WO2010026541A1

WO2010026541A1 - Polarisation mode dispersion emulator apparatus

Info

Publication number: WO2010026541A1
Application number: PCT/IB2009/053845
Authority: WO
Inventors: Andrew William Ralph Leitch; Lorinda Wu; Vitalis Musara
Original assignee: Nelson Mandela Metropolitan University
Priority date: 2008-09-03
Filing date: 2009-09-03
Publication date: 2010-03-11
Also published as: ZA201101617B

Abstract

This invention relates to a polarisation mode dispersion (PMD) emulator apparatus with a fixed mean second order PMD (SO-PMD) and varying mean differential group delay (DGD), the emulator apparatus comprising at least one optical delay line (ODL) operable at least to generate DGD; a sub-emulator comprising a predetermined number of polarisation maintaining fibre (PMF) segments, the sub-emulator being arranged to generate at least a desired amount of fixed SO-PMD; and a polarisation controller (PC) operatively connected to the ODL and the sub-emulator, the PC being arranged and controlled to facilitate operation of the PMD emulator apparatus.

Description

POLARISATION MODE DISPERSION EMULATOR APPARATUS

BACKGROUND OF THE INVENTION

THIS invention relates to a polarisation mode dispersion emulator apparatus.

The phenomenon of polarisation mode dispersion (PMD) is one of the key factors limiting signal transmission in optical network systems. This phenomenon is of concern because of current and future increases in bit- rate transmission from 10 Gbit/s and greater. It will be understood that PMD is defined to first-order (FO) as the differential group delay (DGD) between the fast and slow principal states of polarisation (PSPs) along an invariant unit Stokes vector; and to second-order (SO) as the variation of DGD (Aτ ) and the rotation of PSPs as a function of optical frequency. It follows that in order to understand PMD behaviour and its implications in deployed optical fibres, it is preferable to perform PMD emulation in a controlled environment.

Second-order polarisation mode dispersion (SO-PMD) is dependent on the magnitude of a first-order polarisation mode dispersion (FO-PMD) vector ( τ ), represented as Aτ . It is therefore an object of the invention to provide an emulator at least to investigate the impact of Aτ on SO-PMD experimentally.

SUMMARY OF THE INVENTION

According to the invention there is provided a polarisation mode dispersion (PMD) emulator apparatus with a fixed mean second order PMD (SO-PMD) and varying mean differential group delay (DGD), the emulator apparatus comprising:

at least one optical delay line (ODL) operable at least to generate DGD;

a sub-emulator comprising a predetermined number of polarisation maintaining fibre (PMF) segments, the sub-emulator being arranged to generate at least a desired amount of fixed SO-PMD; and

a polarisation controller (PC) operatively connected to the ODL and the sub-emulator, the PC being arranged and controller to facilitate operation of the PMD emulator apparatus.

The ODL may be configured to generate a FO-PMD vector ( τ_0DL ). The sub-emulator may be configured to generate a FO-PMD vector ( τ , ); and the PC may be configured to align τ , substantially parallel to τ_0DL and parallel to using a controllable PC. The PC may be configured to maintain an angle β between τ_0DL and τ , . In a preferred example embodiment, the PC may be configured to maintain the angle β at 0° at substantially each wavelength thereby to provide SO-PMD independent of FO-PMD changes.

The sub-emulator may comprise a single PMF segment. The PMD emulator apparatus with the single PMF segment may have at least wavelength independent DGD.

Instead, the sub-emulator may comprise a plurality of PMF segments. In particular, the sub-emulator may comprise at least eight cascaded PMF segments.

The PMF segments may be randomly concatenated and may have random birefringence and mode coupling distribution.

Mode coupling sites or mode coupling angles at interfaces between adjacent PMF segments may be arranged to generate SO-PMD.

The PMD emulator apparatus with a plurality of PMF segments has at least wavelength dependent DGD.

The mode coupling angles may be fixed.

Each PMF segment may have a length within twenty percent Gaussian Deviation of a mean length of the sub-emulator.

The ODL may be arranged to generate DGD in proportion to the voltage applied or supplied to the ODL

The ODL may be arranged to generate wavelength independent FO-PMD. -A-

The ODL may be arranged to generate wavelength independent FO-PMD which is greater than FO-PMD generated by the sub-emulator thereby permitting the ODL to control only the DGD whilst leaving SO-PMD substantially unaffected provided β = 0°.

The ODL may be an electrically controllable, reflector-style variable device arranged to induce birefringence.

The PC may comprise at least a half-wave plate constructed from an electro-optic material. The electro-optic material may be selected from a group comprising at least LiNbO₃.

The PC may be computer controlled and may be configured to generate a polarisation angle θ between the ODL and the sub-emulator substantially proportional to a magnitude of a supply voltage to the PC.

The PC may be arranged to provide a polarization angle θ of between O and 180°.

The PC may be controlled to provide a polarisation angle θ that ensures the angle β between the sub-emulator and the ODL FO-PMD vectors is 0° for all wavelengths.

The PC may be configured to rotate any input states of polarisation (SOP) to a certain degree by the application of voltage to the half wave plate.

The PMD emulator apparatus may be configured to allow polarisation- dependent chromatic dispersion (PCD) and principal states of polarisation (PSP)-depolarisation components to be controlled.

The PMD emulator apparatus may be arranged to alter mode coupling angles and to add or remove mode coupling sites. The PMF segments may have a stressed asymmetric fibre core. The PMF segments may have at least two axes with different refractive indices respectively such that light split along these two axes propagate at different group velocities.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a block diagram of a PMD emulator in accordance with an example embodiment;

Figure 2 shows a graphical illustration of a DGD (Δτ) wavelength spectrum;

Figure 3 shows a DGD histogram for the sub-emulator of Figure 1 ;

Figure 4 shows ODL (of Figure 1) characteristics in terms of the mean DGD (<Δτ>) and the mean SO-PMD (<τ_ω>);

Figure 5 shows PMD emulator characteristics for various ΔTODL values for the sub-emulator attached to the ODL;

Figure 6 shows PMD emulator characteristics for various ΔTODL values for the PMF attached to the ODL;

Figure 7 shows a graphical illustration of DGD (Δτ) wavelength spectra; and

Figure 8 shows a graphical illustration of SO-PMD (τ_ω) wavelength spectra.

DESCRIPTION OF PREFERRED EMBODIMENTS In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of an embodiment of the present disclosure. It will be evident, however, to one skilled in the art that the present disclosure may be practiced without these specific details.

Referring to the drawings, a PMD emulator apparatus in accordance with an example embodiment is generally indicated by reference numeral 10. The PMD emulator apparatus 10 (referred to as PMD emulator for ease of reference) comprises components which correspond to the functional tasks to be performed by the PMD emulator 10. In this regard, "component" in the context of the specification will be understood to include an identifiable portion of code, computational or executable instructions, data, or computational object to achieve a particular function, operation, processing, or procedure. It follows that a component need not be implemented in software; a component may be implemented in software, hardware, or a combination of software and hardware. Further, the components need not necessarily be consolidated into one device but may be spread across a plurality of devices.

In particular, the PMD emulator 10 comprises a sub-emulator 12 made up of cascaded polarisation maintaining fibre (PMF) segments 14, a computer controlled polarisation controller (PC) 16 and an optical delay line (ODL) 18 as shown in Figure 1.

The sub-emulator 12 is operable to generate a desired amount of fixed SO- PMD due to the presence of mode coupling. Mode coupling sites are located were two PMF segments 14 meet. It will be appreciated that the mode coupling sites are used to generate SO-PMD (which is both PCD and PSP- depolarisation). However Δτ is also present, thus Δτ and SO-PMD

(τ_ω ) coexist. In order to only change the emulator-(Δτ) a large FO-PMD vector (? ) is therefore introduced by the ODL 18 (the ODL 18 will be explained in a bit more detail below) whilst the angle β = 0° for all wavelengths.

Each PMF segment 14 has a PMD coefficient of ~ 1.5 ps/m. In other words, the PMF used to arrive at the PMF segments has a PMD coefficient of ~ 1.5 ps/m. It will be appreciated that the sub-emulator 12 has a fixed number of PMF segments 14. In a preferred example embodiment, the sub-emulator 12 has eight segments, with random birefringence and mode coupling distribution. The length of each PMF segment 14 lies within 20 % (Gaussian deviation) of a mean length (= 3.9 m) of the sub-emulator 12.

In other example embodiments, the sub-emulator 12 comprises a single PMF segment 14. The single PMF segment 14, typically a 8.15 ps PMF segment, may optionally replace the sub-emulator 12 (not shown). When the sub-emulator 12 is replaced with the single PMF segment 14, the emulator 10 possess Δτ and τ_ω only. In this case Δτ is independent of wavelength.

The PC 16 links the sub-emulator 12 to the ODL 18. In particular, the PC 16 is a half-wave plate constructed from an electro-optic material, typically LiNbO₃. A resultant PC angle is proportional to the magnitude of the supplied voltage. The PC 16 may optionally be set from 0 to 180°. The PC 16 rotates any input states of polarisation (SOP) to a certain degree by the application of voltage to the rotation half wave plate. The best set value is determined from the Concatenation equations. The ODL is an electrically controlled reflector-style variable device that induces difference optical paths (birefringence) such that when light propagate through, the other travels faster than the other. The PMF segments are designed by deliberately inducing high stress in the fibre core; in this case the fibres have an asymmetric core. The cores of the PMFs have two axes with different refractive indices thus light split along these two axes propagates at different group velocity which results in disparity in arrival time (a delay). In this particular example embodiment, the computer controlled PC 16 is typically adjusted till an angle θ is found that ensures the angle β between the sub-emulator and the ODL FO-PMD vectors is equal to 0°. This means these two FO-PMD vectors are parallel. The choice of the angle β = 0° emanated from the Concatenation equations (see below).

It will be noted that the total emulator first order PMD (FO-PMD) and second order PMD (SO-PMD) can be expressed in vector form by the Concatenation equations:

τtot ^τθDL ^{+ R}0DL^τsub (D

¹CO tot Λo ODL ^{+ R}ODIΛJO sub ^"^ODL^tot (2)

where v U,™DL and τ SU ,D are the FO-PMD vectors of the ODL 18 and the sub-emulator 12 respectively, R_ODL '^S ^^e National matrix of the ODL 18, τ_{ω 0DL} and τ , are the SO-PMD vectors for the ODL 18 and the sub-emulator 12 respectively.

In this particular case in order to make the overall SO-PMD of the emulator 10 independent of the ODL DGD, τ_0DL ^χ τ_tot should be null. This is achievable if τ_QDL and τ^ _t are collinear (either the vectors are parallel or anti-parallel). This is deduced from the cross product defined as x O_nmDL ^χ ϊ ^fc _ωtot = v O™DL τ ^u _wtθt sinψ n , where denotes the magnitude of the vectors, ψ (O ≤ ψ ≤ 180°) is the angle separating the vectors and ή is a unit vector (vector with magnitude = 1) perpendicular to the plane containing τ_0DL and τ_tQt . These two vectors in this case are collinear if ψ is O or 180° (resulting in a null vector). This leads to the choice of β = 0° which means τ , is parallel to τ_QDL . Eq. (1) reduces to τ_tot ~ XQ_DL considering the ODL 18 to being the emulator's 10 dominant

FO-PMD vector contributor. In summary, to make the SO-PMD independent of the ODL DGD, β has to be 0°. Since the PMD at each wavelength is represented by a unique vector, the PC 16 needs to be adjusted for each wavelength to ensure the angle β = 0° (obeying the condition TQDL ^x ^tot ⁼ 0) for that wavelength.

Depending on the amount of voltage supplied thereto, the ODL 18 gives an associated DGD. The applied voltage and DGD have a proportional relationship. Typically, the DGD values lie between -60 ps to 60 ps. The negative sign indicates that f is opposite in direction to those that are positive.

An adjustment of the ODL 18 therefore gives an increase or decrease in Δr although there is a low residual SO-PMD present. This is equivalent to a single PMF segment 14 of any Δτ value. The DGD is wavelength- independent and so is τ . It follows (also as previously mentioned) that the emulator 10 with a single PMF segment 14 is wavelength-independent but the emulator 10 comprising the sub-emulator 12 is wavelength-dependent.

As hereinbefore described, the sub-emulator 12 comprises eight cascaded PMF segments 14 which are wavelength-dependent. This therefore results in wavelength-dependent DGD and it becomes statistical in nature (see Figure 2). It follows that the FO-PMD and SO-PMD in the PMD emulator 10 become wavelength-dependent. In the absence of mode coupling only f exists and it is wavelength-independent, whilst both τ and τ_ω coexist when there is a single mode coupling site. In the latter the PSPs are wavelength-dependent, but the DGD is independent of wavelength.

When there is more than one mode coupling site FO-PMD, SO-PMD and higher-order PMD vectors are all present. The DGD between the PSPs show strong wavelength-dependence, as shown in Figure 2, thereby indicating that PSPs are wavelength sensitive. Figure 3 illustrates the occurrence of the sub-emulator DGD, which is extracted from the stochastic DGD characteristics in Figure 2. The sub-emulator DGD histogram approaches the Maxwellian distribution in behaviour.

The PMD emulator 10 is preferably configured such that the PCD and PSP- depolarisation components can be controlled. This is achieved by altering mode coupling angles or by adding mode coupling sites. In this case the mode coupling angles are fixed. However, it will be noted that as the sub- emulator 12 is assembled the number of mode coupling sites is increased from 1 to 7 (plus at the input and output points) and at the same time coupling angles are introduced. The mode coupling sites with varying angles enhance PSP rotations and DGD variations as frequency (or wavelength) varies (the PSP-depolarisation and PCD respectively). It follows that this is how the sub-emulator 12 generates SO-PMD.

Since the ODL 18 provides a wavelength-independent τ ( XQDL ) (that is greater than the τ contributions from the sub-emulator 12 (τ , ). This makes the ODL 18 control only the DGD whilst leaving SO-PMD unaffected provided the angle β in between the τ_0DL ^aπc' ^ h '^s ^°" Maintaining the angle β = 0° angle at each wavelength ensures SO-PMD is independent of the FO-PMD vector changes.

Table 1 (below) is a summary of how mode coupling and birefringence configurations of an emulator affect the overall <Δτ>, Δτ variation with wavelength and <τ_ω>. When the ODL DGD (ΔT_ODL) is greater than the sub- emulator DGD (Δτ_sub) (as- for emulator A), then <Δτ> ~ ΔT_ODL but when Δτ_ODL lies within 20% standard deviation of the mean-Δτ_sub (<Δτ>_sub) (as for emulator B) then <Δτ> will follow the trend of ΔT_ODL- Emulator D shows that the absence of mode coupling results in null SO-PMD. The PMD emulator 10 described in this patent maintains a fixed PCD and PSP-depolarization whilst the wavelength-independent ΔT_ODL changes, i.e. emulator C. The fixed sub-emulator 12 is used to generate a fixed SO-PMD and the ODL 18 to control the DGD.

Table 1 : Summary of the nature of an emulator (comprising of a sub- emulator 12 and ODL 18) and predictable Δτ wavelength spectra, <Δτ> and τ_ω. Emulator type C is under investigation in this paper. The general SO- PMD equation, the concatenation rule and other laboratory experiments assisted in compiling this table

Where: var - variable ΔTO_DL - optical delay line DGD

Δτ_sub - sub-emulator DGD <Δτ>_sub - mean Δτ_sub

In an example embodiment, a Jones matrix eigenanalysis (JME) measurement technique may be used to characterize the DGD and SO- PMD of the PMD emulator 10. The wavelength range 1520 - 1569 nm is typically used with a 0.3 nm wavelength resolution.

Turning to Figure 4 of the drawings, here ODL 18 characteristics in terms of the mean DGD (<Δτ>) and the mean SO-PMD (<τ_ω>) are illustrated with the DGD of the ODL 18 being calibrated (for accuracy). The gradient of the slope is 0.99 which shows there is a slight difference in a set and observed DGD on the ODL 18 (Δτ_set-oDL and Δτ_Ob_S-oDL)- Thus from here onwards (Δτ_set-oDL or Δτ_Obs-oDL will be referred to as the ODL DGD (ΔX_0DL). In addition, there is a presence of low residual SO-PMD (residual- τ_ω). The residual mean-SO-PMD (residual-<τ_ω>) is non-uniform for Δτ_0DL ≤ 18 ps, while the residual-< ^r _ω> remains fairly constant (around 0.24 ± 0.07 ps²) for

20 ≤ ΔT_ODL ^≤ 60 ps. Here it will be noted that the emulator-Δ^r is adjusted and the two τ_ω components of the PMD emulator 10 remain constant. It will be appreciated that if ΔT_ODL ^≤ <Δτ>_sub and β ≠ 0° or 180°, then changes in Δτ_0DL will enhance emulator-Δτ changes with wavelength, thereby affecting the emulator- τ_ω.

Referring to Figure 5 of the drawings, an illustration of how <τ_ω> is maintained constant as <Δτ> is varied is shown. Whilst keeping the sub- emulator 12 configurations fixed (mode coupling and birefringence distribution) and the angle β is always at 0° with wavelength change, ΔTODL was varied from 20 - 60 ps. ΔTODL values therefore provide dominant f such that the emulator-<Δτ> is biased towards ΔTO_DL. The SO-PMD statistics was maintained fairly constant (with a mean-<τ_ω> (<τ_ω>_mean) = 25.4 ± 0.9 ps²) throughout as ΔT_ODL was altered. What was only affected was the emulator- <Δτ>, while the shapes of the DGD wavelength spectra 20 remain unaffected (Figures 7). The PCD therefore remains constant. The DGD wavelength spectra have a <Δτ> value which is always ~ 13 ps above ΔTO_DL. This deviation from ΔTODL is evidenced through the 0.92 gradient and a cutoff <Δτ> of 15.22 ps. Here the DGD range around the emulator-<Δτ> was maintained at ± 11.7 ps. To explain this consider each individual wavelength to possess a unique f , each f is therefore always resolved together with the wavelength-independent dominant ODL f at an angle β = 0°.

Since the mode coupling angles are fixed, although it is assumed that they are uncorrelated (random), the PSP-depolarisation is unaffected. Therefore since both PCD and PSP-depolarisation are unaffected, the emulator-τω is unaffected (20 in Figures 8). It may be deduced from these emulator-τ_ω signatures that PSP-depolarisation and PCD are proportional, with them always changing simultaneously in behaviour (not shown). PSP- depolarisation is the major contributor for SO-PMD compared to PCD. The slight influence on emulator-τ_ω is likely due to the residual-τ_ω that the ODL 18 possesses, as seen in Figure 4, although it has minimal contributions.

Referring now to Figure 6 of the drawings, as a comparison to the PMD emulator 10, the sub-emulator 12 is substituted with a single PMF segment 14 with <Δτ> = 8.15 ps and <τ_ω> = 0.33 ps². Maintaining the angle β at 0° due to PC 16 adjustments as wavelength changes, PMD measurements were performed as ΔTO_DL was varied from 20 - 60 ps. In this case the single segment 8.15 ps fibre is assumed to have a wavelength-independent f and the same applies to the ODL 18. When ΔTODL = O₁ the emulator-<Δτ> is equal to that of the single segment, When ΔTODL is greater than the DGD of the PMF segment 14 the emulator-<Δτ> is close to ΔTO_DL- This is true since the gradient of the slope is almost unit (0.97) and the cut-off <Δτ> is close to zero (2.0 ps) as for the range 20 ≤ ΔTO_DL ≤ 60.

The DGD and SO-PMD spectra of the PMD emulator 10 are shown as 22 in Figures 7 and 8 for various ΔTODL settings. It can be seen that the emulator-<Δτ> is fairly close to ΔTODL. The single mode coupling site and angle β results in the emulator-<τ_ω> remaining low, (Figure 8 in particular) due to negligible PCD and little PSP-depolarisation enhanced by limited PSP rotations with wavelength. The emulator mean-<τ_ω>,<τ_ω>_mean, is 0.45 ± 0.2 ps², when ΔTODL ≥ 20 ps.

The difference in emulator-<Δτ> at each ΔTODL value between Figures 5 and 6 is ~ 13 ps. This agrees closely to the difference between the cut-off emulator-<Δτ> when the single 8.15 ps segment 14 is attached (= 2.0 ps) and that when the sub-emulator 12 is attached (= 15.22 ps) (see Figures 5 and 6).

The invention as hereinbefore described provides a polarisation mode dispersion emulator with fixed SO-PMD but varying FO-PMD to allow researchers for example to investigate the impact of Δτ on SO-PMD experimentally.

The emulator in accordance with the invention conveniently has DGD and constant SO-PMD coexisting together. The SO-PMD or DGD behaviour can advantageously be stochastic or not depending on the configuration of the emulator. This means that one can generate any predetermined SO- PMD and only adjust the mean DGD through simply adjusting mode coupling and later the ODL.

The emulator as hereinbefore described can also assist in designing, investigating or improving PMD compensators.

The emulator can also be used experimentally to investigate which of the two, either DGD₁ SO-PMD or both, has more profound signal degradation effects on propagating light signals.

Claims

1. A polarisation mode dispersion (PMD) emulator apparatus with a fixed mean second order PMD (SO-PMD) and varying mean differential group delay (DGD), the emulator apparatus comprising:

at least one optical delay line (ODL) operable at least to generate DGD;

a sub-emulator comprising a predetermined number of polarisation maintaining fibre (PMF) segments, the sub- emulator being arranged to generate at least a desired amount of fixed SO-PMD; and

a polarisation controller (PC) operatively connected to the ODL and the sub-emulator, the PC being arranged to facilitate operation of the PMD emulator apparatus.

2. The PMD emulator apparatus as claimed in claim 1 , wherein:

the ODL is configured to generate a FO-PMD vector ( %DL )^•

the sub-emulator is configured to generate a FO-PMD vector (^fsub >^{; and}

the PC is configured to align τ , substantially parallel to Ϊ_QDL ^anc' P^ara"^e' to ^usiⁿ9 ^a controllable PC.

3. The PMD emulator apparatus as claimed in Claim 2, wherein the PC is configured to maintain an angle β between τ_0DL and τ , .

4. The PMD emulator apparatus as claimed in Claim 3, wherein the PC is configured to maintain the angle β at 0° at substantially each wavelength thereby to provide SO-PMD independent of FO-PMD changes.

5. The PMD emulator apparatus as claimed in any one of the preceding Claims, wherein the sub-emulator comprises a single PMF segment.

6. The PMD emulator apparatus as claimed in Claim 5, wherein the PMD emulator apparatus with the single PMF segment has at least wavelength independent DGD.

7. The PMD emulator apparatus as claimed in any one of Claims 1 to 6, wherein the sub-emulator comprises a plurality of PMF segments.

8. The PMD emulator apparatus as claimed in Claim 7, wherein the sub-emulator comprises at least eight cascaded PMF segments.

9. The PMD emulator apparatus as claimed in either Claim 7 or 8, wherein the PMF segments are randomly concatenated and have random birefringence and mode coupling distribution.

10. The PMD emulator apparatus as claimed in any one of Claims 7 to

9, wherein mode coupling sites or mode coupling angles at interfaces between adjacent PMF segments are arranged to generate SO-PMD.

11. The PMD emulator apparatus as claimed in any one of Claims 7 to

10, wherein the PMD emulator apparatus with a plurality of PMF segments has at least wavelength dependent DGD.

12. The PMD emulator apparatus as claimed in Claim 10, wherein the mode coupling angles are fixed.

13. The PMD emulator apparatus as claimed in any one of Claims 7 to 12, wherein each PMF segment has a length within twenty percent Gaussian Deviation of a mean length of the sub-emulator.

14. The PMD emulator apparatus as claimed in any one of the preceding Claims, wherein the ODL is arranged to generate DGD in proportion to the voltage applied or supplied to the ODL.

15. The PMD emulator apparatus as claimed in any one of the preceding Claims, wherein the ODL is arranged to generate wavelength independent FO-PMD.

16. The PMD emulator apparatus as claimed in Claim 15, wherein the ODL is arranged to generate wavelength independent FO-PMD which is greater than FO-PMD generated by the sub-emulator thereby permitting the ODL to control only the DGD whilst leaving SO-PMD substantially unaffected provided β = 0°.

17. The PMD emulator apparatus as claimed in any one of the preceding Claims, wherein the ODL is an electrically controllable, reflector-style variable device arranged to induce birefringence.

18. The PMD emulator apparatus as claimed in any one of the preceding Claims, wherein the PC comprises at least a half-wave plate constructed from an electro-optic material.

19. The PMD emulator apparatus as claimed in Claim 18, wherein the electro-optic material is LiNbO₃.

20. The PMD emulator apparatus as claimed in either Claim 18 or 19, wherein the PC is computer controlled and is configured to generate a polarisation angle θ between the ODL and the sub-emulator substantially proportional to a magnitude of a supply voltage to the PC.

21. The PMD emulator apparatus as claimed in Claim 20, wherein the PC is arranged to provide a polarization angle θ of between 0 and 180°.

22. The PMD emulator apparatus as claimed in either Claim 20 or 21 , wherein the PC is controlled to provide a polarisation angle θ that ensures the angle β between the sub-emulator and the ODL FO- PMD vectors is 0° for all wavelengths.

23. The PMD emulator apparatus as claimed in any one of Claims 19 to 22, wherein the PC is configured to rotate any input states of polarisation (SOP) to a certain degree by the application of voltage to the half wave plate.

24. The PMD emulator apparatus as claimed in any one of the preceding claims, wherein the PMD emulator apparatus is configured to allow polarisation-dependent chromatic dispersion (PCD) and principal states of polarisation (PSP)-depolarisation components to be controlled.

25. The PMD emulator apparatus as claimed in any one of Claims 10 to 24, wherein the PMD emulator apparatus is arranged to alter mode coupling angles and to add or remove mode coupling sites.

26. The PMD emulator apparatus as claimed in any of the preceding Claims, wherein the PMF segments have a stressed asymmetric fibre core.

27. The PMD emulator apparatus as claimed in any of the preceding Claims, wherein the PMF segments have at least two axes with different refractive indices respectively such that light split along these two axes propagate at different group velocities.

28. A PMD emulator apparatus substantially as herein described with reference to the accompanying drawings.