Peter A. Wallace

A frequency-modulated continuous wave birefringent fibre-optic strain sensor based on a Sagnac ring configuration

Two-mode fibre interferometers have been shown to be well suited as optical fibre strain sensors because (i) they have a higher resolution than do intensity modulation-based strain sensors and (ii) they have a simple geometry since the reference and signal light beams propagate along a single fibre core. This paper discusses a Sagnac ring configuration of polarization-maintaining fibre strain sensor based on frequency-modulation continuous wave technology wherein the basic sensor layout consists of a frequency-modulated single-mode laser, a 100 metre ring of single-mode birefringent fibre and a PIN photodiode detector. By launching light into both ends of the fibre, propagating and counter-propagating beams may be employed to sense the variations in length due to strain. The sensor exhibits good characteristics including temperature insensitivity over a limited temperature range (C), high resolution ( microstrain), large dynamic measurement range ( microstrain), large signal intensity and good signal contrast.

Development of a quasi-distributed optical fibre pH sensor using a covalently bound indicator

A quasi-distributed pH sensing system was developed to utilize information derived from evanescently excited fluorescence signals originating from sensor sites fabricated on an optical fibre. The sensor sites comprise a section of fibre core exposed by polishing upon which is laid down a sensing film. The film comprises a fluorescent indicator dye immobilized covalently within a hydrogel matrix which is then photopolymerized and attached covalently to the exposed fibre core. Position information is determined from the propagation delay of the returning signals. A system comprising eight sensors spaced at 10 m intervals along an optical fibre has been constructed and characterized. The sensors operate in the region pH 6-pH 8 with a response time of 500 s. The properties of the immobilized indicator dye are found to be similar to those of the dye in aqueous solution. Furthermore, the sensing films so created are found to be resistant to dye leaching.

Reflectometric frequency-modulation continuous-wave distributed fiber-optic stress sensor with forward coupled beams

A distributed optical-fiber stress sensor whose principle of operation is based on the frequency-modulation continuous-wave technique is reported. The sensor consists of a length of birefringent fiber with a mirror attached to one end, a diode laser, and a p-i-n photodiode detector. The intensity and the location of an applied stress are determined simultaneously by detecting the amplitude and the frequency of the beat signal, which is produced by two forward-coupled mode beams. The system was found to have a reasonable spatial resolution of 0.85 m (rms error) in a sensing range of 100 m. The advantages and limitations of the sensor are also discussed.

A distributed optical fibre fluorosensor for pH measurement

Abstract A novel distributed optical fibre pH sensor is reported. The principle of operation of the system is based on the detection of time-resolved fluorescence originating at spatially separated sensor sites. Sodium Fluorescein is used as the pH indicator. The fluorophor is immobilised in porous sol-gel films laid over the fibre core. The distributed system has a spatial range of 100 m and a resolution of the order of 1 m. The pH range of the system is observed to be pH 7–11.

A Multipoint Quasi-Distributed Optical Fiber pH Sensor

This paper reports the development and characterisation of a multipoint quasi-distributed optical fiber sensor for pH measurement. The system is based on a 170 m length of 200 μm core diameter plastic cladding silica fiber where sections of cladding have been removed and replaced with dye immobilised sol-gel glass to form sensing points. Evanescent wave excitation of a dye, immobilised within 2 mm long sections of cladding, enables the pH value of any spillage material to be determined by optical time domain reflectometry along the length of the fiber. The results suggest a spatial resolution of better than 2.5 meters for this fiber system and indicate that this arrangement could form the basis of a practical sensor/actuator system for chemical spillage, provided that suitable dye/analyte combinations are available.

Analysis of cable failure modes and cable joint failure detection via sheath circulating current

Power cables are preferred in urban areas for power transmission and distribution because of their high reliability, environmental friendliness and the visual invisibility. Whilst the volume of underground power cable has been growing steadily, the voltage level of the power cables have also increased significantly in recent years. The increasing volume of high voltage power cables brings about technical challenges to the power system operators and maintenance engineers. One of these challenges is the application of appropriate condition monitoring techniques to detect incipient cable faults and to reduce unplanned outages. This paper aims to analyse the causes, modes and mechanisms, among cable joint failures, and to propose an applicable sheath circulating current monitoring technique with the associated criteria for fault diagnosis. Two joint faults, flooded link box and joint insulation breakdown, are analysed in detail. Finally, a set of criteria is proposed for cable joint fault diagnosis based on the simulation of an 110kV underground power cable system of length 1.5km.

Distributed stress sensor based on a birefringent fiber Sagnac ring

This paper describes a novel configuration of a distributed optical fiber stress sensor based on a Sagnac ring construction. The principle of operation is based on the FMCW technique. The intensity and location of a stress applied to the fiber can be determined simultaneously by detecting the amplitude and the frequency of the beat signal which is produced by two forward-coupled mode beams in the Sagnac ring. The system was found to have a spatial resolution of 1 meter in a sensing range of 50 meters.

A Distributed Frequency Modulation Continuous Wave Fiber Stress Sensor Based on a Birefringent Sagnac Ring Configuration

A new distributed stress sensor, based on a birefringent fiber Sagnac ring configuration and frequency modulation continuous wave technology, has been investigated. The two forward-coupled mode beams in the Sagnac ring, induced by an applied stress at any given point along the fiber length, produce a beat frequency which can be analysed to determine, simultaneously, the magnitude and position of the stress. The system has a reasonable spatial resolution of 1 m in a dynamic sensing range of 50 m.

A variable frequency model of a transformer bushing with localised moisture content

Moisture ingress within a high voltage power transformer bushing can be detrimental to the insulation and causes the majority of bushing failures. In practice, wet bushings are known to have moisture accumulate within the lower region of the condenser. However, traditional power frequency measurements are often too insensitive to adequately evaluate these moisture conditions. Variable frequency analysis can provide a more sensitive estimation of moisture content, but bushing frequency responses are not well documented. In this paper, a simplified bushing model is used to evaluate moisture content within the insulation, for both uniformly distributed and localised wet regions. Numerical software is used to model the dissipation factor and capacitance, as a function of frequency and temperature. It was found that a localised wet region which had a 1.0% moisture content, with an impregnation of between 5% and 20%, was not distinguishable from a uniformly distributed 0.5% moisture content. However, when the localised region moisture percentage increased to 4.0%, a distinction could be made by a steeper response at frequencies of less than 0.1Hz.

Distributed optical fiber chemical sensor

Techniques for distributed optical fiber chemical sensor development were investigated and a model system for pH measurement was developed and, as a result, discrete distribution signals were obtained. pH indicator fluorescein sodium was chosen for this work because of its well known properties and high fluorescent intensity. A low temperature sol-gel glass manufacturing process was utilized to immobilize the indicator onto the optical fiber core. Robust porous glass thin film (approximately 1 micrometers ) was grown on the surface of the optical fiber core with indicator molecules entrapped in the matrix. OTDR was employed to obtain the signal at specific positions along the fiber. A dye laser, pumped by a short pulse N2 laser, produced blue light pulses at 440 nm which were launched into a 1 X 2 optical fiber coupler. A fiber with four sensitive sections was spliced to a 50:50 coupler. The indicator molecules were excited by the blue light via the evanescent wave. Part of the fluorescent light from the indicator molecules was coupled back into the fiber and transmitted back to the coupler. A fast PMT tube was attached to the other arm of the 1 X 2 coupler to detect the fluorescent light. Results were obtained for solutions of various pH value. The system appears to have great potential due to the wide applications in chemical, biochemical, environmental and safety monitoring.