WO2001045304A2

WO2001045304A2 - Per-channel optical amplification using saturation mode

Info

Publication number: WO2001045304A2
Application number: PCT/US2000/033781
Authority: WO
Inventors: Chaoyu Yue; Paul Green
Original assignee: Tellabs Operations, Inc.
Priority date: 1999-12-15
Filing date: 2000-12-15
Publication date: 2001-06-21
Also published as: ATE306150T1; ES2250228T3; CA2394237A1; CA2394237C; EP1240736A2; US7072585B2; AU3635501A; US20040179846A1; DE60023033D1; EP1240736B1; WO2001045304A3; WO2001045304A9; US6735394B1; DE60023033T2

Abstract

An optical communication system includes a plurality of optical channels, each of which passes a single optical wavelength signal. Each of the plurality of optical channels includes an optical amplifier which is controlled to operate at a predetermined output power level independent of channel wavelength and input power level by operating each optical amplifier in a saturation mode. Pumping power for operating each optical amplifier in the saturation mode is supplied from shared optical pumps or a plurality of one per channel optical pumps.

Description

TITLE

PER-CHANNEL OPTICAL AMPLIFICATION

USING SATURATION MODE

Field of the Invention

The invention is in the field of optical

telecommunications, and more particularly, pertains to

an optical communication system in which individual

channel output power levels are equalized independent of channel wavelength and input power level .

Background of the Invention

In Wavelength Division Multiplexed (WDM) optical links it is difficult to assure that signals arriving at each

channel ' s photodetector have a power level that is

within the receiver's dynamic range. Even for simple

point-to-point links, flattening filters are used in the Erbium Doped Fiber Amplifiers (EDFA's), MUX/DEMUX components' profiles of attenuation vs. wavelength must

be trimmed, and the system must be carefully monitored to ensure that large inter-channel differences in concatenated connector and splice losses are not

accumulated.

Typically, all WDM channels are amplified in a single amplifier, with the single amplifier being optimized

for gain flatness. However, there are different power

levels in each channel due to differences in

accumulated channel losses at different frequencies. Variable Optical Attenuators (VOA's) are used in the

respective channels to compensate for the losses. The VOA's require frequent adjustment to maintain required

power levels, and if the power level in a given channel drops below a minimum level, a transponder is required in the line to increase the power level to the required

level .

Thus, there is a need to be able to automatically

readjust the power level on a per-channel basis so that the photodetector at the optical receiver receives a signal with an adequate Optical Signal to Noise Ratio (OSNR) and amplitude to achieve a desired Bit Error Rate (BER) , but not so high a power level that the optical receiver or the electronics to follow are saturated.

Summary of the Invention

In view of the above, it is an aspect of the invention

to adjust the power levels in an optical communication system on a per-channel basis.

It is another aspect of the invention to adjust the power levels in an optical communication system on a per-channel basis by including in each channel an

optical amplifier which is operated in the saturation

mode .

It is yet another aspect of the inventions to adjust the power levels in an optical communication system on a per-channel basis by including in each channel an optical amplifier, with each such amplifier receiving a predetermined pump power for operating each such amplifier in the saturation mode.

It is still another aspect of the invention to connect Optical Line Terminals (OLT's) back-to-back at their respective pass-through interface channels, with each

channel including an optical amplifier, with each such amplifier receiving a predetermined pump power for operating each such amplifier in the saturation mode.

It is still yet another aspect of the invention to adjust the power levels in each output channel from a demultiplexer in a WDM optical communication system on a per-channel basis, with each such output channel including an optical amplifier, with each such amplifier receiving a predetermined pump power for

operating each such amplifier in the saturation mode, with the pump power being provided from either a predetermined power per-channel pump for each amplifier, or a single shared pump which supplies the

predetermined power to each channel amplifier. It is a further aspect of the invention to adjust the power levels in each input channel to a multiplexer in a WDM optical communication system on a per-channel basis, with each such input channel including an optical amplifier, with each such amplifier receiving a predetermined pump power for operating each such amplifier in the saturation mode, with the pump power being provided from either a predetermined power per- channel pump for each amplifier, or a single shared pump which supplies the same predetermined power to each channel amplifier.

It is yet another further aspect of the invention to maximize the number of optical hops in an optical ring network by equalizing the output power level in the respective channels due operating the respective

channel amplifiers at a predetermined power level by operating the amplifiers in the saturation mode.

It is still yet another further aspect of invention to prevent lasing in an optical ring network by operating an amplifier in each channel at a predetermined power level which can't be exceeded, such that one channel can't rob another channel of power due to the one channel ' s wavelength traversing the loop without being dropped.

Brief Description of the Drawings

Fig. 1 is a block diagram of a prior art optical communication system;

Fig. 2 is a block diagram of an optical communication system according to the present invention;

Fig 3 is a block diagram of a WDM optical communication system according to the present invention;

Fig. 4 is a block diagram of one amplifier constituting an optical channel according to the present invention;

Fig. 5 is a typical graph of power- in versus power-out for the optical amplifier 90 shown in Fig. 4; Fig. 6 is a block diagram of a plurality of optical

channels whose optical amplifiers receive pumping power from a shared optical pump;

Fig. 7 is a block diagram of how to couple a plurality of optical pumps to the optical amplifiers of a plurality of optical channels; and

Fig. 8 is a block diagram of a plurality of optical nodes connected in a ring configuration.

Detailed Description

Fig. 1 is a block diagram of a prior art optical communication system 10 in which an optical facility

signal comprising multiple channels of different wavelengths is input on a single fiber 12 to an optical amplifier 14 with flat gain which amplifies the input signal. The amplified optical facility signal is then demultiplexed by a demultiplexer 16 into its constituent wavelengths λl-λm, and is applied to an Optical Cross Connect Switch (OXC) or Optical Add Drop Multiplex (OADM) 18, and then to a multiplexer 20 which multiplexes the wavelengths λl-λm to form an optical facility signal comprising the multiple wavelengths λl- λm which is then amplified by an optical amplifier 22 which is identified to optical amplifier 14, which then outputs the amplified facility signal on output fiber 24. Wavelengths are not shown as being added/dropped in the drawing, however, this is understood by those skilled in the art.

In general, even though the optical amplifiers 14 and 22 have a flat gain, the amplitudes of the individual wavelengths are often different and require adjustment to attempt to equalize the gain of the respective channels. This equalization is typically accomplished using VOA's which are inserted in the respective channels. In addition, the OXC or OADM 18 introduces losses on the order of l-5db, which are reflected in the output power level of the respective channels. If the output power level in a given channel is below a threshold level, an expensive transponder is required to raise the power level above the threshold. Fig. 2 is a block diagram of an optical communication system according to the present invention, in which the output power of each channel is equalized independent of the channel wavelength and input power level. This is accomplished by including an optical amplifier in each channel which is controlled to operate at a predetermined power level, by operating each optical amplifier in a saturation mode. The optical amplifier is termed an "amplet" which is a low-cost optical amplifier using low-cost laser pumps, in comparison to

the amplifier and pumps used for amplifying multiple wavelength facility signals.

In Fig. 2, an optical communication system 30 has an optical facility signal comprising multiple channels of different wavelengths input on a single fiber 32 demultiplexed into its constituent wavelengths λl-λn by a demultiplexer 34, which are then applied to optical amplifiers 36a-36n, respectively in an OXC 37. Although Fig. 3 shows only one input and one output

fiber, each bearing n wavelengths, in general there may be more than one such input fiber and one such output fiber and associated demultiplexers and multiplexers, respectively. The output power level of each of the optical amplifiers 36a-36n is at a predetermined power level independent of channel wavelength and input power level due to those amplifiers also being operated in the saturation mode. This will be described in more detail later with respect to Figs. 4 and 5. The respective amplified channel wavelengths are then applied to the core 38 of the OXC 37, and then the respective wavelengths are applied from the core 38 to

optical amplifiers 40a-40n in OXC 37. The output power

level of each of the optical amplifier 40a-40n are each at a predetermined power level due to those amplifiers

also being operated in the saturation mode. The respective amplified channel wavelengths from OXC 37 are then multiplexed by multiplexer 44 into a multiple channel facility signal which is output on a single

fiber 44.

Fig. 3 is a block diagram of a WDM optical communica- tion system in which OLT's 50 and 52 are connected back-to-back to form an OADM. It is to be appreciated

that there is another OADM (not shown) for optical signal flow in the opposite direction. Demultiplexer 54 and multiplexer 56 are connected back-to-back via the channels including optical amplifiers 58, 60 and 62. A multiple channel facility signal is input on a single fiber 64 and is demultiplexed into its constituent wavelengths λl- λn Rn by demultiplexer 54. Wavelengths λl, λ2 and λ3 are amplified by amplifiers 58, 60 and 62, respectively, and are input to multiplexer 56. Wavelength λ4 is amplified by an optical amplifier 66 and is dropped off at a client equipment 68. Wavelength λn is dropped off at a client equipment 70 without amplification. A client equipment 72 provides a wavelength λ4 to multiplexer 56 via an amplifier 74, and a client equipment 76 provides an unamplified signal λm to multiplexer 56. The multiplexer 56 then outputs a multiple channel facility signal on a single output fiber 78. The client equipment may be any one of a computer, a SONET terminal, a telephone switch, a central office switch for telephones, a digital cross-connect switch, an end device such as a terminal, or the like. Each of the optical amplifiers 58, 60, 62, 66 and 74 are operated in the saturation mode so that their respective output power levels are at a predetermined power level. It is to be appreciated that the channels to client

equipments 70 and 76 may also include optical amplifiers .

Fig. 4 is a block diagram of a single optical channel according to the present invention. An individual wavelength λx is input on a single fiber 82 and passed by an isolator 84 to a coupler 86 which combines λx with the light output λp from a laser pump 88. The laser pump 88 has pumping power sufficient to cause

EDFA 90 to operate in the saturation mode so that its output power level is at a predetermined level. The amplified optical wavelength λx is then passed by an isolator 92 to a single output fiber 9 .

Fig. 5 is a typical graph of power-in (Pi) versus power-out (Po) for the optical amplifier 90 of Fig. 4. It is seen that for an input power level of -30db the output power level is -15db on the steep part of the curve, and for an input power level of -20db the output power level is -5db. Thus, it is seen that for a lOdb difference in input power level there is a lOdb difference in output power level, which difference in power level would have to be subsequently compensated for by a VOA or the use of a transponder in the prior art .

In contrast, it is seen that when operating on or near the flat portion of the curve the output power is substantially the same for different input power levels due to operating on the saturation part of the curve. For example, for an input power level of-lOdb the output power level is - 4db. Thus, it is seen for a

lOdb difference between input power levels of -20db

and -lOdb there is only a ldb difference between the output power levels of -5db and -4db, respectively. Accordingly, it seen that if amplifiers in different channels are each operating in the saturation mode their respective output power levels will be at a predetermined level which is substantially the same level for each amplifier.

This is seen more clearly with respect to Fig. 6 in

which four optical channels for four different wavelengths are shown. Each such channel is identical to the channel 80 shown in Fig. 4, with a shared laser pump 96 providing the same pumping power at λp to each of the isolators 86a-86d, to operate each of the optical amplifiers 90a-90d in the saturation mode so that their respective output power levels are at substantially the same predetermined power level independent of channel wavelength and input power level. It is understood that the shared pump 96 provides the same pumping power to each of the couplers 86a-86d via an optical splitter (not shown) .

Fig. 7 is a block diagram of another pump configuration

in which a plurality of optical pumps are coupled to a plurality of channel amplifiers via a coupler. Channels lOOa-lOOn include optical amplifiers 102a- 102n. Pumping power for the amplifiers 102a-102n are selectively provided by laser pumps 104a- 104m via a MxN

coupler 106 and lines 108a-108n, respectively. The number of channels is equal to N, and the number of pumps is equal to M, where M and N are integers, and M is not equal to N.

For example, if there are 32 channels and each channel requires 20 MW of power, a 4x32 coupler can be used, with each of the 4 pumps providing 160 MW of power. Thus, each pump splits power between 8 of the 32 channels .

In the configuration shown in Fig. 7, one or more of the pumps 104a- 104m may be a spare pump for use in the event of another one of the pumps becoming inoperative.

It is understood that there may be a single pump per channel, with the pump power being the same or

different for the respective amplifiers. If the pump powers are different, it is understood that the

respective amplifiers have different saturation levels.

Also, it is understood that there may be multiple shared pumps used in the practice of the invention. For example, if there are 32 channels there may be 16 pumps, with 2 channels sharing a pump; or 8 pumps with 4 channels sharing a pump; or 4 pumps with 8 channels sharing a pump, and so on.

Fig. 8 is a block diagram of a plurality of optical

nodes 200a-200l connected in a ring configuration. The respective optical nodes may comprise OLT's, OADM ' s , or the like. An optical signal transmission from one node to the next is termed a hop. If the optical nodes are OLT's connected back-to-back according to the prior

art, up to five hops may be made without introduction of a transponder in the lightpath. Thus if an optical signal were transmitted from node 200a to node 200m, a transponder would be required at nodes 200f and 200k.

In contrast, according to the present invention, due to

the equalization of output power level in the respective channels in the optical ring, due to

operating the respective channel amplifiers in the saturation mode, recent modeling results have shown that up to twenty-three hops may be made without introduction of a transponder in the lightpath.

A further advantage that is derived in such an optical ring using amplifiers operating at a predetermined output power level in each of the channels, is the prevention of lasing. Since the power level output of

the amplifiers in the respective channels is constrained not to rise above a predetermined level, a given channel's wavelength that traverses the ring without being dropped can't rob power from another channel, due to the respective output power levels of the amplifiers being held at the predetermined level.

Accordingly, system cost is reduced, as fewer expensive transponders are required. Cost of the optical amplifiers are decreased as less gain is required, VOA's are not required, automatic gain control is not

required and equalization is not required. System level costs are also decreased as simpler software is required since no VOA control is required. Further, an inadvertent ring connection in a given channel will not cause ringing due to the amplifiers in the channel operating in the saturation mode.

In summary, in the apparatus of the present invention each channel in an optical communication system includes an optical amplifier which operates in the saturation mode such that each amplifier has substantially the same output power level independent of channel wavelength and input power level . Although certain embodiments of the invention have been described and illustrated herein, it will be readily apparent to those of ordinary skill in the art that a

number of modifications and substitutions can be made to the preferred example methods and apparatus disclosed and described herein without departing from the true spirit and scope of the invention.

Claims

What Is Claimed Is:

1. An optical communication system comprising: N, where N is an integer, optical channels; input means for inputting a single optical wavelength signal to each of said N optical channels;

N optical amplifiers, with each of said N optical channels including one of said N optical amplifiers; and

control means for operating each of said N

optical amplifiers at a predetermined output power

level .

2. The optical communication system according to Claim 1, wherein said control means operates each of said N optical amplifier at the predetermined output power level by operating each of said N optical amplifiers in a saturation mode.

3. The optical communication system according to

Claim 2, wherein said control means comprises: pump means for supplying an optical signal at a predetermined pumping power level to each of said N optical amplifiers for operating each of said N optical amplifiers in the saturation mode.

4. The optical communication system according to Claim 3, wherein said pump means comprises N pumps, each of said N pumps being connected to a different one of said N optical amplifiers, with each of said N pumps supplying an optical signal at a predetermined pumping power to a one of the N optical amplifiers to which it is connected.

5. The optical communication system according to Claim 3, wherein said pump means comprises:

M pumps, where M is an integer and M<N, with each of said M pumps supplying an optical signal at a predetermined pumping power level; and a MxN coupler for selectively coupling N of said M pumps to said N optical amplifiers, with at

least one of said M pumps being a spare pump.

6. The optical communication system according to

Claim 3, wherein said pump means comprises: a shared pump means for supplying the optical signal at the predetermined power level to each of said N optical amplifiers.

7. The optical communication system according to Claim 6, wherein said shared pump means comprises: a single optical pump for supplying an optical signal at the predetermined pumping power

level; and an optical splitter for applying the optical signal at the predetermined pumping power level from said single optical pump to each of said N optical amplifiers .

8. A wavelength division multiplexed optical

communication system comprising: input means for inputting N, where N is an integer, optical wavelength signals on a single optical

transmission channel; a demultiplexer for demultiplexing the input

N optical wavelength signals onto N optical channels; N optical amplifiers, with each of said N optical channels including one of said N optical amplifiers; and

control means for operating each of said N optical amplifiers at a predetermined output power level .

9. The wavelength division multiplexed optical communication system according to Claim 8, wherein said control means operates each of said N optical amplifiers at the predetermined output power level by operating each of said N optical amplifiers in a saturation mode.

10. The wavelength division multiplexed optical communication system according to Claim 9, wherein said control means comprises : pump means for supplying an optical signal at a predetermined pumping power level to each of said N optical amplifiers for operating each of said N optical

amplifiers in the saturation mode.

11. The wavelength division multiplexed optical communication system according to Claim 10, wherein said pump means comprises N pumps, each of said N pumps being connected to a different one of said N optical amplifiers, with each of said N pumps supplying an optical signal at a predetermined pumping power to a one of the N optical amplifiers to which it is connected.

12. The wavelength division multiplexed communication system according to Claim 10, wherein said pump means comprises :

M pumps, where M is an integer and M<N, with each of said M pumps supplying an optical signal at a predetermined pumping power level; and

a MxN coupler for selectively coupling N of said M pumps to said N optical amplifiers, with at

least one of said M pumps being a spare pump.

13. The wavelength division multiplexed optical communication system according to Claim 10, wherein

said pump means comprises: a shared pump means for supplying the optical signal at the predetermined power level to each of said N optical amplifiers.

14. The wavelength division multiplexed optical communication system according to Claim 13, wherein said shared pump means comprises: a single optical pump for supplying an optical signal at the predetermined pumping power

15. The wavelength division multiplexed optical communication system according to Claim 8, further

comprising: a multiplexer for multiplexing the N optical

wavelength signals on the N optical channels onto a single optical transmission output channel.

16. A wavelength division multiplexed optical communication system comprising: first and second optical nodes connected back-to-back through a plurality of all optical channels in each of said first and second optical nodes, with each of said plurality of all optical channels in at least one of said first and second optical nodes including an optical amplifier for amplifying optical signals at different wavelengths;

and control means for operating each of said plurality of all optical amplifiers at a predetermined

output power level .

17. The wavelength division multiplexed optical

communication system according to Claim 16, wherein

said control means operates each of said optical amplifiers at the predetermined output power level by

operating each of said optical amplifiers in a

saturation mode.

18. The wavelength division multiplexed optical communication system according to Claim 17, wherein said control means comprises: pump means for supplying an optical signal at a predetermined pumping power level to each of said optical amplifiers for operating each of said optical amplifiers in the saturation mode.

19. The wavelength division multiplexed optical communication system according to Claim 18, wherein

said pump means comprises N, where N is an integer, optical pumps, each of said N optical pumps being connected to a different one of said optical amplifiers, with each of said N optical pumps supplying an optical signal at a predetermined pumping power to a one of the optical amplifiers to which it is connected.

20. The wavelength division multiplexed optical communication system according to Claim 18, wherein said pump means comprises:

M pumps, where M is an integer and M<N, with each of said M pumps supplying an optical signal at a predetermined pumping power level; and a MxN coupler for selectively coupling N of said M pumps to said optical amplifiers, with at least one of said M pumps being a spare pump.

21. The wavelength division multiplexed optical communication system according to Claim 18, wherein said pump means comprises : a shared pump means for supplying the optical signal at the predetermined power level to each of said optical amplifiers.

22. The wavelength division multiplexed optical communication system according to Claim 21, wherein said shared pump means comprises: a single optical pump for supplying an optical signal at a predetermined pumping power level;

and an optical splitter for applying the optical signal at the predetermined pumping power level from said single optical pump to each of said optical amplifiers .

23. The wavelength division multiplexed optical communication system according to Claim 16, wherein said first and second optical nodes are each optical line terminals.

24. The wavelength division multiplexed optical communication system according to Claim 16, wherein said first and second optical modes are each add-drop multiplexers .

25. An optical communication system comprising: N, where N is an integer, optical channels; input means for inputting a single optical wavelength signal to each of said N optical channels;

N optical amplifiers, with each of said N

optical channels including one of said N optical

amplifiers; and control means for operating each of said N optical amplifiers in a saturation mode.

26. The optical communication system according to Claim 25, wherein said control means comprises: pump means for supplying an optical signal at a predetermined pumping power level to each of said N optical amplifiers for operating each of said N optical amplifiers in the saturation mode.

27. A wavelength division multiplexed optical communication system comprising: first and second optical nodes connected back-to-back through a plurality of all optical channels in each of said first and second optical nodes, with each of said plurality of all optical channels in at least one of said first and second optical nodes including an optical amplifier for amplifying optical signals at different wavelengths;

and control means for operating each of said plurality of all optical amplifiers in a saturation

mode .

28. The wavelength division multiplexed optical communication system according to Claim 27, wherein said control means comprises: pump means for supplying an optical signal at a predetermined pumping power level to each of said optical amplifiers for operating each of said optical amplifiers in the saturation mode.

29. A method of operating an optical communication system including N, where N is an integer, optical channels, with each such channel including an optical amplifier, comprising: an input step of inputting a single optical wavelength signal to each of said N optical channels; and a control step of operating each optical amplifier at a predetermined output power level.

30. The method according to Claim 29, wherein said control step operates each amplifier at the

predetermined output power level by operating each amplifier in a saturation mode.

31. The method according to Claim 30, wherein said control step comprises: a pumping step of supplying an optical signal at a predetermined pumping power level to each optical amplifier for operating each amplifier in a saturation mode .

32. A method of operating an optical communication system including N, where N is an integer, optical channels, with each such channel including an optical amplifier, comprising: an input step of inputting a single optical wavelength signal to each of said N optical channels;

and a control step of operating each optical amplifier in a saturation mode.

33. The method according to Claim 32, wherein said control step comprises: a pumping step of supplying an optical signal at a predetermined pumping power level to each optical amplifier for operating each optical amplifier in the

saturation mode.

34. A method of operating a wavelength division multiplexed optical communication system including N, where N is an integer, optical channels, with each such

channel including an optical amplifier, comprising: an input step of inputting N optical wavelength signals on a single optical transmission path; a demultiplexing step of demultiplexing the input N optical wavelength signals onto the N optical channels; and a control step of operating each optical amplifier at a predetermined power level.

35. The method according to Claim 34, wherein said control step operates each amplifier at the predetermined power level by operating each amplifier in a saturation mode.

36. The method according to Claim 35, wherein said control step comprises: a pumping step of supplying an optical signal at a predetermined pumping power level to each optical amplifier for operating each amplifier in the

saturation mode.

37. A method of preventing lasing in a wavelength division multiplexed optical ring communication system having a plurality of nodes, with each of said plurality of nodes including N, where N is an integer, optical channels, with each such channel including an optical amplifier, comprising: an input step of inputting a different optical wavelength signal to each of said N optical

channels; and a control step of operating each optical amplifier at a predetermined power level which the amplifier is constrained not to rise above, wherein lasing is prevented when a wavelength

of a given channel transverses the ring without being dropped, since the given channel is unable to rob power

from another channel due to the output power level of each channel being constrained not to rise above the predetermined power level .

38. The method according to Claim 37, wherein said control step operates each amplifier at the predetermined output power level by operating each amplifier in a saturation mode.

39. The method according to Claim 38, wherein said control step comprises: a pumping step of supplying an optical signal

at a predetermined pumping power level to each optical amplifier for operating each amplifier in a saturation mode .

40. An optical communication system comprising: an optical cross connect switch having a plurality of inputs and outputs, with said optical cross connect switch including a core having a

plurality of inputs and outputs, with a plurality of input optical channels being included between the respective inputs of the optical cross connect switch and the inputs of the core, with a plurality of output optical channels being included between the respective outputs of the core and the outputs of the optical cross connect switch, with each of the plurality of optical channels in at least one of the plurality of input optical channels and the plurality of output channels including an optical amplifier for amplifying

input optical signals; and control means for operating each of the

optical amplifiers at a predetermined output power level .

41. The optical communication system according to Claim 40, wherein said control means operates each of said optical amplifiers at the predetermined output power level by operating each of said optical amplifiers in a saturation mode.

42. The optical communication system according to Claim 41, wherein said control means comprises: pump means for supplying an optical signal at a predetermined pumping power level to each of said

optical amplifiers for operating each of said optical amplifiers in the saturation mode.

43. The optical communication system according to Claim 42, wherein said pump means comprises a plurality of optical pumps, each of said plurality of optical pumps being connected to a different one of said optical amplifiers, with each of said plurality of optical pumps being connected to a different one of said optical amplifiers, with each of said plurality of optical pumps supplying an optical signal, at a predetermined pumping power to a one of the optical

amplifiers to which it is connected.

44. The optical communication system according to Claim 42, wherein said pump means comprises:

a MxN coupler for selectively coupling N of said M pumps to said optical amplifiers, with at least one of said M pumps being a spare pump.

45. The wavelength division multiplexed optical communication system according to Claim 42, wherein said pump means comprises: a shared pump means for supplying the optical signal at the predetermined power level to each of said optical amplifiers.

46. The optical communication system according to Claim 45, wherein said shared pump means comprises: a single optical pump for supplying an

optical signal at a predetermined pumping power level; and an optical splitter for applying the optical signal at the predetermined pumping power level from said single optical pump to each of said optical

amplifiers .