Kathryn Atherton

Cavity-enhanced spectroscopy in fiber cavities

We discuss the relative merits of passive and active fiber cavities for ring-down. Ring-down times of approximately 2 micros were recently demonstrated in passive cavities, but operation is restricted to weak evanescent wave interaction. We report on active cavities with amplifiers used for loss compensation, permitting the use of open-path micro-optic cells. Ring-down times of tens of microseconds can readily be achieved and extended to several hundred microseconds in gain-clamped cavities, but relaxation oscillations and system drift impose limits on accuracy and repeatability. A cavity enhancement of 2 orders of magnitude is realistically possible, and sensitivity may be further enhanced if ring-down is combined with established spectroscopic methods.

Optical fibre instrumentation for environmental monitoring applications

We report our research on the development of optical fibre trace gas sensors for environmental applications. We describe the operation of a 64-point fibre-optic methane sensor, which has been installed on a landfill site in Glasgow, UK, where methane is used for power generation as part of the current trend for renewable energy programmes. Although the environmental conditions are harsh, the sensor has performed satisfactorily, detecting methane in the range of ~50 ppm to 100% methane. Another area of our current research is the application of erbium-doped fibre lasers and amplifiers in gas spectroscopy. One system under investigation consists of an all-fibre cavity ring-down loop employing a fibre amplifier for the compensation of loop loss. We have been able to obtain ring-down times as long as 0.2 ms, corresponding to ~1100 pulses in the loop, producing an effective increase in a gas cell length from 5 cm to 55 m. The mode-locked operation of fibre lasers is also under investigation and, using dispersion effects, we demonstrate fine tuning of the wavelength which is important for absorption line scanning, with a typical tuning rate of ~0.014 nm kHz−1 at the third harmonic, closely matching the theoretical predictions. Techniques for extending fibre laser systems to form multi-point, multi-species gas sensors are explored.

Long-term stability of normal condition data for novelty detection

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.

Gas detection by cavity ringdown absorption with a fiber optic amplifier loop

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.

Damage Assessment in Smart Composite Structures: the DAMASCOS Programme

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.

Intra-cavity and ring-down cavity absorption with fibre amplifiers for trace gas detection

We present the design of a fibre optic system for both intra-cavity and ring down absorption measurements in trace gas detection. The system is constructed from standard fibre optic components with a micro-optic gas cell.

Optical fibre sensors and networks for environmental monitoring

Describes the authors work on the development of fibre sensors and networks for monitoring trace gases such as methane, acetylene, carbon dioxide, carbon monoxide, hydrogen sulphide and for detection of spills of gasoline, diesel and organic solvents, all of which are important in environmental and safety management. As an example, a 45‐point fibre optic sensor network has been installed on a landfill site to assess the distribution of methane generation across the site. System operation is based on near‐IR absorption and is currently being extended to monitor other gases such as carbon dioxide and hydrogen sulphide. Concurrently, research is being conducted on fibre lasers for the realisation of multi‐point, multi‐gas monitoring systems. Based on other principles (periodic micro‐bending loss effects), detection of hydrocarbon fuel spills has been demonstrated at multiple locations along the length of a specially designed fibre optic cable using standard optical time domain reflectometry (OTDR) measurements.

Generation and detection of broadband laser generated ultrasound from low-power laser sources

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.

Mach-Zehnder optical fiber interferometers for the detection of ultrasound

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.

Optical fibre sensors for environmental monitoring of trace gases

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.