Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Patent
1987-08-20
1989-04-25
Kyle, Deborah L.
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
367149, 367154, 356362, H04R 2300
Patent
active
048254240
DESCRIPTION:
BRIEF SUMMARY
This invention relates to sensing systems, and in particular to sensing systems for sensing strain or deformation (e.g. elongation or bending) of various members.
Interference in sensing systems due to spurious noise, particularly noise generated within the sensing system itself, can reduce the sensitivity of the system by masking signals to be sensed by the system. This problem is particularly noticeable in sensing systems for sensing acoustic waves, which sensing systems comprise optical fibres, piezoelectric elements or hydrophones which are for use underwater and/or for being towed behind marine vessels.
A sensing system for sensing acoustic waves is described in our co-pending United Kingdom Patent Application (Publication No. 2126820A). In this sensing system a coherent light pulse or coherent light pulse pairs are launched into an optical fibre which is arranged to be subjected to deforming forces such as acoustic waves. The optical fibre is divided into a plurality of sensor elements so that a small proportion of each coherent light pulse transmitted along the fibre will be reflected back along the fibre at the junction between each of the sensor elements. Reflected light pulses are superimposed in the optical fibre, and after leaving the optical fibre, they are passed to a photo-detection means and detector. In response to receiving the pulses, the photo-detection means produces electrical signals indicative of changes in optical path length of the elements due to the incidence of the deforming forces.
The coherent light pulse or pulse pairs are generated by a laser which introduces spurious phase shifts (microphonic effect) on the pulse or pulse pairs due to noise consequent upon external disturbances on the laser. Further, noise generated elsewhere in the sensing system also introduces spurious phase shifts on the pulse or pulse pairs. The sensitivity of the sensing system described in GB No. 2126820A is limited by the susceptibility of the laser to environmental noise. The noise generated by the laser and the noise generated elsewhere in the sensing system can produce phase/frequency modulation on the electrical signals produced by the photo-detection means which can be larger than phase/frequency modulation produced as a result of deforming forces on the sensor elements, and hence the phase modulation produced due to the deforming forces on the sensor elements can be masked.
It is an aim of the present invention to provide a sensing system in which the spurious noise can be compensated for in order that the sensitivity of the sensing system can be improved.
According to the present invention there is provided a sensing system for sensing acoustic waves comprising one or more sensor elements for use underwater, a reference sensor, and compensation means for compensating for noise generated in the sensing system in dependence upon signals provided by the sensor elements and the reference sensor.
The one or more sensor elements may together form an optical fibre for being subjected along its length to deforming forces such as acoustic waves. The optical fibre may be provided along its length with a number of spaced discontinuities which divide the optical fibre into the plurality of sensor elements.
In one embodiment of the present invention, the reference sensor may be one of the sensing elements, the reference sensor being isolated from the deforming forces.
In an alternative embodiment, the reference sensor may be in the form of an isolated optical fibre element which is located remotely from the optical fibre.
In a third embodiment of the present invention the reference sensor may be one of the sensor elements of the optical fibre, which sensor element is arranged to have a different sensitivity from that of the other sensor elements.
In the case where the sensing elements together form an optical fibre, the sensing system may comprise: generating means for producing coherent light pulses for transmission along the sensor elements of the optical fibre and the reference sensor; and a detector
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Lamb Chris
Wade Chris
Eldred John W.
Kyle Deborah L.
Plessey Overseas Limited
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