Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1988-02-16
1989-12-05
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250231R, H01J 516
Patent
active
048854620
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical fibre sensing systems and relates more specifically to such systems comprising an optical fibre sensor which extends over a predetermined path to be monitored by the sensing means for determining changes in temperature, pressure and other parameters at a variably located single point along said path with the position and or magnitude of such changes being measured by the system. The present invention is suitable for measuring temperature, as in the detection of fire or electrical faults or for measuring mechanical strain, as is required for intruder detection or vehicle position sensing, for example.
It is well known in the case of optical fibre gyroscopes comprising a bi-directional optical fibre loop Sagnac interferometer that if a point asymmetrically situated relative to a light source/detector coupling point on the optical fibre loop is subjected to an external phase modulating parameter, such as a temperature change or mechanical strain in the fibre, then a phase change y will be detected by an output detector when the phase change x caused by the parameter varies with time. The phase change detected by the output detector will be proportional to the rate of change of x with time and the distance of the phase modulation point along the optical fibre loop from the centre of the loop, the loop centre being situated such that the lengths of the two fibre paths connecting it to the light source/detector coupler are equal. Thus, any time-varying influence on the optical fibre which is not situated at the centre of the coil will in general cause a phase shift y approximated by ##EQU1## where V is the velocity of light in the optical fibre and (dx/dt) is the rate of change of phase x with time t at a distance Z of the phase modulation point from the centre of the coil.
According to the present invention there is provided an optical fibre sensing system for measuring the location and/or magnitude of an external parameter acting at a single point along an optical fibre loop sensor, in which the system comprises light source means for producing light for propagation around said loop sensor, directional coupler means for coupling said light source means and detector means to said loop sensor and to other optical fibre paths of the system, in which the detector means are arranged for measuring a phase change (x) between light propagating around the loop sensor in opposite directions and for simultaneously measuring a phase change (y) experienced by the light propagating around the loop in one direction, whereby, by computation or otherwise, the rate of change of the phase change x can be derived and by use of the previously referred to algorithm the location and/or magnitude of the parameter can be determined.
The rate of change of the phase change x for determining the factor (dx/dt) may be determined either by electronic differentiation of a voltage or current proportional to y, or it may be computed by a digital electronic computer from the measured values of x as a function of time.
By way of example, the present invention will now be described with reference to the accompanying drawing which diagrammatically depicts an optical fibre sensing system according to the present invention.
Referring to the drawing, the optical fibre sensing system comprises an optical fibre loop sensor 1, which will extend over a predetermined path which is to be monitored for determining a particular parameter acting at a specific point along the fibre sensor 1.
A light source 2 is provided for producing light signals for propagating around the loop sensor 1 in opposite directions by an optical directional coupler 3 which also directs a proportion of the returning light to a first optical detector 4. Optical interference between these light signals propagating in opposite directions around the loop sensor 1 enables a phase change (x) between the two opposite loop paths to be monitored by the optical detector 4. Before the light derived from the light source 2 reaches the optical directional coupl
REFERENCES:
patent: 4545253 (1985-10-01), Avicola
patent: 4632551 (1986-12-01), Pavlath
patent: 4725143 (1986-02-01), Jones et al.
patent: 4755668 (1988-07-01), Davis
patent: 4775216 (1988-10-01), Layton
patent: 4787741 (1988-11-01), Udd et al.
Allen Stephone B.
Nelms David C.
Plessey Overseas Limited
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