GB2184253A - Optical state-of-polarisation modulator - Google Patents

Abstract

An optical measurement system includes a polarisation modulator having an element which is controlled to sweep the output polarisation state such that the time integral of the system photodetector output becomes independent of polarisation. It is shown that, on the Poincaré sphere, the output polarisation states must have a time-weighted distribution symmetrical about the sphere centre; in particular, the output state must sweep around a great circle at a uniform rate. The modulator may require to be preceded by an adjuster to bring the input polarisation state on to a given great circle. Embodiments with optical fibre 15 may use stress-induced linear birefringence 30, coupled if necessary with physical rotation (Figs. 5,6) or rotating stress (Fig. 7); magnetically-induced circular birefringence (Faraday effect) may alternatively be used (Fig. 8). The drive waveforms are triangular. The adjuster comprises two quarter-wave linearly-birefringent loops 29 of adjustable position. The system is a reflectometer using heterodyning of backscattered light (Fig. 1). <IMAGE>

Description

SPECIFICATION Optical state of polarisation modulation This invention relates two optical signal processing in which light is passed through an optical state of polarisation (SOP) modulator that is operated in such a way that the SOP of its output describes a path on the Poincare sphere that exhibits time-weighted symmetryaboutits centre. Such signal processing findsapplic- ation in certain polarisation sensitive systems for the derivation of a detected output signal that is independent of SOP.

For example, in an optical heterodyne or homodyne system coherent lig htfrom two paths is mixed, and the amplitude of the resulting signal depends in part upon the relative SOP's of the two interfering light beams. This dependence can be inconvenient, particularly if the SOP of one ofthe interfering beams is unknown. A particular example isto be found for instance in a coherent light optical fiber reflectometer. In such a reflectometer the backscatter signal from the fibre undertest is mixed with a local oscillator signal.

The SOP ofthe light from the local oscillator can relatively easily be setto a predetermined state, typically a linearly polarised state of a preferred orientation. However the backscattered light is generally of quite inde- terminate SOP. For optimum sensitivity of the reflectometer itwould be necessary to ensure that atthe mixing pointthetwo signals have the same SOP, butifthe SOP ofthe backscattered light is indeterminate there is no obvious way of arranging to modify the SOP ofthe local oscillatoroutputto match it.The optical signal processing of the present invention does not attempt to achieve this match, but it enables the local oscillator output SOPto be manipulated in such a waythatthe mixed signal,when arranged to fall upon a detector, will provide an electrical output signal which is readily convertible to a signal whosetime-averaged value is independent of the mismatch of polarisation states at the optical mixing point.

According to the present invention there is provided an optical state of polarisation (SOP) modulatorwhich modulator incorporates drive means adapted, for an input to the modulator having an optical SOP lying at any point on a given great circle on the Poincaré sphere, to causethe SOP ofthe outputfrom the modulatorto describe a path on the sphere that exhibits time-weighted symmetry about its centre.

The invention also resides in a method of optical processing in which light is transmitted through an optical state of polarisation (SOP) modulator operated to provide for any input SOP lying on a given great circle on the Poincaré sphere an output SOP which describes a path on the sphere that exhibits timeweighted symmetry about its centre, wherein the light entering the modulator is arranged to have an SOP lying on said given great circle.

There follows a description of a coherent light optical fiber reflectometerwhich incorporates an optical SOP modulatorfor operation in a manner embodying the invention in a preferred form. Also described are further examples of modulatorforthe reflectometer. The description refers to the accompanying drawings in which Figure 7 is a schematic representation ofthe reflectometer, and Figure2 is a schematic representation of its SOP adjustor and modulator; Figure 3 is a Poincaré sphere diagram illustrating the mode of operation ofthe modulator of Figure 2;; Figure4is a Poincaré sphere diagram illustrating the mode of operation of an alternative form of modulator, and Figures 5, 6, 7and 8are schematic representations ofexamples ofalternative modulators.

Referring to Figure 1, the light source for a coherent light reflectometer is provided by a laser diode 10.

Light fro this laser, which may incorporate a length offibre (not shown) for line narrowing purposes, isfed bya single mode fibre 11 to a fibre directional coupler 12. The construction ofthe coupler is such asto transmit the majority, typically 80%, ofthe laser lightto a further single modefibre 1 3terminating in an expanded beam collimating lens 14, whilethe remainder is transmitted via a single modefibre 15to afibre directional coupler 16. In the optical path between coupler 12 and 16 are located an optical SOP adjustor17a and SOP modulator 17b. (The nature ofthesetwo last-mentioned optical elements will be described later).

Light emerging from the collimating lens passes through a Bragg acousto-optic optical frequency modulator 18 before being collected by a further collimating lens 19 and launched into a single mode fibre 20. Single mode fibre 20 terminates in a 3 dB fibre directional coupler 21 one output part of which is connected to the fibre undertest 22, while the other is connected to a total absorber 23. The 3dB coupler 21 directs halfthe back-scattered light returning from thetestfibre 22 into a length of single mode fibre 24 connecting the 3dB coupler 21 with the fibre directional coupler 16. Directional coupler 16 is constructed so that the majority, typically 80%, of the backscattered light in fibre 24 is directed to its output port that terminates in a photodetector 25.The other output portterminates in a total absorber 26. Directional coupler 16thus acts asan optical heterodyne mixerthat mixes backscattered light propagating in fibre 24with 'local oscillator' light propagating in fibre 1 from the output ofthe SOP modulator 17b. The photodetectoroutputcurrent is fed via a filter 27 turned to the modulating frequency ofthe Bragg cell 18,typically40 MHz, to an arithmetical processing unit 28, which integrates the square of the output current.

The output current of the photodetector 25 is proportional to the scalar product ELO . EBS, whereto is the optical field ofthe light propagating in fibre 15 (local oscillator), and Ess is the optical field of the light propagating in fibre 24 (backscatter).

In the casethato andE55 are both linearly polarised, and thatthe angle between their planes of polarisa tion is 'A', it is seen that the output current I, of the photodetector 25 is given by I1 = k.lELol I lE85 . cosA. Now if the plane of polarisation of either one of these fields (but not both) is rotated by 90 , a new value of photodetector output current 12 will result which is given by 12 = k .k. {ELo! .I EBS j sin A. It follows therefore that by squaring and summing the output currents II and 12there is formed an output signal which is independentof the angle 'A'.

All possible states of polarisation can be uniquely represented by points on a Poincare sphere, and on the Poincaréspherethisrotation by 90" of the plane of polarisation is represented by a rotation of 1 80" around the equatorial great circle (HPVO = Figure 3) of linearly polarised states. These two linearly polarised states 180 apart on the Poincare sphere sum to the centre ofthe sphere.

So far it has been shown that if one ofthe two interfering beams is switched between two linearly polarised states that sum to the centre ofthe Poincaré sphere, then by summing the squares ofthe resulting photo detector currents it is possible to derive a signal that is independentofthe relative SOP's of the two interfer ing beams. It will be evidentthatthis is also true if the switched SOP beam is not linearly polarised in eitherof its two states that sum to the centre ofthe Poincare sphere, and it can be shown that the relationship still holdsforthe more general case of switching between a set of more than two SOP states that satisfy the condition that the members ofthe set sum to the centre ofthe sphere.Generalising furtherfrom this, it isseen that if a modulator is operated to sweep the SOP of one ofthe interfering beams along a path on the Poincare sphere that exhibits time-weighted symmetry about its centre, for instance a path that sweeps at uniform rate cyclically around a great circle ofthat sphere, then it is possible by integrating the square ofthe photodetec tor output to derive a signal that is independent of the relative SOP's of the two interfering beams.

One way of achieving the requisite modulation is by means ofcyclicallyvarying stress induced linear birefringence in afibre. Forthis purpose the modulator 17b may take theform of a portion ofthefibre 15 which is laterally stressed by a PZT squeezer element as depicted schematically at 30 in Figure 2. The region of this fibre 15 immediately at30 in Figure 2. The region of this fibre 15 immediately 'upstream' ofthe squeezer element is provided with a pair of fibre quarterwave linearly birefringent loops 29 to form the SOP adjustor 17a of Figure 1 for presetting the SOP ofthe lightinputtothe modulatorto an appropriate starting state.Referring to Figure 3, linearbirefringence is represented on the Poincare sphere as a rotation abouta particular (eigen state) axis lying in the plane of the equatorial great circle HPVO of linearly polarised states.

(On this sphere the points H and V represent horizontally and vertically polarised states, the points Land R represent left-handed and right-handed circularly polarised states, and the points P and 0 representthetwo linearly polarised states with polarisation planes inclined at 45"to the horizontal and vertical planes. Forthe purpose ofthis specification quarterwave linear birefringence is defined to mean the birefringence afforded by an element in which the optical path length difference for its two principal directions differs by n#/4 where A is the wavelength ofthe light and n is an odd integer.Similarly half-wave linear birefringence is defined to mean the birefringence afforded by an elementforwhich this difference is n#/2.) Arbitrarily assigning the birefringence (eigen state) axis asthe axis PQ, ifthe strength of the birefringence is given by a rotation of A , then, if light enters the birefringent elementwith an SOP defined by some arbitrary point B, itwill leavethe elementwith the SOP defined by the point D, where the points B and D subtend an angle Ao at the centreof the (small) circle that passes through B and has its centre C lying on the birefringence axis PO. lfthe birefring- ence is stress induced, and is increased from zero up through the value of A"to 360",then the output SOPwill first evolve along the small circle from Bto D, and then all the way round the small circle back to B again.

Uniform procession aroundthis small circle wiil produce time-weighted symmetry about its centre C, but, if the point B does not lie on the greatcirclethrough HLVR, in no way is it possibleto vary the rate of procession so as to produce time-weighted symmetry about the centre ofthe sphere. The loops 29 are therefore adjusted in positionto bring the SOP atthe inputto the squeezer element 30 on to this great circle. The squeezer element30 is then operated to varythe stress applied to the fibre cyclically with an amplitude providing a 360" (full-wave) difference in birefringence between the point of maximum and minimum applied stress.The application of the stress must be in a mannerto provide the required time-weighted symmetry, and may conveniently be achieved by applying the stress as a saw tooth o r a triang ul ar wavefo rm providing uniform exploration.

An alternative to a modulation of SOP that evolves on the PoincarB sphere around a 'line of longitude'and its supplement in orderto producetimeweighted symmetry aboutthe centre ofthesphere, is that provided by modulation that evolves around the equator. This may be achieved by providing stress-induced half-wave linear birefringence and causing the birefringence axisto rotate. Referring again to Figure 3, the birefring ence axis lies in the equatorial plane HPVQ.If the SOP of light input to the birefringent element is linearly polarisedthis SOP is characterised by a pointthatalso lies in this plane, and, since the rotation is through 180" (or 180n", where n is an odd integer), the output SOP is similarly characterised by a point lying inthis plane. Referring now to Figure 4, if the SOP ofthe inputtothe element is given by the point H, andthe birefringence axis EF is instantaneously at some angle A totheaxis HV,then the output SOP is given bythe point Gwhichsubtends an angle of 2A" atthe centre ofthe sphere. Thus the output SOP is seen to evolve around the equatorial great circle at twice the rate at which the birefringence axis is evolving around it.

One way of achieving this half-wave linear birefringence, the direction of whose axis can be suitably mod ulated, is by replacing the squeezerelement30 of Figure 2with afurtherfibre loop 50 (Figure 5)thatintro- duces half-wave linear birefringence and is provided with means (not shown) for articulating the loop through 90" about the common axis 51 ofthe regions ofthe fibre at either end ofthe loop. The fibre loops 29 are still required for presetting adjustment to bring the SOP ofthe light entering loop 50 on totheequatorial great circle of linearly polarised states.Articulation ofthe loop 50 physically through 90 rotates the birefringence axis through 180" on the Poincaré sphere, and hence the required time weighted symmetry may conveniently be achieved using a simple harmonic motion form of articulation.

Instead of arranging forthe loop 50 to be articulated in an oscillatory manner through 90", it can be arranged to rotate continuously at constant speed without inducing progressive twisting ofthefibre. One part icularway of achieving this is depicted in Figure 6. The axis 60 ofthefibre at one end ofthe loop is not exactly coincident with the axis 61 ofthefibre at the other end ofthe loop, but is parallel to and spaced from thefirst axis by a small distance 'd'. The two fibres are moved relative to each otherto maintain this spacing ofthe axes while causing one to move at constant speed around the other in the direction of arrow 62. This causes the loop to execute a similar rotation in the direction of arrow 63.

In the arrangement described above with particular reference to Figure 5, evolution ofthe outputSOP around the equatorial great circle of linearly polarised states was achieved by presetting the positions ofthe loops 29to bring the SOP ofthe light entering the loop 50 two some point ofthatgreat circle while choosingfor loop 50 a size that provides half-wave linear birefringence. An alternative arrangement of these loops for providing the same evolution of output SOP around the equatorial great circle of linearly polarised states involves choosing for loop 50 a size that provides quarter-wave birefringence, and presetting the positions of the loops 29to produce a circularly polarised stateforthe light entering loop 50.

Alternatively the loop 50 can be dispensed with entirely and its place taken, as depicted in Figure 7, bytwo sets 70,71 ofpiezolelectricsqueezerelements acting together on the same piece offibrewhose squeeze axes are to be arranged to be at 45" to each other. These are driven in phase quadrature to produce a rotating field of the appropriate amplitude to produce half-wave or quarter-wave linear birefringence in the fibre according to whetherthe inputSOPfrom the loops 29 is linearly orcircularly polarised.

The modulators of Figures 2,5,6 and 7 have each involed some form of mechanical means operating on the fibre either to squeeze it orto control the movement of a loopformed in it. Such mechanical means can be entirely avoided by making use of thefactthat birefringence elements are intrinsically wavelength depen- dent. This method uses a piece of birefringent polarisation maintaining fibre that is many beat lengths long and involves operating the optical system with a light source that is ramped in frequency (wavelength).

Consider a lengths of such fibre in which the two principal velocities of light areV and KV. Assuming thatata frequencyfthe fibre is exactly L beat lengths long, then s=nvlf=(n+L)kv/f (1) when n is an integer. Neglecting any perturbation introduced by the effects of dispersion, itwill be evident that there is some nearby frequency pf at which the fibre is exactly (L+ 1) beat lengths long,therefore s = mv/pf= (m+L+1)kv/pf (2) where m is another integer.

From equation (1) n(1-k)=LK From equation (2) m(l-k)=(L+l)k Therefore(L+1)lL-- mln = p Thus, for instance, ifthefibre is 100 beat lengths long at some chosenfrequency,then the frequency ofthe light source hasto change by only 1 %from this chosen valueforthe output SOP from the fibre, for anygiven input SOP, to execute a full circle on the Poincare' sphere.The fibre is linearly birefringent, and hencethis circle is a great circlewhen the input SOP lies on the great circle that passes through the circularly polarised states Land Rand the two linearly polarised statesforwhich the polarisation plane is inclined at 45" to the principal axes ofthe birefringentfibre.

An alternative way of producing the requisite modulation without recourse any form of mechanical means operating on the fibre, eitherto squeeze it orto control the movement of a loop formed in it, is illustrated in Figure 8 where use is made of magneticfield induced circular birefringence (Faraday effect). The SOP mod ulator80 is in this instance a Faraday elementwhose magnetic field is adjustable over a rangethatwillvary the induced circular birefringence by onefull wave, oran integral number offull-waves.This modulator element 80 could, at least in principle, be constituted by an optical fibre with an appropriate winding, but at least in part because ofthe much larger Verdet constants currently obtainable in integrated optics structures, it will generally be preferred to employ integrated optics version. In orderforthe operation ofthe modulator 80 to cause its output SOPto evolve around the equatorial great circle of linearly polarised states, the optical inputto the modulator must be linearly polarised. This state is conveniently achieved in the same manneras employed with the previously described modulators, namely by a pairofappropriatelyoriented loops 29 in a length of single mode optical fibre 15. The modulator drive must provide the required time-weighted sym metry. The avoidance of any bias causing the element to dwell at any one angular rotation state morethan at any other implies that the modulation must be such that it exhibits aflat probability density function such as is conveniently provided by a sawtooth, our a triangular, waveform of appropriate amplitudeto induce a peak-to-peak difference in birefringence equal to one or an integral numberoffull waves. It will beapprecia ted thatthese conditions are essentially the same asthose that have to be met in the modulation of the linear birefringence squeezerelement30.

Claims (15)

1. An optical state of polarisation (SOP) modulatorwhich modulator incorporates drive means adapted, for an inputto the modulator having an optical SOP lying at any point on a given great circle on the Poincare sphere, to cause the SOP ofthe outputfrom the modulatorto describe a path on the sphere that exhibits time-weighted symmetry about its centre.

2. An SOP modulator as claimed in claim 1, which modulator is constituted by a length of single mode optical fibre and associated mechanical means adapted to act uponthefibreto provide cyclically varying strain to induce linear birefringence of cyclically varying amplitude.

3. An SOP modulator as claimed in claim 1 which modulator is constituted bya length of single mode optical fibre and associated mechanical means adapted to act upon thefibreto provide cyclically varying strain to induce constant amplitude linear birefringence of rotating azimuth.

4. An SOP modulator as claimed in claim 1, which modulator is constituted by a length of single mode optical fibre and associated mechanical meansadaptedto act upon thefibre,wherein the fibre has been formed into a loop which introduces a strain into the fibre thatinduces linear birefringence equal to an odd integral number of quarter-waves or half-waves, and wherein the associated mechanical means is adapted to articulate cyclicallythe orientation of the loop.

5. An SOP modulator as claimed in claim 1, which modulator is constituted buy a length of single mode optical fibre and associated mechanical means adapted to act upon the fibre, wherein the fibre has been formed into a loop which introduces a strain into the fibre that induces linear birefringence equal to an odd integral number of quarter-waves or half-waves, and wherein the associated mechanical means is adapted to rotate the orientation of the loop.

6. An SOP modulator as claimed in claim 1, which modulator is constituted by a Faraday effect optical element and associated drive means for varying the magneticfield induced circular birefringence ofthe element by one full wave or an integral number offull-waves.

7. An optical homodyne or heterodyne system as claimed in any preceding claim, in which system there is included in the light path of one ofthe light beamsthatare coherently mixed by the system an optical modulator as claimed in any preceding claim.

8. Atandem arrangement of an SOP modulator as claimed in any claim of claims 1 to 6,and, preceding the modulator, an optical SOP adjustorwhose output provides the input to the modulator, which adjustor,for any given inputSOPthereto is capable ofadjustmentto provide an output SOP lying at some point on said given great circle.

9. An optical homodyne or heterodyne system in which there is included in the light path ofoneofthe light beams that are coherently mixed by the system atandem arrangement as claimed in claim 8.

10. An optical fibre time domain reflectometer incorporating an optical system as claimed in claim 7 or9.

11. An optical fibre time domain reflectometer incorporating an optical homodyne orheterodynesystem in which there is included in the light path ofone of the light beams that are coherently mixed bythesystem an SOP modulatoras claimed in claim 1,which modulator is constituted by a length of polarisation maintaining birefringentfibre co-operating with a light source provided with means adapted to vary cyclicallythe frequency of its output.

12. An SOP modulator substantially as hereinbefore described with reference to Figures2to8Ofthe accompanying drawings.

13. Atandem arrangementof SOP adjustor and SOP modulator substantially as hereinbefore described with reference to Figures 2 to 8 of the accompanying drawings.

14. An optical fibre time domain reflectometer substantially as hereinbefore described with referenceto the accompanying drawings.

15. A method of optical signal processing in which light is transmitted through an optical state of polarisation (SOP) modulator operated to provide for any inputSOP lying on a given great circle on the Poincaré sphere an output SOP which describes a path on the sphere that exhibits time-weighted symmetry about its centre, wherein the light entering the modulator is arranged to have an SOP lying on said given great circle.