WO1987005762A1

WO1987005762A1 - Optical data bus and magnetometer

Info

Publication number: WO1987005762A1
Application number: PCT/GB1987/000176
Authority: WO
Inventors: David Alfred Jackson; Julian David Clayton Jones
Original assignee: Kent Scientific And Industrial Projects Limited
Priority date: 1986-03-14
Filing date: 1987-03-13
Publication date: 1987-09-24
Also published as: GB8606353D0

Abstract

Optical apparatus for use as an optical data bus comprises an optical source (2) for launching polarised light into an optically transparent dielectric medium (1), such as, a monomode optical fibre, a reference magneto-optic modulator (3) for producing a change in the polarisation state of the light beam guided within the dielectric medium by Faraday rotation, at least one signal magneto-optic modulator (4, 5, 6) located in proximity to the dielectric medium (1) such that the application of an electrical signal to the signal modulator produces a further change in the polarisation state of the light beam by Faraday rotation, thereby to encode a corresponding signal on the light beam, a detector assembly (9) for producing an electrical signal corresponding to the encoded light beam emerging from the dielectric medium (1), and a processor (10) for decoding the electrical signal from the detector (9) and providing an output uniquely determining the or each encoded signal. The processor (10) may be arranged to supply a servo feedback signal to the reference magneto-optic modulator (3) in orderto maintain the polarisation azimuth of the output light beam from the dielectric medium at a substantially constant angle of rotation. The feedback signal then contains the encoded information. The recovered signal from the or each signal modulator (4, 5, 6) describes the magnetic field at the signal modulator and, hence, the apparatus may also be used as a magnetometer or for the measurement of electrical current.

Description

OPTICAL DATA BUS AND MAGNETOMETER

The present invention relates to optical apparatus which is designed to be used as an optical data bus or for the detection or measurement of magnetic fields and electrical currents. More particularly, the invention relates to such apparatus which exploits the Faraday effect, that is,the rotation of the polarisation azimuth of a beam of linearly polarised light propagating in an optically transparent dielectric medium when subjected to the action of a magnetic field.

It is an object of the present invention to provide an optical data bus which may have information encoded onto a light beam guided within the data bus without interrupting the light beam or having any physical contact with the bus, and in which additional encoders may be fitted whilst the system remains in operation. Such a data bus may be used either as a receive-only bus or as a detector of magnetic fields and electrical currents. The invention consists in optical apparatus capable of being used as an optical data bus or for the detection of a magnetic field or electrical current, comprising an optically transparent dielectric medium having an optical input and output, light source means for producing polarised light and launching it into the optical input of the dielectric medium, a reference magneto-optic modulator arranged to vary the state of polarisation of the light beam guided within the dielectric medium by Faraday rotation, means for producing at least one magnetic field for encoding a corresponding signal on the light beam guided within the dielectric medium by further varying the polarisation state of the light beam by Faraday rotation, detector means for producing an electrical signal corresponding to the encoded light beam at the output of the dielectric medium, and processing means for decoding the electrical signal from the detector means and producing an output corresponding to said at least one magnetic field.

The magnetic field utilising Faraday rotation to encode a signal on the light beam guided within the dielectric medium may be generated by an electrical solenoid wound about or located in proximity to the dielectric medium, by the motion of a magnetic material, by thermal changes in magnetisation, by a magnetic field in free space, or by any other variable magnetic device having at least a magnetic field component substantially parallel to the direction of propagation of the light beam within the dielectric medium.

The reference magneto-optic modulator may be arranged to modulate the polarisation state of the light beam propagating within the dielectric medium either by being interposed in the dielectric medium or by being located in proximity to it. Conveniently, the reference modulator may be in the form of an electrical solenoid surrounding a portion of the length of the dielectric medium. Alternatively, it may be a device which produces modulation by mechanical motion of a magnetised material in the presence of the dielectric medium, or other magnetic device having at least a magnetic field component substantially parallel to the direction of propagation of the light beam within the dielectric medium for inducing changes in the polarisation state of the light beam by the principle of Faraday rotation. The optical input and ouput of the dielectric medium may be disposed at the same end thereof, in which event the opposite end of the medium is reflective. Preferably, the dielectric medium is a low biref ingence mono ode optical fibre which may be twisted to induce additional circular birefringence in the fibre and, hence, reduce the effects of linear birefringence.

Conveniently, the detector means comprises a polarising beam splitter which divides the output beam into two orthogonal linearly polarised components, and photodetector means for detecting these components and producing electrical outputs substantially in quadrature. The beam splitting means may be arranged to terminate the dielectric medium or may be interposed in the medium so as simply to sample part of the encoded light beam. For example, it may be a fibre optic directional coupler interposed in the dielectric medium. The apparatus may be adapted to utilise a closed loop form of signal processing so as to control the operating point of the apparatus. To this end, the processing means may be adapted to provide a feed back signal to control the reference magneto-optic modulator so as to maintain the polarisation azimuth of the optical output at a substantially constant angle of rotation. The feed back current then has the same frequency as the detected or encoded signal and its amplitude is that of the signal current scaled by the turns ratio of the reference and signal magneto-optic modulators.

In this closed loop mode of operation, the apparatus has the advantage that variations in the light source wavelength and the Verdet constant of the dielectric medium do not give rise to "scale factor" errors as is the case with open loop configurations. Although magneto-optic devices utilising -the Faraday effect have been primarily developed for the measurement of large currents, this closed loop method of signal recovery has been found to be equally applicable to the detection and measurement of

-3 relatively small signal currents, for example, 10 A, over a wide frequency range, and hence the technique may also be applied to the realisation of a zero insertion loss data bus.

In the preferred form of the invention, the apparatus utilises reference and signal magneto-optic modulators in the form of simple solenoids through which an optical fibre or other optically transparent dielectric medium extends. These modulators modulate the light beam propagating in the dielectric medium by the principle of Faraday rotation and, in this situation, the amount of rotation J2f of the polarisation azimuth may be expressed in terms of the Verdet constant V, the magnet field strength H, and the distance _1 traversed by the light beam in the dielectric medium during which it is subjected to the magnet field, in the following manner.

Where V = V( ^ ) is dispersive, with A being the vacuum wavelength of the source. Hence, for a simple solenoid comprised of N turns of copper wire

where I is the current flowing in the solenoid. The frequency bandwidth of apparatus according to the invention is limited by electronic rather than optical considerations. Consequently, it is possible to use a plurality of signal magneto-optic modulators on the same optical dielectric medium. Provided that their frequency spectra are distinct, the encoded signals may be individually recovered. For example, in the simple case of a plurality of signal solenoids or coils each carrying sinusoidal currents, the signal from each may be recovered by band-pass filtering. This feature may be exploited to realise a zero insertion loss data bus, where 'each signal solenoid will effectively be a receive-only station. The full range of encoding schemes are available in this technique, including amplitude and frequency modulation, and frequency shift keying.

The signal modulators or receiving stations need not take the form of simple solenoids. Any electromagnet producing a field component substantially parallel to the propagation axis of the dielectric medium will suffice. For example, it would be advantageous to design modulators which may be simply clipped onto the bus without the need to disrupt the dielectric medium itself. 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 block diagram illustrating the basic apparatus of the invention; Figure 2 is a schematic circuit diagram of one preferred embodiment;

Figure 3 are traces illustrating the variation of signal solenoid current and reference or feedback solenoid current with time, in the apparatus of Figure

Figure 4 are traces illustrating the photodetector outputs of the apparatus of Figure 2 when the servo feedback is unlocked and locked, respectivly, and

Figure 5 is a graph illustrating the variation of the magnitude of the servo feedback current with the magnitude of the signal solenoid current. Refering to Figure 1 of the accompanying drawings, the basic apparatus comprises an optically transparent dielectric medium 1, for example, a mono ode optical fibre, a light source -2, for example, a laser light source, coupled to the optical input of the dielectric medium, a reference magnetσ-optic modulator 3 interposed in the light beam guided in the dielectric medium 1 either by interrupting the medium or by being in proximity to it, one or more signal magneto-optic modulators or encoders 4,5,6 disposed in close proximity to the dielectric medium, detector means 9 for sensing the optical output of the dielectric medium and including optical processing and opto-electronic transduction-, and electronic processing means 10, 11 connected to the output of the detector means.

In operation, polarised light from the source 2 is launched into the optical input of the dielectric medium 1 and the magneto-optic modulator 3 produces a change in the state of polarisation of the light beam guided in the dielectric medium by Faraday rotation for example, in response to an electrical signal applied to the modulator 3. The Faraday effect causes the polarisation azimuth of the guided beam to be rotated by an amount proportional to the electrical current supplied to the modulator. The light beam is then guided by the dielectric medium so as sequentially to pass in proximity to the signal modulators 4,5,6. Each of these modulators exploits the magneto-optic or Faraday effect such that the application of a signal to an individual modulator produces modulation in the state of polarisation of the light beam to encode a corresponding signal thereon. The resulting encoded light beam is sampled by the detector 9, which may either serve to terminate the dielectric medium 1 or may simply sample a part of the encoded beam, for example, via a fibre optic directional coupler. The electrical output from the detector is decoded by the processor 10 which provides an output uniquely determining the plurality of encoded signals. This output may be displayed or, alternatively, may undergo further processing, for example, as part of the control system 11. The processor 10 may also serve to supply any necessary signals for operation of the reference magneto-optic modulator 3 to facilitate operation of the apparatus.

In the particular embodiment illustrated in Figure 2, linearly polarised light produced by a laser diode 12, for example, emitting light at 790 nm, and a polarising element 13 is launched into the input end of an optically transparent dielectric medium 14 via a focussing lens 15. The dielectric medium 14 is a length of low birefringence monomode optical fibre twisted, for example, at a rate of 120 rads/m, to induce additional circular birefringence in the fibre and, hence, reduce the effects of linear birefringence. The optical fibre 14 extends through an electrical solenoid or coil 16, constituting a reference magneto-optic modulator, and at least one further electrical solenoid or coil 17, constituting a signal magneto-optic modulator. Only one signal coil 17 is illustrated, although it will be appreciated that more than one coil may be used if desired. The reference and signal coils may, for example, comprise respectively 4300 and 3900 turns of low resistance copper wire.

The light beam emerging from the output end of the optical fibre 14 is focussed by a lens 18 onto a beam splitter 19 which amplitude divides the light and directs it through polarisation analysers 20,21. The latter produce orthogonally polarised light components which are detected respectively by two photodiode detectors 22,23. The photodetector outputs are differentially amplified and integrated by an electronic servo 24 and are fed to the output 25 of the apparatus and back to the reference coil 16.

The apparatus is set such that, with no signal current applied to the signal coil 17, the outputs of the photodetectors 22,23 are effectively at quadrature. The photodetector output signals are differentially combined at the electronic servo, the output of which is fed to the reference coil 16 so that any Faraday rotation of the light beam traversing the fibre 14 owing to an alternating current flowing in the signal coil 17, produces antiphase sinusoidal outputs at the photodetectors. The sinusoidal servo feed back current supplied to the reference coil 16 is hence of the same frequency as that flowing in the signal coil and is fed to the reference coil to cancel out the Faraday rotation produced by the signal coil. The feed back current then has the same frequency as the signal current and its amplitude is that of the signal current scaled by the turns ratio of the reference and signal coils 16,17.

The effect of the servo loop is to maintain the polarisation azimuth of the output beam from the optical fibre 14 at 45° to the polarisation eigen axes of the beam splitter 19. The transfer function of the apparatus is thus maintained at its quadrature point.

The upper trace of Figure 3 shows the sinusoidal current applied to the signal coil 17 shown in Figure 2. The upper trace of Figure 4 shows the corresponding variation of the two photodetector output signals when the electronic servo 24 is switched off. It may be seen that these signals vary in antiphase and that the gain of the receivers is adjusted so that equality of their outputs corresponds to the quadrature condition. The lower trace of Figure 3 shows the current produced in the reference coil 16 when the electronic servo is switched on. As may be seen from the lower trace of Figure 4, the servo effectively maintains the outputs of the photodetectors at a constant and equal value, thus maintaining the apparatus at quadrature, as required. The technique is thus similar to a simple homodyne processing scheme often used for signal recovery in optical fibre interferometric sensors. It will be apparent that the current in the reference coil 16 should be directly proportional to the signal current in the coil 17. This relationship is verified by the data shown in Figure 5, which illustrates the variation of reference current with signal current. The maximum current which can be measured depends only on the current limit of the servo electronics, and the ratio of turns in the reference and signal coils.

When a plurality of signal coils 17 are used in conjunction with the optical fibre 14, the feed back current supplied to the reference coil 16 by the servo electronics 24 contains the encoded information. For example, if the signal frequencies are w ^", w„, w^ etc., then the feed back current contains components which are harmonics of w, . The separate signals may therefore be recovered using conventional electronic demodulation schemes. The form of the coding may be either amplitude or frequency modulation and the encoded information may be either analogue or digital in form. It is clear that the recovered signal from an individual signal coil or encoder 17 describes the magnetic field at that coil. This apparatus may thus be used as a magnetometer or for the measurement of electrical current by detecting the magnetic field which produces such current.

Whilst particular embodiments have been described, it will be understood that modifications can be made without departing from the scope of the invention as defined by the appended claims. For example, the apparatus described above may alternatively be used with a heterodyne signal processing system. In this case, a periodically varying signal is applied to the reference magneto-optic modulator 3 or 16. Moreover, whilst the operation of the apparatus has been described in terms of a linear optical configuration, it will be apparent that other arrangements are possible. For example, the distal end of the optical fibre 14 may be reflective, so that both the light source 12,13 and the detectors 19-23 may be positioned at the input end of the fibre.

Claims

1. Optical apparatus capable of being used as an optical data bus or for the detection of a magnetic field or electric current, comprising an optically transparent dielectric medium (1,14) having an optical input and output, light source means (2,12/13) for producing polarised light and launching it into the optical input of the dielectric medium, a reference magneto-optic modulator (3,16) arranged to vary the state of polarisation of the light beam guided within the dielectric medium by Faraday rotation, means

(4-6,17) for producing at least one magnetic field for encoding a corresponding signal on the light beam guided within dielectric medium by further varying the polarisation state of the light beam by Faraday rotation, detector means (9,19-23) for producing an electrical signal corresponding to the encoded light beam at the output of the dielectric medium, and processing means (10,11,24) for decoding the electrical signal from the detector means and producing an output corresponding to said at least one magnetic field.

2. Apparatus according to claim 1, wherein the means for producing at least one magnetic field comprises an electrical solenoid or other magneto-optic modulator (4-6,17) disposed about or located in proximity to the dielectric medium (1,14).

3. Apparatus according to claim 1 or 2, wherein the reference magneto-optic modulator is interposed in the dielectric medium (1,14) or is located in proximity to it.

4. Apparatus according to claim 1, 2 or 3, wherein the optical input and output of the dielectric medium (1,14) are disposed at the same end thereof and the opposite end of the medium is reflective.

5. Apparatus according to any preceding claim, wherein the dielectric medium is a low birefringence mono ode optical fibre, preferably, twisted to induce additional circular -birefringence in the fibre and thereby reduce the effects of linear birefringence.

6. Apparatus according to any preceding claim, wherein the detector means comprises a polarising beam splitter arrangement (19,20,21) which divides the output light beam into two orthogonal linearly polarised components, and photodetector means (22,23) is arranged to detect said components and produce electrical outputs substantially in quadrature.

7. Apparatus according to any preceding claim, wherein the detector means is arranged to terminate the dielectric medium or is interposed in the medium for sampling only a part of the encoded light beam.

8. Apparatus according to any preceding claim, wherein the processing means (12,24) is adapted to provide a feed back signal to control the reference magneto-optic modulator (3,16), whereby to maintain the polarisation azimuth of the output light beam at a substantially constant angle of rotation, and wherein means senses the feed back current to provide a measure of the frequency of an encoded signal and its amplitude.