AU668324B2

AU668324B2 - An optical fiber telecommunications method, a link using the method, and a pumping system for four-wave mixing in particular for the link

Info

Publication number: AU668324B2
Application number: AU77668/94A
Authority: AU
Inventors: Jose Chesnoy
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1993-11-09
Filing date: 1994-11-08
Publication date: 1996-04-26
Anticipated expiration: 2014-11-08
Also published as: FR2712445B1; EP0652653A1; AU7766894A; FR2712445A1; CA2135259A1

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority 0 o Related Art: soq Name of Applicant.

Alcatel N.

$to Actual Iniventor(s): so 4 Jose Chesnoy Address for Service., PHILLIPS ORMONDE, FITZPATRICKC Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: AN OPTICAL FIER TELECOMMUNICATIONS METHOD01, A LINK USING THE METHOD, AND A PUMPING SYSTEM FOR FOUR-WAVE MIXING IN PARTICULAR FOR THE LINK Our Ref 388880 POP Code:, 1501/78784 The following statement is a full description of this invention, including the best method of performing it knoon to applicant(s):

I

AN OPTICAL FIBER TELECOMMUNICATIONS METHOD, A LINK USING THE METHOD, AND A PUMPING SYSTEM FOR FOUR-WAVE MIXING IN PARTICULAR FOR THE LINK The present invention relates to optical fiber telecommunications. It relates more particularly to implementing long-distance links using such fibers for transmitting high data rates.

BACKGROUND OF THE INVENTION The cost of the fibers required to make up the transmission line of such a link constitutes a large part of the total cost of the link. That is why numerous telecommunications networks use fibers that offer the advantage of being relatively cheap, but that suffer from th4 drawback of having non-negligible chromatic dispersion at the wavelengths used. Such fiberc are referred to below as "standard" fibers. Attempts to increase the data rate or the distance of data transmission at such wavelengths and using such fibers come up against the problem the signals being deformed by dispersion in the fibers. More specifically, at a typical wavelength of 1.55 pm, such deformation limits the transmission of an (NRZ) amplitudemodulated signal to about 90 km at 10 Gbit/s. Therefore, it poses a major problem.

e* Known solution$ to that problem use passive compensation of the dispersion. They lead to very high costs.

Another solution has been proposed by an article entitled COMPENSATION OF FIBRE CHROMATIC DISPERSION BY .SPECTRAL INVERSION R.M. Jopson, A.H. Gnauck and R.M0 Derosier ELECTRONICS LETTERS ist April 1993 vol. 29 No. 7.

That solution consists in inverting the optical phase of a wave carrying the signal midway along tha transmission path. For that purpose, a "four-wave mixing" phenomenon is used which produces phase conjugation. More specifically, an incident wave modulated by the signal to be transmitted and referred to below as the "upstream wave" is mixed with a continuous and powerful pump wave in a mixing fiber. The optical frequency vo of the pump wave, which frequency is referred to below as the "pump frequency", must coincide with a neutral frequency vo which cancels the dispersion of the mixing fiber which, for that purpose, must be of the dispersion-shifted type. Furthermore, the polarization of the pump wave must be linear and parallel to that of the upstream wave, A downstream wave is then generated with phase inverted relative to the upstream wave, and the optical frequency va of the downstream wave is symmetrical to the optical frequency vs of the upstream wave about the neutral frequency vo. Therefore, the downstream wave can be filtered prior to being amplified and prior to propagating along the following line segment which is made up of ordinary fibers.

That solution proves difficult to apply to a practical implementation of an optical link, in particular a longdistance high data rate link. The power of the downstream wave generated in that way varies strongly and quickly. On 20 reception, such power variations prevent the transmitted data from being restored correctly, i.e. they give rise to a high error rate.

OBJECTS AND SUMMARY OF THE INVENTION Particular objects of the present invention are to make it possible: to reduce the error rate and/or the cost of an optical fiber link; to increase the length and/or the data rate of the link; for that purpose, to compensate more effectively for the effects of the chromatic dispersion of ordinary fibers by means of a four-wave mixing phenomenon generating a downstream wave having phase inverted relative to an upstream wave; and more generally, to increase the utility of such a phenomenon by freeing the power of the downstream wave from variations which appear when such a method is implemented under conditions that are far removed from those of a laboratory.

To these ends, the invention provides in particular an optical fiber telecommunications method, in which method light waves carrying a signal to be transmitted are guided along a line made up of optical fibers having chromatic dispersion, and in which method a four-wave mixing phenomenon is caused at at least one intermediate point along the line by pump light injected into a mixing fiber so as to compensate for the effects of the dispersion, wherein the pump light is constituted by two pump waves, the polarizations of the two waves being orthogonal to each other, their optical frequencies being substantially mutually symmetrical about a neutral frequency cancelling the chromatic dispersion of the mixing fiber.

By using the two pump waves, the power of the downstream wave is made independent from the polarization of the upstream wave. In this way, the power of the downstream wave does not vary as a result of variations in the polarization of the upstream wave. Therefore, the invention offers a major advantage in all cases in which the polarization of the upstream wave has random variations.

Such is the case in particular at an intermediate point along a long-distance optical link.

BRIEF DESCRIPTION OF THE DRAWING :ogo A more detailed description of how the present invention may be implemented is given below by way of nonlimiting example and with reference to the diagrammatic figures of the accompanying drawing. When the same element is shown in more than one figure, it is given the same reference. In the accompanying drawing: Figure 1 is a view of an optical link of the invention; Figure 2 shows a light-wave spectrum appearing in a known four-wave mixing assembly; and Figure 3 shows a light-wave spectrum appearing in a four-wave mixing assembly included in the lik shown in Figure 1.

MORE DETAILED DESCRIPTION The optIcal fiber link given by way of example includes the following elements: A trinsmitter I receiving a signal S representing data to be transmitted. The transmitter responds by transmitting a departure wave Wi constituted by an optical carrier wave which is modulated to carry the signal. The wave has a modulation spectrum SWi whose width LM is representative of the data rate of the signal. Typically, the polarization of the wave is linear.

An optical line L having an input 2 for receiving the departure wave, and an output 3 remote from the input for responding by restoring an arrival wave W2 which Is formed from the departure wave so that it also carries the signal S.

A receiver 4 receiving the arrival wave W2 for :**responding by restoring the signal S.

The line L has a succession of segments Ti, 8, T2 optically connected together in series between the Input 2 :00404 20 and the output 3, and constituted by optical fibers of the :000*0line. The fibers are capable of guiding waves whose optical frequencies lie in a spectral range DS of the line, the 0:00range including the frequencies vs and Va of the departure 00 0 wave and of the arrival wave. The succession of segments includes an upstream dispersive segment Ti and a downstream dispersive segment TZ, in which segments the optical fibers of the line are standard fibers. The fibers are such that the two segments have same-sign chromatic dispersions in the US spectral range of the line. The dispersions give rise to progressive offsets between various spectrum components of the waves that are guided by the line. The dispersions also cause the signal restored at the output of the line to have dispersion degradations which must be at least limited so as to limit the error rate of the transmission.

For that purpose, the line further includes at least one mixing assembly 6 interposed optically in series between the upstream dispersive segment TI and the downstream dispersive segment T2. The assembly itself includes a mixing segment 8 typically constituted by a mixing fiber having zero chromatic dispersion for a neutral frequency vo situated in the spectral range of the line. An input 10 of the mixing segment receives an upstream wave Wi which is a light wave formed from the departure wave for carrying the signal S. The upstream wave is supplied at the output of the upstream segment TI with an upstream carrier frequency vs lying in the spectral range of the line. An output 12 of the mixing segment 8 supplies a downstream wave W2 at an input of the downstream segment T2, which downstream wjave has a carrier frequency vai lying in the spectral range of the line, and from which downstream wave the arrival wave is to be formed. More particularly, in the link given by way of example, the upstream wave is constituted directly by the departure wave itself, after the departure wave has been subjected to attenuation and amplification accompanied in :e particular by ranidom variations in its polarization, in an initial fraction caf the length of the line, which fraction constitutes the segmnent Ti. Likewise, the arrival wave is cunstituted directly by the downstream wave after the downstream wave hlas travelled over a final fraction of the length of the line, which final fraction constitutes segment Since the length of the mixing f iber is very short compared with the total length of the line, each of the two fractions (initial and final) represents subotantially half of said total length.

The mixing assembly further includes a pumping system, *itself including the following elements: 0A pump generator for supplying pump light spectrally centered in the. vicinity of the neutral frequency, which light has power that is sufficient to give rise to nonlineat effects in the mixing segment.

6A pump coupler 14 injecting the pump light into the mixing segment 8 in the same direction as the upstream wave Wl at a pump injection point 16. This gives rise to the four-wave mixing phenomenon, This phenomenon generates the downstream wave W2 with a carrier frequency va and a modulation spectrum SW2 that are symmetrical to the carrier frequency vs and to the modulation spectrum SW1 of the upstream wave Wl about the pumping light. At the same time, the phenomenon imparts phase conjugation resulting in phase inversion between the downstream wave and the upstream wave.

'A mixing output filter 18 for transmitting the downstream wave only.

in a known four-wave mixing assembly implemented under laboratory conditions, the upstream wave has a controlled constant direction linear polarization. The pumping light is then chosen to have an optical frequency equal to the neutral frequency Vo of the mixing fiber and a linear polarization parallel to that of the upstream wave.

According to the present invention, the pump generator includes two pump sources Gl, 02 supplying the pumping light in 'a 1 e form of two pump waves having respective optical f requencies 2, v P2 that are symmetrical about a middle pump frequency Vo. The middle frequency is equal to, or at least In the vicinity of, the neutral frequency Vo of the mixing segment 8. The two pump waves aleo have respective orthogonal linear polarizations V1, V2, The pump coupler 14 inject~s the two pump waves together into the mixing segment Go: 8. Under these conditions, the power of the downstream wave W2 generated by the four-wave migdng phenomenon becomes independent from the polarization of the upstream wave Wi at the pump injection point 16.

More precisely, the middle pump frequency must be such S. *~'that, at any point along its length, the mixing segment has two chromatic dispersions that are equal in absolute terms and that have opposite signs for two frequencies that are symmetrical to each other about the middle frequency, one of the two frequencies being the carrier frequency of the upstream wave, and the other frequency being the carrier frequezity of the downstream wave.

Typically, the chromatic dispersion of known optical fibers varies linearly as a function of optical frequency sgo that the above condition stating that the two chromatic dispersions must be opposite for two frequencies that are symmetrical about the middle pump frequency is equivalent to a simpler condition stating that the middle pump frequency must be equal to the neutral frequency of the mixing fiber.

To prevent the '5ignal boing degraded by intermodulation products, the two pump frequencies vPl, vP2 have a difference VPl vP2 that is greater than the width LM of the modulation spectrum of the departure wave.

Preferably, the two pump waves P1, P2 have substantially the same power Regardless of whether or not they are equal, each of the two po-ers is preferably greater than 1 niW, eog. lying in the range 1 niW to 5 mW.

The mixing segment preferably has a length greater than 1 km, e.g. about 10 km, Typically, it is constituted by a single optical fiber.

Preferably, the mixing output filter 18 has rejection greater than about 20 de for the pump waves Pl, P2 and the upstream wave Wi.

Advantageously, the pumping system includes an optical amplifier 20 kor amplifying both the two pump waves P2 and also the upstream wave Wi. Typically, the line L includes other amplifiers such as 22. It may further *000 include a plurality of mixing assemblies distributed over its length.

On manufacturing the pumping system for a future optical link, at least one of the two pump sources G1, e.g. single-frequency semiconductor lasers, and the mixing output filter 18, e.g. of the Fabry-.Perot type, are chosen to be tunable. Such tunability enables in particular the middle pump frequen~cy no to be subsequently made equal to the neutral frequency of a mixing fiber 6 (once the neutral frequency has been defined).

in the above-described example, the pump frequencies VP1, VP2 lie in the range delimited by the upstream carrier frequency vs and the downstream carrier frequency va, i.e.

the difference vP2 vPl between the pump frequencies is 8 Jess than the dif~erenc~ Va vs between the carrier frequencies. However, ~t I.s to be understood that the differente vP2 VP1. cQuld be greater than Va vs.

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Claims

9.a *0 9r* 9 *9C 99 9 A 4 k A BS TR A CT Light waves carrying a signal to be transmitted are guided along a line made up of optical fibers having chromatic dispersion. To compen-late for the effects of tho dispersion, a four-wave mixing phenomenon is caused at at least one intermediate point along the line by pump light injected into a mixing fiber*. According to the invention, the pump light is constituted by, two pump Waves, the polarizations of the two waves being orthogonal to each other, their optical frequencies being substantially mutually symmetrical about a neutral frequency cancelling the chromatic dispersion of the mixing fiber. The invention is applicable in particular to long-distanoie links. 0,.