WO1994021059A1

WO1994021059A1 - Fibre optic link with polarisation control

Info

Publication number: WO1994021059A1
Application number: PCT/GB1994/000409
Authority: WO
Inventors: James David Hobby; Donald Harry Maguire Kings
Original assignee: Cookson Group Plc
Priority date: 1993-03-03
Filing date: 1994-03-03
Publication date: 1994-09-15
Also published as: GB9304353D0

Abstract

Light is transmitted from a local station (4) to a remote station (2) via an optical fibre (7) to supply a device (1) with light of constant polarisation state and intensity. The effects of external perturbation on the polarising state of transmitted light through fibre (7) are compensated by a mechanical transducer (18) acting on the fibre at the local station (4), a polarising beam splitter (11) being provided at the remote station and deflecting light through a second fibre (13) to a detector (14) at the local station. Fluctuation in intensity of light received via the second fibre (13) is used to generate a feedback signal acting on the transducer (18). The effects of external perturbations acting on the fibre (7) to produce variation in intensity of the transmitted light are similarly compensated by modulating the output of light source (3). A non-polarising beam splitter (30) at the remote station is used to sample the received intensity via a third fibre (32) and the intensity of the light source (3) is modulated accordingly by means of a detector (34) and feedback circuit (36). The method is useful in remote powering of fibre optic sensors over extended distances.

Description

Fibre Optic Link with Polarisation Control

This invention relates to a method of transmitting light through an optical fibre to a remote station such that in the received light the intensity and state of polarisation are preserved in the presence of external influences acting on the fibre.

It is known that external influences acting on an optical fibre such as electric fields and mechanical stresses can induce anisotropy in the material forming an optical fibre which can result in perturbation to the state of polarisation.

There are circumstances under which light received at a remote station is required to have constant intensity and polarisation state, such as for example in the case of an optical sensor, where it may not be practicable to locate a suitable light source in close proximity to the sensor due to environmental conditions.

According to the present invention there is disclosed a method of transmitting light from a local station to a remote station through an optical fibre comprising the steps of controlling the polarisation state of transmitted light entering a local end portion of the fibre at the local station, sensing the polarisation state of light received from a remote end portion of the fibre at the remote station, generating a first feedback signal representative of changes in the polarisation state of the received light induced by external influences acting on the fibre and using the first feedback signal to vary the polarisation state of the transmitted light such that any induced change in polarisation state is compensated by an equal and opposite change in polarisation state of the transmitted light. An advantage of such a method is that the received light remains substantially unperturbed by environmental effects such as vibration experienced by the fibre. In a preferred embodiment of the present invention, a device at the remote station is to be supplied with linearly polarised light of fixed polarisation angle, the method comprising the steps of transmitting linearly polarised light of constant intensity and controllable polarisation angle, splitting the received light at the remote end of the fibre into X and Y components of mutually orthogonal fixed polarisation angles, supplying the X component to the device at the remote station, sensing the intensity of the Y component, generating a first feedback signal representative of the intensity of the Y component and using the first feedback signal to control the polarisation angle of the transmitted light so as to annul any variation in intensity of the Y component to thereby maintain the intensity of the X component substantially constant.

An advantage of this method is that the polarisation state of the X component is necessarily constant and the intensity of the X component is invariant with respect to external influences acting on the fibre affecting the polarisation state of transmitted light.

Preferably the received light is split into X and Y components by means of a polarising beam splitter.

Conveniently the polarising beam splitter is arranged such that the polarisation angles of both X and Y components are angularly displaced relative to the polarisation angle of the transmitted light by respective angles not equal to 90 or a multiple thereof. This ensures that non zero steady state values of the X and Y component amplitudes are maintained.

Preferably the Y component is conducted from the remote station to the local station by means of a second optical fibre.

Preferably the intensity of the Y component is sensed by means of a first detector at the local station operable to generate an electrical signal representative of the intensity of the Y component. Conveniently the first feedback signal is generated by operation of a first comparator having an output proportional to the difference between the a first reference voltage and the electrical signal representative of the Y component. Preferably the polarisation angle of the transmitted light is varied by means of a transducer driven by the first feedback signal and operable upon the local end portion of the fibre to induce birefringence in the material of the fibre. Conveniently the transducer is operable to apply a mechanical force to the local end portion of the fibre so as to vary the polarisation angle by stress birefringence.

The method in accordance with the present invention may additionally include the step of sensing the intensity of the light received from the remote end portion of the fibre at the remote station, generating a second feedback signal representative of changes in the intensity of the received light induced by external influences acting on the fibre and using the second feedback signal to vary the intensity of the transmitted light such that any induced change in intensity is compensated by an equal and opposite change in intensity of the transmitted light. This has the advantage of removing the effects of any perturbation acting on the fibre transmitting light to the remote station which would cause fluctuation in intensity in the received light. Such fluctuation would if present in the received light result in simultaneous fluctuation in both X and Y components, the fluctuation in the Y component being interpreted by the first detector erroneously as indicating that the polarisation state of the transmitted light had been perturbed.

Conveniently the intensity of the received light is sensed by dividing out a fixed proportion of the received light by means of a non-polarising beam splitter and transmitting the portion to a second detector at the local station via a third optical fibre. Conveniently the second feedback signal is generated by operation of a second comparator having an output proportional to the difference between the output of the second detector and a second reference voltage. Conveniently the intensity of the transmitted light is varied by modulating the intensity of the light source using the second feedback signal.

Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings of which: -

Figure 1 is a schematic representation of the transmission of light from a local station to a remote station having a device to be supplied with linearly polarised light in accordance with the present invention;

Figure 2 is a schematic diagram showing the construction of the first feedback circuit.

Figure 3 is a schematic representation of an alternative arrangement for transmitting light from a local station to a remote station and including control of the transmitted light intensity; and Figure 4 is a schematic diagram showing the construction of a second feedback circuit for use in the arrangement shown in Figure 3.

In Figure 1 a device 1 is located at a remote station 2 indicated generally by broken lines, the device being an optical sensor requiring for its satisfactory operation a constant supply of linear polarisation having constant intensity and polarisation angle. A laser light source 3 is located at a local station 4 indicated generally by broken lines and positioned at some distance from the remote station 2. Linearly polarised light 5 generated by the light source 3 is collected and directed into a local end portion 6 of a single mode optical fibre 7 by means of an optical element 8 and the transmitted light is conducted by the fibre to a remote end portion 9 of the fibre at the remote station 2. A collimator 10 collimates the received light at the remote station 2 and directs the received light into a polarising beam splitter 11.

The polarising beam splitter 11 splits the received light into X and Y components of mutually orthogonal fixed polarisation angles. The polarising beam splitter 11 is set up such that the X and Y components have polarisation angles which are angularly displaced relatively to the polarisation angle of the received light but with the amplitude of the X component being greater than that of the Y component. The polarising beam splitter is adjusted however so that the Y component is greater than zero. The X component is directed to the device 1 and necessarily has a fixed polarisation angle by virtue of the beam splitter 11. The Y component is collected by an optical element 12 and focused into a second optical fibre 13 of the multimode type which extends from the remote station 2 to the local station 4 where it is coupled to a first detector 14 operable to produce an electrical signal 15 representative of the intensity of the Y component.

The signal 15 is fed to a first feedback circuit 16 operable to generate first electrical feedback signal 17 which drives a transducer 18.

The transducer 18 is an electromechanical transducer clamped on to the local end portion 6 so as to apply a mechanical stress to the fibre which is proportional to the first feedback signal 17.

The first feedback circuit 16 is shown schematically in Figure 2 and consists of a first comparator 19 arranged to compare the first detector signal 15 with a voltage reference V Ref and to produce a comparator output 20 proportional to the magnitude to the difference between the first detector signal and V ref. The first comparator output 20 is amplified by a high tension amplifier 21 which in turn is connected to the transducer 18.

In the absence of external influences acting on the fibre 7 intermediate the local station 4 and the remote station 2, the X and Y components will remain equal in intensity since the polarisation of the received light will remain constant. Polarisation of the received light may not necessarily be identical to that of the transmitted light exiting the transducer 18 due to the inherent birefringent properties of the fibre 7.

In the presence of an external influence inducing a positive incremental change in polarisation . state of the received light, the intensity of the X component will incrementally increase and the intensity of the Y component will incrementally decrease. This decrease in the Y component intensity is sensed by the detector 14 resulting in a positive output voltage appearing at the first comparator output 20. The transducer is then actuated to apply an incremental stress to the fibre 7 in a sense which will incrementally change the polarisation state of the transmitted light by a negative amount equal in magnitude to that of the positive increment associated with the external influence. This compensating rotation returns instantaneously the polarisation state of the received light to its original polarisation angle and restores the intensities of the X and Y components to their steady state values. The intensity of the X component as supplied to the device 1 thereby is maintained substantially constant.

Other forms of polarised light source may be used in place of the laser light source 3 such as for example a light emitting diode having an output polarised by a suitable polarising element.

The embodiment described with reference to Figures 1 and 2 is set up such that the amplitude of the X component is greater than that of the Y component. There may be circumstances where it is appropriate for the amplitude of the Y component to be arranged to be greater than that of the X component.

The transducer 18 shown in Figure 1 has a linear portion of fibre to which a mechanical stress is applied. Alternative arrangements for stressing the fibre in a manner proportional to signal 17 may be utilised such as for example an arrangement in which a length of optical fibre is helically wound on a cylindrical mandrel comprising a piezoelectric transducer operable to radially expand and contract the mandrel.

Alternatively the polarisation angle of the transmitted light may be controlled other than by mechanically induced stress birefringence and in particular may be achieved using a transducer producing polarisation change by an applied electric field (Kerr effect or Pockels effect) or by an applied magnetic field (Faraday effect) .

The embodiments described above with reference to Figures 1 and 2 may be modified as follows with reference to Figures 3 and 4 in which corresponding reference numerals are used to those of preceding

Figures where appropriate for corresponding elements.

Light of constant polarisation angle is delivered to device 1 from the light source 3 in the manner described above but additional compensation is provided for the effects of any external influence acting on the fibres 7 resulting in fluctuation in light intensity received at the remote end portion 9.

Received light at the remote end portion 9 passes from the collimator 10 into a non-polarising beam splitter 30 comprises a non-polarising beam splitting cube.

The non-polarising beam splitter 30 is arranged to divide out a fixed proportion of the received light and deflect this divided portion of light via a second optical element 31 into a third optical fibre 32 of the multimode type via which the divided portion of light is conducted back to the local station 4.

The remainder portion 33 of the received light exits the non-polarising beam splitter 30 and is input to the polarising beam splitter 11.

At the local station 4, the third optical fibre 32 is connected to a second detector 34 arranged to produce a second detector output signal 35 which is proportional to light intensity. The second detector output signal is therefore representative of the intensity of light received at the remote station 2 and is input to a second feedback circuit 36 where the signal is compared with a second reference voltage V ref2 as shown in Figure 4 by means of a second comparator 37. The second comparator 37 produces a second feedback output signal 38 which is used to control the intensity of the light source 3 such that any deviation in the intensity of light received at the remote station is cancelled by a equal and opposite change in the intensity of light produced by the light source 3.

Perturbations due to external effects acting on the fibre 7 are found in practice to be predominently fluctuations in polarisation so that fluctuation in received intensity will in many instances not be evident. In those instances however where intensity fluctuation is evident, it is important to remove the effects of intensity fluctuation by means of the method described with reference to Figures 3 and 4 since intensity fluctuation may otherwise be sufficient to cause malfunction of the first feedback arrangement described with reference to Figures 1 and 2 since variation in intensity of light received at the remote end portion 9 will cause variation in the signal received at the first detector 14 thereby erroneously actuating the transducer 18.

The non-polarising beam splitter 30 may alternatively comprise a pellicle beam splitter or a 1 x 2 fibre optic splitter. The device 1 in the above embodiments is a fibre optic sensor for remotely sensing current in a mains power distribution system, the device typically being located in a pylon at a distance of perhaps one or two kilometers from the local station 4.

Claims

CLAIMS :

1. A method of transmitting light from a local station (4) to a remote station (2) through an optical fibre (7) comprising the steps of controlling the polarisation state of transmitted light entering a local end portion (6) of the fibre at the local station, sensing the polarisation state of light received from a remote end portion (9) of the fibre at the remote station, generating a first feedback signal (17) representative of changes in the polarisation state of the received light induced by external influences acting on the fibre and using the first feedback signal to vary the polarisation state of the transmitted light such that any induced change in polarisation state is compensated by an equal and opposite change in polarisation state of the transmitted light.

2. A method as claimed in claim 1 wherein a device (1) at the remote station is to be supplied with linearly polarised light of fixed polarisation angle, the method comprising the steps of transmitting linearly polarised light of constant intensity and controllable polarisation angle, splitting the received light at the remote end of the fibre into X and Y components of mutually orthogonal fixed polarisation angles, supplying the X component to the device at the remote station, sensing the intensity of the Y component, generating the first feedback signal so as to be representative of the intensity of the Y component and using the first feedback signal to control the polarisation angle of the transmitted light so as to annul any variation in intensity of the Y component to thereby maintain the intensity of the X component substantially constant.

3. A method as claimed in claim 2 wherein the received light is split into X and Y components by means of a polarising beam splitter (11) .

4. A method as claimed in claim 3 wherein the polarising beam splitter is arranged such that the polarisation angles of both X and Y components are angularly displaced relative to the polarisation angle of the transmitted light by respective angles not equal to 90° or a multiple thereof.

5. A method as claimed in any of claims 2 to 4 wherein the Y component is conducted from the remote station to the local station by means of a second optical fibre (13) .

6. A method as claimed in claim 5 wherein the intensity of the Y component is sensed by means of a first detector (14) at the local station operable to generate an electrical signal (15) representative of the intensity of the Y component.

7. A method as claimed in claim 6 wherein the first feedback signal is generated by operation of a first comparator (19) having an output (20) proportional to the difference between the a first reference voltage (V ref) and the electrical signal representative of the Y component.

8. A method as claimed in any of claims 6 and

7 wherein the polarisation angle of the transmitted light is varied by means of a transducer (18) driven by the first feedback signal and operable upon the local end portion of the fibre to induce birefringence in the material of the fibre.

9. A method as claimed in claim 8 wherein the transducer is operable to apply a mechanical force to the local end portion of the fibre so as to vary the polarisation angle by stress birefringence.

10. A method as claimed in any preceding claim including the step of sensing the intensity of the light received from the remote end portion of the fibre at the remote station, generating a second feedback signal (38) representative of changes in the intensity of the received light induced by external influences acting on the fibre and using the second feedback signal to vary the intensity of the transmitted light such that any induced change in intensity is compensated by an equal and opposite change in intensity of the transmitted light.

11. A method as claimed in claim 10 in which the intensity of the received light is sensed by dividing out a fixed proportion of the received light by means of a non-polarising beam splitter (30) so as to form a divided portion of the received light and transmitting the divided portion to a second detector (34) at the local station via a third optical fibre (32) .

12. A method as claimed in claim 11 in which the second feedback signal is generated by operation of a second comparator (37) having an output (38) proportional to the difference between the output of the second detector and a second reference voltage (V ref2) .

13. A method as claimed in any of claims 10 to 12 wherein the intensity of the transmitted light is varied by modulating the intensity of the light source using the second feedback signal.