CA2257495C - Optical transmission systems using optical amplifiers and wavelength division multiplexing - Google Patents

Abstract

The invention provides a method and system for transmitting very high bit rates over an optical link, using unidirectional and bidirectional WDM
technology. The 1550 nm window for optical transmission is spatially separated into two bands, "Red" and "Blue", and the channels in each band are selected so that respective wavelengths present a substantially equal gain tilt. A
fiber amplifier designed according to spatial separation and wavelength selection of the invention is disclosed, along with multiple span WDM network topologies.

Description

WO 97/S0203 PCI/C~96/00810 OPrlCALTR~ S il~.~ SYSTeMS USING omc~L ~s ~ND
WAVELENGTH L lvl~lON MULTIPLEXlNG

BACKGROUND OP THE INVENI~ON

Field of the Invention The invention is directed to optical transmission ~yS~ s and methods for Ir-~-c~ s;~n of information over optical networks, and more particularly to optical trAn~ni~Csi~n systems and methods that use optical amplifiers and wavelength division multiplexing.

Background Art The ~Jel[O~ AnCe of standard reach optics limits the spacing between the line terminAting eqllir~erlt (LTE) and regenerating equipment to a~ro)~ AtPly 80 km (20 dB at 0,25 dB/km) on non-dispersion shifted or dispersion shifted optical ffber. The 80 km limitation is caused by physical degradation of the transmitted optical signal due to optical dispersion and optical attenuation. The dispersion and attenuation limits, res~e.lively, can both be extended to beyond 80 km with the introduction of external modulation, use of dispersion shifted optical fiber, optical amplifier technology and wavelength division multiplexing (WDM) technology.
When designing multiband trAn~mi~sion ~y~lellls, three significant issues to consider are the number of amplifiers required (1), gain tilt (2), and ~l~otecLon (3).
(1) Optical amplifiers are e~ sive units and so the number and types of units required to implement a given data connection is an important design parameter for an optical network.
There are three general types of optical amplifiers: post-amplifiers that connect to a tr~n~mltter to boost the output ~o~vel; line amplifiers that amplify along the route; and preamplifiers that improve the sensitivity of optical receivers. These different types of amplifiers provide dilferent output powers, use different input power ~ levels, and generally have different noise figure requirements.
One way of reducing the n-~mher of optical amplifiers is to use the wavelength division multiplexing (WDM) technology. Use of the WDM technology reduces the strands of optical fiber cable needed to establish a communication link, and it can provide manifold capacity WO 97/!;0203 rCI'/CA96100810 f-yr~nQ~n on ~icting fiber links. Its potential for routing c~ lc is equally i,~.~orla,lt.
For eY~mrle, the WDM filters pe.~l~l. the flm~hlon of coupling the pump source laser wavelength to the ~biuln doped fiber. Three-S port WDM filters are currently used to couple multiple wavelengthsinto and out of the transn i~Sion ffber.
A four-port WDM coupler for implementing a bidire~iorl~l optical amplifier module using a unidirectional optical amplifier is rli~lQserl in United States Patent No. 5,452,124 (Baker, issued 10 "~t~l.ber 19, 1995 and assigned to Williams Telecornm~lnirA~ions Group).
Isolators are also equipment used in WDM SySl~ s, and they function to allow an optical signal to pass in a single direction. If optical isolators are used internal to an optical amplifier, then they 15 make the ~mplifier an inherently unidirectional device. It is known to use isolators inside gain regions of an optical amplifier. United States Patent No. 5,280,549 (Barnard et al, issued on Januar-y 18, 1994 and assigned to National Research Council of ('~nAA~) discloses a frequency depen-l~nt optical isolator which allows signals to pass in one direction 20 only, so that two signals may be i~ol~te~ according to their frequencies.

(2) The use of ~,~,iu", doped fiber arnplifiers (EDFA) for multichannel, bidirechon~l tran~ sion is current practice. Of great importance in network applications is the configuration of the optical amplifier and what signal wavelength to use in conjunction with the 25 pump wavelength. P~ec~llce the ~mp!ifier gain is not y~lfeclly flat for all irlco~in~ wavelengths, the precise wavelengths to use is a function of the gain vaAations of the different available pumps. Gain tilt is a signific~nt issue to consider when ~1esi~ning multiband transmission sybl~ s. Gain tilt measures the change in the profile of t-h-e gain for 30 each transmission l~h~nn~l at the actual value of the gain of the amplifier module, with re~ecl to the gain profile at the nomin~l value of the gain, i.e. at the value for which the amplifier is ~esignecl In other words, the gain tilt function varies with link loss. This function depends only on the physics of the dopant in the host fiber glass, and is 35 of interest when signals of more than one ~ nnel or direction share the same fiber.

CA 02257495 l998-l2-03 WOg71S0203 rcr/cAs6/ooslo No chemical soll~ticns have yet been found for addfessi.lg the gain tilt problem. Do~r.~, fluoride, etc can help flatten the gain profile, but do not solve the tilt. Electronic solutions are c~ lly under investigation.
One solution is "gain ~l~mpin~ which means m~int~inirlg the amplifier gain constant on all ch~nnels with an idler or lasing.
Ho~.~.,ei, this soh~tion requires use of twice the number of laser pumps to provide the necessary extra photo~.
Another sol~ti~n is '1OSS p~A~lin~", which irnplies tuning the loss of each span to match the nominal value for the amplifier or, in other words, to operate all amplifiers of the link at their nolnin~l gains.
This solution has the disad~lz t~ge of re~ ir...g attenuators to be placed in each span, and is not very robust in the presence of variations in losses and optical l,owers in the system over time and temperature.
"Gain clamping" methods combined with "loss p~lin~"
slightly i~ JrOVe the system's robustness, at the price of much more expensive pump lasers.
Another solution to address the gain tilt problem is to use an adjustable optical filter. The relative loss between different 20 wavelengths could then be r ~ Rte~l by a mechanical or electrical control. The best location for such a filter is inside the amplifier. The fflter requires adjustability in the field or, better yet, to be continuously controlled by measuring each wavelength power level. These filters may become more affordable in a few years, but they are currently very 25 eA~nsive and thereror~ not used.
The prior art fails to provide cost effective solutions for amplification of bidirectional multi-channel optical signals. In addition, efre~t;ve ir~pl~mentation of four-port WDM fflters is difficult because of the loss introduced by the filter, gain tilt and ~oteclion 30 problem~. The prior sol-~ti~I s and configurations are not concerned with control of the gain tilt or with p r~lection of tr~nCTni~sion in multi-rl ~nnel amplifiers.

(3) To ensure the desired av~ ity of network connections is maint~ e~l, it is shndard practice in the telecomnlunication industry 35 to implement re~ n~l~nt equipment so that should one unit fail, another can be rapidly swilcl-cd into place. This is called protection swit~hing. The number and the type of amplifiers that need to be held WO 97/S0203 PCT/C~96/00810 as repl lc~nt unib is also in.~ nt. Reducing the number of dif~ t types of e.~ .e,~t in a network reduces the number of types of spare units and, thro~lgh sPndqrd sparing statistics, siE~ifi~Antly reduces the total number of spare units that the network o~rator must purchase and have readily at hand.
Four general types of l,;oteclion switching are known: "1~1"
,role.Lon, wl.erel"~ one set of e~lui~lent protectc another set of equi~ ~nt on a m~t~hed pair basis; "1:N" ~-ote.lion, whereby one set of equipment l,.otecls N other sets; "ring" yroleclion; and "cross-10 connect" p;otection P~o~e.lion ~.otocols can be configured as "bidirectional switching" and "unidirection~l switching". The protection protocol has nothing to do with the direction of trar.cmis6ion on the fiber; it is just the switching ~fotocol type. Telecommunication traffic may be 15 bidirectional in nature, as for example, voice circuits, or unidirection~l, as for example, CATV sign~ls~ Bidirection~l traffic means that the data is transmitted in both directions. Bidirectional also means that while a given circuit is interrupted in one direction, there is min;m~l penalty to interrupt the other direction of the same circuit.
A unidirectional 1,rolec1ion switch switches only one direction of a circuit, namely, that dire~lion requiring protection if only one direction is degraded. In contrast, a bidirectional ~;ote~tion switch tries to switch both directions together in all cases.
A ring topology with "1+1" protection offers si~nific~nt 25 advantages in compa ison with a linear, or "1:N" topology. When more than one wavelength is carried by one optical amplifier and only one signal at a time can be },.vle~ted, such as in a "1:N" system, then when that amplifier fails some of the signals will not be ~7ro~e~ted.
This severely impairs the availability of circuits carried within those 30 sigJ-ls. The methods below allow signals with multiple wavelengths through one ~mrlifi~r to be efflciently protected.
Electrical, and soon optical, cross-connects can implement the above and more general ~roteclion or restoralion topologies.
Ho~.~vel, cross-connects are not generally as fast to ~rotecl as trans-nicsion equipment implenlentirlg the above three ~roteclion methods, and are therefore generally used to implement restoration rather than protection.

WO g7/S0203 PCTtC~g6/00810 SUMMARY OP ~HE INVEN'IION
It is an object of the present invention to provide multiple wavelength trAncm. i~io~ systems with optical amplifiers and wavelength division m~ iplexing, which reduce or alleviate some or 5 all of the drawbacks encountered with the prior art solutions.
It is another object of this invention to provide an ill~ro-ved approach to the design of multi-~ h~nnel tr~ncmicsion SyS~l~S for allowing mllltiple wavelengths to be amplified by the same optical amplifier and tr~ncmitte~l over the same optical fiber.
Still another object of this invention is to provide multiple wavelength systems with a re~-~re~ I llm~er of amplifiers and other optical components such as dispersion compPns~ior modules, which allows the S)~St_.l- to tolerate more loss from the outside plant fiber cable.
Yet another object of the invention is to provide multiple wavelength systems with a reduced number of optical components which implicitly ~luire a re~-lce-l number of spare components for protection f~ ities.
Accordil~gly, the invention is directed to a wavelength division 20 multiplexing (WDM) optical ~n~rlifjer module comprising a first peripheral gain region for amplifying a first multi-channel optical signal; first splitter means connected to the first peripheral gain region for spatially separating the first multi~ nel optical signal into a first band optical signal and a seccn~l band optical signal according to the 25 wavelength; a central gain region for re~ivillg the first band optical signal and the secor~d band optical signal, separately amplifying same, and providing an amplified first band optical signal and an amplified second band optical signal; serond splitter means for recombining the amplified first band optical signal with the amplified second band 30 optical signal into a seconrl multi-rhqnnel optical signali and a second peripheral gain region connected to the secon~l splitter means for amplifying the secol ~1 multi-ch nnel optical signal.
According to ano~er aspect, the invention comprises an optical bidirectional regenerator for multi-~ h~nr~el telecommunication using 35 wavelength division multiplexing, comprising first splitter means for spatially separating a first multi-rh~nnDl optical signal into a first band optical signal and a second band optical signal according to the WO 97/S0203 PCI/CA96/008~0 wavelength; a bidirectional regenerator mo-llllP for receiving the first band optical signal and the second band optical signal from the ffrst splitter means, separately amplifying same, and providing an ~mplifiell first band optical signal, and an amplified secon~ band optical signal;
5 and secor~l splitter means for recQ~nhining the amplified first band optical signal with the amplified second band optica} signal into a second multi-rh~nnPl optical signal.
The invention also provides a method for tr~rlsrnittin~ multi-l~h~nnel optical ~ ls bel~n a first and a seconA site connected by an 10 optical tr~ncmi~ion path inc~ ing a fiber amplifier, co~ .ising the steps of spatially ~~araling the gain region of the fiber amplifier into a first gain region and a second gain region; associating the first gain region with a first tr~nCInission band and the second gain region with a second transmission band; transporting a first optical signal in the first 15 transmission band; and transporting a second optical signal in the second transmission band.
In addition, the invention provides for a method for transmitting multi-channel optical signals between a first and a second node of a transmission link comprising the steps of defining a first 20 tr~r~cmi~sicln band ~ssori~te~l with a first direction of tr~nsmi~sion and a second transrnission band associated with the opposite direction of transn~icsion; transferring, I,elweell the first node and a first end of a fiber span, a first multi-channel optical signal comprising a first optical signal S1m, where m~ [1,M] and M is an illteger, in the first 25 transmission band, and a secon-l optical signals S2k, where k~ [1,K] and K is an integer, in the second tr~n~mi~.cion band; spatially separating on the fiber span the first optical signal S1m along a first unidirectional route, and the second optical signal S2k along a seco~l unidirectional route, in accordance with the wavelength; separately amplifying the 30 first optical signal S1m and the second optical signal S2k; and combining the first optical signal and the second optical signal to form a secon~i multi-~AnnPl optical signal and transferring same between the tr~ncn~ission link and the second node.
A major advantage of the optical trancmi~sion systems according 35 to this invention is that multiple wavelengths may be implemented in the 1550 nm window of the band of an Erbium doped fiber amplifier (EFDA) without severe gain tilt impairments. This allows reduction of WO g7/S0203 PCIICA96100810 the nllmhPr of dirre.~,lt types of e~lui~luent in a nel~.o.~, which in turn reduces the number of types of spare units, and through standard sparing statistics, si~ifie~tly reduces the total number of spare units that the network ol,elalor must purchase and have readily at hand.
For P~mple, four ch~nnelc per direction may be su~l.u. led with a configuration using spatial separation and WDM wavelength choice according to this invention.
Another major advantage of the optical tr~ncmicsion sy~l~.,.s according to this invention is that reduced fiber counts need to be used for deploying such systerns, which is especially important when there is a lack of fibers on a given route. As a result, ~ C~icsion sys~ s accordil.g to this invention also reduces the system cost, when more than two wavelengths are amplified per amplifier. The re~ e~l impact of the loss of the other optical components allows the syslem to tolerate more loss from the outside plant fiber cable.

BRIEF DESCRlmON OP T~ DR~WINGS
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the ~re~l~ed embo~limentc, as illustrated in the appended drawings, where:
Figu~e lA sho ws a configuration of a ~a,.smission ~tLm using three-port WDM splitters;
Figure lB shows a configuration of a tr~n~micsion system using a four-port WDM splitter;
Figure 2A shows the gain for two channels when the amplifier operates at the norninal gain;
Figure 2B illustrates the gain for the two channels of Figure lA
when the amplifier module operates at a gain lower than the nominal gain;
Figure 2C is a s~h~m~tiC graph showing the gain tilt per dB of gain change for the bands of u-leresl, Figure 3 illustrates a block diagram of an optical amplifier using spatial separation;
Figure 4 is a block diagram of a bidire~ion~l regenerator;
Figure 5 shows three eY~mplPc of selection of the wavelength for multi-channel optical tr~n~micsion systems;

WO 97/S0203 rCT/CA96/00810 Figure 6 is a m~ iple span optical tr~nsmi~sion sy~
configuration using common pre/post WDM amplification modules;
Figure 7A is a paired optical tr~n~mi~sion system showing the econoTny in the fiber spans for worl~lg and prote.tion links of a 1:1 linear configuration;
Figure 7B illuslrdles paired two st;~cl~ed, two fiber rings using WDM amplifier nno~t-les; and Figure 7C is a paired four-fiber ring configuration using WDM
amplifier modules.
DES~ OF TH~ r~hh~V EMsoDIM~T
WDM couplers or splitters are filters used to separate or combine optical si~n~lc acco.~iing to their wa-velength. For example, a WDM
coupler is used to couple the light from the laser source into the gain region of an EDFA. These couplers are very useful in multi-rh~nnel optical s~ s, however, the isolation requirementc in the WDM
could be â severe issue due to the multi-path i"te,fer~ l-ce from imperfect isolation of the splitters. Three-port conventional WDM
couplers may be used in a configuration as illuskated in the example of Figure lA. Figure lA shows a bidirectional conr~ec~ion between sites A
and B where rh~nnelc of wavelengths ~ 1 and A 2 use the same fiber 1.
WDM sFlitter 2 at site A connects trarlcTnit~r Tx1 to fiber 1 through ports 4 and 3, for launching optical signal Sl of wavelength ~1 on fiber 1. WDM splitter 2 also connects fiber 1 tO receiver Rx2 through ports 3 and 5, for ~lire~t;l~g optical signal S2 of wavelength A 2 from fiber 1 to receivel Rx2. Simil~rly~ WDM sp}itter 6 is connected with port 7 to fiber 1 and with port 8 to transmitter Tx2 for launching optical signal S2 on fiber 1, while port 9 is connecte~ to receiver Rx1 for directing the optical signal S1 from aber 1 to receiver Rx1.
The four-port filter ~lisrlose~ in the previously i~l~ntifie~l U.S.
Patent No. 5,452,124 (Baker) may be used for obtaining bidirectional - amplification using a unidirectional amplifier. A bidirectional system configuration using such a four-port WDM splitter is illus~ated in Pigure lB. Sites A and B have a similar configuration with that of Figure lA. Optical signal S1 of wavelength ~ 1 travels from site A to site B, while optical signal S2 of wavelength A2 travels in an opposite direction of propagation, namely from site B to site A. The four-port WO 97/50203 PCl~/CA96/00810 WDM splitter 10 is provided at a site C, logelhel with a unidirectional optical amplifier 15. ~litler 10 routes the traffic so that both channels have the same direction in the gain portion of the amplifier 15. Thus, signAIs S1 and S2 rec~iv~d at ports 11 and 12, ~es~eclively, are routed in 5 the same direction to exit at port 13, which is connecte~ to the input port of optical AmFlifier 15. The amplified signals S1 and S2 arrive at port 14 of the WDM filter 10 in the same direction of trar~C~nip~;on and are launched on fiber 1 in the ap~lo~,.iate direction, ac~vrding to the wavelength A 1 or ~ 2-The r l~mher of channels and the wavelength of each channel are i~ t design ~arameters for optical transmission systems. It has been noted that the gain of an optical amplifier rhAn~s when the power at the input is ~lir~er~nt (lower or higher) than the nominal power for which the ~n~r~ pr was designed. This change in the gain 15 with the input power, .l~firle.l as "gain tilt", is also dependent on thechannel wavelength. As an example, Figure 2A shows the gain of tr~ncmis~ion ch~nnels 1 and 2 when the amplifier module operates at the norninal gain. Figure 2B shows the changes in the gain of channels A l and A 2 when the actual value of the gain is lower than the nominal 20 value. Operating at a higher gain would produce a profile with the G},~o~ite slope.
Today's optical amplifier te~ hnol~gy works well for single wavelength amplification. However, the gain curve across the 1550 nm passband of installed, non-dispersion fiber is not flat, especially in the 25 1530 to 1542 nm range. Figure 2C shows the change in gain at each wavelength if the gain at 1545 nm is increased by 1 dB. For example, the gain at 1532 nm would go up 1.8 dB for each 1.0 dB increase at 1545 nm.
This is 1.8-1.0=0.8 dB of tilt/dB of change in span loss.
WDM is not generally used in the 1530-1542 nm range because of 30 the gain peak with silica-based EDFAs which causes per-channel output power to vary greatly in this region and seriously affects signal gain outside this region when several amplifiers are ~acc- le-1 The current solutions to address this ~.o~lcm include selectively varying the input power for each channel, or staying in the region of flat gain. The first 35 solution causes a significant reduction of the output power of the individual channels, and makes the span engineering ~liffic~lt. The second solution limits the number of channels that can be multiplexed, .

WO 97/S0203 PCT/C~96/00810 and also pi~v~,.ls the reuse of the large ~mhe~efl base of OC-48 transmitters, without the ~ itiorl of e~ ive wav~ gth adapters.
Accor.ling to this invention, the optical Amrlifj~r gain region of about lS28 nm to 1560 nm is split into two bands: "Blue" for 1528 to 1543 mn, and "Red" for 1547 to 1560 nm. The two bands are spatially separated for amplificAticn The wavelength within a band is chosen to be in the regions of similar gain tilt, or to have mini~nllm variation of the gain tilt/dB in the res~eclive "Red" or "Blue" band. This is especially iln~o~ lant for the "Blue" gain region, which has large variations in gain tilt, as shown in Figure 2C.
Figure 3 illustrates a block diagram of a bidirectional fiber Amplifier, as an example an erbium doped fiber ~mplifier (EDFA) 15 using spatial separation accor.ling to this inv~..Lion. In this disclosure, the terms "active optical fiber" and "gain region" ~lP~ign~te a length of 15 optical fiber doped with a rare earth elemPnt and pumped with coupled waves for amplifying optical si~nals As shown in Figure 3, the optical signals traveling within the central section of optical amplifier 15 are grouped in two bands, each for a direction of transmission in this example. The traffic in the Red 20 and Blue bands is physically separated on fibers 17 and 19 using a first three-port WDM splitter 21. S;gJ~A1C on fibers 17 and 19 are then recor.-hine-l by a second three-port WDM splitter 23. As a result, the amplifier has two peripheral gain spans 25 and 29, and two central gain lengths 27 and 31. The signAl$ travel in both directions at the periphery 25 of the amplifier in the gain spans 25 and 29 and in one direction in the central lengths 27 and 31. Isolators 33, 35, and 37 are also provided and oriente~l so as to obtain bidirectio~l operation between the bands.
Because the "Red" and "Blue" gain lengths are spatially separated, separate power control per direction is ~lefelably used, through 30 separate saturation of gain and through control of separate pump powers. }~mbo~lim~nts of the amplifier module 15 where the sign~lc travel in the same direction may also be designed, in which case separate or cornmon control per band may be used.
This architecture significantly reduces the effect of gain tilt 35 between the two bands and multi-path interference from im~.fe~l isolfltion of the two splitters. By using different directions in the central region of the EDFA, and also two spatially separated bands, the WO 971S0203 PCT/C~96100810 isolation requirements in the WDM splitters 21 and 23 are achievable.
In ~ on~ by SelJardling the bands within the ~ iripr and providing gain stages outside the separation, the noise figure and output power penalty due to the losses of the WDM couplers are re~
The optical fiber exhibits different dispersion characteristics en~lin~ on the wavelength used for tr~ns~icsion. For st~n~rd single mode fiber, the two main tr~n~miCsiQn windows of interest are at 1310 nm for non-dispersion shifted optical fiber, and 1550 nm for dispersion shifted optical fiber. When non-dispersion shifted optical fiber is used, the l.a~,sll-itter should ~.eÇerably be equipped with dispersion co.l.~..sating modules (DCM) for reducing the significant tr~ncn ission impairment at high bit rates.
Figure 4 is a block diagram of a bidirectional regenerator built in accordance with the spatial separation method of the invention.
15 Inserting a four-port WDM splitter 41 into a fiber carrying bidirecPo~ ~l WDM sign~l~ separates the incoming s~ lc a~~ordi..g to their direction of propagation. The signals in the "Blue" band arrive, for example, at port 11 on fiber 1', and the signals in the '1~ed" band arrive at port 12 on fiber 1. These signals exit at port 13 and are routed by 20 WDM splitter 41 in the same direction from port 13 to the input of a preamplifier 42, then to a dispersion compensating module 43. A
three-port WDM splitter 44 separates the signals accu~dil.g to their wavelength and routes them to receivefs Rx1 and Rx2 of block 40.
After re~..cration or add-drop multiplex functions are performed in 25 block 40, the signals are passed to two trancmitPrs Tx1 and Tx2. From the two trar ~mitters~ the sigrl~lc are combined with a three-port WDM
splitter 45, passed through another dispersion compensation module 46 if nee~ , and then amplffled by a post-amplifier 47. The four-port WDM splitter 41 receives the amplified signals at port 14 and inserts 30 these sign~l~ into the two fibers in the ay~lol,liate directions.
Pigure 4 also illustrates how the configuration compensates for the loss introduced by the WDM couplers and DCMs presently av~ilable SPlertion of the wavelengths within the Red and Blue bands is 35 also important in multi-channel systems for further reducing the gain tilt. Pigure 5 shows three examples of selection of the wavelength within a band, for obtaining up to eight wavelengths evenly split WO 9~/S0203 12 PCT/C~96/00810 L~t.,~e~n the two directiorlc. In the first e~ le, two channels in the "Blue" band and two ch~nnel~ in t-h~e "Red" band are sP1e~1e~l~ The wavelength of the first "Blue" ~hAnn~l iS ~ 1=1533 nm and that of the second "Blue" rhsnnel is A2=1541 nm. The wavelength of the first 5 "Red" channel is A3=1~49 nm and that of the second "Red" channel is ~4=1557 nm. The Lrr..~.ce in gain tilt for the two "Blue" ~h~nnelc is relatively large, and it is shown in Figure 2C at 0.65, while the difrer~.lce in gain tilt for the two '1~ed" channels is 0.1. A gain tilt of 0.65 will cause ~ub~ s in the system.
The secoll~ example shows another s~lection with two channels in the "Blue" band and two ch.~nn~ls in the "Red" band. The wavelength of the "Blue" channels is A 1=1528.7 nm and A.2=1533.5 nm having ayyroY;~ tely the same gain tilt of 1.75. The wavelength of the "Red" channels is A 3z1552.5 nm and A 4z1557.4 nm having a 15 diflLr~lce in gain tilt of a~,~.o~imately 0.1, and thus very little diflere..ce. This small gain tilt will not cause problems in the system of this example, because the wavelengths of the channels were sPlect~i to have a matched gain tilt.
The third example shows a system with eight ch~nn~lc~ with four channels in the "Blue" band and four ch~nn~ls in the "Red" band.
The wavelength of the "Blue" channels is ~ 1=1528.7 nm, l2=1530.3 nm, ~3=1531.9 nm and ~4=1533.5 nm, having a variation in the gain tilt of a maximum of 0.1. The wavelength of the "Red" channels is ~5=1550.1 nm, ~6=1552.5 nrn, ~7=15~4.9 nm and Ag=1557.4 nm, with a difference in gain tilt of a~,fo/~ t~ly 0.1.
This method of selection of the wavelength of a trancmission channel allows multiple wavelengths to be ampliffed in the Blue band of an erbium doped fiber amplifier without severe gain tilt impairments.
In addition, active equalization between wavelengths may also be ~Llail.ed by adjusting the tr~n~mitte~ power and also by providing in-line equalization devices when they become avAil~hle.
If multiple t~h~nnels are tr~ncn itte~3 on a single fiber span and amplified in the same mo~ e or mod~ s, the nurnber of amplifiers, fiber spans, and other optical colllponents between two terminals can be minirr~i7e~l. There are several ways of achieving this reduc~ion in fiber spans and amplifiers based on spatial separation of the WO 971S0203 PCI'IC~96/00810 trAn~mi~sion bands acco~ ,g to the direction of propagation, and on the p~oper selection of signal wavelength. Some examples will be pr~senteA in the following.
A receiv~ typically requires -SdBm of optical power and 5 overloads at 0 d8m, and a l~ c~itpr is typically required to launch into the o~hi~lP plant fiber +7 or ~10 dBm. These design parameters make it difficult or i...~ossible to design an optical Amp!ifier which sim~ltAneously operates as a pre- and post-ampliaer. In A~ldition, these requirements are a8,giavated by the presence of gain tilt. To obtain 10 these power levels, the ~Ull~t transmitters are provided with an optical amplifier configured as a post-ampliffer and the receivers are provided with an optical amplifier configured as a pre-amplifier.
However, a single fiber Amrlifj~r may operate simultaneously as a pre- and post-Amrlifier ac~or~il.g to this invention, by providing 15 separate power control per direction and selecting the wavelength of the chAnnelc for minimi~.ing the gain tilt.
Figure 6 shows a configuration with bidirectional amplifiers operating as line ampliffers and as combined pre- and post-amplifiers.
In this conffguration, there are four si~lc, A l~ ~ 2, A 3 and A 4, in the 20 "Red" band, and one signal, ~ 5, in the "Blue" band. The DCM 43 and the WDM splitter 2 at site A are separated from the optical cable 62 by the gain of the pre/post-amplifier 60. Similarly, the DCM 45 and the WDM splitter 6 at site B are ~parated from the optical cable span 63 by the gain of the pre/post-amplifier 61. This reduces the impact of the 25 optical loss of these components on noise and on power level.
Optical amplifiers 60 and 61 are provided with separate power control per direction, so that amplifier 60 o~erates as post-amplifier for trAnf.mitt~rS Tx1-Tx4 and as pre ,-nnFl;fier for receiver Rx. SimilArly~
Amplifier 61 operates as a pre-A...~Jlifier for rece.v~.s Rxl to Rx4 and as a 30 post-amplifier for transmitter Tx.
A~,o~.iate selectiQn of wavPl~r~tll colnhine 1 with the spatial separation of the ~h~nn~l$ acco~ g to this invention, allows for fur~er ~ plifit~A~tion of the configuration of a multi-channel ~y~te~
Thus, if the nl-mber of transmission ch~nllels in one direction equals 35 the number of rhAnnel~ in the opposite direction, a cost el~eclive network may be obtained by reducing the n~lml~e- of proleclion links.

WO 97/S0203 rCr/C~g6/00810 Figures 7A, 7B and 7C illustrate systems with an equal number of S~ c in both bands, here called bidire.lioal pairing. Thus, for each trar s~itpr receivei path in one direction, there is a recei~,~r-tr~nsn~itter path in the ol,~Gsite direction. This configuration has s~ fic~nt advantages and reduoes the ntl~h~r of ~;ote-lion links.
With an a~o~liate flattening filter, a WDM amplifier module as shown in Figure 3 can be ~eSiKr~l to su~l~orl an equal number of wavelengths in both dir~lions. It is rec~,lu,~ended that working and protection groups be pro~,;.s one(~ through separate amplifiers in order to ~ .Lt exposure to a single amplifier failure.
Figure 7A is a balanced bidirertior~l optical t~ sil,ission ~yste~
showing the economy in the fiber spans for working and ~rote.~on links of a linear configuration of a single 1:1 system. One set of amplifiers 52 and 53 is equipped to accommo~te the working traffic on fiber 1. The ~rolection trafflc takes place on a s.l,arate route 20 using amplifiers 54, 55, 56, and 57. No more amplifiers are necessary to support a second 1:1 system since the amplifiers support two "Red"
band and two "Blue" band wavelengths. Further systems may be provicioned to use the same fiber and amplifiers, provided that a~,rop-;ate wavelengths are available and the link budget can be met.
C'o~pA-ed to a system that does not use this pairing, such as a unidirectional WDM system, a paired bidirectional system requires fewer ch~nnels to carry and ~.ote.~ a given amount of data. Fewer channels means fewer fibers, fewer amplifiers, and fewer terminal electronics required to carry the same data.
Figure 7B is a configuration of tWo-st~rl~e~l~ two-fiber rings using WDM amplifier modules as shown in Figure 3. Because of the bidirectional nature of the amplifiers, only one fiber per span is required to implement a base system. No more amplifiers are thereafter required to implement a second system. Further ring Sy~ S can be provisione~1 to use the same fiber and amplifiers, if a~,o~.;ate spatial separation and paired wavelength selection are used, as ~iscllsse~l in connection with Figures 2C and 5, and the link budget can be met.
Pigure 7C is a four-fiber ring configuration using the WDM
amplifier modules. A four-fiber ring may be converted to a "two-fiber ring with span switching". This differs from a normal two-fiber ring in WO 97/S0203 15 I'CTICA96/00810 that the four-fiber ring protocol is impl~merlte-l, allowing such features as span switching.
While the i~v.ntion has been described with refe,~.lce to particular example embodiments, further mo~ifi~tiQns and 5 impro~ .ents which will occur to those skilled in the art may be made within the purview of the appended claims, without departing from the scope of the invention in its broader aspect.

Claims (14)

WHAT IS CLAIMED IS:

1. A method for transmitting multi-channel optical signals between a first and a second node (A, B) of a transmission link connected over an optical fiber, comprising the steps of:
defining a first transmission band associated with a first direction of transmission and a second transmission band associated with the opposite direction of transmission;
selecting K forward channels within said first transmission band and M reverse channels in said second transmission band, all said channels having a substantially similar gain tilt/dB;
transferring, between said first node (A) and node (B), a first multi-channel optical signal comprising all said K forward channels in said first transmission band, and a second multi-channel optical signal comprising all said reverse channels in said second transmission band;
providing a WDM optical amplifier module (52) between said node (A) and said node (B) and spatially separating the gain region of said optical amplifier into a first and a second gain region; and separately amplifying said first multi-channel optical signal and said second multi-channel optical signal in said first and second gain regions separately.

2. A method as claimed in claim 1, wherein said step of defining comprises selecting said first band in the blue region of the spectrum and selecting said second band in the red region of the spectrum.

3. A method as claimed in claim 1, wherein said step of defining further comprises selecting a first sub-band in said first band and a second sub-band in said second band, each sub-band characterized by a substantially constant gain tilt/dB.

4. A method as claimed in claim 3, wherein all said K forward channels of said first multi-channel optical signal are selected in said first sub-band and all said M reverse channels of said second multi-channel optical signal are selected in said second sub-band.

5. A method as claimed in claim 1, wherein said step of amplifying further comprises separately controlling the gain of said first multi-channel optical signal and said second multi-channel optical signal.

6. A method as claimed in claim 1, wherein said step of selecting comprises:
preparing a gain tilt/dB graph for said first band; and selecting on said graph said first band wavelengths to have substantially similar gain tilt values on the abscissa.

7. An optical bidirectional regenerator for multi-channel telecommunication using wavelength division multiplexing, comprising:
first splitter means (44) for spatially separating a first multi-channel optical signal into a first band optical signal and a second band optical signal according to the wavelength;
a regenerator module (40) for receiving said first band optical signal and said second band optical signal from said first splitter means, separately amplifying same, and providing an amplified first band optical signal, and an amplified second band optical signal; and second splitter means (45) for recombining said amplified first band optical signal with said amplified second band optical signal into a second multi-channel optical signal.

8. A regenerator as claimed in claim 7, wherein said regenerator module comprises a first regenerator module (RED Tx, Rx) for amplifying said first band optical signal and a second regenerator module (BLUE Tx, Rx) for amplifying said second band optical signal.

9. A regenerator as claimed in claim 7, further comprising:
a first unidirectional optical amplifier (42) connected to said first splitter means for pre-amplifying said first multi-channel optical signal; and a second unidirectional optical amplifier (47) connected to said second splitter means for pre-amplifying said second multi-channel optical signal.

10. A regenerator as claimed in claim 7, further comprising a WDM
router (41) connected to an optical transmission link for reconfiguring a bidirectional input signal into said first multi-channel optical signal and for reconfiguring said second multi-channel optical signal into a bidirectional output signal, which is an amplified version of said bidirectional input signal.

11. An optical multi-channel telecommunication system using optical amplification and wavelength division multiplexing, comprising:
K first band transmitters (Tx1-Tx3) at a first site (A), a transmitter T k, where k ~ [2,K] and K is an integer, for launching an associated first optical signal S1k of a wavelength selected in a first transmission band associated with a direction of transmission;
M second band receivers (Rx) at said first site (A), a receiver R m, where m ~ [2,M] and M is an integer, for receiving an associated second optical signal S2m of a wavelength selected in a second transmission band associated with an opposite direction of transmission;
first splitter means (2) for routing said K first optical signals S1 k and said M second optical signals S2m between an optical transmission path (62,52,53,63), said first band transmitters T k, and said second band receivers R m, according to the wavelength;
M second band transmitters (Tx) at a second site (B), a transmitter T m for launching said associated second optical signal S2m;

K first band receivers (Rx1-Rx3) at said second site (B), a receiver R k for receiving said associated first optical signal S1k; and second splitter means (6) for routing said K first optical signals S1k and said M second optical signals S2m between said optical transmission path (62,52,53,64), said K first band receivers, and said M second band transmitters, according to the wavelength.

12. A system as claimed in claim 11, wherein said optical transmission path comprises a plurality of WDM optical amplifier modules (52,53) series connected on a working fiber (W) for amplifying said K first optical signals and said M second optical signals.

13. A system as claimed in claim 12, wherein K=M.

14. A method for transmitting multi-channel optical signals between a first and a second site connected by an optical transmission path including a fiber amplifier, comprising the steps of:
spatially separating the gain region of said fiber amplifier into a first gain region and a second gain region;
associating said first gain region with a first transmission band and said second gain region with a second transmission band;
selecting K forward channels within said first transmission band and M reverse channels in said second transmission band, all said channels having a substantially similar gain tilt/dB;
transporting a first multi-channel optical signal in said first transmission band; and transporting a second multi-channel optical signal in said second transmission band.