The work described here concerns the development of fiber sensors and networks for monitoring trace gases such as methane, acetylene, carbon dioxide, carbon monoxide and hydrogen sulphide, all of which are important in environmental or safety monitoring. A 45-point fiber sensor network using a single DFB laser source has been installed on a landfill site to assess the distribution of methane generation across the site, with detection levels from < 100 ppm to 100% methane. The system is currently being extended for carbon dioxide and hydrogen sulphide monitoring. Concurrently, fiber lasers sources are under investigation to provide a single source for several gases using techniques such as mode-locked operation for interrogation of multi-point systems and ring-down spectroscopy for high sensitivity measurements.
Cavity ring-down is investigated as a technique to increase the sensitivity of optical fiber gas sensors. The ring-down cavity consists of an optical fiber loop containing a micro-optic cell and an erbium-doped fiber amplifier. The erbium fiber amplifier introduces gain into the cavity to increase the ring-down times and therefore the system sensitivity. This paper reports an investigation of the system sensitivity.
This paper presents the basic principles of the laser generation of ultrasound (LGU) through thermal conversion and illustrates the approaches to its use in material evaluation using the broadband features of the source. Traditional LGU involves high energy optical pulse irradiation which often induces surface damage, especially in carbon or glass fiber composites. We therefore expand the concept into low power excitations using laser diode sources. This enables excitation without damage but requires coded temporal signals. Arrays of semiconductor laser sources can also produce very broadband acoustic signals, both temporally and spatially. Piezoelectric sources are usually the opposite constrained in both space and time. This basic observation opens new avenues of material investigation, some of which feature in this paper.
The DAMASCOS (DAMage Assessment in Smart COmposite Structures) project is a European Union funded program of work bringing together a number of academic and industrial partners throughout Europe. The aim of Damascos is to apply new ultrasonic detection and generation techniques integrated within the structure, together with advanced signal processing to realize damage assessment and ageing characterization in composite structures. This paper describes the background, experimental findings and future applications of the technology as the project moves into its final phase.
Ultrasonic Lamb waves have been proved to be a potentially desirable method for quick interrogation of large plate structures. Integrating fiber optic sensors offer great potential to monitor large areas due to their geometric flexibility. To date this potential has not been widely exploited and there have been only a few reports of non-destructive testing with integrating fiber optic sensors. In this report we present our preliminary measurements for the interrogation of hole defects in plate-like materials with surface-bonded integrating fiber optic sensors. Due to the signal integrating characteristics of fiber sensors very complicated signals were obtained. Therefore signal processing is needed to interpret waveforms for flaw detection.
This paper discusses using the multi-pass techniques of cavity ring-down and intracavity spectroscopy (ICLAS) to increase the sensitivity of optical fibre gas sensors. An experimental fibre ioop system has been built with standard optical fibre components and a micro-optic cell. An erbium amplifier is included in the fibre loop. For the cavity ring-down the amplifier allows the fibre loop to mimic the high finesse cavities used for conventional, bulk optic ring-down by compensating for the system losses. The amplifier acts as the gain medium for the intracavity set-up.
The simultaneous detection of in-plane and out-of-plane ultrasound displacements is crucial for the interpretation of Lamb wave mode pattern and their interaction with defects within the plate. In this report we present our preliminary experiments for the simultaneous measurement of these two components. A two-channel fibre optic interferometer system has been built which enable the measurements of in-plane and out-of-plane displacements separately and simultaneously. One channel is a Michelson and the other a modified Michelson interferometer or a Mach-Zehnder interferometer. The Michelson interferometer allowed direct measurement of the absolute out-of-plane displacement while the modified Michelson interferometer measured the in-plane displacement. The Mach-Zehnder interferometer measured both components. This two-channel fibre optic interferometer allows a directly calibrated measurement of the two components of displacement simultaneously and offers a great insight into ultrasonic flaw interrogation in plate-like materials.
As a technique of diagnosing failure in structures and systems, the method of novelty detection shows considerable merit. The basis of the approach is simple: given measured data from normal condition of the structure, the diagnostic system builds an internal representation of the system normal condition in such a way that subsequent departures from this condition can be identified with confidence in a robust manner. The success or failure of the method is contingent on the accuracy of the description of normal condition. In many cases, the normal condition data may have quite a complex structure: for example, an aircraft may experience a wide range of ambient temperatures in the course of a single flight. Also, the operational loads experienced by the craft as a result of flight manoeuvres may have wide-ranging effects on the measured states. The object of the current paper is to explore the normal condition space for a simple benchmark monitoring system. The said system uses Lamb-wave inspection to diagnose damage in a composite plate. Both short-term and long-term experiments are carried out in order to examine the variations in normal condition as a result of run-in of the instrumentation and variations in ambient temperature. The exercise is not purely academic as the fiber-optic monitoring system is a serious candidate for a practical diagnostic system.
Ultrasonic Lamb waves have been extensively investigated for non-destructive testing of materials. Embedded or surface bonded optical fiber, acting as the signal arm of a Mach- Zehnder interferometer, is one method previously utilized to detect the Lamb waves. Optical fibers therefore have potential as permanent sensors for structural monitoring of damage and defects in materials. A greater understanding of the ultrasound interaction with the optical fiber sensor will bring application closer. In order to probe this interaction we built a two channel interferometer allowing ultrasound traveling through a material to be monitored simultaneously by a Mach-Zehnder interferometer and also a Michelson interferometer. The Michelson interferometer allows a non- constat measurement to be made of the absolute surface displacement associated with an ultrasonic Lamb wave. Comparison of the ultrasound signals detected by the two different interferometers provides a greater insight into the detection mechanism and sensitivity of the Mach-Zehnder interferometer. The work is then extended to look at embedded fibers in composite materials and damage detection.