GB2234828A

GB2234828A - Optical polarisation state controllers

Info

Publication number: GB2234828A
Application number: GB9003140A
Authority: GB
Inventors: Timothy Andrew Large
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1989-08-09
Filing date: 1990-02-12
Publication date: 1991-02-13
Anticipated expiration: 2010-02-12
Also published as: GB2234828B; GB8918177D0; GB9003140D0

Abstract

A polarisation controller is formed by a pair of optical fibre polarisation beam splitters (10, 11) are connected in tandem using two linking pieces (12,13) of single mode optical fibre, one of the fibres being wrapped around a piezoelectric transducer (14). Two such controllers (30a, 30b) may be connected in tandem with the principal axes of one at 45 DEG to those of the other so as to form a device which for a given input state of polarisation (SOP) can provide any output SOP. Such a tandem arrangement is employed as a component of an SOP-matching local oscillator signal generator for coherent detection of light. In an alternative form of controller the second optical fibre polarisation beam splitter (11) is dispensed with, and instead, the linking piece 13 is connected as an optical feedback path linking one of the outputs of the beam splitter to one of its inputs. <IMAGE>

Description

Optical Polarisation State Controllers This invention relates to optical polarisation state controllers, and particularly, though not exclusively, to such controllers in which the optical path lies in optical fibre. The control of the state of polarisation (SOP) of an optical signal is an essential part of many optical sensor and coherent optical communications systems.

Examples of bulk optics devices employed to modulate the SOP of a light beam are given for instance in United Kingdom Patent Specifications Nos. 1 245 760, 1 292 205, 1 350 266, 1 262 885 and 1 368 598. Examples of fibre-optics based devices are given for instance in United Kingdom Patent Applications Nos. 2 184 251 A, 2 184 252 A and 2 184 253 A. An SOP controller is also one subject matter of a paper by C.N. Pannell et al entitled 'Monomode Fiber Modulators: Frequency and Polarisation State Control', Fibre and Integrated Optics Volume 7 pp 299 - 315 to which attention is directed. The SOP modulator of that paper relies upon differential propagation effects induced in a high birefringence fibre by stretching it.

Controllers of the present invention have certain features in common with those modulators, but, by arranging to separate the light into two separate optical waveguides, one is able to make the controller much more sensitive to the effects of applied stress through being able to apply the stress solely to one of those paths so that the effect of the stress is realised in total rather than have the stress applied to a birefringent fibre so that. it changes both its principal refractive indices to provide an effect that is proportional to the difference between those two changes.

According to the present invention there is provided an optical polarisation state controller having an optical waveguide input feeding an optical waveguide polarisation beam splitter which divides a signal applied to said input into two orthogonally polarised components, one of which components propagates through a variable phase delay transducer before being substantially completely recombined with the other component in a single passage through an optical waveguide polarisation beam splitter to form a single entity feeding an optical waveguide output.

In one form the controller employs two optical fibre polarisation beam splitters connected in series, while in another form only one splitter is employed and a feedback path incorporating the variable delay device links one of the outputs of the splitter to one of its inputs.

Two such controllers of either form may be connected in tandem with a relative orientation such that light emerging from the second, or sole, polarisation beam splitter of the first controller plane polarised with its plane of polarisation aligned with one of the principal axes of that beam splitter is incident upon the first, or sole, polarisation beam splitter of the second controller plane polarised with its plane of polarisation at substantially 450 to the principal axes of that beam splitter. If plane polarised light is then arranged to be incident upon the first, or sole, polarisation beam splitter of the first controller with its plane of polarisation at substantially 450 to the principal aXes of that polarisation beam splitter, then it is possible to show that all possible states of polarisation (SOP's) of the output of the tandem arrangement can be arrived at by appropriate choice of the phase delays produced by the transducers of the first and second polarisation state controllers. It can also be shown that this property holds, not only for light launched into the first, or sole, polarisation beam splitter of the first controller at substantially 450 to the principal axes of that beam splitter, but for any SOP which, when represented on the Poincare sphere, lies on the great circle whose normal intersects the sphere at the points representing linearly polarised states substantially coinciding with the principal axes of the beam splitter.

One particular application for such a tandem arrangement of polarisation controllers is as a component of an SOP-matching local oscillator signal generator of a coherent light detection system. In such a system a coherent source is employed to inject light into the tandem arrangement with an appropriate SOP, and an associated feedback path is provided for the transducer of each polarisation controller of the tandem arrangement, each of which feedback paths is adapted to regulate the mean level of bias applied to its associated transducer in a manner to minimise the component of the coherent detected output in phase with an amplitude modulation of the bias applied to that transducer. In this way the SOP of the local oscillator signal output from the tandem arrangement is automatically caused to become locked with that of the information signal with which it is being coherently mixed, and thus minimise any loss of detection efficiency attributable to polarisation mismatch. It will be noted that this locking of the two SOPs is accomplished without analysing the SOP of the incoming information signal.

There follows a description of polarisation state controllers embodying the invention in a preferred forms. Also described is a tandem arrangement of two such controllers together with a description of the use of that tandem arrangement as a component part of an SOP-matching local oscillator signal generator of a coherent light detection system. The description refers to the accompanying drawings in which: Figure 1 is a diagram of an optical polarisation state controller.

Figure 2 is a Poincaré sphere diagram, Figure 3 is a diagram of a tandem arrangement of two Figure 1 type optical polarisation state controllers, Figure 4 is a diagram of a coherent detection system incorporating an SOP-matching local oscillator signal generator incorporating the tandem arrangement of Figure 3, and Figure 5 is a diagram of an alternative form of optical polarisation state controller.

A component part of the optical polarisation state controller of the form depicted in Figure 1 is an optical fibre polarisation beam splitter; indeed two such splitters are incorporated into a single polarisation state controller of this particular form.

A preferred method by which such optical fibre polarisation beam splitters may be constructed is described in United Kingdom Patent Specification No. 2,150,703 B to which attention is directed. The essence of this method is that two single mode optical fibres are stranded together, and are then subjected to a succession of stretching operations in which the fibres are longitudinally traversed under longitudinal tension through the short hot zone of a flame that produces localised heat-softening of the fibres. The stretching of the fibres is arranged to produce a coupling zone extending over many tens of wavelengths.

With each successive traverse the coupling is strengthened. As a result of the lack of circular symmetry in the coupling region, the coupling strength for light plane polarised in the direction containing the cores of both fibres in not quite the same as for that plane polarised in the orthogonal direction. The progressive stretching is terminated when the two coupling strengths have got 'out-of-step' with each other to the extent that light of one particular plane of polarisation launched into one fibre at one end of the splitter emerges substantially exclusively from one fibre at the other end, whereas light of the orthogonal plane of polarisation launched into the same fibre emerges substantially exclusively from the other fibre.

By analogy with the properties of light propagation in uniaxial crystals, these two planes of polarisation are termed the principal axes of the polarisation beam splitter.

Referring to Figure 1, two such optical fibre polarisation beam splitters 10 and 11 are connected together by two lengths 12 and 13 of single mode fibre, one of which is wrapped one or more turns around a cylindrical piezoelectric element 14. The second polarisation beam splitter 11 needs to be oriented so that its principal axes substantially coincide with the respective planes of polarisation of light received by it from the first polarisation beam splitter 10 via the two linking lengths of optical fibre 12 and 13. Under these circumstances, light launched into one fibre of the first beam splitter will emerge substantially exclusively from one fibre of the second beam splitter.

The first polarisation beam splitter separates the light into two orthogonally polarised components which the second polarisation beam splitter then recombines. Thus it is seen that the propagation of light through the polarisation controller of Figure 1 is somewhat analogous with the propagation of normally incident light through an uniaxial crystal plate cut parallel to its optic axis. When light is normally incident upon such a plate it is divided into orthogonally polarised O-ray and E-ray components which are recombined as the light emerges from the far side of the plate. Since the two components propagate with different velocities through the plate, there is a slippage of phase between the relative phase of the two components at entry into the plate and the relative phase of the two components at their emergence from the plate. The birefringence of a particular crystal plate is a measure of the proportional difference in propagation velocity of the two components, and thus the thickness of the plate will determine the magnitude of phase slippage. If for instance the thickness provides a phase slippage of My/2, then the plate is termed a quarter-wave plate. In the case of the optical fibre polarisation state controller of Figure 1, the two fibres 12 and 13 will normally be of identical construction, and so, in the absence of strain, the propagation velocities of of the two orthogonally polarised components will be the same.

In this instance phase slippage results instead from having a difference in optical path length between fibre 12 and fibre 13. Moreover this difference in optical path length is electrically controllable by means of the piezoelectric element 14.

A slippage in phase means that in general the output SOP will not be the same as the input SOP. The changes in SOP that result from phase slippage are conveniently illustrated by reference to a Poincaré sphere, for instance as depicted in Figure 2. On this sphere the points H and V represent horizontally and vertically polarised states, the points L and R represent left-handed and right-handed circularly polarised states, and the points P and Q represent two linearly polarised states with polarisation planes inclined at 450 to the horizontal and vertical planes. Each possible orientation of linearly polarised state is represented by some point on the equatorial great circle through HQV and P. Each possible state of elliptically polarised light is similarly represented by some point on the sphere lying between the equatorial great circle through HQVP and the two poles L and R. On the Poincar sphere a phase slippage is represented by a rotation about an eigenaxis which intersects the sphere in two stationary points. These two stationary points represent the two SOPs that are unaffected by changes in magnitude of phase slippage. In the case of the polarisation state controller of Figure 1, these two stationary points are clearly the linearly polarised states coinciding with the two principal axes of the first polarisation beam splitter. Light of either of these two SOPs launched into this beam splitter is not divided between the two fibres 12 and 13 but is launched exclusively into one or other. It is clear therefore that no change in relative optical path lengths between fibres 12 and 13 will, under these particular circumstances, have any effect upon the SOP of the light emerging from the second beam splitter. Arbitrarily assigning these two principal axes as horizontally and vertically polarised states, the eigenaxis of the Poincar sphere is the axis through HV. A phase slippage of is represented as a rotation through on the sphere about the axis HV. Thus if the sign of the phase slippage is such as to be represented by a clockwise rotation viewed along the HV axis, then if light is launched into the controller with an SOP represented by the point P, then the light emerging from the controller will emerge with an SOP represented by the point P' on the great circle through LPR and Q lying at an angle e round from P. Correspondingly - the input SOP was represented by the point S, then the output would be represented by the point S' lying at an angle 0 round from S on the small circle through S whose normal lies on the HV axis.

In Figure 3 there is depicted a tandem arrangement of two optical polarisation state controllers 30a and 30b, each constructed as described above with reference to Figure 1. The two con=rollers are directly coupled with a relative orientation such that the principal axes of the second polarisation beam splitter lla of the first polarisation state controller 30a are at 450 to the principal axes of the fi-st polarisation beam 10b splitter of the second polarisation state controller 30b. With this arrangement, if the eigenaxis of the first controller 30a passes through H and V on the Poincaré sphere of Figure 2, the eigenaxis of the second controller 30b passes through P and Q.

If light is launched into the first polarisation state controller 30a of the tandem arrangement with an SOP defined by any point a- the great circle QLP and R, it can be shown that, ty appropriate choice of bias applied respectivel to the piezoelectric elements 14a and 14b of the two controllers 30a and 30b, it is possible to provide the output from the second polarisation beam split=r llb of the second controller 30b with any desired SOP Suppose for instance that the desired output SOP is given by the point T, and that the input SOP to the tandem arrangement is given by the point Q, then an octput SOP given by the point T can be arranged by biasing the piezoelectric element 14a to provide, for the first controller, a phase difference equivalent to a rotation ea. The output SQP of the first controller is therefore given by the point T'. Then piezoelectric element 14b is biased to provide for the second controller a phase difference equivalent to a rotation bx , thereby providing the requisite output SOP given by the point T.

One particular application for a tandem arrangement of polarisation controllers as described above, with particular reference to Figure 3, is as a component of an SOP-matching local oscillator signal generator of a coherent light detection system. Such a system is depicted in Figure 4. An information signal from a transmitter 40 at some remote location is conveyed over single mode fibre 41 to the detector location where it is coherently mixed with a local oscillator signal from a local oscillator source 42 using a 3dB single mode optical fibre coupler 43. The resulting signals appearing on the two outputs of the 3dB coupler are detected on a pair of detectors 44 to provide an IF signal output at 45. The amplitude of the IF signal depends upon the degree of polarisation match of the two optical signals being mixed in the 3dB coupler 43. The amplitude is at a maximum if there is an exact match of SOPs, but is reduced to zero if they are orthogonal. Typically both the transmitter 40 and the local oscillator source 42 emit light with a well defined SOP, but unless the transmission path 41 is constructed in polarisation maintaining fibre, the SOP of the signal from the transmitter 40 is liable to arbitrary change in its passage along the transmission path, and this change is liable to vary with time. For this reason the tandem arrangement of two polarisation controllers 30a and 30b of Figure 3 is interposed between the local oscillator source 42 and the 3dB coupler 43. The two piezoelectric elements 14a and 14b are biased using associated bias drive units 46a and 46b. Each bias unit provides a variable voltage bias upon which a small fixed amplitude modulation is superimposed. The small amplitude modulation modulates the local oscillator signal SOP applied to the 3dB coupler, and hence modulates the amplitude of the IF signal. The amplitude of the resultant modulation of the IF signal is at a minimum when the SOPs of the two signals being mixed in the 3dB coupler are matched.

Accordingly the IF signal is fed to synchronous detection circuitry 47 which detects the amplitudes of the components of the IF signal resulting from the two modulations impressed by the piezoelectric elements 14a and 14b. The detected amplitudes are employed to provide feedback signals applied on electrical feedback paths 48a and 48b to control the level of voltage bias applied to the respective elements in such a way as to minimise the modulation components in the IF signal, and thus secure a match of the SOPs being mixed in the 3dB coupler 43.

The optical polarisation state controller of Figure 1 is of the type that employs two optical fibre polarisation beam splitters connected in series, and this has been the type of controller illustrated in the tandem arrangement of Figure 3 and the coherent detection system of Figure 4. These optical fibre polarisation beam splitters can all be replaced with another type of controller, a type now to be described with reference to Figure 5, that employs only one polarisation beam splitter.

The controller of Figure 5 has a single optical fibre polarisation beam splitter 50 with two inputs 51, 52 and two outputs 53, 54. Either one of these outputs, say output 54, is connected to either one of the outputs, say input 52, via an optical fibre feedback single mode optical fibre link 55 which is wrapped one or more times round a cylindrical piezoelectric element 14.

In the case of the controller of Figure 1 an orientational requirement has to be satisfied, namely that the second polarisation beam splitter 11 needs to be oriented so that it combines the signals propagating in the links 12 and 13 in a single output.

In the case of the controllers of both Figure 1 and Figure 5 light of an arbitrary SOP is divided into two components by passage through a polarisation beam splitter, and these two components have subsequently to be recombined into a single entity launched into a single output fibre of the controller. In the case of the controller of Figure 1 this recombination is achieved by satisfying an orientational requirement relating the principal axes of the second polarisation beam splitter to those of the first. In the case of the controller of Figure 5 this combination is similarly achieved by satisfying an orientational requirement, in this case the requirement that the light launched into the link 55 is launched back into the input 52 with the state of polarisation that ensures that in a single passage through the beam splitter this light is substantially completely launched into the output 53.

An essential part of the polarisation state controllers of both Figures 1 and 5, and hence also of the tandem arrangement of Figure 3 and of the coherent detection system of Figure 4 is the optical waveguide polarisation beam splitter. This has taken the form of an optical fibre waveguide polarisation beam splitter, but it is to be understood that the invention is not limited exclusively to devices which employ polarisation beam splitters constructed in optical fibre form, but extends to devices employing other forms of polarisation beam splitters such as that described by K. Thyagarajan et al. in an article entitled "Integrated-optic polarization - splitting directional coupler', Optics Letters Volume 14, No. 23 pages 1333-5 (December 1st, 1989). Non-fibreoptic waveguide polarisation beam splitters are also described by A.R. Vellekoop et al., Journal of Lightwave Technology Volume 8, No. 1, pages 118-124 (January 1990) and by N. Goto et al., Electronics Letters Volume 25, No. 25, paper 1732-4 (7th December 1989), to which attention may be directed.

Claims

CLAIMS.

1. An optical polarisation state controller having an optical waveguide input feeding an optical waveguide polarisation beam splitter which divides a signal applied to said input into two orthogonally polarised components, one of which components propagates through a variable phase delay transducer before being substantially completely recombined with the other component in a single passage through an optical waveguide polarisation beam splitter to form a single entity feeding an optical waveguide output.

2. An optical polarisation state controller as claimed in claim 1, wherein said optical waveguide polarisation beam splitter which divides the signal applied to said input of the controller is connected optically in series with said optical waveguide polarisation beam splitter which combines said one component with said other component in a single passage therethrough.

3. An optical polarisation state controller as claimed in claim 1 wherein said optical waveguide polarisation beam splitter which divides the signal applied to said input of the controller is also said optical waveguide polarisation beam splitter which combines said one component with said other component in a single passage therethrough.

4. An optical polarisation state controller having an optical waveguide input and an optical waveguide output, and including first and second optical waveguide polarisation beam splitters connected optically in tandem by two lengths of optical waveguide with a relative orientation of the beam splitters such that light of any polarisation state launched into one waveguide of the first beam splitter emerges substantially exclusively from one waveguide of the second beam splitter, wherein one of said lengths of optical waveguide incorporates a variable phase delay transducer by means of which the differential phase delay of the two lengths is rendered adjustable, wherein the optical waveguide input of the controller feeds the first polarisation beam splitter, and wherein the second polarisation beam splitter feeds the optical waveguide output of the controller.

5. An optical polarisation state controller having an optical waveguide input and an optical waveguide output, and including an optical waveguide polarisation beam splitter having first and second inputs and first and second outputs, and an optical feedback path linking the second output with the second input via a variable phase delay transducer, wherein the optical waveguide input of the controller feeds the polarisation beam splitter by way of its first input, and wherein the optical waveguide output of the controller is fed by way of the first output of the polarisation beam splitter.

6. An optical polarisation state controller as claimed in any preceding claim, wherein the phase delay transducer is constituted by a piezoelectric transducer mechanically coupled to a portion of optical waveguide.

7. An optical polarisation controller as claimed in claim 6 wherein the portion of optical waveguide is a portion of optical fibre wrapped around the piezoelectric transducer.

8. An optical polarisation controller as claimed in any preceding claim wherein the or each polarisation beam splitter is an optical fibre polarisation beam splitter.

9. An optical polarisation state controller substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

10. An optical polarisation state controller substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.

11. A tandem arrangement of two optical polarisation state controllers as claimed in any preceding claim, which two controllers are interconnected with a relative orientation such that light emerging from the optical waveguide output of the first controller plane polarised with its plane of polarisation aligned with one of the principal axes of the polarisation beam splitter feeding that output is launched into the optical waveguide input of the second controller plane polarised with its plane of 0 polarisation at substantially 45 to the principal axes of the polarisation beam splitter fed by that input.

12. A tandem arrangement of two optical polarisation state controllers, which tandem arrangement is substantially as hereinbefore described with reference to Figures 1, 2 and 3 of the accompanying drawings.

13. A tandem arrangement of two optical polarisation state controller, which tandem arrangement is substantially as hereinbefore described with reference to Figures 1, 2, 3 and 5 of the accompanying drawings.

14. An SOP-matching local oscillator signal generator of a coherent light detection system, which generator includes a coherent light source optically coupled with a tandem arrangement as claimed in claim 11, 12 or 13 so as to be adapted to launch light into the tandem arrangement with an SOP which, when represented on the Poincar sphere, lies on the great circle whose normal intersects the sphere at the points representing linearly polarised states substantially coinciding with the principal axes of the first polarisation beam splitter fed by the optical waveguide input of the first polarisation controller of the tandem arrangement, wherein an associated feedback path is provided for the phase delay transducer of each polarisation controller of the tandem arrangement, each of which feedback paths is adapted to regulate the mean level of bias applied to its associated phase delay transducer in a manner to minimise the component of the coherent detected output in phase with an amplitude modulation of the bias applied to that transducer.

15. An SOP-matching local oscillator signal generator as claimed in claim 14, wherein the light that is launched into the tandem arrangement is plane 0 polarised at substantially 45 to the principal axes of the first polarisation beam splitter of the first polarisation controller of the tandem arrangement.

16. An SOP-matching local oscillator signal generator as claimed in claim 14 or 15, wherein the two phase delay transducers are amplitude modulated in phase quadrature.

17. An SOP-matching local oscillator signal generator substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.

18. An SOP-matching local oscillator signal generator substantially as hereinbefore described with reference to Figures 1 to 5 of the accompanying drawings.