High-speed, solid state, interferometric interrogator and multiplexer for fiber Bragg grating sensors

We report on the design and prototyping of a robust high-speed interferometric multiplexer and interrogator for fiber Bragg grating sensors. The scheme is based on the combination of active WDM channel switching and passive, instantaneous interferometry, allowing the resolution of virgin interferometric interrogators to be retained at MHz multiplexing rates. In this article the system design and operation are described, and a prototype scheme is characterized for three sensors and a multiplexing rate of 4 kHz, demonstrating a noise floor of 10 nε/√Hz and no cross-sensitivity. It is proposed that the system will be applicable to demanding monitoring applications requiring high speed and high resolution measurements across the sensor array

Impact detection techniques using fibre-optic sensors for aerospace & defence

Impact detection techniques are developed for application in the aerospace and defence industries. Optical fibre sensors hold great promise for structural health monitoring systems and methods of interrogating fibre Bragg gratings (FBG) are investigated given the need for dynamic strain capture and multiplexed sensors. An arrayed waveguide grating based interrogator is developed. The relationships between key performance indicators, such as strain range and linearity of response, and parameters such as the FBG length and spectral width are determined. It was found that the inclusion of a semiconductor optical amplifier could increase the signal-to-noise ratio by ~300% as the system moves to its least sensitive. An alternative interrogator is investigated utilising two wave mixing in erbium-doped fibre in order to create an adaptive system insensitive to quasistatic strain and temperature drifts. Dynamic strain sensing was demonstrated at 200 Hz which remained functional while undergoing a temperature shift of 8.5 °C. In addition, software techniques are investigated for locating impact events on a curved composite structure using both time-of-flight triangulation and neural networks. A feature characteristic of composite damage creation is identified in dynamic signals captured during impact. An algorithm is developed which successfully distinguishes between signals characteristic of a non-damaging impact with those from a damaging impact with a classification accuracy of 93 – 96%. Finally, a demonstrator system is produced to exhibit some of the techniques developed in this thesis

Optical fibre sensors - applications and potential

Fibre optic sensors have progressed considerably during the past few years and are now establishing their potential as very real contenders in the environmental, structural monitoring and industrial sensing areas. This paper will explore some examples of these emerging applications and analyse the benefits which optical fibre technology offers within these measurement sectors. We shall then continue to explore emerging prospects which offer new opportunities for future research and exploitation

Solid-state interferometric interrogator and multiplexer for high-speed dynamic and absolute FBG wavelength measurement

We present a solid-state FBG array interrogator and multiplexer capable of determining absolute FBG wavelengths and of providing high-speed, high-resolution static and dynamic measurements. Using a described procedure, deployable on multiplexing passive-interferometric schemes, the system is able to determine initial sensor wavelengths and thereafter track wavelength changes with interferometric resolution. The scheme allows high-resolution interrogation of FBG sensor arrays to be applied to many industrial applications, where previously the lack of combined absolute and quasi-static wavelength measurement precluded the use of interferometric techniques. Using a preliminary laboratory embodiment, we demonstrate a wavelength determination accuracy of <0.3 nm and a measurement resolution of 10 fm/√Hz, and propose pathways to improved performance and miniaturisation

Application of Fiber Optic Instrumentation

Fiber optic sensing technology has emerged in recent years offering tremendous advantages over conventional aircraft instrumentation systems. The advantages of fiber optic sensors over their conventional counterparts are well established; they are lighter, smaller, and can provide enormous numbers of measurements at a fraction of the total sensor weight. After a brief overview of conventional and fiber-optic sensing technology, this paper presents an overview of the research that has been conducted at NASA Dryden Flight Research Center in recent years to advance this promising new technology. Research and development areas include system and algorithm development, sensor characterization and attachment, and real-time experimentally-derived parameter monitoring for ground- and flight-based applications. The vision of fiber optic smart structure technology is presented and its potential benefits to aerospace vehicles throughout the lifecycle, from preliminary design to final retirement, are presented

Fiber Optic Sensors for Structural Health Monitoring of Air Platforms

Aircraft operators are faced with increasing requirements to extend the service life of air platforms beyond their designed life cycles, resulting in heavy maintenance and inspection burdens as well as economic pressure. Structural health monitoring (SHM) based on advanced sensor technology is potentially a cost-effective approach to meet operational requirements, and to reduce maintenance costs. Fiber optic sensor technology is being developed to provide existing and future aircrafts with SHM capability due to its unique superior characteristics. This review paper covers the aerospace SHM requirements and an overview of the fiber optic sensor technologies. In particular, fiber Bragg grating (FBG) sensor technology is evaluated as the most promising tool for load monitoring and damage detection, the two critical SHM aspects of air platforms. At last, recommendations on the implementation and integration of FBG sensors into an SHM system are provided

High temperature sensing with single material silica optical fibers

We present recent developments in high temperature sensing using single material silica optical fibers. By using a single material fiber, in this case a suspended-core fiber, we avoid effects due to dopant diffusion at high temperature. This allows the measurement of temperatures up to the dilatometric softening temperature at approximately 1300°C. We demonstrate and compare high temperature sensing in two configurations. The first exploits a small section of single material fiber spliced onto a length of conventional single mode fiber, which operates through multimode interference. The second utilizes a type II fiber Bragg grating written via femtosecond laser ablation.Stephen C. Warren-Smith, Linh V. Nguyen, Heike Ebendorff-Heidepriem, and Tanya M. Monr

Long Period Gratings in Multicore Optical Fibers for Directional Curvature Sensor Implementation

[EN] Multicore optical fibers are especially attractive for the fabrication of curvature and shape sensors due to the spatial distribution of the different cores. Fiber Bragg gratings have been used in the past for the implementation of these sensors, however, despite their inherent properties, they have a very limited sensitivity. In this paper, we study the use of long period gratings (LPGs) for the implementation of a directional curvature sensor. We inscribed a set of three different LPGs in a seven core optical fiber using a selective inscription technique. We inscribed a single LPG in the external cores and an array of three LPGs in the central core. We have characterized the proposed sensor for strain, torsion, and curvature magnitude and direction. The proposed sensor shows a linear response for curvature magnitudes from 0 to 1.77 m(-1) with a maximum curvature sensitivity of -4.85 nm/m(-1) and shows a near sinusoidal behavior in all the cores with curvature directions from 0 degrees to 360 degrees. The sensor shows a good insensitivity to strain. The torsion in the multicore optical fibers can be detected and measured using the maximum attenuation of the LPGs in the external cores.This work was supported in part by the Sistema Nacional de Garantia Juvenil under Grant PEJ-2014-A-75865 (Promocion de Empleo Joven e Implantacion de la Garantia Juvenil 2014, MINECO), in part by the Ministry of Economy and Competitiveness under Project TEC2014-60378-C2-1-R, in part by the FINESSE the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Action Grant 722509, and in part by the Generalitat Valenciana under Grant APOSTD/2016/015.Barrera Vilar, D.; Madrigal-Madrigal, J.; Sales Maicas, S. (2018). Long Period Gratings in Multicore Optical Fibers for Directional Curvature Sensor Implementation. Journal of Lightwave Technology. 36(4):1063-1068. https://doi.org/10.1109/JLT.2017.2764951S1063106836

Review on Strain Monitoring of Aircraft Using Optical Fibre Sensor

Structural health monitoring of aircraft assures safety, integrity and reduces cost-related concerns by reducing the number of times maintenance is required. Under aerodynamic loading, aircraft is subjected to strain, in turn causing damage and breakdown. This paper presents a review of experimental works, which focuses on monitoring strain of various parts of aircraft using optical fibre sensors. In addition, this paper presents a discussion and review on different types of optical fibre sensors used for structural health monitoring (SHM) of aircraft. However, the focus of this paper is on fibre bragg gratings (FBGs) for strain monitoring.  Here, FBGs are discussed in detail because they have proved to be most viable and assuring technology in this field. In most cases of strain monitoring, load conditioning and management employs finite element method (FEM). However, more effort is still required in finding the accurate positions in real time where the sensors can be placed in the structure and responds under complex deformation