| Summary: | 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.
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