AU690181B2

AU690181B2 - An optical communications system having a polarization scrambler

Info

Publication number: AU690181B2
Application number: AU12247/95A
Authority: AU
Inventors: Gerard Bourret; Vincent Letellier; Dominique Vidal
Original assignee: Alcatel Submarcom SA
Current assignee: Alcatel Submarcom SA
Priority date: 1994-02-18
Filing date: 1995-02-15
Publication date: 1998-04-23
Anticipated expiration: 2015-02-15
Also published as: EP0668672A1; AU1224795A; CA2142771A1; FR2717331B1; JPH09233029A; FR2717331A1

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: V. V Name of Applicant: Societe Anonyme dite Alcatel Submarcom Actual Inventor(s): Vincent Letellier Geiard Bourret Dominique Vidal Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: AN OPTICAL COMMUNICATIONS SYSTEM HAVING A

SCRAMBLER

POLARIZATION

Our Ref 399680 POF Code: 1501/247717 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1- 26/02 '06( U 7:03 F AX W1 0014 1807 P11LU4APS ORMONfl AIPO- CW~hL1SS'NL:A Q00u2 iA AN OPTICAL COMMUNICATIONS SYSTEM HAVING A POLARIZATION

SCRAMBLER

The present invention generally relates to optical commfluications, in particular those in which -the readability of a signal delivered at the output of the system is adversely affected by a phenomenon related to the degree of polarization presented by a light wave which has carried the signal through the system. More generally, the present invention relates to those of such systems in which a defect might appear that is related to a degree of polarization of a light wave conveyed by 'the system.- Degree of polarization is a magnitude that is conventional in physics, and that is defined as a mean value over a time period. For example, it is equal to 100* when a wave remains linearly polarized in a V. direction that does not vary over that period, or when such a wave remains completely circularly polarized in one direction during that period. It might be zero, or it might take intermediate values if the polarization varies during that- period.

It is also known that the polarization of a wave can 25 be characterized by the position of a representative 0 0 :0 point on a sphere that may be referred to as the "Poincar6 sphere".

Typically, this invention applies when a carrier 'wave that is polarized rectilinearly or highly elliptically interacts with anisotropic active centers of an amplifier medium. Typically, such centers are constituted by erbium ions Er3+. They are excited by an optical pumping wave so as to cause optical gain to appear and to be maintained, thereby compensating for losses to which the carrier wave is subjected along the line. Typically, such centers are dispersed in a glass matrix in random manner with respect to their angular positions. Their anisotropy is such that each of them interacts mainly with the waves that are polarized in a direction defined by the angular position of the center.

This direction may be referred to as the "main amplification direction" of the center.

The consequences of the anisotropy of the active centers of the amplifier medium are described below in simplified manner by considering only two populations that are uniform with respect to their main amplification directions. To simplify matters further, the main amplification directions are considered to be totally selective, i.e. a light wave interacts with such a population only insofar as the wave has a component that is polarized in the main amplification direction of the S 15 population.

The main amplification direction of the first population is that of the polarization of the carrier wave. The main amplification direction of the second ogeeo S"population is orthogonal to that of the first population.

As a result of the amplification, the first population is strongly de-excited, thereby limiting the gain applied to the carrier wave and therefore to the signal. That portion of the noise whose polarization is orthogonal to that of the carrier wave is amplified with a higher gain by the second population, provided that its power remains lower than that of the carrier wave combined with any ee..o noise having the same polarization as the wave. In this way differential gain appears. Along a line including a succession of such amplifiers, the differential gain causes a progressive and damaging decrease in the signalto-noise ratio. That is why it has been proposed to use a polarization scrambler for modulating the polarization of the carrier wave so that, in each amplifier, the signal is amplified by both of the first and second above-mentioned populations having active centers.

Such a scrambler is disclosed in an article entitled "Observation of new polarization dependence effect in

I

3 long-hau' optically amplified system" by M.G. Taylor, OFC/IOOC'93, Postdeadline paper, PD5, pp 25-28. That known scrambler uses an acousto-optical modulator and operates at a frequency of 85 Mhz. It is difficult to implement.

An object of the present invention is to make it simple to implement a polarization scrambler that is effective.

According to one aspect of the present invention there is provided an optical communications system having a polarisation scrambler, the scrambler of the system including an electro-optical phase modulator for imposing a recurrent phase variation on a light wave carrying a signal to be transmitted, wherein said phase modulator includes a material that exhibits a birefringent effect, two principal axes of the modulator sloping relative to a polarization presented by said light wave at the input of the modulator, so that said recurrent phase variation causes a recurrent variation in a polarization presented by the wave at the output of the modulator.

S 15 According to a further aspect of the present invention there is provided an optical communications system having a polarization scrambler, the system including: o a transmission assembly receiving a signal to be transmitted and :0 responding by transmitting a light wave constituting a primary modulated wave and carrying the signal in the form of modulation of a parameter of the wave, so that the parameter constitutes a carrier parameter, the primary modulated wave having polarization; an optical path having an input receiving said primary modulated wave, the path also having an output rE sponding by restoring an output wave carrying said signal, at least one member included in series in the path being sensitive to a degree of polarisation of a wave that it receives, and that constitutes an input wave of the member, the member having a characteristic time and being capable of causing a defect to appear, the defect increasing with a degree of polarization presented by the input wave of the sensitive member over a period whose duration is equal to the characteristic time; a polarization scrambler included in series in the path upstream from said sensitive member, the scrambler causing the polarization of the wave it is CA\WINWORDODYFPECn\12247-95.DOC passing through it to vary, so as also to cause the polarization of said input wave of the sensitive member to vary, the variation being caused during said characteristic time so as at least to limit said defect; and a receiver receiving said output wave so as to restore said signal; wherein said polarization scrambler includes: an electro-optical phase modulator receiving the primary modulated wave in a polarized form, the modulator having two principal axes that slope relative to the polarization of the wave, so as to impart a phase offset between two light vibrations that are parallel to the two characteristic axes and that are propagating through the modulator, the phase offset being controlled by an excitation voltage applied to the modulator, the two vibrations together forming a line input wave at the output of the modulator; and an excitation source supplying said excitation voltage and causing it to vary recurrently so as to impose angular sweeping on the polarization of the 15 line input wave, the variation being caused with recurrence times that are not significantly longer than said characteristic time, and with an amplitude that is large enough to limit said defect.

S: of This recurrent variation in polarization constrains the representative point of the polarization to travel along an arc of the Pointcar6 sphere. The variation So 20 is such that the degree of polarization of the wave is decreased significantly over the recurrence period.

A preferred embodiment of the present invention is given below by way of non-limiting example and with reference to the diagrammatic figures of the accompanying drawings. When the same element is shown in more than one figure, it is designated by the same reference. In the accompanying drawings: Figure 1 is an overall view of an optical communications system having a polarization scrambler in accordance with the present invention; and C'\WNWOR D\DYFSPECI\12247-95.DOC 02/03 '08 MON 00,'55 VAX 61:3 0614 1867 PIII4P ORHONDB #44 AIPO C0MISVNBR I0I02 4 Figure 2 is a view of the polarization scrambler of the system shown in Vigure 1.

As shown in Figure 1, the communications system given by way of example includes various elements that are known per set: Firstly, there is a transmission assembly 2. This assembly receives a signal to be transmitted S. it responds by transmitting an optical-type primary modulated wave WM carrying the signal in the form of modulation of one of the parameters of the wave. This parameter is referred to below as the "carrier parameter". Typically, it is constituted by the amplitude, the frequency, or the phase of the wave WM.

For example, the transmission assembly 2 includes a fixed-frequency semiconductor laser transmitter 4 injecting a carrier wave WP at a wavelength of 1,550 rn into an optical fiber B. Preferably, this fiber is a polarization-maintaining optical fiber. An amplitude modulator 6 is connected to the transmitter via the optical fiber 8. The amplitude modulator 6 is controlled by an electrical control circuit 10 receiving the signal S. In this example, the carrier parameter is the amplitude of the wave delivered at the output of the amplitude modulator 6.

The modulated wave WM has polarization that remains substantially constant. This polarization may result, for example, from the wave being polarized rectilinearly by the transmitter 4.

In known manner, the system then includes an optical path having an input 14 and including both a p olarization scrambler 22 (described below), and also a transmission line 12. The transmission line receives an optical-type line input wave WE which is formed by the scrambler 22 from wave WM. The line also has an output 16 which also constitutes an output of the optical path, and which responds by irastorIng an output wave WS carrying the signal. Typically, the line is constituted by optical fibers such as 18.

Generally, the present invention is advantageously applicable when firstly certain members included in the line are sensitive to the degree of polarization of the wave that it transmits, and secondly those members cause defects to appear as a result of their sensitivity and in a manner that is dependent on characteristic times of the members. More precisely, this sensitivity is such that, if the line input wave has a non-zero degree of polarization over a length of time in the vicinity of or longer than the characteristic time, then the degree of polarization produces a defect that increases with the degree of polarization. For example, the defect may S 15 affect the signal-to-noise ratio of the line output light wave.

:0 More particularly, the line 12 is, for example, S0" several thousands of kilometers long. Typically, it then 0 includes amplifiers such as 20 distributed over its length and constituted by optical fibers doped with optically pumped erbium ions. Both by spontaneously ""0"emitting noise and also by amplifying pre-existent noise, 0 the amplifiers then constitute noise generators capable S"of causing differential gain to appear as described 00 25 above. In this way, the amplifiers can produce a defect o constituted by an induced decrease in the signal-to-noise ratio of the line output wave. Therefore, they constitute members that are sensitive to the degree of polarization as mentioned above. The characteristic time of each amplifier is substantially its gain saturation time.

The polarization scrambler 22 is organized so as to avoid or limit the differential gain. It receives the primary modulated wave WM at its input. It restores wave WE at its output while modifying the polarization of the wave that is passing through it. This modification is performed at a scrambling frequency whose reciprocal constitutes a scrambling period which is chosen to be shorter than said characteristic time. The modification is organized so as to reduce the degree of polarization of the wave over the scrambling period. In this way, the modification at least limits the induced decrease in the signal-to-noise ratio of the line output wave.

Finally, the receiver 24 receives the line output wave WS so as to restore said signal S.

As shown in Figure 2, and in accordance with the present invention, the polarization scrambler includes the following elements: A polarizer 34. The polarizer receives the primary modulated wave WM via an optical fiber 28 which is preferably a polarization-maintaining optical fiber.

e The polarizer restores the light therefrom in the form of a wave WP. The wave propagates therealong along a light propagation axis 30. Preferably, the polarizer has a high level of polarization. It may be applied directly to the above-mentioned phase modulator. In certain cases, the polarizer may be omitted.

A phase modulator 26. This element receives the primary modulated wave WP via an optical fiber 29 which is preferably a polarization-maintaining optical fiber.

The phase modulator restores the light therefrom in the form of the line input wave WE which is received by another optical fiber 32. The wave propagates therealong along the light propagation axis 30. The phase modulator is constituted by a waveguide formed in a lithium niobate crystal. The characteristic axes of the crystal are preferably at an angular position of 45° to the characteristic axes of the polarization-maintaining optical fiber 29. In a first configuration, the cut of the lithium niobate crystal may be an x-cut or a y-cut with propagation along the z-axis. This configuration may be chosen so as to reduce the polarization mode dispersion of the phase modulator. In a second configuration, a z-cut is chosen with propagation along ~1111 1 I the x-axis or along the y-axis. This configuration may be chosen so as to reduce the control voltage required for controlling the phase modulator.

SFinally, an excitation source 36 applying an excitation voltage to the phase modulator 26. This source causes the excitation voltage to vary substantially periodically with a frequency that constitutes said scrambling frequency. The scrambling frequency must be at least in the vicinity of and preferably greater than the reciprocal of the characteristic time of the amplifiers. Typically, the scrambling frequency is greater than 100 Hz.

The excitation voltage of the modulator 26 induces a difference between the optical path lengths as seen by S 15 two light vibrations angularly positioned along respective ones of the two characteristic axes of the S"modulator. The difference results in angular sweeping being imposed on the polarization of the wave that is S" passing through the modulator. The amplitude of the sweeping increases with the amplitude of the excitation voltage.

The amplitude of the periodic variation in the excitation voltage is chosen so as to impose a S"predetermined sweeping amplitude on the polarization of the wave during each scrambling period. This amplitude is chosen to be large enough to obtain the desired olooo limitation or zeroing of the degree of polarization of the line input wave WE, thereby also limiting or preventing the induced decrease in the signal-to-noise ratio. Typically, the degree of polarization is chosen to be less than Advantageously, the relationship governing the variation in the voltage is chosen to be triangular so as to impose an angular sweeping speed that is substantially constant.

Claims

1. An optical communications system having a polarisation scrambler, the scrambler of the system including an electro-optical phase modulator for imposing a recurrent phase variation on a light wave carrying a signal to be transmitted, wherein said phase modulator includes a material that exhibits a birefringent effect, two principal axes of the modulator sloping relative to a polarization presented by said light wave at the input of the modulator, so that said recurrent phase variation causes a recurrent variation in a polarization presented by the wave at the output of the modulator.

2. An optical communications system having a polarization scrambler, the system including: a transmission assembly receiving a signal to be transmitted and °i responding by transmitting a light wave constituting a primary modulated wave 1 and carrying the signal in the form of modulation of a parameter of the wave, so that the parameter constitutes a carrier parameter, the primary modulated wave having polarization; an optical path having an input receiving said primary modulated wave, ithe path also having an output responding by restoring an output wave carrying 0 said signal, at least one member included in series in the path being sensitive to a degree of polarisation of a wave that it receives, and that constitutes an input wave of the member, the member having a characteristic time and being S:°o capable of causing a defect to appear, the defect increasing with a degree of polarization presented by the input wave of the sensitive member over a period whose duration is equal to the characteristic time; a polarization scrambler included in series in the path upstream from said sensitive member, the scrambler causing the polarization of the wave it is passing through it to vary, so as also to cause the polarization of said input wave of the sensitive member to vary, the variation being caused during said characteristic time so as at least to limit said defect; and a receiver receiving said output wave so as to restore said signal; wherein said polarization scrambler includes: C:\W]NWORDDYFSPEC12247-95.DOC I an electro-optical phase modulator receiving the primary modulated wave in a polarized form, the modulator having two principal axes that slope relative to the polarization of the wave, so as to impart a phase offset between two light vibrations that are parallel to the two characteristic axes and that are propagating through the modulator, the phase offset being controlled by an excitation voltage applied to the modulator, the two vibrations together forming a line input wave at the output of the modulator; and an excitation source supplying said excitation voltage and causing it to vary recurrently so as to impose angular sweeping on the polarization of the line input wave, the variation being caused with recurrence times that are not significantly longer than said characteristic time, and with an amplitude that is 5 large enough to limit said defect.

3. A system according to claim 2, wherein said polarization scrambler further includes a polarizer placed at the input of said phase modulator and t, 15 having a transmission axis angularly positioned at substantially 45 degrees relative to the characteristic axes of the modulator.

4. A system according to claim 2, wherein said optical path includes an optical fiber transmission line including a succession of optical fiber amplifiers :having respective saturation times, each of the amplifiers constituting one of said sensitive members and generating noise, said characteristic time of the member being C:AWNWOMRDDYFISPEC122475.DOC constituted by the saturation time of the amplifier, said defect being constituted by an induced decrease in the signal-to-noise ratio at the output of said line.

5/ A system according to claim 4, wherein said excitation source is a periodic voltage source whose frequency constitutes a scrambling frequency and is greater than 100 Hz.

6/ A system according to claim 5, wherein said periodic voltage varies in the form of a triangular-wave signal.

7/ An optical communications system having a polarization scrambler, substantially as herein described with 15 reference to and as illustrated in the accompanying 0 drawings. DATED: 14th February, 1995 PHILLIPS ORMONDE FITZPATRICK Attorneys for: oS SOCIETE ANONYME DITE ALCATI J- SUBMARCOM ""401-U# •555o5 -1 -I ABSTRACT The polarization scrambler of the system receives an amplitude modulated wave. It includes a polarizer and a birefringent electro-optical phase modulator. The voltage from an excitation source controls the modulator. It varies recurrently so as to cause the polarization of the wave output by the modulator to sweep angularly over the Poincare sphere. This wave is transmitted via an optical fiber line including amplifiers. The saturation times of the amplifiers are shorter than the recurrence period of the voltage, thereby limiting their differential gain. The invention is applicable to telecommunications. *o tt'6 l A