FIELD OF THE INVENTION

The present invention relates to an optical sensor which may be used for the measurement of two parameters, such as pressure and temperature.

BACKGROUND OF THE INVENTION

Many industrial processes require knowledge of a variety of parameters in order to control productivity and quality of the process effectively. Intensity sensors, in which the parameter to be measured results in a change of magnitude of a source signal, are relatively simple devices but are usually unsuitable for very accurate measurements because the source signal cannot be adequately stabilised. An example of an intensity sensor is described by European Patent Application EP 0 144 509 A3 "Fiber Optic interferometer transducer" in which it is taught how a dual-path polarimetric interferometer may be constructed so that, when suitable means convert isotropic pressure to anisotropic radial forces on an optical fibre, a useful signal results, which may be used as a measure of the isotropic pressure.

One method of compensating for source and other unwanted optical intensity changes in an intensity optical sensor is taught by D.E.N.Davies, J.Chaimowicz, G.Economou and J .Foley, "Displacement sensor using a compensated fibre link", Proceedings Optical Fibre Sensors '84, Stuttgart. pp387-390, 1984. In this example, the source power is split and the light traverses two separate paths before being re-combined on an optical detector. The source light is moάu ,. -£ and a delay is placed in one of the paths such that there is a sign.--.want phase shift at the modulation frequency of one path compared with the other. The intensity sensor is also placed in one of the paths. In this way, the output of the sensor is encoded as being relative to the reference path and a variety of effects which are equivalent to fluctuations in the source power are eliminated. The amount of phase delay experienced by the modulation of the light signal is dependent upon the modulation frequency, and this effect may be used to measure a second physical parameter separate from that being measured by the intensity sensor.

SUMMARY OF THE INVENTION

An aim of the present invention is to provide apparatus for measuring two physical parameters, preferably pressure and temperature, in an accurate and convenient way.

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BRIEF DESCRIPTION OF THE INVENTION

According to the present invention, there is provided apparatus for the measurement of two physical parameters, which apparatus comprises sensing means, modulation means for varying the modulation frequency of the intensity of a source of light, means for conveying the light from the light source to and from the sensing means, and detection means for detecting the intensity of the light from the sensing means as a function of the modulation frequency, the sensing means being such that the amplitude and phase of the modulation of the light traversing the sensing means are determined by the modulation frequency as well as by the physical parameters to be measured, and where the phase is substantially affected by one parameter, preferably temperature, and the amplitude is substantially affected by the other parameter, preferably pressure.

In general the phase Φ of the modulation of the light will be a function of both temperature T and pressure P, that is Φ = f(T,P). Similarly, the amplitude A of the modulation of the light will be a different function of both temperature T and pressure P, that is A = g(T,P). Thus the temperature and pressure values may be obtained from T = F(A,Φ) and P = G(A,Φ) where F and G are functions of the measured amplitude and phase of the modulation of the light. Accordingly, the sensing means should be designed such that the functions F and G exist. Often, the functions f and g take a simple form such that Φ = a.T + b.P and A = c.T + d.P, from which the functions F and G may be found using well-known methods for solving simultaneous equations.

Preferably the sensing means is an optical fibre sensor and, more preferably, is a pressure and temperature optical fibre sensor.

The light source means may be a light emitting diode, and the modulation means may be frequency synthesiser whose output determines the drive current of the light emitting diode.

The detection means may be a photodetector.

The apparatus may include demodulation means. The demodulation means may be a vector voltmeter.

The means for conveying light to and from the sensing means may be two optical fibres. Alternatively, the means for conveying light to and from the sensing means may be a single optical fibre.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described solely by way example and with reference to the accompanying drawings in which:

Figure 1 is a diagram of a two parameter sensor according to th present invention:

Figure 2 is a diagram of the variation in amplitude of the modulati of the detected light with modulation frequency; Figure 3 is a diagram of a preferred embodiment of the prese invention, in which optical fibres are used;

Figure 4 is a diagram of a preferred embodiment of the prese invention in which an all optical fibre pressure and temperature sensor provided; and Figure 5 is a diagram of a preferred embodiment of the prese invention, in which an all optical fibre pressure and temperature sensor provided using a single optical fibre link between the sensing means an the source and detection means.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 is an embodiment of the invention, in which a light source is intensity modulated by modulation means 2 and light is transmitted splitting means 3, in which the light is separated into two separate pat

4 and 5. The light traversing path 4 is delayed by sensing means in t form of delay sensor means 6 relative to the second path 5, in which t intensity is varied by an intensity sensor means 7 which is sensitive some external parameter. The magnitude of delay resulting from the del sensor means 6 is dependent on a different external parameter. The t paths are recombined by coupler 8 and the light transmitted back detection and demodulation means 9, in which an electrical signal generated which depends upon the amplitude of the modulation of t incident light. Computing and controlling means 10 is used to change t modulation frequency and measure the resulting modulation amplitude the detected optical signal. Figure 2 shows a typical plot of output sign as a function of the modulation frequency in which the frequency has be changed to a number of different values and a mathematical curve 1 fitted to the measured amplitude values 1 1. The well known Nyqui sampling criterion is used to determine the minimum number of differe measured amplitude values 1 1 that are required to describe properly t curve 12. For example, where the curve 12 is sinusoidal, which it oft is, a minimum of three measured amplitude values 11 are required for o cycle of the curve 12, preferably equally spaced over the curve 12.

The desired output from the intensity sensor 7 may be obtained from the curve 12 in a known manner, using the computing and controlling means 10. The desired output from the delay sensor means 6 may be obtained by measuring the magnitude of the time delay D, whose value is dependent upon an external physical parameter. One method comprises noting the modulation frequency fl, at the minimum 13 of the curve 12, and f2, at the maximum 14 of the curve 12. The differences in phase angles of the modulation signal passing through the delay sensor means 6 and the intensity sensor 7 are given by:

Φl = 2.π.D.fl and Φ2 = 2.π.D.f2 at the two frequencies fl and f2.

The signal received by the demodulation means 9 will have a maximum when Φ2 is equivalent to an even integral number of half cycles of modulation, and will have a minimum when Φl equivalent to an odd integral number of half cycles of modulation. Thus the difference between Φl and Φ2 will be one half cycle, or π radians, and D may be computed from:

π = Φ2 - Φl = 2.π.D.(f2 - fl) and so 2.D = l/(f2 - fl)

from which a measure of the external parameter may be deduced from prior or subsequent calibration.

Alternatively, the modulation frequency may be varied by small amounts about an amplitude minimum 13 so as to seek out the frequency at which the signal amplitude is exactly a minimum 13. At this point, the amount of delay D will be an integer number of half wavelengths of the modulation frequency, and so, the exact value of D may be determined if the approximate value is known to half a wavelength. Alternatively and equivalently, the modulation frequency at which a signal maximum 14 occurs may be sought and D determined in a similar way.

In an alternative embodiment of the present invention, and with reference to Figure 1, the delay sensor means 6 and the intensity sensor means 7 are placed in the same path 4 as each other, and the other path 5 is then preferably insensitive to external effects.

Figure 3 shows a preferred embodiment of the present invention, in which a dual parameter sensor 15 is provided, and is connected by optical fibre cables 16 and 17, whose length may be many kilometres. Light from the light source 1. modulated by the modulation means 2 is transmitted to an optical fibre coupler 18 by the optical fibre 16. Light passes through the delay sensor means 19, in --vhich the delay experienced by the light is

dependent upon an external factor, is reflected at mirror 20. returns back through the delay sensor means 19. and returns to detection and demodulation means 9 via the coupler 18 and the optical fibre 17. The other part of the source light passes through an intensity sensor 21, is reflected at a mirror 22, and returns to the detection and demodulation means 9 via the intensity sensor 21 , the coupler 18 and the optical fibre 17.

In a preferred embodiment of the present invention, the delay sensor means 19 is simply a piece of optical fibre whose length is chosen relative to the wavelength of the modulation of the light within the said optical fibre so as to provide adequate sensitivity to the external parameter to be measured. Two such parameters are elongation and temperature of the fibre.

Figure 4 shows another preferred embodiment of the present invention, in which all the sensing components are optical fibres, in which the state of polarisation of the light is controlled, and in which the external parameters measured are pressure and temperature. A dual parameter sensor 23 is connected by optical fibre cables 16 and 17, whose length may be many kilometres. Light from the light source 1, modulated by the modulation means 2 is transmitted to an optical fibre coupler 18 by the optical fibre 16 via depolarising means 24. Light passes through optical fibre polarising means 25, and a length of optical fibre 27, and is then reflected at a mirror 26. The light returns back through the optical fibre 27 and the polarising means 25, and returns to the detection and demodulation means 9 via the coupler 18 and the optical fibre 17. The other part of the source light passes through an optical fibre 28, an optical fibre polarising means 29, and a birefringent sensing means 30, before being reflected at a mirror 31. The light passes back through the birefringent sensing means 30, the polarising means 29, and the optical fibre 28, and then returns to the detection and demodulation means 9 via the coupler 18 and the optical fibre 17. The combination of the polarising means 29, the birefringent sensing means 30 and the mirror 31 implements a polarimetric interferometer in which the light intensity is modified by a change to the birefringence of the birefringent sensing means 30, preferably externally applied pressure. The lengths of the connecting optical fibres 27 and 28 are chosen such that the differenc between them creates an optical path length delay of an amount which may be measured by changing the modulation frequency according to th present invention. The refractive index and physical lengths of optical fibres are inherently temperature dependent and so the optical lengths o the optical fibres 27 and 28 are normally dependent upon temperature an so the change in delay with temperature makes a convenient temperatur sensor. In addition, an embodiment of the invention may includ enhancement of the temperature sensitivity of the optical fibre by mean

of a suitable coating, or any other means. The components of the dual parameter sensor 23, as described in Figure 4. are preferably arranged so that they are all at a similar temperature, in which case the temperature measured by the delay sensor may be used for the temperature compensation of the intensity sensor.

Figure 5 shows another preferred embodiment of the present invention, in which sensing means 41 is an all optical fibre dual parameter sensor where the two physical parameters measured are preferably pressure and temperature. Sensing means 41 is connected by a single optical fibre link 33, whose length may be many kilometres, to the light source 1 and detection and demodulation means 9. Light from the light source 1, modulated by modulation means 2 is transmitted to polarising means 34 and partially reflective mirror 35 via depolarising means 24, optical fibre coupler 32, and optical fibre link 33. Light is partially reflected by the mirror 35 and the remaining part passes through a length of fibre 36, polarising means 37, which preferably selects the same polarisation state as polarising means 34, and optical fibre sensing means 38, and is then reflected back from a mirror 39. The optical fibre sensing means 38 is such that its birefringence is sensitive to a physical parameter to be measured, preferably pressure, and the optical fibre sensing means 38, the polarising means 37, and the mirror 39 form a polarimetric sensor 40. The light reflected from the mirror 39 passes back through the optical fibre sensing means 38, the polarising means 37 and the optical fibre 36, and reaches the partially reflective mirror 35 where part of the light is transmitted and part is reflected. This back and forth reflection between the mirrors 35 and 39 creates multiple optical path length delays where their relative optical path length delays depend on the physical length of the fibre 36, and their relative amplitudes are determined by the reflectivity of the mirror 35 and the light intensity returning from the polarimetric sensor 40. The reflected light returns to the detection and demodulation means 9 via the optical fibre link 33 and the coupler 32. The optical path length delay and the response of the polarimetric sensor 40 may be measured by changing the modulation frequency of modulation means 2 according to the present invention. Thus both a physical parameter which alters the optical length of optical fibre 36, preferably temperature, and a physical parameter which alters the behaviour of polarimetric sensor 40, preferably pressure, may be measured. The otherwise unused output fibre 42 of the optical fibre coupler 32 is preferably index matched to minimise undesirable back reflections, but more preferably it may be used to monitor the characteristics of the light source 1 using additional detection means not shown.

The embodiments of the present invention described herein, including Figures 3, 4 and 5, do not exclude the addition or exclusion of

optical components in order to enhance the performance of the apparatus. For example, an optical isolator may be placed in the optical path between the light source means 1 and the depolarising means 24 in Figure 4 in order to prevent any reflected light returning to the light source means 1, or the polarising means 34 may be removed from the embodiment described in Figure 5.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Thus, for example, the mirrors may be a variety of types of mirror, including metal film, dielectric film, refractive index differences, or gratings within an optical fibre, and where the reflectivity of the mirror is not necessarily 100%. Partially reflecting mirrors may be inserted into an optical fibre waveguide by depositing a dielectric coating onto the end of the fibre and then splicing to another length of fibre, as taught by T.Yoshini, K.Kurosawa, K.Itoh, and T.Ose, "Fiber-Optic Fabry-Perot interferometer and its sensor applications", IEEE J. Quantum Electron., Vol. QE- 18, pp.1624- 1632, 1982. Alternatively, partially reflected mirrors may be formed by writing a grating into the fibre as taught by G. Meltz, W.

Morey, and W. H. Glenn, "Formation of Bragg Gratings in optical fibres by transverse holographic method", Opt. Lett.,14, ρp.823-825, 1989.