The present invention relates to interferometric apparatus which is adapted to respond to a magnetic field and, more particularly, to such apparatus which is based on a Sagnac interferometer and which is designed for use as an optical fibre data ring or optical magnetometer for detecting magnetic field and measuring electrical current- Optical fibre links are commonly used as part of high speed data networks. In most applications, the fibre acts as a "point-to-point" link between a transmitter and a receiver. In other applications, the optical fibre may be used in conjunction with either conventional optical components, such as, beam splitters, or fibre optic directional couplers to form a data ring. The common disadvantage of such systems is that new data transmitters can only be added to the fibre optic data network by "breaking" the optical fibre link and connecting a new transmitter at the break point. It is one object of the present invention to overcome this disadvantage of existing data networks employing optical fibre links and to provide a novel form of fibre optic data bus in which a propagating light beam can be encoded directly by an external electrical or electronic signal without the necessity of either "breaking" into an optical fibre link or having any physical contact with the fibre. Such a novel data bus may be used either as a receive only data ring or as detector of magnetic fields and electrical currents. The data may be extracted from the data bus using either homodyne or heterodyne techniques.

The measurement of electrical currents by optical techniques provides a number of important practical advantages. This is particularly true in heavy electrical engineering applications where problems are associated with the isolation of high line potentials, and with the saturation of transformer cores by large currents. A number of techniques based on the use of fibre optic sensing elements has been described, all of which are based on the Faraday effect; that is, in dielectric media, bound electrons experience a radial force in response to an applied magnetic field. The effect of this is to produce a difference between the propagation constants for left and right circularly polarised components, thus the plane of polarisation is rotated. The amount of rotationΔφof the polarisation azimuth may be expressed in terms of the Verdet constant, V, the magnetic field strength, jϊ, and the distance which the light beam traverses in the medium subject to the magnetic field, __; , such that

where V = V(λ.) is dispersive. A linearly polarised light beam with polarisation azimuth ΔΦ can be considered as a combination of two orthogonal (left and right) circularly polarised beams with a phase difference of 2Δφ between them, which in the Jones calculus (see paper entitled "A new calculus for the treatment of optical systems" by R. C. Jones in J.Cpt. Soc.Am, 31, 488 of 1941) is expressed as

COSΔφ _iΔ<** i -iΔΦ r n (2) sinΔφ

The Faraday rotation, ΛO is basically the resultant differential phase retardation between the right and left circularly polarised components of the beam due to the magnetically induced circular birefringence of the medium. Since the magnitude of Λφ is a linear function of both the path length and the magnetic induction, a measurement of the magnetic field strength, _H, can be achieved by recording the observed azimuth rotation, φ , of the linearly polarised beam. A general difficulty is experienced in the use of this technique in that the magnitude of the Verdet constant is relatively small for most optical materials. In particular, the magnetically induced circular birefringence may be obscured by other sources of birefringence. For example, in an early implementation of the technique (see paper entitled "Optical fibres for current measurement applications" by A. M. Smith published in Optics and Laser Tech., February 25, 1980), the Faraday rotation in a length of single-mode optical fibre was measured from the ratioing of the recorded output intensities after the emergent beam was analysed into two orthogonal linear polarisation states, using a Wollaston prism. This technique suffers from the disadvantage that the effect of linear birefringence of the sensing medium on the state of polarisation of the beam and the resultant change in the ellipticity of the polarisation state is also measured, which is not magnetic field dependent. Both linear and circular birefringences of the sensing medium can be altered by external fields sucn as temperature, strain or anisotropic pressure. Hence, the system transfer function is strongly dependent on ^r .uch fields and variations in them. In genera] , low birefringence fibre sensing elements are used. The low

linear birefringence requirement is an essential one, since the magneto-optically induced azimuth rotation alternates in sign at half fibre beat length intervals, β /2 , yielding a zero net value over one whole beat length or a multiple of it. The beat length, <-?, is given by /Δ. n, where ^ is the vacuum wavelength of the light and Λ n is a measure of the linear birefringence of the fibre expressed by (n _f - n ) where n _^ . , n are the refractive indices of the fast and slow polarisation eigen modes of the fibre respectively. Therefore, only fibres whose beat length ^ > 2 < where ^ is the length of the sensing region, can satisfactorily be employed. However, the variations in the linear and circular birefringences caused by temperature changes and the resulting phase errors affect the performance of such systems, especially vvhen they are used to measure quasi-direct currents.

In accordance with the present invention, a reciprocal path interferometric technique is used which is based on a Sagnac interferometer and in which a beam of light is amplitude divided and launched in counter-propagating paths into a single loop of single mode optical fibre, or a coil of such fibre forming the sensor. The optical intensity of the interferometer output is directly dependent on the Faraday rotation of the polarisation azimuths, after the two beams are recombined. In such an arrangement, because non-magnetically induced circular birefringence (optical activity) is a reciprocal effect, it is cancelled by common mode rejection. Furthermore, the effects of linear birefringence are reduced, as more fully described below. For these reasons, the invention is ^' much less dependent on environmental perturbations, such as, temperature and vibration, than hitherto known

systems. This advantage is particularly valuable in the measurement of direct electric currents, where long term stability is of paramount importance.

The invention enables the use of a low coherent light source. Moreover, an electronic synthetic heterodyne signal processing technique may be used and this obviates the requirement of opto-mechanical adjustment to set the instrumental operating point and has an effectively infinite dynamic range. With a heterodyne signal processing system, the measurand current is recovered from the phase modulation of an electrical or electronic carrier. Such processing is desirable because it provides a linear response function over an effectively infinite measurement range. In combination with the optical configuration utilised by the invention, this signal processing system does not rely on the adjustment of mechanical components (such as polarisers) to set the operating point of the system. The system may be based on the use of a piezo-electric modulating element which provides a dynamic phase bias in a manner analogous to that previously used with fibre optic gyroscopes.

Whilst a heterodyne signal processing system is specifically described in connection with the measurement of electrical current, it will be apparent that it is equally applicable to the measurement of any magnetic fields.

As described above, the Faraday effect is t'.e magneto-optically induced circular birefringence in a medium. Hence, the transfer matrix of a finite element of an isotropic dielectric, possessing both linear and circular birefringence, can be written as a product of two Jones matrices describing the linear and the circular birefringences of the medium

COS Φ -s i nΦ -i Δ/2 0

T = s i nΦ COS Φ i Δ/2

( 3

where Δ- and express the degree of linear and circular birefringence respectively , and

Φ = Φ Δφ (4)

where ₀ is the intrinsic circular birefringence (optical activity) of the fibre and is the magneto-optically induced birefringence. The irradiance of an optical beam, I, transmitted through such a medium is given by

i o i j ^f ] . [T x] (5)

where X is the Jones vector describing the incident beam, and " + " implies the Hermitian conjugate. Hence,

I = fn(Φ _o ,Δφ,Δ) 6)

assuming attenuation to be implicit. Both φ ₀ and Δ are affected by external fields such as temperature, strain, anisotropic pressure, etc, hindering a reliable quasi-steady measurement of ΛΦ ,and producing noise and operating point drifts. To overcome this problem a ring configuration is used in which reciprocal effects such as birefringence of the fibre are reduced by common mode rejection;- Presented below is an analysis of the ring ronfiguration in which the fibre is considered as a linear array of N dielectric elements each possessing finite linear and circular

birefringence whose individual transfer matrices are of the kind given in equation (3). The magnetic field is taken to be uniform along the sensing region.

In accordance with equation (3) the system transfer matrix for the clockwise, T^ , and the anti¬ clockwise, T _f , propagating beams can be written as

where

rJ± __

with φ . ==.' ^___-_ ' φ = total circular birefringence of the fibre

and Δ -= -rr ; Δ ≡ total linear birefringence of the fibre

and N is a large integer; φ . = ₀ - + _ - describes the degree of circular birefringence experienced by the clockwise propagating beam which goes through a positive, +Δ ., polarisation azimuth rotation, and

Φ- 3 ■ = ^r oj• - describes the degree of circular birefringence experienced by the anti-clockwise propagating beam which goes through a negative,-.Δ .. polarisation azimuth rotation.

The interferometric output intensity function is then given by

( 8

where the order in which the two counter propagating beams pass with jth element of the fibre is respected. Provided

oj ^Φ o(N-j+l)

and

it can be shown that the intensity function, I, reduces to

I = X ^f [2I + R(2Δφ) + R(-2Δψ)]X

(10)

Δ<f = [ Δ j=l ^*

where R(±2 _ ) are the rotation matrices through ±2 Δφ, and I_ is the identity matrix. The interferometric irradiance, hence, becomes

I Q 1 + cos(2Δ<f) (11

and is independent ofφ and Δ and their temporal variations. The conditions outlined in equation (9) refer to the preservation of symmetry in the Sagnac interferometer. This means that the effect of any external fields such as temperature changes or vibration is greatly reduced and takes a differential form expressed by

where «Φeis the overall error in the recorded interferometric phase due to the effect of the error field, M . , on the fibre element j, Δ M , and n., £ . 1 1 j j are the refractive index and the physical length of the jth element respectively. And in the case of an asymmetric birefringence existing along the ring, its transfer function can be shown to be given by

I o. 1 + a cos[2Δφ + p]

(13

where a = ^• cos( _ ) is an amplitude modulation term with£= __ (A j.- Δ\. W—j.+ I . ) being the differential linear birefringence of the ring experienced by the counter propagating beams, and p = is a phase error term, with φ = 4 - (Φ ^ψ _ψ o_ι.• ~-Φ ^v _v ^v oo.(,(N.N..,.--π].-++,, 1,1.,)'. ⁾ being the differential circular birefringence of the ring. Hence both effects are greatly reduced from having an integral form, as in previous configurations, to having a differential one.

In order that the present invention may be more readily understood, reference will now be made to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of a data ring and • homodyne signal processing system according to the invention,

Figure 2 is a schematic diagram of a closed loop magnetic field or current detector and homodyne signal processing system according to the invention, Figure 3 is a schematic diagram of a data ring or magnetic field or current detector and heterodyne signal processing system according to the invention,

Figure 4 is a schematic diagram illustrating another embodiment of the invention for magnetic field and current detection,

Figure 5 is a diagram illustrating the two electronically produced carriers utilised in the embodiment of Figure 4, after being squared up with 50% duty cycles (solid line: no current: dashed line: D.C. current applied),

Figure 6 is a graph illustrating the results achieved with the apparatus of Figure 4 for direct current measurements using both a laser and an LED source, Figure 7 is a graph illustrating the variation of Verdet constants with the square of light source frequency,

Figure 8 is a graph illustrating the results achieved with the apparatus of Figure 4 for alternating current measurements for both laser and LED light sources, and

Figure 9 is a graph illustrating the environmental insensitivity of the apparatus of Figure 4.

Referring to Figure 1 of the drawings, the data ring comprises a single loop 1 of optical fibre which is illuminated by a solid state light source 2 which may be either a solid state laser or a light emitting diode (L.E.D.). The light source 2 is connected to the optical fibre loop by a fibre optic directional coupler 3. The input light from the source is polarised and amplitude-divided into two beams at the directional coupler, which beams propagate in clockwise and anti-clockwise directions within the fibre loop. The latter is contained along most of its length in a protective sheath of, for example, flexible plastic or aluminium. After propagating through the fibre loop 1 these two beams mix coherently at the directional coupler 3 giving rise to an optical intensity which depends on the optical path difference between the clockwise and anti-clockwise beams. This optical interference signal is detected by a photodiode 4.

One technique for transferring information into the ring is as follows. A small electrical coil 5 is wound about the optical fibre of the loop 1 such that a current signal of frequency £ _ ■, flowing through the coil produces a magnetic field with the field direction along the direction of propagation of the counter- propagating beams in the fibre loop. Hence, the polarisation azimuths of the counter-propagating optical beams within the fibre loop 1 are affected as they pass through the coil, by reason of the Faraday effect, such that the azimuth of each beam is rotated. The degree of rotation of each beam depends on the amplitude of the signal f _s . . When these co-rotating azimuth modulated optical beams arrive at the directional coupler 3 they combi _e coherently and produce an optical interference signal which is

amplitude modulated with a frequency spectrum containing harmonics of fs ι and including ^Ό a maj ^■ -or component at a frequency 2f . A plurality of ^s 1 electrical coils similar to 5, that is, coils 6, 7, etc may be used to induce signals into the loop 1 and, if their frequencies are different, that is, f , f etc then the electrical current of the photodiode output will contain signals at frequencies 2f ,

2f , 2f etc. ^Ξ 2 ^S 3 The interaction between the light beams and the or each magnetic field is a linear effect. Hence, the data ring can be used to recover signals generated in electrical coils 5, 6 and 7. Analogue data can be recovered from the variation in the amplitude of the recovered signals at, for example, 2fa_.1 , 2f Z3 _ , 2fi.3 etc. Digital data can be impressed on the light beams by frequency switching the input current signals between f's ₁ to f" s _ >, f's ₂ to f"s ₂ etc. The information contained in these signals may then be recovered uniquely by passing the output signal from the photodiode 4 through separate band pass filters 8 centered at 2fs-| , 2fs ₂ etc with bandwidths 2f' s * to

2f" _f 2f to 2f" etc, provided the frequency ranges ^s 1 ^s 2 ^s 2 of the filters do not overlap. To increase the interaction between the electrical currents in the coils 5, 6, 7 etc, material of high magnetic permeability may be included at each coil location. The current carrying conductors need not be wrapped about the optical fibre. Any configuration giving a field component parallel to the fibre axis will suffice. Also, any technique giving a time dependent magnetic field may be used to encode data into the ring, for example, by mechanical motion of a magnetic material or electrical circuit.

Figure 2 illustrates an embodiment for detecting magnetic fields where the direction of the unknown magnetic field has a component along the direction of the optical fibre ring or loop 1. The magnetic field may be generated either by an electrical coil 10 wound about the fibre of the loop, as illustrated, or by a magnetic field in free space.

As in the previous embodiment, input light from a solid state optical source 2 is polarised and amplitude-divided into two beams at the fibre optic directional coupler 3, and these two beams, propagate in clockwise and anti-clockwise directions about the fibre loop 1. After propagating through the fibre loop, the two beams are recovered and mixed coherently by the directional coupler 3 and the resulting interference signal is detected by the photodiode detector 4.

The signal processing system includes a servo control arrangement comprising an electrical coil 11 wound about the fibre of the loop 1 , a difference amplifier having inputs connected to the outputs of the photodiode 4 and a second photodiode detector 13, which monitors the output of the light source 2, and having its output connected, via a current amplifier, to supply a feedback signal to the coil 11, as well as a signal to the next stage in the signal processing system. Servo control 12 includes the difference amplifier and current amplifier.

If the magnetic field, for example, produced by the coil 10, is composed of both an oscillatory part of magnitude A at frequency F and a steady state part of magnitude B, then the output signal at the detector 4 will contain signals at frequency 2F related to A and .t Fm related to B. In addition, there can be another component of amplitude C, again at frequency F , caused

by any non-reciprocity in the ring. The amplitude of C is environmentally sensitive and represents a source of error in the measurement of A and B.

The system illustrated in Figure 2 operates as follows. The output signal from the photodiode 4 is fed into one input of the difference amplifier, the output of which is electronically integrated and fed to coil 1 via the current amplifier. The other input to the difference amplifier is derived from the photodiode 13, the output signal of which is adjusted such that it corresponds to the 'quadrature' point in the interferometer's transfer function. Hence, the feedback signal supplied to the coil contains a DC component, which represents signal components B and C, and an alternating component at F directly proportional to A. The constant of proportionality is obtained, for example, by applying a magnetic field of known magnitude to the system. This method of signal recovery can also be used if the interferometer is subject to more than one magnetic field and can therefore be used as an alternative method of signal recovery for the data ring.

In many applications of the fibre optic data ring, it will be important to recover data as a signal superimposed on a high frequency electrical or electronic carrier rather than at the base band frequency, as in the case of the previously described homodyne systems. An arrangement for recovering data in this manner is illustrated in Figure 3. It includes the basic ring configuration shown in the previous embodiments including a single optical fibre loop 1 , a solid state light source 2, a directional coupler 3, a photodiode detector 4 and data coils 5, 6, 7. The electronic carrier is produced by incorporating a

dynamic phase shifter into the optical fibre loop 1 close to the directional coupler 3. The dynamic phase shifter may, for example, be a Bragg cell or a piezo-electric element (around which part of the fibre loop 1 is wound) . In one implementation of such a system based upon a piezo-electric phase modulator 15, the latter is driven sinusoidally by an electronic oscillator 16 at a frequency f , and the output of the photodiode 4 is fed through an electronic switch 17, which is. synchronously driven from the oscillator 16.

Th- resulting signal is band pass filtered at 2f to produce an electronic carrier at 2f . This technique of phase modulation and carrier production is known to those skilled in the art of fibre optic gyroscopes. As described above, for the case of the homodyne data ring, when the co-rotating azimuth modulated optical beams arrive at the directional coupler 2 they combine coherently to produce a phase modulated signal. This signal, centred about the base band, is then transposed onto the electronic carrier producing a phase modulated carrier of centre frequency 2f by the combined action of the oscillator 16 and the electronic switch 17. If a s ■ig ^nal current of amplitude As„ι of frequency f _s is applied to a signal coil then the ^" resulting carrier sidebands are at frequency f _p ± f _s with an amplitude proportional to A _s - . When the coils

5, 6, and 7 are modulated by currents at different frequencies f _s , f _s _ , with amplitudes A _s , A _s , A _s etc, the output carrier signal contains signals at fp ±f,_3-| ,fp ± fϊ-2 and f,F_ with amplitudes proportional to A_ , A_ , A etc. These signals can ^b 1 ^a 2 ^3 then be demodulated by conventional electronic methods, such as, a phase locked loop 18.

A' piezo-electric phase modulator is particularly

suited for data recovery where the bandwidth required for the system is less than 100kHz; for higher bandwidths the Bragg cell modulator is preferred.

The heterodyne recovery system described with reference to Figure 3 may also be used as a detector of electric currents and magnetic fields. A typical application is where it is necessary to detect the amplitude A and frequency f of mains current under both normal conditions and when there is the possibility of large surge currents due to overloads or surges in the supply. In this type of application only a single signal coil is used, for example, the coil 5.

In normal operation a signal related to the amplitude and frequency of the current may be recovered with a phase locked loop detector. To enable measurements of the output signal associated with large surge currents an additional high speed digital phase tracker is incorporated into the system.

Initial results with these systems have shown that the dynamic range exceeds 10

The embodiment illustrated in Figure 4 comprises a coil 20 formed from single mode optical fibre 21 wound about a 30 cm diameter circular aluminium former with 45 turns. The fibre 21 is also twisted, for example, at a rate of approximately 120 rads/m in the process of winding onto the former to induce circular birefringence, thereby suppressing the quenching effect of linear birefringence in the fibre. An electrical coil 22, for example, a copper wire coil, is wound about the optical fibre coil to form a toroid with 580 turns. Polarised light from a solid state light source 23 is amplitude divided and launched into opposite ends of the coil 20 by a beam splitter 24 and lenses 25 so that light beams are propagated in opposite directions

about the coil. These counter-propagating light beams are recovered and mixed at opposite ends of the coil by the lenses 25 and beam splitter 24 and the resulting optical interference signal is detected by a photodiode 26 which produces an electrical signal corresponding to the interference signal and feeds it to signal processing means 27. Similarly to the embodiment described with reference to Figure 3, adjacent one end of the coil, the optical fibre 21 is wound about a piezo-electric phase shifter 28.

Using equation ( 1 ) , the amplitude of Faraday rotation,ΔΦ , in the case of a toroid 22 is given by

^N w where = D, H = nl and n = ^~~~ with D = toroidal diameter, N = toroidal turns, N _j = fibre turns and 1 = current flowing through the toroid. Since H and are co-linear,Δφ is then given by

As shown in equation (11), a direct measurement of the Faraday rotation of the polarisation azimuth,Λφ , is possible by recording the interferometric output intensity. Clearly, this technique suffers from the fact that the interferometer transfer function is a non-linear function of the Faraday rotation, that is I o_ 1 + cos(2 Δφ ). To overcome this problem the magneto-optically induced azimuth rotation is recovered using a pseudo-heterodyne detection scheme with a wide dynamic range and a linear scale factor, developed for fibre gyroscopes (see aper entitled "Pseudo-heterodyne detection scheme for fibre gyroscope" by A. D. Kersey,

A. C. Lewin, D. , A. Jackson, published in Electronic Letters, 20, 368 of 1984 and a paper entitled "Phase reading, all fibre optic gyroscope" by B. Y. Kim and H. J. Shaw, published in Opt. Letts., 9, 378, of 1984), in which the piezc—electric phase modulator 28 incorporated close to one end of the fibre coil 20, is driven with a modulation signal A sin ϋ t, where A is the peak modulation amplitude and ω is the modulation frequency. It can be shown that the interferometric output then becomes

I α 1 + a cos[Asinωt + p]

p = tarr ¹ [-iεtan(φ+2Δς) - ε'tan(δ)]

(16) a ² = cos ² δ[cos ² (φ÷2Δφ) - ε ² sin ² (φ+2 _Δ φ)]

⁺ ε' ² sin ^2δ cos ² (φ+2Δφ) + 2iεε'sinδcosδsin +2Δ ) cσs(φ ₊ 2 _Δ φ _. )

where (2-e ² )

and ε .' = —eΞ ²

(2-e ² ) with e being the eccentricity of the elliptical state of the beam. In the limit of small differential linear and circular birefringence, the above expressions for the amplitude and phase modulation terms reduce to

p = tan ^_1 [-iεtan(2Δφ)] (17) a = .[cos ² (2Δφ) - ε ² sin ² (2Δφ)] ⁱ

In the present embodiment, the state of polarisation of the two beams propagating clockwise and anti-clockwise

- 1 9-

around the coil 20 has been measured and found to be elliptical with eccentricity e = 0.94, which using equation (16) yield a scale factor ε= 0.6. Hence, the interferometric irradiance function will exhibit an amplitude modulation of up to 2% for currents of the order of 1 A flowing through the toroid 22. The observed phase modulation, p, is also affected by the scale factor and is smaller in magnitude as given in equation (17); the experimental results should, therefore, be corrected accordingly.

An important feature of the configuration shown in Figure 4 is the fact that the two interferometer paths are, by necessity, exactly equal to each other, and consequently, the coherence requirements of the source 23 are relaxed. The present embodiment has been demonstrated using both HeNe laser and a lower coherence light emitting diode. Light from a linearly polarised source was amplitude divided at the beam splitter 24, and launched into opposite ends of the fibre coil 20. The piezo-electric phase modulator 28 incorporated close to one end of the fibre coil was driven with a sinusoidal wave at a frequency t- ^ of 19 kHz to provide a high frequency carrier. The two counter propagating phase modulated beams interfere at the beam splitter 24 and the resultant interference fringes are detected at the photodetector 26. The photodetector output is processed at 27 by band pass filtering synchronously gated segments of the signal to give two sine wave carriers at twice the modulation frequency, the phases of which vary by equal magnitudes but opposite signs in response to an applied field, j^, to the ring (see Figure 2). As shown in equation (11), the effect of an applied magnetic field is to induce a phase retardation, 2 Δ , of the carrier, where Δφ is

the magnitude of the Faraday rotation of the azimuth of the polarisation state, determined by equation (15).

Hence, measurements of quasi-steady electric and magnetic fields are possible by direct measurement of the induced relative phase shift in the two carriers using a phase analyser.

A periodic electrical signal applied to the apparatus in the form of alternating current through the toroid 22, causes phase modulation of the carrier at the signal frequencyω (=50 Hz, mains, in present experiments), which appears as sidebands about the

2ωmcarrier. The amp ^~ litude of the fundamental harmonic of this sideband as observed with a spectrum analyser is directly proportional to the amplitude of the applied signal.

Figure 6 illustrates the results achieved with the apparatus of Figure 4 for DC measurements obtained using a multi-longitudinal mode 5 mW HeNe laser and an LED source, at wavelengths of 633 nm and 780 nm respectively, using the scale factor obtained in the previous section after polarisation state considerations. Clearly, from equation (15), the gradient of the straight line yields the Verdet constant at each wavelength. Thus, Verdet constants of V(633 nm) =• 5.35 x 10~ ⁶ rads A ^~1 and V(780 nm) = 3.34 x

10 rads A were obtained, in good agreement with previous work (see paper entitled "Optical fibre

Faraday rotation current sensor with closed loop operation" by A. D. Kersey and A. Dandridge, published in Electronics Letters, 21, 464 of 1985) and exhibit

2 the expected ^ dependence of V (see paper entitled

"dispersion of Verdet constant in stree-birefringent silica fibre by J. Noda, T. Hosaka a _d Y. Sasaki published - in Electronics Letter, 20,22 of 1984)

where is the source frequency. Figure 7 is a plot of the obtained Verdet constants [V(633 nm) and V(780 nm), present work; V(830 nm), see Kersey et al, 1985 as above] versus *tf . Figure 8 shows the recovered sideband amplitudes of the modulated carrier versus the applied AC signal amplitude, again using HeNe and LED sources. It can be seen that in both cases, a highly linear output is obtained. In the present tests, the upper limit on the maximum detectable signal is set by a phase change of 2~\ radians, to avoid ambiguity due to the periodic nature of the interferometer transfer function, equivalent in the present tests to an applied current of approximately 100 A; the lower limit is determined by the signal processing scheme used. Phase sensitive signal processing schemes have been developed for use with fibre optic interferometric sensors capable of phase resolutions in the range of 1 m rad to 1 μ rad. In the present tests, the minimum detectable current was ~ 0.1 mA. Figure 9 shows plots of the recorded interferometric phase and the temperature to which the fibre coil 20 as a whole was subjected in time. As it can be seen, no appreciable change in the interferometric phase was observed when the fibre coil temperature was increased by about 15°K and allowed to cool freely. This illustrates the greatly reduced temperature dependence of the coil configuration, making it suitable for sensor applications outside the benign laboratory environment. As well as te aperature fluctuations, the effects of other fields such as vibration or pressure which act on the optical fibre linear and circular birefringence are compensated, provided that they act uniformly on the fib_.e coil, and are reduced if a field gradient exists along the fibre

-22-

path, since, as described above, their effect will take a differential form. It is also clear that this technique may be used simultaneously to recover a number of current signals applied to the electrical coil 22, and that these currents may be resolved provided that they are of different frequencies. In this sense, the instrument may be configured as a listen-only optical data bus. This technique has a number of important advantages over previously proposed optical data rings. Chief amongst these are the facts that, firstly the transmitting stations are simply magneto-fibre optic transducers, and are consequently non-intrusive, and may be connected without physically disrupting the waveguide; and secondly, that each transmitter draws no optical power from the ring. Therefore, the number of transducers is limited only by the available frequency bandwidth.

From the foregoing, it will be apparent that the embodiment of Figure 4 may be used as a magnetometer for the measurement of direct and alternating electrical currents, and may be used with a heterodyne signal recovery scheme which exhibits excellent linearity over a wide dynamic range. The pseudo-reciprocity of the arrangement confers environmental insensitivity and allows cheap low coherence optical sources, to be used. The technique also has the potential of being applied to the production of an optical data ring with a large number of non-in.rusive transmitters, but without significant attenuation.

Whilst particular embodiments have been described it will be appreciated that modifications may be made without departing from the scope of the invention as defined by the appended claims..