WO1998037396A1

WO1998037396A1 - A method and system for simulating pmd using incremental delay switching

Info

Publication number: WO1998037396A1
Application number: PCT/US1998/003489
Authority: WO
Inventors: John A. Fee; Niall A. Robinson
Original assignee: Mci Communications Corporation
Priority date: 1997-02-25
Filing date: 1998-02-25
Publication date: 1998-08-27
Also published as: AU6437998A

Abstract

The simulator is a means for controlling quality of optical receivers. The receivers are tested to determine to what extent the receivers are effective at PMD equalizing by various degrees of PMD effect. The speed with which the receiver accommodates various degrees of PMD could also be used as a degree of effectiveness of the receiver. Then, if better PMD characteristics were desired, the PMD equalizer are added to the receiver as necessary to improve receiver capability. A polarization beam splitter separates the optical data signal into first and second orthogonally polarized optical signals. A first variable time delay element provides a first incremental propagation delay for the first polarized optical signal. A second variable time delay element provides a second incremental propagation delay for the second polarized optical signal. The first and second variable time delay elements consist of a series of optical switches optically interconnected by different incremental lenghts of optical fiber.

Description

A Method and System for Simulating PMD Ualng Incremental Delay Switching

BBtctcffra nd of the ∑r teritiovi

5

Field of - hus Invention

The present invention generally relates to fiber optic iransmiasi m and communication f information, and more specifically to a solution to the problem of Polarization Mode fθ Dispersion.

.Related Art FotarizaUort-Ktode Disperaion (PMD)

Fiber optics technology is revolutionizing the 15 te/ecommunicatioris field. The main driving force iβ the promise of extremely high communications bandwidth. At high, ba dwidt s, a. single beam of modulated laser light can cany vast amounts of information - equal to hundreds of tho isands of phone calls or hundreds of video channels.

However, pulse broadening limits the effective bandwidth and propagation distance of an optical communication signal. Because of the inherent dispersive nature of an optical fiber mediurα, ail portions of a light pulse do not travel the same 2.i distance through an optical fiber causing pulse broadening.

Figure 1 illustrates how pulse broadening arises from varying light propagation delays which eventually distorts light output. Digital input pulses lOO arc input to an optical fiber medium 30 HO, The. amplitude-modulated pulses are generated by a 37396 - ³ -

modulated laser source, such as a direct-modulated laser or an externally-modulated laser.

Different portions of a light pulse encounter varying propagation delays arising from-, inter alia, the varying lengths of reflected paths within optical fiber 110, For clarity, three paths arc illustrated which coi respond to a relatively straight, βhυrt path 100a, a reflected, intermediate length path 100b, and a relatively long, reflected, path lOOc. Due to the varying propagation delays, aee, e.g., the Δt delay in arrival time b ween 100a and 100b, the combined optical output is distorted- Thus, a photoreceptor detecting the output pulses 100a- lOOc will generate a distorted output 120.

As shown in Figure 2, such pulse broadening can lead to intersymbol interference. "Pulse broadening" is called "dispersion" or "spreading" because of the non-uniform way in which parts of the incident signal 20O propagate through a dispersive fiber medium. In a mild form of dispersion, the transitions between ON and OFF stateβ observed at a receiver a e not as abrupt and distinct as the transitions that were originated by a transmitting laser. More severe blurring in the time domain limits the useful bandwidth of the path.

Jn Figure 2, dispersion effects l ve broadened two closely spaced pulses to the extent that they are almost indistinguishable, as indicated by a question mark in the output signal 22a. This will cause an in oπiαatioπ bit to be received O 98/37396 - 3 •

erroneously, with perhaps disastrous results on network communication and customer dissatisfaction.

Several refinements have been made to reduce dispersion and > increase the useful bandwidth. First, single-mode fiber was. developed having a slender core such that there is essentially only a single light path through the fiber. Secondly, die distributed feedback (DFBJ laser was developed with an extremely narrow distribution of output wavelengths. This iϋ technique minimizes chromatic dispersion caused by the iact that different wavelengths traverse the length of the fiber over icuiger periods of time. Finally, a dispersion-shifted fiber material was produced to minimize the increased toιe-v wavelength dependency at a specific wavelength of fifteen is hundred and fifty nm. common in telecommunication applications.

Cum lativel , recent improvements in fiber materials and transmitter devices have reduced pulse dispersion and 20 mcreased working bandwidth. "Lightwave" technology has advanced at such a pace that the bandwidth capabilities have mo e than doubled every two years. As a result, working bandwidths, expressed in terms of digital bit~per-secσnd rates, have escalated from 500 Mbps to 10 Obps.

Problem Solved by the Invention

These progressively more exotic refinements have brought the technology to a new bandwidth barrier; Polarization-Mode Dispersion (PMD). Previously,, PMD was insignificant in magnitude relative to other dispersive effects, but now if is a Imiiting factor- It is well known that light can be polarized and that, for a given beam of light, this polarization can be expressed in terms of two orthogonal axes that are normal to the axis of propagation. As a beam of light propagates through a fiber, the light energy present along one such polarization may leak into the other polarization.

This leakage would normally be of litde consequence (lightwave i-eceivers will detect both polarizations), except that real world fibers carry different polarisations at slightly dilϊerent time delays due to reflection. This effect can be on the order of 10-20 picoseconds (ps) in 100 km fiber and becomes important when the modulating pulses are 50-100 ps. in width. To complicate matters, the polarisation dispersion within a given fiber changes s a function of time and temperature. Therefore, an effective PMD compensation mechanism must monitor and adapt to the changes sα as to keep PlVfD to a nunimum.

To nullify the effects of PMD, researchers have suggested application of an adaptive compensation device in an optical path at the receive end just before the receiving transducer. TϊΛese compensators typically employ a detector for analyzing - s -

Che relative partitioning and delay of the incoming signal along two orthogonal polarizations. The compensators correct a data signal by purposefully adding delay selectively to one polarization or anotiber. A controller interprets the findings of the delay analyzer and iruanipulates adjustable delay elements so as to compensate for the potarusatioii-dependent delay differences caused by the imperfect fiber transmission path. However, these techniques are not practical in telecommunication applications, such as, long-haul optical fiber communication.

The variable delay elements are usually optical fibers that are cither heated or squeezed to alter their propagation characteristics. While these elements are adaptable to laboratory electronic control techniques, they are inadequa te in terms of rcproducibility and predictability of response. They are also impractical for use in a commercial traffic-bearing fiber network wherein recovery time following an equipment or power failure should be rninimized. See, e.g., Ozeki, -et al, "Polarization-mode-dispersion equalisation experiment using a variable equalizing optical circuit controlled by a pulβe- waveform*compaτison algorithm ' 0±*C '94 Technical Digest. paper Tu 4, pp. 62-64; Ono, et al, "Polarization Control Method for Suppressing Polarisation Mode Dispersion Influence in Optical Transmission Systems*. Journal ofLigktwaυ

Technology. Vol. 12, No. 5, May 1994, pp. 89-91; Takahaai, et aL, "Automatic Cornperta&tion Technique for Timewige Pfucώating Polarisation Mode Dispersion in In-line Amplifier Systems", Electronics Letters, Vol. 30, No. 4, Feb. 1994, pp. 348- 49; and WO 93/09454, Rockwell, Marshall A.; Liquid Crystal Optical Waveguide Display System {each of which is incorporated in its entirety herein by reference) -

What is needed is a method and system for testing the ability of various receivers to withstand the effects of PMD. Then, the receivers cculd be mixed or altered to obtain the required rttsults.

•Summary of tfυe Invention

The present invention provides a system and method for testing the ability of arious receivers to withstand the effects of PMD and compensating for polarisation mode dispersion (PMD) in an optical data signal using optical switch elements to provide incremental delays between different polarization modes of the optical data signal.

In a preferred embodiment, a number of receivers can be tested utilizing the same test pattern to compare PMD compensation characteristics, thereby providing a uniform basis of comparison. The simulator can also be utilised for quality control, e.g, by gathering time variant dispersion data from a computer on a network and utilized this information in the simulation system controller to test all of the other circuits against what was actually measured.

To provide the time delays, for example, a polarisation mode O 98/3

separator separates the optical data signal into first and second orthogonally polarized optical signals. A first variable switching delay clement provides a first incremental propagation delay for the first polarized optical signal, A second variable switching 5 delay element provides α second incremental propagation delay fcr tlie βecoπd polarized optical signal A controller controls optical switches in the first and second variable switching delay elements to set first and second incremental propagation delays.

1 In particular, the first and second polarised optical signals are incrementally delayed relative to one another so as to compensate for polarization mode dispersion. A beam combiner then combines the first and second polarized optical signals to form an optical output data signal which can be detected

IT accurately and reliably by a receiver without the effects of polarization mode dispersion. In this way, optical data, signals can be transmitted over greater distances along a long-haul fiber optic dispersive medium at even greater bit-rates and bandwidth.

21)

Compeurcd to other known technologies, the present invention is more reliable and provides a method and system for delivering mote uniform receivers. This is a particular advantage in a mission-critical higji data rate optical communications network. 25 The application and commercialization of this invention is very timely as the optical network technologies are approaching the PMD barrier, O 98/37396

Further features and advantages of the present invention, as well as the structure and operation of various embodiments nf the present i ventio , are described in detail below wzth refererjve to the accompanying drawings.

5

Brief Description of the Figures The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the lft principles of the invention and to enable a person skilled in the pertinent art to make and use the invention, in which:

Figure 1 illustrates pulse broadening through a dispersive optical fiber; 15

Figure 3 shows rhe mtarsymbol interference caused by pulse broadening;

F-gure 3A shows an optical communication link having an 20 automatically controlled PMD compensator in accordance with a preferred embodiment;

Figure 3B is a detailed block diagram of the PMD compensator shown in Figure 3 A in accordance with a preferred embodiment;

25

Figure 4- shows a controlled, variable, incremental delay switching element using optical switches and fixed delay ele e ts in accordance with a preferred embodiment; O 98/37396 " ^{9 *}

Figures 5A, SB, 5C and BD illustrate examples of optical switches in accordance with a preferred ernbodirnentj

^■5 Ftgure 6 is a block: diagram of a PMD simulator in accordance with prefeιτed embodiment; and

Figure 7 is a block diagram illustrating a system that utilizes the PMD simulator of Figure 6 in accordance with a preferred IP embodiment.

The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or fuiictloπally similar elements. 15 Additionally, the left-moat digϊt(sj of a reference number typically identifies the drawing in which the reference number first appears.

Detailed Description of the Preferntd Bmbodimtents zo A PMD compensator in accordance with a preferred embodiment is based on delay elements that use optical switching to add well-defined increments of delay to polarized portions of an optical signal. An apparatus in ajcewtda ce with a preferred embodiment: is depicted in Figures 3A and 3B of the attached

25 drawings.

Fi&ure 3A shows a typical end-to-end network path, A 8/37396 - ιo -

uanβmitter 30 transmits an optical data signal through an optical fiber 310 for detection by a receiver 350, For example, transmitter .300 can be a DFB laser modulator or any other type of modulated light source for producing an optical data signal. Optical fiber 310 is a long-haul, single-mode, dispersion-shifted fiber approximately one-hundred kilometers in length. More generally, any type of optical fiber can be used. Additional fibers, Line .amplifiers, and/or repeaters can also be included between the transmitter 300 and receiver 350. Receiver 350 can be any suitable photodetector for detecting the modulated optical data signal.

A poiarύation-mode dispersion (PMD) compensator 320 is provided along optical fiber 310 near the receiver 350. According to the present invention, the PMD compensator 32 equalizes PMD which further increases bandwidth and tranamisaion range. For example, given the presence of PMD compensator 320, transmitter 300 can transmit modulated laser data on the order of one to one-hundred Gb/s (gigabits/ se ), or more, over a one-hundred km. singl mode fiber without jnterβyrnbol interferencs caused by PMD. Receiver 350 can detect the output reliably and accurately in a tel^omraunicarion environment.

Figure 3S shows a block diagram of the components of the PMD compensator 320 in accordance with one embodiment of the present invention. Optical paths are indicated generaJiy by a. loo *p al Aong the transmission path. The optical data signal /37396

traveling through optical fiber 310 enters beam splitter 332 as Optical input 31S. A portion of the optical input 315 is diverted to a delay detector 327- The delay detector detects delays between two orthogonal polarization modes of the detected light, The operation and implementations of such delay detectors is well-known and need not be described in further detail.

The majority of the optical input 315 passes through beam splitter 322 to a polarization-dependent beam splitter 334. The polarization-dependent beam splitter is a type of polarization mode separator which separates the optical input 313 into two optical signals that are orthogonally polarized with respect to one another. The two polarised signals travel along two separate paths leading through respective variable switching delay elements 326 and 338, Each of the variable switching delay elements 326 and 328 provides a respective incremental delay to optical signals passing therethrough to equalize or compensate for PMD- After passing through the delay elements 326 and 328, the two orthogonally polarized optical signals are recαmbined by beam combiner 340. An optical output 345 having little o^ no polaiissation-mode dispersion is then returned to optical fiber 310 and/or receiver 350.

The degree of increnaental delay, if any, imparted by the variable svritdbing delay elements 326 and 328 is manipulated by control signals received over respective control lines 331 and 332 from a controller 330. Controller 330 receives data output frtfm the delay detector 327 repreBenting the magnitude of PMD over an output line 329. Controller 330 then processes the data and generates control signals for the variable switching delay elements 336 and 328 to counteract the PMD effects of the long transmission path along fiber 310. The control signals

5 may be of any conventional form including electrical light or electro-magnetic depending on the nature of the control lines 331 and 332. In particular, the controller 330 sets optical switches within the variable switching elements 326 and 328 to apply an incremental relative propagation delay between the to orthogonally poIartTJed signals which compensates for the delay detected by delay detector 327. Because PMD is compensated through optical switching, an extremely fast response time to detected delay, e.g., on the order of nanoseconds, can be achieved that is independent of the degree of delay adjustment.

!J5

Note that in Figure 3B. two delay elements are applied so that either polarization can be retarded with respect to the other. Λnαther variation can use a single delay element to provide a relative propagation delay. Also, one or more rotatable

?a polarizers can be used at the polarization beam splitter 324 to seiect a polariisation component at any degree of rotatioti as the two optical signals passing through the switching delήy elements 326 and 328 do not necessarily have to be orthogonally polarized. zs

An important feature in accordance with a preferred embodiment is the implementation of the variable switcliing deJ^ elements 326 and 328. Each of these switching delay 8/37396

element* have a similar structure and operation. Accordingly^ only o e switching delay element need be described in deb-u

Figure 4 shows a detailed example of one variable switching delay element 236 used in the present invention. A seriea of optical switches SW0-SW6 are connected in stages by different incremental lengths of fiber. At each stage, an optical switch SW0-SW6 can awitch an optical signal over a reference fiber segment or a delay fiber segment which is longer than the reference fiber seg ent so aβ to introduce an incremental propagation delay. Depending upon how each switch SWO-SW6 is εet, an optical output 427 can be delayed by different incremental time intervala.

For example, switch SWO is a 2 X 2 optical cross-connect switch having four ports A tp D. In one switch configuration, switch SWO caxi simultaneously connect port A to port C and connect port B to port D. Optical input 425 would exit port C and travel along a reference fiber segment 440 c.φtτiencing no relative propagation delay, e.g., 0 picoseconds- Alternatively. SWO can switch optical input 42S to pass from port A to exit port D for transmission over a delay fiber segment 441. The delay fiber segment 441 is longer than the reference fiber segment 440 by a predetermined amount calculated to introduce a one picosecond propagation delay compared to the transit time for light traveling through the reference fiber segment 440.

Figure 4, the switching stageβ introduce 37396

progressively more delay. In particular, the arrangement of Figure 4 uaea delay values that progress geometrically by powers of two from one to thirty-two picoseconds for the individual stages. However, the switching delay element 42^β can represent any integral number delay value from zero picoseconds to sixty-three picoseconds by varying the switched state of the optical switches S 0-SW6. Many other value assignments for the propagation delay are certainly possible and have been contemplated. The use of many low picosecond values is preferred for high data rate communications because the θtepwise introduction of large delay values can cause momentary signal disruption. Thus, for extremely high bit rates the range of PMD compensation and quantisation step* will be relatively small.

Thus, the above propagation delays in the switching delay clement 426 are illustrative and can be varied by adding more or less stages and changing the reference fiber lengths. Further, at each switching stage, other optical delay elements ceui be used in combination with or instead of the optical delay segments to apply a propagation delay. .Also, multi-port cross- connect swi itches having more than two ports on a side, e.g., N X N optical cross-connects where N « 3, 4, 8, or more, can be used to allow multiple delay fiber segments to introduce a greater variety of incremental delays at each stage.

Each of the optical switches SW0-SW6 te controlled through control lines 435 based on a control input 431 output from

Several possible constructions of the individual 2 X 2 optical s itches SWO to S 6 are shown in Figures 5A to 5D. Figure 5A shows one 2 X 2 switch 500 having four Semiconductor Optical Amplifiers (SOAs) 802, 504, S06 and 508 at each port Λ to D respectively. A 3db (loss) coupler 505 interconnects optical paths between the pair of SOAs 502, S04 and the pair of SOAs S06 and So^β. Aa is well known, these optoelectronic SOA devices can provide optical gain and switching capability by adjusting the electrical bias current at a gate that drives the devices. For example, if SOA 602 is biased "on," an optical signal at port A can pass through SOA 50Jt to 3db coupler 505. The optical signal may be further switched (and amplified) to exit port C and/ or D with little or no loss by txtrning on SOA SOtf arid SOA 50S respectively_. 8/

Another X 2 switch variation is shown in Figure SB having lour couplers G23, 325, 53T and 529 at each port A to D to form four optical paths cπoes-coπnected between the ports A to D. Pour SOAs 522-528 are provided for β itching and amplifying optical signals passing along the four optical p?» hs. Although more couplers are used, this configuration has the advantage that A-D and B-C connections are possible without njbdng optical signals input at ports A and B.

Figure 5C shows a further variant that separates switch SOO into two half-switches 540 and 560. Half-switc 540 has two SOAs 942. 544 at ports A and B connected to a 3db coupler

543. Half-switch 560 has two SOAs 566 and 56β at ports C and D connected to a 3db coupler 565. The two 3db couplers 645 and 546 optically couple the two half-switches 540 and S60. A st e with such a half-switch at either end forma a modular unit. This can be useful from a design and implementation standpoint. Aβ with the other switch designs, tlus arrangement can be made lossless by using the SOAs 6 2,

544, 666 and 568 Lo provide some gain.

Finally, Fi ure 51? shows a switch 580 having well-known Mach-Z^hnder electro-optic switches 582 and 586 to route the optical signals between ports A to D under the influence of electrical field gradients. A fixed gain block 584, βuch as, an optical amplifier, is added to compensate for any inherent coupling loβs of the Mach-Zchnder switches 582, 586. Figure 6 is a block diagram of a PMD simulator in accordance with a preferred embodiment. As described above, since light signads transmitted through a single mode fiber experience polarizat ion that is best described in terms of a bnrføon al component and a vertical component, each component travels on a parh normal to the axis of light propagation The transmission of these signal components is affected separately by causing them to be less than perfectly synchronised. This

IP problem is referred to aβ Polarisaαon-Mσde Diapersioxi (PMD). PMD results in broadened signals that are difficult for α receiver to properly read and interpret. Thus,, there exists a requirement to test the ability of various receivers to withstand the effect of PMD and keep signals within the limits of the receivers.

Mj uie 6 Illustrates such a βolution. The PMD simulator illustrated in Figure 6 receives a beam of optical light at the Polarisation Beam Splitter (PBS) 600 and splits the beam of light into a pair of signals TE 610 and TM 620. The signals arc

20 diverted to individual variable time delay elements 630 and 631. The amount of delay i« controlled by a time variant controller 65D to introduce the appropriate amount of delay to optimize die performance a id compensation to the effects of PMD. Then, the signals are joined at the polarization beam

7 joiner 640 with appropriate compensation having been achieved. One of ordinary skill in the art will readiry comprehend that the variable time delay elements can utilize any of the techniques discussed herein or any other technique 8/37396

to introduce the appropriate amount of delay into the systcm.

In another exnbodiment of the invention, the simulator system breaks die signal into at least two polarization modes, and inserts variable time delay elements into each of the paths as described above. Then, the artificial PMD signal is sent to a receiver, and the controller 650 varies the frequency and the amount of dispersion that each of the modes have as they come out of d e fiber, A number of receivers are *tested" using the same test pattern resulting from the controlled modes to compare rjerformance and maintain uniformity of the signal. The simulator can alao be used to maintain quality control, for example, by gathering time-variant dispersion data from a computer on the network and using this date in the simulation controller 6SO to test the other circuits against an actual measurement.

Figure 7 is a block diagram illustrating a system that utilizes the FMD simulator of Figure 6 in accordance with a preferred embodiment. Processing commences when a signal is transmitted over the S er optic link as shewrx in function block 700. As the signal travels along the transmission route, and is optically switched to one of the fiber optic receivers (730 - 770), a PMD simulator 710 (such as the simulator shown in Figure 6) is utilized to analyze the characteristics of the signal as it is processed by the fiber optic receivers (73θ - 770). The purpose of the PMD simulator 710 is to "test" the receivers to compare ej^ rma ce and maintain uniformity of the signal. The PMD 8/37396 ^{• lfl} "

simulator 710 can also be used to maintain quality control, for example, by gathering timo-variant dispersion data from a computer on the network and using this data in the simulation cf trollcr to test the other circuits against an actual measurement.

The present invention is described in the example environment of a fiber communication network. Description in these terms is provided for convenience only. It is not intended that the invention be limited to appHcatiαn In this example environment. In fact, after reading the following description, it will become apparent to a person skilled in the relevant art how to implement the invention in alternative environments.

Wl die various embodiments of the present invention e been described above, it should be understood that they have been presented by way of example only, and not limitation, f will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

• LAOtfS

Having thus described our invention, what we claim as new, and desire to secure by Letters Patent i»:

i 1 An apparatus for simulating polariza ion mode dispersion ?■ in an o tical data signal traveling through an optical fibe*

3 to one or more receivers, comprising.

4 (aj a transmitter that transmits a uniform optical signal s through the optical fiber to the one or more receivers;

6 (b a receiver of the one or more receivers including an

7 unknown degree of polarization mode equalization; and ^β (c) a ø easur g device that detects a degree of polarization 9 mode equalization of the receiver.

J 2. The system of claim 1, wherein the optical fiber comprises 2 a commun ation lin .

J 3. The system of claim 1, including logic tiiat stores the

2 degree of polarization mode equalization for each of the

3 one or more receivers.

i 4. The s s em of claim 3, further comprising:

2 a combiner that couples the one or more receivers to

3 produce a required polarization mode equalization.

i 5. The system of claim 1, wherein the receiver includes a

2 polarization mode separator with a polariaation-dependent

3 beam splitter. i 6. The system of claim 1, wherein the measuring device

? detects the speed ot polarization mode equalization for the i one or more receivers,

i 7, The system of claim 1, wherein each receiver includes a 2 x 2 optical switch for switching between a reference fiber

3 segment and a respective delay fiber seg ent to provide a

4 relative incremental propagation delay.

J 8. The system of claim 7, wherein at least one of said 2

2 optical switches includes a plurality of semiconductor i optical amplifiers, each semiconductor optical amplifier providing at least one of optical gain and switching action

5 based on a control signal output from said controller.

j 9. The system of claim 7, wherein at least one of said 2 2

2 optical switches includes txvo Madi-Zehnder electro-optic

3 switches interconnected by an optical, amplifier,

1 10. The system υf claim 1, wherein variable switehing delay

2 eleiπents comprising a plurality of optical switches a optically interconnected in series by different incremental lengths of optical fiber are added to the receiver to

5 improve polariaation mode equalization.

O 98/37396 ^* '

i 1 1. A method for simulating polarization mode dispersion in

² an optical data signal traveling tfirough an optical fiber to

'i one or more receivers, comprising the steps of:

^•' (ft) transmitting a uniform optical signal through the optical

5 fiber to the one or more receivers' d (b) receiving an unknown degree of polarization mode equalization at a receiver; and

B (c) detecting a degree of polarization mode equalization of the

9 receiver utilizing a measuring device attached to the o receiver.

1 12. The method of claim 11, wherein the optical fiber

2 comprises a communication link.

1 13. The method of claim 1.1, including the step of storing the

2 degree of polarization mode equalization for each of the t one or more receivers.

1 14. The method of clai 13, further comprising the step of

2 coupling the one or more receivers to produce a iβq ired

1 polarization mode equalisation.

t LS. The method of claim 11, wherein the receiver includes a

2 polarization mode separator with a polarization-dependent

3 beam splitter. i 15. The metliod of claim. I, wherein the measuring device detects the speed of polarisation mode equalization for the onϋ or more receivers.

1 17. The method of claim 11, wherein each receiver includes a

2 2 x 2 optical switch for switching between a reference

3 fiber segment and a respective delay fiber segment to

4 provide a relative incremental propagation delay.

i 16. The method of clarm 17, wherein at least one of said 2 2 optical switches includes a plurality of semiconductor

? optical amplifiers, each semiconductor optical amplifier

4 providing at least one of optical gam and switching action

5 based on a control signal output from said controller.

) 10. The method of claim 17, wherein at l<ϊast one of βaid 2 X ?. optical switches includes two Mach-Zehnder electro-optic

:? switches interconnected by an optical amplifier.

\ 20, τfi« method of cia m 1 i, wherein variable switching delay

2 elements comprising a plurality of optical switches

3 optically interconnected in series by different incremental lengths of optical fiber are added to the receiver to

5 improve polarization mode equalization.