structural damage.Nevertheless, this volume is an impressive collection of background and examples of real applications in heavyweight engineering. It adds significantly to the claim that fibre optic sensors have at last arrived.Peter Foote

The field of communications has been revolutionized by the advent of optical fiber. Optical fiber now connects most of the world carrying a vast amount of information through a very limited physical medium. To the materials engineer, optical fiber technology has stimulated interest in a new type of micro-sensor application, where the size, weight, and the ability to integrate the sensor into a material structure play a major role. Areas of interest currently include materials characterization, cure monitoring, and structural health monitoring. The fiber Bragg grating (FBG) is such a sensor technology, creating an optical strain gauge within the core of an optical fiber through the use of a wavelength specific filter. As the FBG experiences induced strain along its major axis, its light signal indicates the amount of strain with great accuracy and sensitivity. In FBG applications, where loading may occur in all directions, complex changes take place in the FBG signal. As these changes impact the usefulness of the FBG as a strain sensor, this thesis endeavors to study and demonstrate the effect of transverse load on the FBG structure. A review of the basics of optical fiber fabrication, light transmission, and optical fiber sensor technology is presented first, followed by a detailed discussion of the FBG's manufacturing and operation. The crux of the thesis is presented with the results of experiments conducted to characterize the FBG signal in both unloaded and transversely loaded configurations. Polarization and wavelength based experiments and analyses are conducted to detect changes in the FBG signal. The results of the study are used to propose cause and effect relationships between transverse loading, its effect on the material properties of the FBG, and changes in the FBG signal.

Optical fibres (OF) with inscribed arrays of fibre Bragg gratings (AFBG) are interesting for different types of fibre devices such as a fibre sensors and fibre lasers and optical signal management. The most advanced method of manufacturing such AFBG is to write them directly in-line during the fibre drawing process. This gives the possibility of making a huge number of fibre Bragg gratings (FBG) in a single OF. The parameters of these AFBG could be constant or variable along the OF length. We present results of an investigation AFBG produced during fibre drawing process. Typical parameters of such AFBG are: working wavelength into 1550 nm band, spectral width 0.2mm (3dB level), reflective contrast (excess to Rayleigh backscattering level) up to 45 dB. We use polymer or aluminum as AFBG coating. AFBG can be written either in single or multi core preforms. OF with fibre Bragg gratings A(FBG) arrays written over a long length and in-line metal coating have been made for the first time. The optical parameters of the FBG arrays are tested by the optical frequency domain reflectometer method in a wide temperature range, demonstrating no degradation in reflection at heating up to 600C for a fibre with Al coating. Moreover, array of metal coated FBG can not be erased thermally. The mechanical strength of the developed fibre is practically the same as for “ordinary” OF with similar coating, showing the absence of the influence of FBG writing process on fibre strength. Some possibilities of the developed AFBG use in advanced sensor systems with special resolution about 10 mm, Brillouin random fibre laser with linewidth less 40 kHz are considered.

In October 2013, the 22nd International Conference on Optical Fibre Sensors was held in Beijing, attracting about 500 participants with 417 presentations. The conference began in 1983 in London, and in the subsequent 30 years has defined the subject. The conference is held approximately every 18 months, and rotates between three world regions: Asia/Pacific, Europe and the Americas. The conference is not ‘owned’ by any learned society or professional institution, but is organized by a self-sustaining international steering committee. This special feature represents the sixth occasion on which Measurement Science and Technology has published papers based on a development of a cross-section of work presented at the conference.The subject of optical fibre sensors has its beginnings in the enabling technologies of the optical fibre itself and the development of laser technologies suitable for practical use in demanding real-world applications. But the real driver for the subject in its early years was in the development of systems for defence applications, most notably for strategic-grade sea-bed hydrophone arrays for submarine detection, and the optical fibre gyroscope (the community has recently celebrated the 35th anniversary of its earliest publication) for aerospace navigation. Both applications continue to be important, but now with extensive civil applications: hydrophones for oil exploration and reservoir monitoring and management, and fibre gyroscopes for applications ranging from those requiring low cost and mass production (such as industrial robots and in agricultural machinery) to the most exotic and highest performance for space applications.The articles in this special feature exemplify the principal themes of the subject: enabling technologies, application-specific developments and systems considerations. In recent years, perhaps the most important—indeed, dominant—enabling technologies have been based on structuring of fibres: longitudinally, as in Bragg gratings, or transversely, using the science of metamaterials to produce microstructured fibres (e.g. photonic crystal fibres). In-fibre gratings continue to provide new types of sensor based on wavelength encoding, or for wavelength control for specialized sources or detection techniques. Microstructured fibres, meanwhile, provide materials with dispersion characteristics unattainable with conventional materials, as well as otherwise unfeasible physical characteristics that can be tailored to specific sensing applications. Examples of these types of technologies can be found in the following articles.The fields of application of optical fibre sensors, even if restricted to those presented at the conference, would be too lengthy to enumerate here. However, in this issue there are examples from medicine, transport, chemical sensing and electric power distribution, amongst others. An important advantage conferred by optical fibre sensors is the ability with which they can be multiplexed to form large arrays, interrogated via a single fibre, a topic that forms the subject of a number of papers in the issue. Lastly, as fibre sensors become the technology of choice in widespread applications, the issue of formal measurement standards begins to become important, and it is evidence of the maturity of the field that the subject is addressed in one of the papers published here: optical fibre sensors can now surely be said to have progressed from the physics laboratory to become a mainstream engineering reality.

Optical fiber sensors (OFS) such as fiber Bragg gratings (FBGs) are passive and versatile sensor devices that have been used to perform a wide range of physical and chemical measurements. However, the performance and functionality of the OFS are limited by its passivity. This letter demonstrates an active fiber sensor scheme for multiparameter measurements. FBG-based optic fiber sensors are fabricated in an optical fiber core by femtosecond laser direct writing scheme. Guided light scattered out from laser-induced nanograting in fiber core is harnessed by on-fiber coating. Energy conversion films coated on the optical fiber convert scattered optical energy into thermal energy to heat the FBG sensor above ambient temperatures for active measurements. Functionalities of optically heated fiber sensor array are demonstrated for both temperature and liquid level measurements at the room temperatures.

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction Optical fibers have, in the past few years, begun to gain acceptance as permanently installed downhole sensors, although trials were carried out as long ago as 1990.1 Optical-fiber sensors are fully passive, totally immune to electromagnetic radiation, and small enough that, in certain cases, they can be installed deep in a well after the completion itself has been run. The strong E&P industry interest in optical sensors derives mainly from the expectation of a higher reliability than conventional gauges because of the absence of active electronics in the downhole device. The technology that these devices exploit was developed mainly for the telecommunications industry during the past 35 years.2 The optical attenuation of the best fibers is well below 0.2 dB/km (equivalent to a signal reduction by a factor of 10 for every 50-km of propagation path), and a usable information transmission rate is well in excess of 1 Tbit/s baud rate. Thus, optical fibers are a well-developed, high-bandwidth, low-loss transmission medium. The telecommunications industry has standardized on a fiber diameter of125 µm that is sufficiently small to be flexible, yet retaining reasonable strength. A conventional optical fiber consists of a central core (typical diameter of5 to 50 µm) surrounded by a cladding of slightly lower refractive index. In general, both regions are made from silica, modified by the addition of materials to tailor the refractive index profile and the dispersion of the structure thus formed. A coating is applied to the fiber (typically to an outer diameter of 250 µm) to protect the surface from scratches, which will weaken the fiber, and to buffer it from microbending that would cause light to be lost. For oilfield use, higher levels of fiber protection for temperature, chemical attack, and mechanical abrasion and crushing are required. In increasing order of effectiveness, coatings have been manufactured from ultraviolet curable acrylates, silicone rubber, fluorated polymers, polyimides, and metal. In general, a metallic tube, similar to a 1/4-in. diameter controlline, further protects the coated fiber. Commercially available sensors can be categorized into those that aim to replace conventional permanently installed devices (such as pressure gauges and flow meters) and replaceable, distributed measurements [such as a distributed temperature system (DTS)]. Pressure Sensors There are several ways in which an optical fiber can be turned into a pressure sensor. Two methods currently in use are fiber Bragg gratings (FBG)and Fabry-Perot interferometry. An FBG3 is a device consisting of a short piece of fiber, which has a periodic modulation of the refractive index longitudinally along the fiber axis. Primarily, FBGs are narrow-band reflectors that transmit all wavelengths with little loss, except for a narrow range centered on a wavelength that matches the pitch of the index modulation. Reflection bandwidths as low as 1 GHz can be manufactured, and FBGs are used as filtering devices in the telecom industry. More sophisticated FBGs are used as band-pass filters or as dispersion-compensating devices. For sensor applications, FBGs work by stretching the fiber in which the grating is inscribed, thus altering the wavelength of the reflected light. The Fabry-Perot technique4 uses the interference pattern setup by the reflection of laser light from two reflectors, such as the cleaved ends of fibers placed in close proximity (approximately 50 µm) and sealed together in a 350-µm diameter silica tube. The gap between the mirrored fiber ends varies mechanically by fluid pressure acting externally on the tube causing the interference pattern set up between the mirrored ends of the fiber to change with the imposed external pressure. Unfortunately, neither of these systems yet provides the pressure resolution and accuracy available in the current generation of quartz electrical gauges, as shown in Table 1. Neither has their reliability yet proved superior, although only a few gauges have been installed and it is probably too early to make a direct comparison with electrical equivalents. Installation requires that the gauge be in a mandrel, allowing transmission of reservoir pressure to the sensor through a bellows. The mandrel is installed on the production tubing in a fashion similar to electrical-pressure-gauge mandrels. Thus, the whole downhole instrumentation must be installed with the completion.

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.

OFS-19 was held in April 2008 in Perth, Australia, with Professor David Sampson (University of Western Australia) as General Chair assisted by Technical Programme Co-Chairs Professor Stephen Collins (Victoria University, Australia), Professor Kyunghwan Oh (Yonsei University, Korea) and Dr Ryozo Yamauchi (Fujikura Ltd, Japan). 'OFS-19' has once again affirmed the OFS series as the leading international conference for the optical fibre sensor community. Since its inception, in London in 1983, and under the leadership of an international steering committee independent of any learned society or professional institution, it has been held approximately every eighteen months. The venue nominally rotates from Europe, to the Americas, and thence to Asia and the Pacific. OFS-19 demonstrated the continuing vigour of the community, with some 240 papers presented, plus 8 tutorials; submissions and attendance were from 29 countries, with a little over half coming from the Asia-Pacific Region.In recent years, it has become a tradition to publish a post-conference special issue in Measurement Science and Technology, and these special issues offer a representative sample of the current status of the field. In the 25 years since OFS began, many of the early ideas and laboratory-based proof-of-principle experiments have successfully evolved into highly developed instrumentation systems and commercial products. One of the greatest success stories has been the optical fibre Bragg grating. Its exquisite intrinsic sensitivity to temperature and strain has led to an expanding niche in structural monitoring, especially in civil engineering. It has formed the 'beach-head' for penetration of optical fibre sensors into the oil and gas industry, initially in the harsh environment of down-hole monitoring. Latterly, it has paved the way for new applications of one of the earliest fibre optic sensors, the fibre hydrophone, which is now making its mark in sub-sea seismic surveying. Additionally, distributed fibre sensors, based on Raman or Brillouin scattering, are beginning to be deployed for remote and sub-sea infrastructure monitoring. Western Australia enjoys a booming oil and gas sector, and so OFS-19's Special Session entitled Oil & Gas: Current Practice–Future Opportunity was timely and locally relevant.An innovation at OFS-19 was turning the traditional first day's Workshop into a Tutorial Workshop delivered under the title Optical Fibre Sensors: Enabling the Next Generation, Stretching the Present Generation. International experts delivered a set of eight tutorials, covering both fundamentals and cutting-edge advances, to a large proportion of the conference delegates (the tutorials are available for download at obel.ee.uwa.edu.au/OFS-19).This special issue amply demonstrates in microcosm the breadth of the field of optical fibre sensors, with papers concerning applications in the oil and gas industry, in water and air quality, in civil engineering, as well as new sensors, sensor systems and methods for sensing. In addition, there are papers concerning sensor fabrication and calibration, as well as components of sensing systems.Several papers and topics are worthy of mention. The engineering of nanostructured materials promises much in many fields, including sensing in general. Thus, it is not surprising to find that nanotechnology is in evidence in the field of fibre sensing (Jarzebinska, Viegas). Microengineered mechanical structures also promise much for sensing and the exquisite 'head-of-a-pin' engineering of a cantilever on a fibre end-face is an elegant and versatile platform demonstrated here for refractometry (Alberts). The field has always provided fertile ground for new ideas, and this issue proves no exception. For example, three papers deal with new ways of solving the well-known issue of decoupling temperature from strain in fibre Bragg gratings (Guo, Nguyen, Yam). The ultimate endpoint for research in such a practical field is a useful deployed sensing system, and the oil and gas industry is the focus for four papers in this issue (Aref, Jackson, Mignani, Possetti).We hope that this special issue helps to further developments in the field of optical fibre sensors and would like to thank all the contributing authors and reviewers for making it possible. We also thank the staff at IOP Publishing for their support and in ensuring timely publication. OFS-20 will be held in Edinburgh, Scotland, 5--9 October 2009 (www.ofs20.org), with a corresponding special issue planned.

A fibre Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fibre that reflects particular wavelengths of light and transmits all the others. As such, FBGs can be used as inline optical filters to block certain wavelengths, or as wavelength-specific reflectors. Applications include optical fibre communications, sensors and fibre lasers. This book addresses the critical challenge of developing Fibre Bragg Gratings (FBGs) for applications as sensors in harsh and space environment. Coverage ranges from the basic principles through design, fabrication, and testing to their industrial implementation. A thorough review includes the in-depth examination of the FBGs properties and the most important developments in devices and applications. A particular emphasis is given to the applications of fibre optic sensors in the space environment, which is characterized mainly by vacuum, high thermal gradients, mechanical vibrations and various types of cosmic radiation. The book concludes with a summary and overview of challenges faced by FBG technology. The book is supplemented by an extensive survey of published papers, books and conference reports. As an added benefit, the book is structured in such a way as to provide useful and in-depth training and skills development to graduate/undergraduate students, specialised engineers, and academic/industrial experts.

The long-span and high-speed optical transmission networks require high-quality optical cables and components to reduce the detrimental effects of polarization-mode dispersion (PMD) and polarization-dependent loss (PDL) on system performance. In addition, many high-accuracy fiber-optic sensing networks also require low polarization-dependent components. As a result, polarization-insensitive transmission fibers and components have been pursued recently. A well-established technique to produce low-PMD and PDL fibers is to spin the fiber during the fiber drawing process, known as spun fibers. As key optical components, fiber Bragg gratings (FBGs) play important roles in optical fiber communications and fiber sensors. However, FBGs written into traditional single-mode fibers exhibit relatively high polarization dependence due to the birefringence introduced by side UV exposure in FBG fabrication process. In this paper, we report for the first time the characteristics of FBGs written into spun fibers that were formed by spinning fibers with certain intrinsic birefringence. Distinct properties of FBGs in spun fibers are found in the experiments, compared to those in traditional single-mode fibers. Based on Jones matrix method, the FBG spectra are simulated and compared with the measured ones. Preliminary experimental results indicate that the spun fiber gratings suffer lower PMD under the same fabrication conditions. Moreover, our theoretical analysis shows that the PMD and PDL characteristics of spun fiber gratings can be further improved by optimizing the structures of spun fibers.

This study proposes a novel optical fiber Bragg grating (FBG)-based load cell sensor using 3-D printing technology. It explores the impact of the infill density and pattern on the FBG’s temperature and mechanical strain sensitivity. Two FBGs were used, one for strain measurement, FBG L, and the other for temperature compensation, FBG T. Conducted measurements show that the strain-sensitive FBG L and strain-free FBG T exhibited a similar average temperature sensitivity value of around 10.5 pm/°C with a good linearity value of 99.92%. This will also allow the unstrained FBG T to effectively compensate for FBG L’s temperature changes during load measurement. Under varied load conditions, load cell sensors with lower infill density, such as 20%, displayed more significant average wavelength shift and increased strain sensitivities compared to those with higher infill density, such as 80%, due to having more voids and hollow sections within its actuator. This facilitates easier deformation, thus inducing more significant stretching on FBG L. Regarding the infill pattern, the gyroid pattern outperformed the tri-hexagon in both average wavelength shift and strain responses, showing its suitability for precise measurements under diverse loads. Therefore, this work demonstrates the critical role of the infill density and pattern in the performance of an efficient load cell sensor using FBGs.

Multiplexed optical fibre sensors were embedded into a carbon-fibre-reinforced-preform during the industrial production of a full-sized, one-piece tail cone assembly for a regional jet aircraft. Optical fibre Fresnel sensors monitored both the infusion of the resin, via measurement of the refractive index-dependent attenuation in the reflected light signal, and the degree of cure of the resin, via measurement of the chemical cure reaction-dependent change in refractive index. The resin cure was also monitored by optical fibre Bragg gratings (FBGs) fabricated in high linearly birefringent optical fibre, which measured through-thickness strain development, while FBGs in standard single mode optical fibre measured longitudinal strain development. The magnitudes and profiles of the transverse and longitudinal strains developed during the curing process were consistent across different locations on the tail cone. Typical transverse and longitudinal strains, related to cure reaction-induced shrinkage, were −1500 ± 17 μϵ and −500 ± 5 μϵ, respectively. Post-production, the same embedded FBG sensors were used subsequently to monitor structural strains when the tail cone was subjected to vacuum pressure loading. The longitudinal strains measured using the embedded FBG sensors were generally in good agreement with the longitudinal strains measured by the surface-bonded resistance foil strain gauge (RFSG) sensors, both qualitatively and quantitatively. The in-plane transverse and circumferential strains, oriented collinearly, were measured by the embedded FBGs and appropriately oriented surface-bonded RFSG sensors, respectively, and were, qualitatively, in good agreement.

Railway tracks are the direct supporting structures of the trains, which are vulnerable to produce large deformation under the temperature stress or subgrade settlement. The health status of track is critical, and the track should be routinely monitored to improve safety, lower the risk of excess deformation and provide reliable maintenance strategy. In this paper, the distributed optical fiber sensor was proposed to monitor the continuous deformation of the track. In order to validate the feasibility of the monitoring method, two deformation monitoring tests on one steel rail model in laboratory and on one real railway tack in outdoor were conducted respectively. In the model test, the working conditions of simply supported beam and continuous beam in the rail model under several concentrated loads were set to simulate different stress conditions of the real rail, respectively. In order to evaluate the monitoring accuracy, one distributed optical fiber sensor and one fiber Bragg grating (FBG) sensor were installed on the lower surface of the rail model, the strain measured by FBG sensor and the strain calculated from FEA were taken as measurement references. The model test results show that the strain measured by distributed optical fiber sensor has a good agreement with those measured by FBG sensor and FEA. In the outdoor test, the real track suffered from displacement and temperature loads. The distributed optical fiber sensor installed on the rail can monitor the corresponding strain and temperature with a good accuracy.

The physical quantities involved in a triaxial testing device have mostly been monitored with electric sensors. These sensors are currently subject to short circuit when submerged under water and electromagnetic interference (EMI). Waterproofing and EMI noise filtration have often been a challenge to the triaxial test set-up. These drawbacks can be substantially minimized when using optic fiber sensors. The optic fiber Bragg grating (FBG) sensors have the additional advantage of being partially distributive where multiple sensors can share the same signal transmission line. Taking advantage of these unique capabilities, the authors explored the possibility of converting all pressure/force and linear displacement transducers in a triaxial testing device into FBG based sensors. A series of shearing tests on unsaturated and saturated soil specimens were carried out using the new FBG sensored triaxial testing device. In most cases, the measurement of physical quantities was paired with electric sensors so that the results can be compared. This paper describes the principles of the individual FBG sensor designs and demonstrates their applications in triaxial testing.