Patent Publication Number: US-2004052524-A1

Title: Method of power control in an optical communication system

Description:
[0001] The present invention concerns a method of power control in an optical communication system; the invention also relates to an optical communication system operating according to the method.  
       [0002] Optical communication systems are known and comprise a number of spatially distributed nodes interconnected through optical fibre waveguides, the waveguides operable to convey information bearing radiation between the nodes. Such systems often employ wavelength division multiplexing (WDM) techniques so that communication traffic propagating between the nodes is modulated onto one or more radiation components occupying corresponding mutually different wavebands. The wavebands are frequently referred to as channels.  
       [0003] When such systems are operating, the number of channels in use can be dynamically changing as WDM add/drop multiplexers and WDM cross-connects at the nodes are reconfigured under software control to add or remove active channels. Dynamic changes can also arise because of channel failures arising from optical parts being disturbed or damaged, for example during maintenance procedures.  
       [0004] The systems are designed to maintain mutually similar radiation power in active channels propagating along wav guides in the systems so that optical devices such as pumped erbium doped fibre amplifiers (EDFAS) incorporated therein are not subjected to sudden input power fluctuations nor to excess radiation concentrated in specific channels. As EDFAs are inherently non-linear devices, disparity in relative channel radiation powers input to such EDFAs can cause accentuation of channel radiation power differences.  
       [0005] It is therefore conventional practice in the nodes of the systems to include feedback loops to monitor and regulate radiation power associated with the channels to ensure that they are of mutually similar power. Moreover, it is also conventional practice when outputting radiation from nodes to emit into associated waveguides as much radiation power as possible and to apply attenuation at nodes subsequently receiving the emitted radiation. This practice is adopted in order to try to obtain as high a signal-to-noise ratio as possible in the systems and hence improve their traffic carrying capacity. Additionally, for design simplicity, it is also conventional practice to include an attenuator at each receiver node to regulate the power of received radiation applied to an optical detector thereat; such design simplicity enables the attenuator, the detector and an associated power control feedback loop to be co-sited at the receiver node.  
       [0006] The inventor has appreciated that the amount of radiation power presently emitted in conventional optical communication systems is sufficient to cause optical non-linear effects in optical fibre waveguides interconnecting nodes. In long-haul optical fibre waveguide paths approaching 100 km in length, such non-linear effects occur primarily in regions of the paths closest to where radiation is launched thereinto on account of attenuation along the waveguide paths reducing the power of the radiation at regions of the paths remote from where the radiation is launched thereinto. Moreover, such non-linear effects can result in four-way mixing, cross phase modulation and similar mutual interactions between radiation components. In some communication systems including long-haul optical fibre waveguide paths approaching  100  on in length without optical repeaters, radiation emitted thereinto can be 100 mW or more. The inventor has further appreciated that the non-linear effects can themselves be bandwidth limiting to the systems. Moreover, when emitted radiation power levels exceed 1 W in a communication system optical fibre waveguide, waveguide reliability can deteriorate because there is sufficient power to physically damage the waveguide in the event of a defect arising in the waveguide; standing wave patterns can arise which causes extensive damage along major parts of waveguides.  
       [0007] The inventor has further appreciated that it is important in future high-performance optical communication systems to control the total power of composite WDM radiation emitted into optical fibre waveguides of the systems; this is a departure from conventional practice where excess received power is controlled using input attenuators. Such control is necessary to reduce interactions arising from waveguide non-linear optical phenomena and to circumvent reliability problems when excess radiation power is emitted into the waveguides. The excess power can severely damage the waveguides in the event of one or more defects occurring therein, for example a waveguide split or localised point of power absorption therein.  
       [0008] According to a first aspect of the present invention, there is provided a method of power control in an optical communication system, the system comprising a plurality of nodes coupled together by optical waveguiding means for guiding communication traffic bearing radiation between the nodes, the method including the steps of: regulating the power of the communication traffic bearing radiation at the first node to provide optical output radiation; emitting the optical output radiation through the waveguiding means to a second node of the system; measuring radiation power of the output radiation received at the second node to generate corresponding power indicative data; communicating the power indicative data to controlling means operable to generate error data for regulating the optical output radiation measured at the second node to a predetermined power level; and communicating the error data to the power regulating means at the first node for controlling the regulating means so that the radiation power of the output radiation received at the second node is stabilized substantially at the predetermined level at which optical non-linearities are reduced to less than a predetermined threshold in the waveguiding means.  
       [0009] The invention provides the advantage that it possible to reduce optical non-linear effects in the waveguiding means by operating the communication system according to the method.  
       [0010] The aforementioned predetermined threshold is defined as a threshold at which system performance, for example bit error rate, is not substantially limited by non-linear optical phenomena arising in the waveguiding means but by other factors in the system, for example polarisation mode dispersion.  
       [0011] Preferably, the method is applied in an optical communication system wherein the communication traffic bearing radiation is wavelength division multiplexed into a plurality of channels.  
       [0012] In the method, it is beneficial that at least one of the power indicative data and the error data are communicated in a supervisory channel associated with the channels bearing the communication traffic. Use of the supervisory channel circumvents a need to provide alternative communication pathways for conveying at least one of the power indicative data and the error data.  
       [0013] In one form of the method, it is preferable that the controlling means is located at the first node. Such an arrangement enables the regulating means and the controlling means to be collocated at the first node with the monitoring means located at the second node, the second node therefore only providing a power monitoring function for the method.  
       [0014] Alternatively, the controlling means can be located at the second node; however, such an arrangement may often require more information to be conveyed between the nodes compared to collocating the regulating means and the controlling means.  
       [0015] Conveniently, the first node includes monitoring means for determining the number of active wavelength division multiplexed channels present in the communication bearing radiation and varying the predetermined level in response to the number of active channels. Varying the predetermined level in response to the number of active channels enables a compromise to be reached between circumventing optical non-linear effects in the waveguiding means and maintaining signal-to-noise ratio.  
       [0016] The predetermined level is preferably varied substantially as a linear function of the number of active channels so that radiation power per active channel is maintained substantially constant at the second node in operation. Alternatively, the predetermined power level is beneficially maintained substantially constant when one or more channels in the output radiation are active; such a substantially constant output power received at the second node ensures that components such as erbium doped fibre amplifiers in both the first and the second nodes are operating at nominally constant power.  
       [0017] Beneficially, the controlling means is operable to set the regulating means to an attenuation greater than −20 dB when none of the channels are active. This attenuation applied when none of the channels are active reduces the amount of optical noise injected into the waveguiding means.  
       [0018] It is also preferable in the method that: the regulating means is operable to attenuate independently radiation of each channel propagating therethrough; and radiation in each channel is monitored individually at the second node so that the controlling means is capable of independently adjusting attenuation of each channel at the first node to substantially equalize radiation power present in the channels which are active.  
       [0019] Such independent control of the channels enables the system to perform channel leveling for output radiation received at the second node, thereby correcting for differential frequency-dependent attenuation phenomena occurring in the waveguiding means.  
       [0020] In order to simplify the system, it is preferable in the method that the regulating means is provided by an optical amplifier whose forward gain is controllable by the error data modulating optical pumping power applied to the amplifier.  
       [0021] In a second aspect of the present invention, there is provided an optical communication system comprising a plurality of nodes coupled together by optical waveguiding means for guiding communication traffic bearing radiation between the nodes, the system further comprising: power regulating means for regulating radiation power of the communication traffic bearing radiation at the first node to generate corresponding output radiation; emitting means at the first node for emitting the output radiation into the waveguiding means for propagation to a second node of the system; radiation power measuring means for measuring radiation power of the output radiation received at the second node after it has been conveyed through the waveguiding means and for generating corresponding power indicative data; and controlling means for receiving the power indicative data and using it to regulate the radiation power measured at the second node to a predetermined level by generating corresponding error data and communicating the error data to th regulating means for controlling the regulating means so that the radiation power measured at the second node is stabilized substantially at the predetermined level at which optical non-linearities are reduced to less than a predetermined threshold in the waveguiding means.  
       [0022] Conveniently, the communication bearing radiation is wavelength division multiplexed into a plurality of channels. Such wavelength division multiplexing provides the system with a potentially relatively large communication traffic communicating capability. Moreover, use of wavelength division multiplexing enables the system to provide the communication bearing radiation with a supervisory channel for communicating at least one of the error data and the power indicative data between the first and second nodes. Such a supervisory channel circumvents a need to include alternative communication pathways for conveying supervisory information within the system.  
       [0023] Preferably, the controlling means is co-located with the regulating means at the first node. The co-location enables the error data to be communicated within the first node. Alternatively, the controlling means can be located at the second node so that the measuring means and the controlling means are co-located for communicating the power indicative data within the second node.  
       [0024] Advantageously, the first node includes monitoring means for determining the number of active wavelength division multiplexed channels present in the communication bearing radiation and varying the predetermined level in response to the number of active channels. Varying the predetermined level in response to the number of active channels enables the system to improve signal-to-noise ratio whilst circumventing effects due to optical non-linearity in the waveguiding means.  
       [0025] The controlling means is preferably operable to vary the predetermined level substantially as a linear function of the number of active channels so that the radiation power per active channel remains substantially constant in operation. Such a constant level of radiation power per active channel ensures that the signal-to-noise ratio of each active channel is maintained as the number of active channels is varied. Alternatively, the controlling means is beneficially operable to maintain the predetermined power level substantially constant at the second node when one or more channels in the output radiation are active; the substantially constant predetermined power level ensures that devices such as optical amplifiers in the nodes can operate at nominally constant pumping power.  
       [0026] Preferably, the controlling means is operable to set the regulating means to an attenuation greater than −20 dB when none of the channels are active. This attenuation reduces optical noise being injected into the waveguiding means when none of the channels are active, and thereby reduces bit error rate occurrence within the system.  
       [0027] In order to compensate for frequency-dependent optical attenuation phenomena in the waveguiding means, the regulating means is preferably operable to independently regulate radiation power of radiation associated with each channel propagating therethrough, and the monitoring means is operable to monitor radiation power of each channel independently at the second node so that the controlling means is capable of individually adjusting radiation power of each channel at the first node to substantially mutually equalize radiation power of the channels which are active. Beneficially, the active channels are mutually equalized in radiation power to within 6 dB. 
     
    
    
     [0028] Embodiments of the invention will now be described, by way of example only, with reference to the following diagrams in which:  
     [0029]FIG. 1 is a schematic illustration of a first embodiment of the invention, the embodiment comprising two optical communication system nodes mutually interconnected through associated optical fibre waveguides; and  
     [0030]FIG. 2 is a schematic illustration of a second embodiment of the invention, the embodiment comprising two optical communication system nodes mutually interconnected through associated optical fibre waveguides. 
    
    
     [0031] Referring to FIG. 1, there is shown a part of an optical communication system  10  comprising first and second nodes  20 ,  40  respectively. The first node  20  is included within a dotted line  30  and the second node  40  is included within a dotted line  50 . The first node  20  is connected to the second node  40  through an optical fibre waveguide  60  for conveying communication traffic from the first node  20  to the second node  40 . Likewise, the second node  40  is connected to the first node  20  through an optical fibre waveguide  70  for conveying traffic from the second node  40  to the first node  20 . The nodes  20 ,  40  include identical components for communicating to one another.  
     [0032] Communication between the nodes  20 ,  40  will now be described in general overview.  
     [0033] The first node  20  receives up to N optical input radiation signals and multiplexes them to provide first composite radiation. The first node  20  then amplifies the first composite radiation, and subsequently attenuates the resulting amplified first composite radiation and finally adds a first monitoring signal thereto to provide first output radiation for emitting into the waveguide  60  to the second node  40 . The first node  20  also receives second output radiation from the second node  40 , extracts a second monitoring signal therefrom, amplifies the second output radiation and then demultiplexes the amplified second radiation to provide up to N optical output signals. The node  20  employs the second monitoring signal to regulate attenuation of the first composite radiation so as to provide a predetermined first output radiation received power level at the second node  40 . Such regulation also takes account of attenuation occurring in the waveguide  60 .  
     [0034] Likewise, in a reciprocal manner, the second node  40  receives up to N optical input radiation signals and multiplexes them to provide second composite radiation. The second node  40  then amplifies the second composite radiation, and subsequently attenuates the resulting amplified second composite radiation and finally adds the second monitoring signal thereto to provide the second output radiation for emitting into the waveguide  70  to the first node  20 . The second node  40  also receives the first output radiation from the first node  20 , extracts the first monitoring signal therefrom, amplifies the first output radiation and then demultiplexes the amplified first output radiation to provide up to N optical output signals. The node  40  employs the first monitoring signal to regulate attenuation of the second composite radiation to provide a predetermined second radiation received power at the first node  20 . Such regulation also compensates for attenuation occurring along the waveguide  70 .  
     [0035] The first node  20  monitors power in the second output radiation received thereat to generate the first monitoring signal. Similarly, the second node  40  monitors power in the first output radiation received thereat to generate the second monitoring signal. Thus, the first node  20  monitors the second output radiation it receives from the second node  40  and provides feedback to the second node  40  for it to regulate its attenuator so that the second radiation is maintained at a predetermined power level as monitored by the first node  20 . Likewise, in a reciprocal manner, the second node  40  monitors the first output radiation it receives from the first node  20  and provides feedback to the first node  20  for it to regulate its attenuator so that the first radiation is also maintained at a predetermined power level as monitored by the second node  40 . As a consequence of regulating the output radiation in the nodes  20 ,  40  to a regime where optical non-linear phenomena in the waveguides  60 ,  70  are substantially circumvented, performance of the system  10  is thereby enhanced.  
     [0036] Component parts of the nodes  20 ,  40  will now be described in further detail. The nodes  20 ,  40  are configured in an identical manner and include similar component parts; on account of the similarity, only component parts of the first node  20  will be elucidated. Component parts of the first node  20  will be identified by a qualifier ‘a’ whereas corresponding component parts of the second node  40  will be identified by a qualifier ‘b’.  
     [0037] The first node  20  comprises an optical multiplexer  100   a  including N optical input ports for receiving up to N input radiation signals. The multiplexer  100   a  comprises an optical output port which is conveyed through an erbium doped fibre amplifier (EDFA)  110   a  and then through an attenuator  120   a  to a first optical input port of a coupler  130   a . A second optical input port of the coupler  130   a  is connected to an optical output port of a supervisory channel data inserting unit  140   a . An optical output port of the coupler  130   a  is connected to the fibre waveguide  60  for coupling the first output radiation from the coupler  130   a  to the second node  40 . The N input ports are also connected to corresponding optical input ports of an array of power detectors  150   a  whose power monitoring electrical outputs are conveyed to a power controller  160   a . An electrical output of the controller  160   a  is connected to an electrical control input of the attenuator  120   a  for controlling attenuation provided by the attenuator  120   a . A further electrical input to the controller  160   a  is connected to an electrical output of a supervisory channel data extracting unit  170   a  whose optical input is connected to a first optical output port of a coupler  180   a . A second optical output port of the coupler  180   a  is coupled through an EDFA  190   a  to an optical input port of an optical demultiplexer  200   a . The demultiplexer  200   a  includes N optical output ports at which up to N radiation signals are output in operation. The second optical output port of the coupler  180   a  is also conveyed to an optical input of a power monitor  210   a  The power monitor  210   a  includes an electrical output which is connected to an electrical input of the data inserting unit  140   a . An optical input port of the coupler  180   a  is coupled to the fibre waveguide  70 .  
     [0038] Radiation conveyed along the fibre waveguides  60 ,  70  is in WDM form where the N radiation signals are included in mutually different wavelength ranges, each signal occupying a range of wavelengths corresponding to its associated channel. Moreover, the radiation in the waveguides  60 ,  70  also includes radiation corresponding to one or more supervisory channels, the supervisory channels being used to convey, amongst other supervisory information, radiation power data for use in controlling the attenuator  120   a . The supervisory channels are set at wavelength ranges different to those associated with the N radiation signals. If necessary, the supervisory channels can occupy a wavelength range corresponding to a channel N+1, namely following monotonically from channels 1 to N used to convey communication traffic.  
     [0039] Operation of the first node  20  will now be described in detail with reference to FIG. 1.  
     [0040] The second node  40  operates in a similar reciprocal manner. The first node  20  receives input radiation from the node  40  via the fibre waveguide  70 . The radiation propagates to the coupler  180   a  whereat it is split into first and second components, the first component passing to the data extracting unit  170   a  and the second component coupling to the power monitor  210   a  and also via the EDFA  190   a  to the demultiplexer  200   a . Radiation received at the demultiplexer  200   a  is filtered and directed to respective optical outputs depending upon radiation wavelength The power monitor  210   a  measures total power in radiation output from the second port of the coupler  180   a  to provide corresponding power data which it passes to the data insertion unit  140   a . The unit  140   a  inserts the power data into the supervisory channel present in radiation output from the first node  20  along the fibre waveguide  60  to the second node  40 ; the second node  40  thereby establishes total radiation power received at the first node  20  and its power controller  160   b  compares the total radiation power with a predetermined power level and proceeds to generate an error signal for adjusting its attenuator  120   b  to stabilize the total power measured at the first node  20  at the predetermined power level. The first component of radiation from the coupler  180   a  passes to the data extraction unit  170   a  which extracts therefrom supervisory information supplied from the second node  40  relating to total power in radiation receive thereat from the first node  20  as measured by the power monitor  210   b . Total power information passes from the extraction unit  170   a  to the power controller  160   a  which also receives radiation power information from the array  150   a ; power information from the array  150   a  is used by the power controller  160   a  to set an appropriate attenuation for the attenuator  120   a , for example depending upon the number of optical inputs of the multiplexer  100   a  receiving radiation and thereby being active. The power controller  160   a  forms part of a feedback loop and generates an error signal for adjusting attenuation provided by the attenuator  120   a  to maintain the total power in radiation received at the second node  40  to a predetermined power level. In a first operating regime, the predetermined power level can be maintained constant irrespective of the number of active channels provided that at least one channel is active. Alternatively, in a second operating regime, the predetermined power level can be made variable as a linear function of the number of active inputs to the multiplexer  100   a  in order to maintain radiation power per active channel substantially constant, for example within a 6 dB error margin.  
     [0041] It is preferable that up to N input radiation signals received at the nodes  20 ,  40  are mutually equalised in power prior to being input to the multiplexers  100   a ,  100   b ; such equalisation is necessary to circumvent the EDFAs  10   a ,  110   b  accentuating radiation of certain relatively more powerful channels. If necessary, optical power leveling units can be included in the nodes  20 ,  40  and configured to precede the multiplexers  100   a ,  100   b.    
     [0042] In a modified version of the nodes  20 ,  40 , the EDFAs  110   a ,  110   b  can be modified to provide a variable amplification function, thereby circumventing a need to include the attenuators  120   a ,  120   b  and hence simplifying the nodes  20 ,  40 ; gain provided by the EDFAs  110   a ,  110   b  can, for example, be controlled from the power controllers  160   a ,  160   b  by adjusting pumping power applied to the EDFAs  110   a ,  110   b . Moreover, the EDFAs  190   a ,  190   b  can be omitted to provide a further simplification of the nodes  20 ,  40  provided that sufficient radiation power is received for the demultiplexers  200   a ,  200   b  to output sufficient radiation power at their optical output ports.  
     [0043] A power control feedback loop provided in part by the power controller  160   a  in the node  20  is arranged to have a relatively long time constant, for example greater than one second. Such a relatively long time constant is chosen to try to avoid transient overshoot in the feedback loop from occurring. Most changes in radiation power received at the nodes  20 ,  40  from one another arise, other than by deliberately inserting and removing channels, as a result of environmental temperature changes which occur gradually, for example over a time period of minutes. Thus, the nodes  20 ,  40  are effective at compensating for varying power losses occurring along the fibre waveguides  60 ,  70  as well as fluctuations in optical gain provided by the EDFAs  110   a ,  110   b.    
     [0044] As described earlier, in a first operating regime, the nodes  20 ,  40  are normally operated to maintain received radiation power at a predetermined power limit. Alternatively, in a second operating regime, the aforementioned predetermined level can be made variable for each of the nodes  20 ,  40  depending upon the number of active channels being conveyed between the nodes  20 ,  40 ; the power controllers  160   a ,  160   b  are capable of determining the number of active channels from power information received thereat from the detector arrays  150   a ,  150   b  respectively. If there are no active inputs conveying input radiation to the multiplexers  100   a ,  100   b , their respective power controllers  160   a ,  160   b  can be operable to apply a relatively high attenuation, for example in the order of −35 dB and at least greater than −20 dB, at their associated attenuators  120   a ,  120   b  to prevent the attenuators  120   a ,  120   b  being set to minimum attenuation and thereby injecting optical noise into the fibre waveguides  60 ,  70 . The first regime ensures that the EDFAs  110   a ,  110   b  are operating a nominally constant power output. The second regime ensures that radiation associated with each WDM channel conveyed through the fibre waveguides  60 ,  70  is of nominally constant power.  
     [0045] The present invention is capable of being implemented in alternative embodiments to that shown in FIG. 1. Referring now to FIG. 2, there is shown part of a communication system indicated by  300 , the part  300  comprising first and second nodes. The first node is indicated by  310  and is included within a dotted line  320 . Similarly, the second node is indicated by  330  and is included within a dotted line  340 . The first node  310  is connected to the second node  330  through an optical fibre waveguide  350  for conveying communication traffic from the first node  310  to the second node  330 . Likewise, the second node  330  is connected to the first node  310  through an optical fibre waveguide  360 .  
     [0046] Component parts included in the nodes  310 ,  330  for use in mutually communicating along the fibre waveguides  350 ,  360  are identical and similarly configured. These parts will now be described using a qualifier ‘a’ to refer to a component in the first node  310  and a qualifier ‘b’ to refer to a component in the second node  330 .  
     [0047] The component parts and their interconnection will now be elucidated for the first node  310 , Similar component parts and interconnection pertain to the second node  330 . The first node  310  includes an optical multiplexer  400   a  comprising N optical input ports for receiving up to N input radiation signals and an optical output connected through an EDFA  410   a  to an optical input port of an optical attenuator  420   a . An optical output port of the attenuator  420   a  is coupled to a first input port of an optical coupler  430   a . The coupler  430   a  comprises a second input port which is coupled to an optical output port of a data inserting unit  440   a . As well as being connected to the multiplexer  440   a , the N input ports are also conveyed to an optical detector array  450   a , each port having its corresponding detector in the array  450   a . Electrical measurement outputs from the array  450   a  are coupled to electrical inputs of the supervisory channel inserting unit  440   a.    
     [0048] The first node  310  further includes an optical demultiplexer  460   a  comprising N optical output ports and an optical input port coupled to an optical output port of an EDFA  470   a . The EDFA  470   a  incorporates an optical input port which is connected to a first output port of an optical coupler indicated by  480   a . A second output port of the coupler  480   a  is coupled to an optical input port of a power monitor  490   a . An input port of the coupler  480   a  is connected to a first output port of an optical coupler  500   a . A second output port of the coupler  500   a  is coupled to an optical input port of a data extracting unit  510   a . Lastly, an input port of the coupler  500   a  is connected to the fibre waveguide  360 .  
     [0049] The first node  310  additionally comprises a power controller  520   a  which is connected to receive power monitoring output data from the power monitor  490   a  and also a signal presence output from the data extraction unit  510   a  An electrical output from the power controller  520   a  is coupled to an electrical input of the data inserting unit  440   a . Finally the data extracting unit  510   a  includes a first signal presence output which is connected to an electrical input of the power controller, and also includes a second output which is coupled to a control input of the attenuator  420   a.    
     [0050] Operation of the nodes  310 ,  330  will now be elucidated in general overview.  
     [0051] The first node  310  receives up to N optical radiation signals at the multiplexer  400   a  and multiplexes them to provide first composite radiation which is subsequently amplified by the EDFA  410   a . The amplified composite radiation propagates to the attenuator  420   a  which controllably attenuates it to provide corresponding first output radiation which passes through the coupler  430   a  to the fibre waveguide  350 . The first output radiation propagates to the second node  330  and is received at the coupler  500   b  thereof. The coupler  500   b  diverts a portion of the radiation received thereat to the data extracting unit  510   b  and another portion via the coupler  480   b  to the power monitor  490   b  and to the EDFA  470   b . The EDFA  470   b  amplifies radiation received thereat from the coupler  480   b  and outputs corresponding amplified radiation to the demultiplexer  460   b . The demultiplexer  460   b  filters radiation received thereat from the EDFA  470   b  and thereby separates out radiation components according to their wavelength to associated N optical outputs.  
     [0052] Radiation received at the node  330  via the fibre waveguide  350  is diverted through the coupler  480   b  to the power monitor  490   b  which measures power received thereat and generates corresponding power indicative data. The power indicative data is subsequently passed to the power controller  520   b . The power controller  520   b  functions as part of a feedback loop for controlling attenuation provided by the attenuator  420   a  to stabilize radiation power as measured by the power monitor  490   b  at a predetermined level. The power controller  520   b  outputs error data for adjusting attenuation provided by the attenuator  420   a , the error data passing to the supervisory channel data inserting unit  440   b  which inserts the error data into a supervisory channel associated with second output radiation emitted from the second node  330  into the fibre waveguide  360 . The second radiation propagates to the first node  310  and a portion of the radiation is coupled through the coupler  500   a  to the supervisory channel data extracting unit  510   a  which isolates the error data generated in the power controller  520   b  and passes it to the control input of the attenuator  420   a.    
     [0053] The detector array  450   a  monitors radiation power in the radiation signals input to the multiplexer  400   a  and passes corresponding radiation power indicative data to the data inserting unit  440   a  which outputs the power indicative data in appropriate digital format onto the first output radiation which propagates to the second node  330 , for example via the supervisory channel. The power indicative data is used in the nodes  310 ,  330  to determine which of the N inputs to the multiplexer  400   a  are active. The power indicative data is retrieved at the second node  330  by the data extracting unit  510   b ; the data is conveyed to the power controller  520   b  for use therein for calculating appropriate error data for determining attenuation to be provided by the attenuator  420   a  for achieving a predetermined radiation power level as monitored by the power monitor  490   b.    
     [0054] In a similar manner to the nodes  20 ,  40 , in a first operating regime, the predetermined power level can be maintained by the power controllers  520   a ,  520   b  at a constant level, irrespective of the number of active inputs to the multiplexers  400   a ,  400   b . Alternatively, in a second operating regime, the predetermined power level can be varied as a linear function of the number of active inputs as determined by the detector arrays  450   a ,  450   b . The first regime ensures that the EDFAs  410   a ,  410   b  are operating at nominally constant power, whereas the second regime ensures that radiation power per WDM channel conveyed through the fibre waveguides  350 ,  360  is nominally constant. When none of the input ports to the multiplexers  400   a ,  400   b  are active, their associated power controllers  520   b ,  520   a  respectively can set their associated attenuators  420   a ,  420   b  to provide a relatively high attenuation, for example in the order of −35 dB and at least greater than −20 dB, such high attenuation preventing the attenuators  420   a ,  420   b  otherwise being set by their feedback loops to provide minimum attenuation resulting in significant optical noise being injected into the fibre waveguides  350 ,  360 . Such optical noise can, for example, result in an increase in system communication traffic bit error rate.  
     [0055] It will be appreciated from the foregoing that in both the nodes  20 ,  40  as well as the nodes  310 ,  330  power control is achieved by stabilising received radiation power by way of feedback loops and omitting input attenuators to ensure that only a requisite amount of radiation power is output to the waveguides  60 ,  70 ,  350 ,  360 , thereby keeping to a minimum optical non-linearities arising in the waveguides. There arises a threshold level of radiation power below which system performance does not improve as the radiation power output to the waveguides  60 ,  70 ,  350 ,  360  is reduced; other factors, for example polarisation mode dispersion or chromatic dispersion, at the threshold level begin to dominate system performance. In comparison, it is conventional practice to emit as much power as possible into waveguides and then to dissipate excess power at locations of radiation reception; such an approach results in greater fibre waveguide non-linearities arising than necessary.  
     [0056] It will also be appreciated by one skilled in the art that modifications can be made to the nodes  20 ,  40 ,  310 ,  330  without departing from the scope of the invention. For example, one or more of the attenuators  120   a ,  120   b ,  420   a ,  420   b  can be a multichannel attenuator allowing independent attenuation adjustment for each WDM channel present in radiation propagating therethrough. In this respect, one or more of the power monitors  210   a ,  210   b ,  490   a ,  490   b  can be modified to measure radiation power present in each WDM channel received thereat. Such a modification enables the feedback loops provided in the nodes  20 ,  40 ,  310 ,  330  not only to reduce optical non-linearities in the fibre waveguides  60 ,  70 ,  350 ,  360  but also to provide a power leveling function. The power leveling function can be used to substantially mutually equalize power present in the active channels; equalization is defined as corresponding to a mutual power difference of less than 6 dB between the active channels. Such power leveling circumvents power hogging to certain more prominent channels from occurring in the EDFAs  190   a ,  190   b ,  470   a ,  470   b . Moreover, the power leveling also compensates for any wavelength dependent attenuation effects which may occur within the fibre waveguides  60 ,  70 ,  350 ,  360 .  
     [0057] With regard to N, namely the number of input ports to the multiplexers  100   a ,  100   b ,  400   a ,  400   b , and also the number of output ports of the demultiplexers  200   a ,  200   b ,  460   a ,  460   b , N is preferably in a range of 8 to 128 to make the nodes  20 ,  40 ,  310 ,  330  compatible with future optical communication systems.  
     [0058] The nodes  20 ,  40 ,  310 ,  330  can form part of an optical communication system wherein the nodes  20 ,  40 ,  310 ,  330  function as cross-connects and add/drop multiplexers.