WO2004063782A1

WO2004063782A1 - Device, system and method for emulating polarization mode dispersion of optical fibers

Info

Publication number: WO2004063782A1
Application number: PCT/EP2003/000089
Authority: WO
Inventors: Benedetto Riposati
Original assignee: Telecom Italia SpA
Current assignee: TIM SpA
Priority date: 2003-01-08
Filing date: 2003-01-08
Publication date: 2004-07-29
Anticipated expiration: 2005-07-08
Also published as: AU2003201957A1

Abstract

Present invention relates to a method for emulating Polarization Mode Dispersion or PMD of an optical fiber, to a device that emulates the distortion given by the PMD and to a system comprising the device (12) of present invention. The device (12) comprises, connected in series, a first variable Differential Group Delay (DGD) component (101), a rotator (102) and a second Differential Group Delay (DGD) component (103). The device (12) through the rotator (103) which is able to selectively rotate a first variable PMD vector generated by the first DGD component (101) in relation to a second variable PMD vector generated by the second DGD component (103) permits to reproduce a wide range of PMD values of first and second order. Advantageously, the device (12) requires a limited number of parameters for generating a wide range of PMD values as well as an easy control.

Description

"DEVICE, SYSTEM AND METHOD FOR EMULATING POLARIZATION MODE DISPERSION OF OPTICAL FIBERS" TECHNICAL FIELD

Present invention relates to a method for emulating Polarization Mode Dispersion or PMD of an optical fiber, to the system able to allow its application and to the device that emulates the distortion given by the PMD.

Polarization mode dispersion is a phenomenon of distortion of the optical signal that propagates in single mode optical fibers, i.e. optical fibers that carry signals in a fundamental propagation mode constituted by a pair of degenerate modes with orthogonal polarization. This phenomenon is, as is well known, linked to the dual degeneration of the fundamental mode of the optical fibers. In fact, construction imperfections in real fibers (core ellipticity, external stresses, etc.) entail different group velocities for the two degenerate modes of the optical signal transmitted at one end of the fiber and hence distortions of the signal received by a receiving device (receiver) located at a second end of the fiber.

The PMD of an optical fiber is represented or quantified through Differential Group Delay or DGD and Principal States of Polarization or PSP.

In particular, PMD is typically studied in the Stokes space, where States of Polarization are represented by Stokes vectors and transformations of Polarization are represented by a Mueller Matrix.

In more detail, PMD is usually described in the Stokes space in term of a PMD vector Ω, having the direction of the Principal State of Polarization (PSP) and the magnitude equal to the DGD. In a real fiber the vector Ω varies with the wavelength λ (or equivalently with the frequency f) . Typically, at a given frequency f, this dependency ^~" is described in term of dependency from angular frequency ω

COPY and is defined as modulus |Ω| at a determined value ω₀ (first order PMD or DGD) and as modulus of first order derivative | dΩ/dω | at ω₀ (SOPMD: Second Order PMD). BACKGROUND ART

As known, in order to evaluate or certify characteristics of optical rice/transmission systems, it is useful to verify, in a laboratory environment, the effects of different values of PMD on the rice/transmission systems .

Such an evaluation or certification can be obtained by emulating in the laboratory environment possible values of PMD through the use of devices named PMD Emulators or PMDE that, as known, are able to reproduce controlled PMD variations as obtainable through real optical fiber installations .

Therefore, as known, the PMD Emulators must be capable of generating a wide range or first and second order PMD.

From US Publication 2001-0024538 it is known a PMDE which uses "multiple birefringent wave-guiding sections that are interconnected (each other by means of rotatable connectors) to have adjustable polarization-changing connectors. The polarization of light transmitting from one section to another adjacent section can be modified differently between different adjacent sections according to a distribution function to produce one PMD state to represent one possible PMD state of a real PMD fiber. The connectors can be adjusted to produce different sets of polarization modifications to produce different PMD states to represent different possible PMD states of the real PMD fiber. "

The known PMDE requires a plurality of sections or stages that necessitate to be separately controlled through rotatable connectors or polarization controllers. Therefore the known PMDE is complex and substantially difficult in use.

In fact, the known device, in order to vary the PMD in a wide range of values, as required, for example, for certifying optical systems operating in a range of 10-40 Gbit/sec, requires an high number of sections in sequence, typically at least 6, and the separate control of each of said sections for varying the PMD.

Moreover, due to the high number of sections connected in sequence, the PMD values can be hardly reproduced because even a small variation on one section provides an high variation of the PMD, as could be easily understood by an expert on the field. DISCLOSURE OF THE INVENTION

Object of present invention is a device, system and method for emulating PMD of a real fiber, simpler than that disclosed by known prior art.

Moreover object of present invention is a system and device which, with respect to the known prior art, presents the advantage of easiness of the control .

This object is achieved by the device, system and method for emulating PMD of a real fiber as claimed.

In particular, this object is achieved by the device in accordance with the present invention which, by means of a polarization controller or rotator and two variable DGD components, is able to emulate the distortion given by the PMD of a real fiber. BRIEF DESCRIPTION OF DRAWINGS

The above and other features of the present invention will be better understood from the following description of a preferred embodiment of the invention, which is intended purely by way of example and is not to be construed as limiting, taken in conjunction with the accompany-ing drawings , where : Figure 1 is a block diagram of a measurement setup using a PMDE; and

Figure 2 is a block diagram of the PMDE in accordance with the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

With reference to Figure 1 , a measurement system (system) 10 for evaluating or studying in a laboratory environment the effects of PMD on optical systems comprises connected in series the following devices: an optical transmitter (transmitter) 11, a PMDE 12 and an optical receiver (receiver) 13.

Moreover, the system 10 comprises a performance measuring instrument (measuring instrument) 14 which is connected to the .devices 11 and 13 for respectively controlling each or at least one of said devices 11 and 13.

The transmitter 11, of known type, is constituted, for example, by the transmitter Model LCM 155-64N of the company Nortel and is able to generate modulated optical signals, for example, in a range of 10 to 40 Gbit/sec, to be sent to the PMDE 12.

The receiver 13, of known type, is constituted, for example, by the receiver Model RCV 7002AN of the company HITACHI and is able to receive the optical signals after they have traveled through the PMDE 12.

The measuring instrument 14, of known type, comprises, for example, a Bit Error Rate (BER) tester including an error detector Model MP1764A and a pattern Generator model MP1763 of the company Anritsu and is able to send control commands both to the transmitter 11 and the receiver 13 in order to evaluate, in a known way, the number of bit erroneously detected by the receiver.

As those skilled in the art could easily understand, according to further embodiments of present invention- -the measuring instrument 14 could comprise, for example, an oscilloscope connected solely to the receiver 13 and able to evaluate the number of bit erroneously detected by the receiver.

Moreover, as those skilled in the art could easily understand, the system 10 could comprise, interposed between the devices 11, 12 and 13, other optical devices, of active and/or passive type, as for example optical amplifiers, fiber sections, optical attenuators, polarization scramblers, without departing from the architecture of the system 10, as disclosed.

The PMDE 12 comprises, connected in series, the following optical components or units: a first variable Differential Group Delay (DGD) component 101 (Fig.2), a variable polarization rotator or controller (polarization controller or rotator) 102 and a second variable Differential Group Delay (DGD) component 103. Moreover, the PMDE 12 may comprise, according to a further embodiment, a driving section or component 104, connected through control connections of known type to the components 101, 102, and 103 and able to selectively control the above devices 101, 102, and 103.

The first and second DGD components, 101 and 103, comprise, for example, optical devices of known type having variable birefringence as the model DINADELAY of the Company General Photonics, able to apply to the two orthogonal polarizations of the optical signal two different group delays with a difference which can be assigned within a determined range.

The polarization controller or rotator 102 comprise, for example, a Uniphase Hermetic Lithium Νiobate Polarization Controller of the company JDS able to rotate the polarization of the input optical signal by a variable angle . The PMDE 12, according to present invention, is able to generate, as can be easily verified through the following explanation, a wide range of first and second order PMD.

In fact, by tacking as reference, for sake of simplicity, the following PMDE 12 configuration wherein: the two variable DGD components, 101 and 103 respectively, have their birefringence axes aligned to each other, let's say in the x direction, so that their PMD vectors are: [Δ^"-i,0,0];

- the rotator 102 has the axis of rotation orthogonal (in the Stokes space) to the axes of the variable DGD components, 101 and 103; the PMD vector Ω can be calculated as follows:

Aτ₂ 1 0 0 cos(26>) -sin(2< ) 0 Δr, Δr₂ + cos(26>)Δτ, 1 0 + 0 cos(-»Δ r₂ ) - sin(ωΔ τ₂ ) sin(26>) cos(2< ) 0 0 Δr, cos(ωΔτ₂)sin(2-?) 0 0 sin(-»Δr₂) cos(ωΔr₂) 0 0 1 0 Δτ, sin(ωΔ--₂)sin(2-?)

Wherein M₂ and M_R are the Mueller matrix of the second variable DGD component 103 and of the rotator 102, respectively, and θ is the angle of polarization rotation of the rotator 102.

The magnitude of first and second order PMD, according to the above expression, is:

I Ω I = DGD = ^Aτ² + Aτ₂ ² + Δr,Δr₂ cos(20)

SOR - = Δτ,Δr₂ sin(20)

Therefore, by varying the values of Δti, Δτ₂ and θ , by means of the first DGD component 101, the second DGD component 103 and the rotator 102, respectively, it is possible to independently set the desired values of DGD and SOPMD and consequently emulate first and/or second order PMD in a laboratory environment .

The values of Δτ_x, Δτ₂ and θ , can be controlled, for example, by voltage signals sent by the driving section 104 to the components 101, 102 and 103, respectively. Suitably, the driving section 104 may be realized by means of a micro-controller based electronic device of known type, for example the TMS 320F243 of the company Texas Instruments. As those skilled in the art could easily understand, according to further embodiments of present invention the driving section 104 can comprise devices for generating signals, for example mechanical or thermal signals, able, in general, to selectively modify the values of Δτi, Δτ₂ and θ , without departing from the architecture of the PMDE 12, as disclosed.

As reference, present invention has been disclosed by keeping, for sake of simplicity, a determined PMDE 12 configuration, in particular a determined alignment of the PMDE components, 101, 102 and 103, respectively, but, as those skilled in the art could easily understand, similar results can be achieved by means of other reciprocal orientations of the three components 101, 102 and 103, respectively.

The operating principle of the system 10 (Fig.l, Fig.2) and device 12 as described above is as follows. The concatenation of a first DGD component 101, a rotator 102 and a second DGD component 103 makes it possible to selectively adjust the PMD of first and/or second order by selectively operating on the three components . In fact, by rotating through the rotator 102 a first variable PMD vector, generated by the first DGD component 101, in relation to a second variable PMD vector, genera-ted by the second DGD component 103, it is possible to generate PMD values in a wide range of values, as for example, the range required for evaluating PMD in a range of 10 to 40 Gbit/sec.

In particular, by selectively controlling the variable vectors respectively generated by the first and second DGD components, 101 and 103, it is possible to obtain by rotating the respective polarization angles of the above variable vectors, a wide range of values of DGD and SOPMD. For example, to test a 10 Gbit/s optical transmission system, values of DGD from 0 to 70 ps and of SOPMD from 0 to 1500 ps/THz can be obtained by using the device according to present invention.

One of the advantages of the PMDE 12, with respect to the prior art, is, therefore, the limited number of parameters required for generating a wide range of PMD values- as well as the easiness of the control . In fact, due to the disclosed architecture it is necessary to set maximum three parameters for generating a wide range of PMD values while known prior art requires to control, as a general rule, more than three parameters, and, as typical rule, at least 6 parameters.

Obvious modifications or variations are possible to the above description, in the dimensions, shapes, materials, components, circuit elements, connections and contacts, as well as in the details of the circuitry and of the illustrated construction and of the method of operation, without thereby departing from the spirit of the invention as specified in the claims that follow.

Claims

1. Method for emulating at least first order Polarization Mode Dispersion or PMD values of an optical fiber characterized by the step of selectively rotating a first variable vector representative of a first determined PMD value in relation to a second variable vector representative of a second determined PMD value .

2. Method as per claim 1, characterized by the step of selectively controlling said first and said second variable vector.

3. Device for emulating Polarization Mode Dispersion or PMD of an optical fiber characterized by a first component (101) having a first determined birefringence axis and able to apply first variable Differential Group Delay or DGD to an optical signal in order to generate a first optical signal having a determined DGD; a second component (103) having a second determined birefringence axis and able to apply second variable Differential Group Delay or DGD to an optical signal; - a third component (102) interposed between said first (101) and said second (103) component and able to rotate the first determined birefringence axis with respect to said second determined birefringence axis in order to generate an optical signal having at least a first order determined PMD value.

4. Device as per claim 3, characterized by a driving component (104) connectable to said first (101) , to said second (103) and to said third (102) component for selectively controlling said first (101) , said second (103) and said third (102) component.

5. Device as per claim 3 or 4 , characterized in that - said first (101) and said second (103) component comprise optical DGD type devices having variable birefringence.

6. Device as per claim 3, 4 or 5 , characterized in that said third component (102) comprises an optical polarization controller type device.

7. Device as per claim 3, 4 or 5 , characterized in that

- said third component (102) comprises an optical rotator type device .

8. Device as per claim 4, 5, 6 or 7 , characterized in that

- said driving component (104) comprise a micro-controller based device .

9. System for emulating Polarization Mode Dispersion or PMD of an optical fiber comprising at least a transmitter (11) able to transmit optical signals;

- at least a receiver (13) able to receive optical signals; and characterized by

- a device (12) for emulating PMD as claimed in claims 3 to 8 and interposed between said transmitter (11) and said receiver (13) .

10. System as per claim 9, characterized by

- at least a measuring instrument (14) able to control said at least a transmitter (11) or said at least a receiver (13) .