Fengzhong Dong

FIBRE OPTIC TECHNIQUES FOR REMOTE SPECTROSCOPIC METHANE DETECTION FROM CONCEPT TO SYSTEM REALISATION

Abstract Spectroscopic measurement of methane gas concentrations using absorption lines in the near infra red (the 1.67 μm region) has been demonstrated by several research teams. The detection technique has the advantage of access through optical fibres which are very transparent in this spectral region and the disadvantage of relative insensitivity compared with the fundamental absorption lines in the 3–4 μm area. This paper will describe a particular realisation of a fibre optic, highly multiplexed (up to 128 points) realisation of a methane gas detection system designed for safety monitoring applications and detection up to the lower explosive limit (5% by volume). This implementation is currently undergoing site trials and has combined advanced technological development especially in the signal processing domain with applications engineering, in particular with emphasis on the compatibility between the system performance, both technical and economic and potential applications. This approach which has involved close collaboration between the university research group and the specialist industrial provider of gas measurement systems promises to realise a cost effective and practical measurement system with state of the art technical performance.

Design of a fibre optic multi-point sensor for gas detection

Abstract We report the design of a multi-point fibre optic methane sensor using a DFB laser source with a branched fibre network and micro-optic cells. Measurements are performed through derivative spectroscopy, with line scanning and digital signal processing, to give sensitivities down to a few ppm metre. The form of the derivative signal obtained from the system is modelled theoretically and compared with the experimental signal. The main limitation in the signal to noise ratio of the system is due to interference effects (etalon fringes) from the cells and we show how these effects may be minimised.

Sub-picosecond studies of the third-order optical nonlinearities of - toluene solutions

The third-order optical nonlinearities of - toluene solutions are studied employing transient degenerate four-wave mixing (DFWM) and z-scan techniques using 0.5 ps laser pulses at 497 nm. The concentration dependence of the of the solution and both the sign and the magnitude of the real and imaginary parts of the third-order susceptibility of are obtained. The molecular second hyperpolarizability of is found to be . The role of nonlinear absorption and its influence on the DFWM signal is discussed. Finally a comparison of our experimental data with other data in the literature is given.

SUB-PICOSECOND RESONANT THIRD-ORDER NONLINEAR OPTICAL RESPONSE OF AZOBENZENE-DOPED POLYMER FILM

The dynamics and the sub-picosecond resonant third-order nonlinearity of azobenzene-doped polymer films are investigated by means of transient degenerate four-wave mixing and z-scan techniques. The third-order susceptibility χ(3) and the dephasing time T2 are measured to be (6.3±0.6)×10−12 esu and (1.0±0.1)×10−12 psec, respectively. The creation of a new intermediate state related to a conformational deformation mechanism is supported by experimental observations.

High order nonlinear optical response of fullerene solutions in the nanosecond regime

Abstract High order optical nonlinearities of C 60 - and C 70 -toluene solutions are studied in the nanosecond time regime, employing the transient degenerate wave mixing technique. The intensity dependence of the high order signals of the solutions is investigated. The ratios |χ eff (2n + 3) |χ eff (2n + 1) | are determined to be of the order of 10 −6 and 10 −5 for C 60 and C 70 respectively.

Detecting Ultrasound Using Optical Fibres

Optical fibre techniques for detecting ultrasonic waves are, in principle, well established. The ultrasonic wave could impose a pressure and/or strain field on an optical fibre in its path and this pressure and/or strain modulation would in turn be observed as a change in optical delay along the fibre length. The delay change may be detected interferometrically, polarimetrically or through using the induced strain changes in Bragg gratings. The resulting fibre optic detection system can perform as well in terms of signal to noise ratio over specified bandwidths as more established piezoelectric ceramic detectors. Additionally fibre systems can also perform some signal processing, particularly beam forming, and have inherently much higher spatial and temporal bandwidth than the piezoceramic precursor. This paper explores some of the options available for the use of fibre optics in detecting ultrasound and demonstrates the elements of basic signal processing techniques including examples of source location (equivalent to fault detection) and optical fibre beam forming. Whilst these functions can be performed using piezoceramics, and indeed can be made more adaptable through electronic element by element processing on large arrays, fibre optic systems in contrast offer substantial benefits in terms of a much simplified array interconnect (only the fibre itself is required) and material compatibility especially for high performance high strain range mechanical systems.

High-order nonlinear optical response of -toluene solutions in the sub-picosecond regime

High-order optical nonlinearities of -toluene solutions were observed employing the transient degenerate four-wave mixing technique using 0.5 ps laser pulses at 497 nm. The intensity and fullerene concentration dependence of fifth- and seventh-order signals were investigated. The effect of the nonlinear absorption on the nonlinear optical response is examined. The ratios and are determined to be of the order of .

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.

Fibre optic polarimetric detection of Lamb waves

In this paper we describe the detection of Lamb waves by monitoring the changes in the polarimetric state of the light emerging from an optical fibre which has been either bonded to, or incorporated into, a sample plate. Results have been obtained using both isotropic materials, such as polymer sheets, or anisotropic materials such as carbon fibre composites. These results demonstrate that polarimetric analysis has the potential to provide an alternative method of detecting Lamb waves and also provides a possible method for investigating the nature of the interaction between the acoustic wave and the light propagating through the fibre.

Development of a 32-element fiber optic hydrophone system

The structure and performance of an all polarization maintaining optical fibre hydrophone element is described and a sensitivity of -158±1.5dB is achieved. A 32-element, spatially multiplexed system is constructed with a noise-equivalent sound pressure of ~3.58x10-4 Pa per square root Hz at 1kHz and the major results of sea trials indicate that the system is useful for research and industrial applications.