The use of fibre Bragg gratings for ultrasonic Lamb wave detection and source location

There is an increasing interest in the use of ultrasonic Lamb waves for materials characterisation and the detection of structural defects. Fibre Bragg gratings offer some significant advantages over more traditional transducers such as piezoelectrics for ultrasound detection and are also dual purpose, since the same sensors can also be used for strain mapping.. We describe the use of fibre Bragg gratings to detect ultrasound signals and thereafter to determine both their direction of propagation and source location. The main aspects that will be concentrated upon will be – optimising FBG interrogation techniques; maximising the efficiency of strain transfer from the sample to the FBG sensors; describing the directional response of FBGs; how this allows them to be configured into rosettes to determine the direction of the received ultrasound signal and, finally, how an array of 2 or more rosettes can be used to determine the source of the ultrasound

A comparison of 3 optical systems for the detection of broadband ultrasound

There are many applications of ultrasound in the field of material properties' evaluation and structural health monitoring. Here we will consider the detection of broadband laser generated ultrasound taking as an example acoustic emission as simulated by the pencil break test. In this paper three optical methods of detecting these ultrasound signals are compared; these are polarimetry, fibre Bragg gratings and vibrometery. Of these, the first two involve the bonding of a fibre sensor to the sample, whilst the vibrometer is a non-contact instrument that measures out-of-plane displacements. FBGs respond to the inplane strains associated with an ultrasound wave whilst the polarimeter detects birefringence produced by pressure waves acting normal to the fibre. The sensitivities of the systems are compared and their relative merits are discussed. It will also be shown that the polarimetric responses of symmetric and antisymmetric Lamb waves differ, which opens up the possibility of learning more about the nature of an acoustic signal using this technique than can be determined simply from the measurement of in-plane or out-of plane displacements alone

Near infrared spectroscopy for fibre based gas detection

Gas sensing systems based on fibre optic linked near infra red absorption cells are potentially a flexible and effective tool for monitoring accumulations of hazardous and noxious gases in enclosed areas such as tunnels and mines. Additionally the same baseline technology is readily modified to measure concentrations of hydrocarbon fuels - notably but not exclusively methane, and monitoring emissions of greenhouse gases. Furthermore the system can be readily implemented to provide intrinsically safe monitoring over extensive areas at up to ~250 points from a single interrogation unit. In this paper we review our work on fibre coupled gas sensing systems. We outline the basic principles through which repeatable and accurate self calibrating gas measurements may be realised, including the recover of detailed line shapes for non contact temperature and / or pressure measurements in addition to concentration assessments in harsh environments. We also outline our experience in using these systems in extensive networks operating under inhospitable conditions over extended periods extending to several years

Low power laser generated ultrasound : signal processing for time domain data acquisition

The use of low power modulated laser diode systems has previously been established as a suitable method for non-destructive laser generation of ultrasound. Using a quasi-continuous optical excitation amplified by an erbium-doped fibre amplifier (EDFA) allows flexible generation of ultrasonic waves, offering control of further parameters such as the frequency content or signal shape. In addition, pseudo-random binary sequences (PRBS) can be used to improve the detected impulse response. Here we compare two sequences, the m-sequence and the Golay code, and discuss the advantages and practical limits of their application with laser diode based optical excitation of ultrasound

Low power laser generated ultrasound : signal processing for time domain data acquisition

The use of low power modulated laser diode systems has previously been established as a suitable method for non-destructive laser generation of ultrasound. Using a quasi-continuous optical excitation amplified by an erbium-doped fibre amplifier (EDFA) allows flexible generation of ultrasonic waves, offering control of further parameters such as the frequency content or signal shape. In addition, pseudo-random binary sequences (PRBS) can be used to improve the detected impulse response. Here we compare two sequences, the m-sequence and the Golay code, and discuss the advantages and practical limits of their application with laser diode based optical excitation of ultrasound

Optical acoustic detector based on a fiber Fabry-Perot interferometer

We describe a novel optical acoustic detector based on a bias-controlled fiber Fabry-Perot interferometer. The detector has a broad bandwidth from 10 Mhz to a few gigahertz and higher sensitivity than conventional systems, which are useful for noncontact characterization of microsamples based on laser ultrasound

High frequency acoustic detector based on fiber Fabry-Perot interferometer

We describe a novel optical acoustic detector based on a bias-controlled fiber Fabry-Perot interferometer. Broad bandwidth as 15 MHz similar to a few GHz and higher sensitivity than that of conventional systems were demonstrated. The minimum detectable phase was 4x10(-8) rad/Hz(1/2), which was 3 times larger than the quantum-limited value

RF subcarrier based fiber strain sensor for monitoring composite gas storage tanks

In this report we will present some preliminary results on a RF sub-carrier based fibre strain sensing system for on-line monitoring of composite high pressure storage tanks for natural gas or hydrogen. By employing a temperature compensation scheme to the system developed has shown stable performance with better than 20μm length resolution and comparable long-term drift. The system uses readily available components and could be used for low-cost high volume applications such as on-line continuous monitoring

Structural damage identification using multifunctional Bragg grating sensors: II. Damage detection results and analysis

Structural health monitoring has become a respected and established discipline in engineering. Health monitoring involves the development of autonomous systems for continuous monitoring, inspection and damage detection of structures with minimum involvement of labour. The ultimate goal of structural health monitoring is to increase reliability, improve safety, enable light-weight design and reduce maintenance costs for all kinds of structures. The identification of structural damage is therefore a key issue in structural health monitoring. The scope of this paper is to present the results of testing a system for the identification of structural damage based on fibre Bragg grating sensors. The basic idea is to use fibre Bragg gratings as acoustic receivers of ultrasonic Lamb waves. The layout of such a damage identification system is introduced and its theoretical limits are studied numerically and experimentally. The set-up for damage identification experiments is described and the results of initial experiments introducing damage detection based on the analysis of Lamb wave signals are presented. The results for the Bragg grating sensors are then compared to the results of established technology for Lamb wave detection using piezoceramic transducers

The use of fibre Bragg grating sensors for damage detection and location in structural materials

We describe a novel method of damage location using ultrasonic Lamb waves detected by fibre Bragg gratings. The gratings are configured in a rosette configuration in order that the direction of an incoming wave may be determined. Location of holes was determined by finding the intersect point of the reflected ultrasound signals obtained from two rosettes. Accurate results are obtained when optimum geometries source transducers and fibre gratings are use