Browsing by Author "Tatum, R. P."
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Item Open Access Fibre optic chemical sensing using Langmuir-Blodgett overlay waveguides(Cranfield University, 1998-04) Flannery, D.; Tatum, R. P.; Ashwell, G. J.Fibre optic chemical sensing has been demonstrated using side-polished single mode optical bre, evanescently coupled to chemically sensitive Langmuir-Blodgett overlay waveguides. The sensors exhibit a channel dropping response centred on a wavelength dependent upon the thickness and refractive index of the overlay waveguide. It has been shown that chemically sensitive organic dyes prove to be suitable materials for forming the overlay waveguide whereas the Langmuir-Blodgett deposition technique provides the required overlay thickness control. A brief optic pH sensor has been demonstrated by depositing a merocyanine dye onto a side-polished optical fibre which exhibits a change in transmission of 9.7 ± 0.8 dB pH" at a working wavelength of 750 n and 4.2 ± 0.5 dB pH`1 at 780 nm, with a shift in the channel dropping centre wavelength of 18.8 ± 0.8 n pH`1. The response time of the sensor to a step change in pH has been measured to be ~ 20 s. The advantage of being able to chemically engineer the organic materials used to form the overlay waveguides has been demonstrated by manufacturing a second pH sensor designed to operate at longer wavelengths using a second, modified merocyanine dye. The overall wavelength response of this second sensor to pH has been shown to be non- linear but a sensitivity of 23.3 ± 0.3 n pH`l is observed over a linear region. Finally, i order to investigate the non-linear response, a computer model based on the Kramers- Kronig relations has been devised and shows good agreement with experimental values. lItem Open Access Optoelectronic time-division multiplexing for multicomponent laser doppler anemometry(Cranfield University, 1996-10) Lockey, R. A.; Tatum, R. P.This thesis describes the investigation of time-division multiplexing for use in mUlticomponent laser Doppler anemometry (LDA). To make two- or three- dimensional velocity measurements of a flow, two or three separate velocity measurements in different, non-collinear or non-coplanar directions respectively must be taken from which the flow vector can be calculated. It is necessary to separate each velocity channel. Conventionally, the separation is carried out by optical techniques, such as separating the signal from each channel by wavelength or polarisation, with one detector per channel. In time division multiplexing, each channel is switched On and off in turn. A single detector records a stream of interleaved pulses, and each channel is extracted by taking every other or every third pulse. The envelope of the amplitudes of the pulses varies at the Doppler frequency. A novel system has been demonstrated which uses a single laser diode source and a single detector. The distribution of the pulses to each channel and the delay between each channel is carried out using a optical fibre network with fibre delay lines, with a consequent reduction in 'electronic complexity. The characteristics of the optoelectronic components used in the system are tested, and two signal processing schemes are investigated. A two-dimensional Doppler difference fibre-linked probe was constructed to demonstrate the technique. Hybrid instruments using two Doppler difference channels and a reference beam channel to measure orthogonal velocity components offer the possibility of avoiding the errors introduced by non-orthogonal systems. A pulsed-diode reference beam anemometer based on optical fibres and suitable for inclusion in a time-division multiplexed hybrid instrument was tested.