Abstract:
A wavelength division multiplexed (WDM) apparatus is provided for a bidirectional dense WDM optical fiber communication network with cost-effective and efficient allocation of the resources available at each network node. For each fiber link, the invention uses a WDM signal composed of channels propagating in opposite directions and a band optical add-drop multiplexer (OADM) to isolate or combine a band to the WDM signal spectrum. As a result, the invention significantly reduces the number fiber connections and filtering equipment required with respect to each direction of transmission.

Description:
This application is a continuation application of U.S. patent application Ser. No. 09/289,969, filed on Apr. 13, 1999 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to dense wavelength division multiplexed (WDM) optical fiber communication systems and more particularly to bidirectional optical networks. 
     BACKGROUND ART 
     In a WDM optical communication system, client signals are transmitted at different carrier wavelengths. The client signals are combined into a WDM signal using a multiplexer and transmitted along a single optic fiber. The WDM signal is typically composed of several channels of information with each channel corresponding to one of the client signals. When the WDM signal is received by the receiver terminal, the individual channels are separated by a demultiplexer and processed by optical receivers. 
     By partitioning and maintaining multiple wavelengths in a single fiber link, WDM technology can provide extended transport capacity and is an obvious fit for transmitting high volumes of traffic. This is particularly true for long distance communications schemes. As the demand for bandwidth is continuously rising, current WDM systems are now designed to provide even more bandwidth capacity. 
     Current WDM systems are not always designed solely with greater transport capacity. In some cases, the transport capacity provided by currently available WDM systems is adequate. When bandwidth is abundant, cost often becomes the prime consideration. In metropolitan environments, for example, the bandwidth requirements of local systems are typically lower than those of long distance WDM networks and therefore it is often desirable to reduce the initial cost of deploying a WDM network albeit at the expense of bandwidth. 
     In addition to greater bandwidth capacity, WDM systems are often designed with other attributes such as, for example, bidirectional operation and/or increased reliability to reduce link failures. However, these requirements does not permit a cost-effective and efficient allocation of the resources present in the network. 
     In conventional unidirectional WDM systems for example, the installation of at least two fiber links is required to achieve transmit and receive operations. Additional fibers are also necessary in both unidirectional and bidirectional WDM systems to “protect” the working fibers in the event of a link failure. Current protection configurations which require the installation of additional fiber links between nodes include dedicated protection (1 protection fiber for each fiber link also referred to as 1:1 protection), shared protection (1 protection fiber for N fiber links or 1:N protection) and ring protection. Therefore, in order to reduce the initial cost of a WDM optical network, it may be desirable to reduce the number of fiber connections used. 
     In addition to this ineffective use of fibers, the accommodation of multiple fiber links necessitates replicating some of the filtering equipment required at each node. In particular, the installation of a unidirectional or bidirectional WDM system such as those described above requires duplicating some of the filtering apparatus for each fiber used. The equipment unique to each fiber link and present at each node typically includes an add-drop multiplexer (ADM) which itself is comprised of a series of multi staged narrowband filters with high figures of merit (FOM). ADMs are typically used to achieve communication between nodes along a main fiber. Briefly described, an ADM is used in a WDM network to filter out and reroute at least one channel of the WDM signal (hereinafter the “drop channels”) while the remaining channels (hereinafter the “through channels”) travel through the ADM. The ADM can also be used to “add channels” to the WDM signal, using wavelengths that have been vacated as a result of channels being dropped at the ADM or at ADMs earlier in the transmission path. As more than one channel usually needs to be accessed at a network node, multichannel ADMs are used and are typically designed with cascaded filters such that each ADM filter is adding or dropping a channel to or from the WDM signal. The multichannel ADMs contribute significantly to the cost of a WDM optical network, particularly in the case of unidirectional local area networks where numerous multichannel ADMs are used. Duplicating this filtering equipment for each fiber link may prove to have a major impact on the initial cost of the WDM network. 
     In bidirectional networks, more complex multichannel ADMs are required which also has a substantial bearing on cost. In particular, for a given channel count, the multichannel ADMs used in bidirectional systems have twice the number of cascaded filters per fiber than the multichannel ADMs used in unidirectional networks. This higher number of filters results in higher optical loss on each fiber and necessitates optical amplifiers, thus also considerably increasing the system cost. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to obviate or mitigate one or more of the above-identified disadvantages and shortcomings of current WDM systems. 
     This invention provides a bidirectional dense wavelength division multiplexed (WDM) optical fiber communication network with a cost-effective and efficient allocation of the resources available at each network node. 
     In a preferred embodiment, the invention uses for each fiber link a bidirectional WDM signal composed of channel signals propagating in opposite directions (transmit and receive). At each node, a band optical add-drop multiplexer (OADM) is used to select a first optical band from the WDM signal spectrum for isolating a designated number of optical channels capable of carrying a first set of interleaved transmit and receive channel signals while the remainder portion of the WDM signal spectrum is passed through. A multichannel coupling unit then separates the selected first optical band into the individual optical channels carrying the first set of interleaved transmit and receive channel signals. Each node is then able to communicate bidirectionally with other nodes of the WDM network over the first optical band of the WDM signal by receiving the receive channel signals in a receiver unit and generating the transmit channel signals in a transmitter unit. 
     According to the present invention, the band OADM also operates to combine a second optical band to the remainder portion of the WDM signal spectrum to form a new bidirectional WDM signal also for bidirectional communication with other nodes of the WDM network. To obtain this second optical band, the band OADM is connected to a second multichannel coupling unit where the optical channels carrying a second set of interleaved transmit and receive signals are combined to form the second optical band. The transmit channel signals of the second set of interleaved transmit and receive signals are also generated by the transmitter unit while the receive channel signals are received in the receiver unit for bidirectional communication with other nodes of the network via the new WDM signal. 
     By comparison with existing unidirectional configurations, the present invention advantageously reduces the number of filters and fibers required by half for the first 50% of the channels by interleaving channels within a band, therefore reducing the setup cost for low channel count systems. For high channel count systems, full fill capacity is maintained and at full fill, no additional cost is incurred. 
     Yet another advantage of the present invention over existing unidirectional configurations is that the number of ADM nodes per fiber is reduced therefore improving the optical link budgets and deferring the requirements for optical amplifiers. 
     By comparison with existing bidirectional topologies of comparable channel density, another advantage of the present invention is the resulting reduction in filtering requirements therefore further reducing the cost of the WDM system. In particular, by interleaving channels within a band, the effective channel spacing between adjacent channels of that band is reduced while the associated crosstalk is maintained to an acceptable level. As the effective channel spacing between adjacent channels is reduced, a corresponding reduction in the figure of merit (FOM) of the filters required is achieved which results in lower filtering costs. Alternatively, for a fixed FOM, the effective spacing between adjacent channels within a band can be reduced for providing a higher bandwidth capacity per band. 
     One particular embodiment of the present invention comprises a band OADM for a bidirectional WDM network for processing first and second bidirectional WDM signals each of a respective spectrum formed of multiple optical channels for carrying channel signals in a first direction and channel signals in a second direction. The band OADM is operative for isolating a first optical band from the first WDM signal spectrum, isolating a second optical band from the second WDM signal spectrum, and passing through a remainder portion of the first WDM signal spectrum to form a remainder portion of the second WDM signal spectrum, wherein each of the first and second optical bands contains a respective first set of optical channels comprising at least one optical channel for carrying a channel signal propagating in the first direction and at least one other optical channel for carrying a channel signal propagating in the second direction. Each of the first and second WDM signal spectrum may beneficially be formed of a respective first plurality of optical channels for carrying respective channel signals propagating in the first direction and a second plurality of optical channels for carrying respective channel signals propagating in the second direction, wherein the first plurality of optical channels is interleaved with the second plurality of optical channels. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in greater detail with reference to the accompanying diagrams, in which: 
         FIG. 1A  is a block diagram of a conventional bidirectional wavelength division multiplexed (WDM) ring network; 
         FIG. 1B  is a block diagram of the multichannel add-drop multiplexer of the bidirectional WDM ring network of  FIG. 1A ; 
         FIG. 2  is a block diagram of a bidirectional WDM ring network according to an embodiment of the present invention; 
         FIG. 3A  is the band optical ADM (OADM) of the bidirectional WDM ring network of  FIG. 2 ; 
         FIG. 3B  is a wavelength plot of the wideband demultiplexer transfer function of the band OADM of  FIG. 3A ; 
         FIG. 4A  is a diagram of the narrowband multiplexer/demultiplexer (mux/demux) of the bidirectional WDM ring network of  FIG. 2 ; and 
         FIG. 4B  is a wavelength plot of the transfer function of the narrowband mux/demux of  FIG. 4A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Current dense wavelength division multiplexed (WDM) fiber optic communication networks which use multiple fiber links to provide additional functionality such as bidirectional communication or network connectivity protection are diverse in topology and configuration. The bidirectional dense WDM optical ring network (hereinafter the “bidirectional WDM ring network” or “the bidirectional WDM network”) is an example of a WDM network with multiple fiber links. A conventional bidirectional WDM ring network is shown in  FIG. 1A . This bidirectional WDM ring network uses a first fiber link  4  for providing clockwise communication between a plurality of nodes (only three shown) generally indicated by  1 ,  2 ,  3  and a second fiber link  5  for providing counter-clockwise communication between the same nodes  1 ,  2 ,  3 . 
     Each node  1 ,  2 ,  3  incorporates a pair of add-drop multiplexers (ADMs) generally indicated by  8 ,  9 ,  10 ,  11 ,  12 ,  13 , an optical transmitter unit  7 ,  17 ,  21  comprising a plurality of optical transmitters (not shown) and an optical receiver unit  6 ,  16 ,  20  comprising a plurality of optical receivers (not shown). The nodes  1 ,  2 ,  3  each communicate with the rest of the bidirectional WDM ring network via two WDM signals, each respectively propagating in one of the first and second fibers  4 ,  5 . 
     The WDM signals used for communication in the bidirectional WDM ring network of  FIG. 1A  are each formed of N optical channels and these N optical channels each carry a respective channel signal propagating at particular wavelength. For clarity in describing the bidirectional WDM ring network of  FIG. 1A , the WDM signals will hereinafter be only referred to as comprising N optical channels without any reference to the associated channel signals. It is understood that by referring to the optical channels, reference to the associated channel signals is implied. 
     For clockwise communication, a first WDM signal comprising N optical channels respectively propagating at a particular wavelength is used in the first fiber  4  to transmit data between the ADMs  8 ,  11 ,  12  which are interconnected along the first fiber  4  in a ring configuration. Briefly stated, each ADM  8 ,  11 ,  12  operates to extract (or drop) a desired set of channels K (only three shown) from the WDM signal propagating in the first fiber  4  and feed these K channels to the corresponding optical receiver unit  6 ,  16 ,  20  where each of the K channels is received by a respective one of the plurality of optical receivers contained therein. The channels N–K which are not extracted by a given node  1 ,  2 ,  3  are passed through that particular node  1 ,  2 ,  3  and combined (or added) with a new set of channels K′ generated by the associated optical transmitter unit  7 ,  17 ,  21  to form a new WDM signal for transmission over the first fiber link  4 . 
     For counter-clockwise communication, a second WDM signal of N optical channels is used along the second fiber  5  to transfer data between the ADMS  9 ,  10 ,  13  which are also interconnected in a ring configuration. Similarly to the ADMs  8 ,  11 ,  12  used for clockwise operations, each ADM  9 ,  10 ,  13  functions to extract (drop) a desired set of channels L from the second WDM signal, pass through the remaining N-L channels and add thereto a new set of channels L′ each generated by a respective one of the optical transmitters contained in the associated optical transmitter unit  7 ,  17 ,  21  to form a new WDM signal propagating on the second fiber  5 . 
     The nodes  1 ,  2 ,  3  of the bidirectional WDM ring network of  FIG. 1A  and more particularly the ADMs  8 ,  11 ,  12  used for clockwise communication and the ADMs  9 ,  10 ,  13  used for counter-clockwise communication all have an identical architecture and mode of operation and will be now described below with reference only to the ADM  8  used in the clockwise transmission path. For this description, reference is now made to  FIG. 1B  where the ADM  8  of  FIG. 1A  is shown in more detail. 
     The ADM  8  of  FIG. 1B  is comprised of a plurality of cascaded demultiplexers  29 ,  30 ,  31  (only three shown) connected at one end (demultiplexer  31 ) to receive the WDM signal and respectively coupled to the optical receiver  6  of  FIG. 1A . These cascaded demultiplexers  29 ,  30 ,  31  are connected in series with a plurality of cascaded multiplexers  24 ,  25 ,  26  (only three shown) which are respectively connected to the optical transmitter unit  7  of  FIG. 1A  to produce the WDM signal (at multiplexer  24 ) for further transmission along the first fiber  4 . 
     In operation, the ADM  8  receives the WDM signal composed of N channels which are propagating in the clockwise direction along the first fiber  4  and extracts from it a desired set of channels K while the remaining N–K channels are passed through. The extracted channels K are conveyed to the optical receiver unit  6  (see  FIG. 1A ) for further processing and rerouting. The ADM  8  also operates to combine a new set of channels K′ generated by the optical transmitter unit  7  of  FIG. 1  with the through channels N–K for further transmission in the clockwise direction and along the first fiber  4 . 
     Typically, conventional multiplexers and demultiplexers such as those used in the ADM  8  of  FIG. 1A  are narrowband optical filters, each operating at a fixed wavelength such that only a particular channel occupying a narrowband centered around a particular wavelength can be extracted from or combined with the WDM optical spectrum. As noted above, these narrowband optical filters are connected in series and therefore cause optical losses on the channels N–K passed through. In order to reduce the through loss, a band optical (hereinafter the “band OADM”) can also be used in combination with the narrowband optical filters. The band OADM is further described below in reference with  FIGS. 2 ,  3 A and  3 B. 
     For the ADM  8  of the bidirectional WDM ring network of  FIG. 1A , each demultiplexer  29 ,  30 ,  31  operates to extract one of the desired K channels from the WDM signal for rerouting and further processing by the corresponding optical receiver unit  6  ( FIG. 1A ) while the cascaded multiplexers  24 ,  25 ,  26  respectively function to couple one of the new channels K′ generated by the optical transmitter unit  7  ( FIG. 1A ) into the first fiber  4 . Usually, the new channels K′ are added to the through channels N–K using wavelengths which have not yet been used or wavelengths that have been vacated as a result of the channels being dropped by one of the demultiplexers  29 ,  30 ,  31  or by other demultiplexers located earlier in the transmission path. By coupling these new channels K′ into the first fiber  4 , the new channels K′ are therefore combined to the through channels N–K to form the new WDM signal for clockwise transmission to other nodes  2 ,  3 . 
     Referring now to  FIG. 2 , there is shown a bidirectional WDM ring network which uses a single fiber link  32  according to an embodiment of the present invention. The bidirectional WDM ring network is composed of a plurality of nodes generally indicated by  33 ,  34 ,  35  (only three shown) which are interconnected via the fiber link  32 . 
     For achieving bidirectional communication between the nodes  33 ,  34 ,  35 , a bidirectional WDM signal of a defined spectrum formed by N contiguous optical channels is used in the fiber link  32 . Each of the N optical channels carries a respective channel signal propagating in a particular direction along the fiber link  32 . For clarity, the bidirectional WDM signal used in the bidirectional WDM ring network of  FIG. 2  will hereinafter be referred to as a WDM signal comprising N optical channels without any reference to the associated channel signals. However, it is understood that by referring to the optical channels, reference to the associated channel signals is implied. 
     The N optical channels of the WDM signal are wavelength interleaved such that the channel direction of transmission alternates with the ascending order of channel wavelength. As an example, the WDM signal propagating in the fiber  32  has the even numbered channels propagating in one direction, and the odd numbered channels propagating in the opposite direction. 
     The nodes  33 ,  34 ,  35  of the bidirectional WDM ring network embodiment illustrated in  FIG. 2  implement two functions. First, each node  33 ,  34 ,  35  operates to isolate a first band comprising a set of K contiguous and interleaved channels from the WDM signal optical spectrum. At each node  33 ,  34 ,  35 , K is chosen to be an even number such that the first band isolated from the WDM signal optical spectrum contains an even number of channels. As an example, for the WDM ring network of  FIG. 2 , K is set to 4 (K=4) such that each node  33 ,  34 ,  35  has a first band containing four interleaved channels. To further illustrate this,  FIG. 2  also shows the four channels isolated at node  33  as being respectively labelled channel  1  (ch 1 ), channel  2  (ch 2 ), channel  3  (ch 3 ) and channel  4  (ch 4 ) and interleaved with ch 1  and ch 3  propagating in one direction and ch 2  and ch 4  propagating in the opposite direction. In this particular example, ch 1  and ch 3  are referred to as receive channels while ch 2  and ch 4  are referred to as transmit channels. 
     At each node  33 ,  34 ,  35 , the respective first band isolated is then separated into its constituent transmit and receive optical channels ch 1 , ch 2 , ch 3 , ch 4 . Each node  33 ,  34 ,  35  is then able to communicate bidirectionally with other nodes  33 ,  34 ,  35  of the bidirectional WDM ring network over the first optical band of the WDM signal via the transmit and receive channel signals ch 1 , ch 2 , ch 3 , ch 4 . 
     The second function performed by the nodes  33 ,  34 ,  35  is to respectively combine the remainder portion of the WDM signal optical spectrum (hereinafter the “through band”) containing the interleaved channels N–K with a second band containing a new set of K′ interleaved channels to form a new bidirectional WDM signal also for bidirectional communication with other nodes  33 ,  34 ,  35  of the WDM network. Similarly to the selection of K, K′ is also selected to be an even number such that the second band to be combined to the through band contains an even number of channels. For example, the embodiment shown in  FIG. 2  has K′=4 and therefore the second band at each node  33 ,  34 ,  35  also has four interleaved channels. Again, to further clarify this,  FIG. 2  shows as an example, the four channels forming the second band combined at node  33  to the through band as being respectively labelled ch 1 ′, ch 2 ′, ch 3 ′ and ch 4 ′. Similarly to ch 1 , ch 2 , ch 3  and ch 4  of the first band, these four new channels ch 1 ′, ch 2 ′, ch 3 ′ and ch 4 ′ are interleaved such that ch 1 ′ and ch 3 ′ are propagating in one direction and ch 2 ′ and ch 4 ′ are propagating in the opposite direction. For this particular example, ch 1 ′ and ch 3 ′ are referred to as transmit channels while ch 2 ′ and ch 4 ′ are referred to as receive channels. 
     Node  33  of the bidirectional WDM ring network of  FIG. 2  uses the same spectral range for the first band and the second band. Within that spectral range, the new set of interleaved channels ch 1 ′, ch 2 ′, ch 3 ′ and ch 4 ′ (K′=4) of the second band is combined to the WDM signal spectrum using the same wavelengths than those used by the channels ch 1 , ch 2 , ch 3 , and ch 4  of the first band. 
     In order to perform the band isolation/combination function described above, each node  33 ,  34 ,  35  incorporates a band OADM generally indicated by  36 ,  37 ,  38 . The band OADMs  36 ,  37 ,  38  are interconnected in a ring configuration along the fiber link  32 . 
     At each node  33 ,  34 ,  35 , the band OADM  36 ,  37 ,  38  is connected to a first and second multichannel coupling unit  39 , 40 ,  41 , 42 ,  43 , 44  also referred to as narrowband multiplexer (mux)/demultiplexer (demux) with each narrowband mux/demux  39 , 40 ,  41 , 42 ,  43 , 44  connected to an optical receiver unit  45 ,  48 ,  50  comprising a plurality of optical receivers (not shown) and an optical transmitter unit  46 ,  47 ,  49  comprising a plurality of optical transmitters (not shown). 
     The nodes  33 ,  34 ,  35  and more particularly the band OADMs  36 ,  37 ,  38  and narrowband mux/demux  39 , 40 ,  41 , 42 ,  43 , 44  arrangements used all have an identical architecture and mode of operation and will now be described below with reference to a single band OADM  36  and its associated narrowband mux/demux  39 ,  40 . 
     The band OADM  36  of  FIG. 2  consists of a wideband demultiplexer  51  for isolating the first band from the WDM signal spectrum and a wideband multiplexer  52  for combining the second band to the WDM signal spectrum to form the new bidirectional WDM signal. More specifically, the wideband demultiplexer  51  is externally coupled to node  35  to the WDM signal. The wideband demultiplexer  51  is also externally connected to the first narrowband mux/demux  39  which, in turn, is coupled to both the optical receiver unit  45  and the optical transmitter unit  46 . The second narrowband mux/demux  40  is also coupled to both the optical transmitter unit  46  and the optical receiver unit  45  and to the wideband multiplexer  52 . In addition to being coupled to the second narrowband mux/demux  40 , the wideband multiplexer  52  is also connected to the wideband demultiplexer  51  to form the new WDM signal. 
     Preferably, the band isolating and combining functions are each performed by a wideband optical filter.  FIG. 3A  provides an example of a possible implementation of the band OADM  36  of  FIG. 2  in which wideband optical filters are used. More specifically, the band OADM  36  illustrated therein has a wideband optical filter  53  externally coupled to node  35  ( FIG. 2 ) to receive the WDM signal and also to the first narrowband mux/demux  39  ( FIG. 2 ). Internally, this wideband optical filter  53  is connected to an optional clean-up filter  55  which is, in turn, coupled to another wideband optical filter  54 . The clean-up filter  55  may be used to remove any vestiges of the band dropped from the WDM signal by the wideband optical filter  54 . This prevents interference with the added channels, which could otherwise arise since the added channels may use the same carrier frequencies as were used by the dropped channels. 
     For further information relating to the architecture of the band OADM described above, reference may be made to copending U.S. patent application Ser. No. 09/208,465, entitled “Multichannel Optical Add/Drop Multiplexor/Demultiplexor”, filed Dec. 10, 1998, in the name of D. Danagher. The disclosure of this application is hereby incorporated herein by reference. 
     Referring now back to  FIG. 2 , the operation of the wideband demultiplexer  51  together with the narrowband mux/demux  39  to isolate the first band and separate the first band isolated into its constituent channels ch 1 , ch 2 , ch 3 , ch 4  is identical to the operation of the wideband multiplexer  52  and the narrowband mux/demux  40  for combining the channels ch 1 ′, ch 2 ′, ch 3 ′, ch 4 ′ to form the second band and combining the second band to the WDM signal spectrum and will now be described below with reference to the wideband demultiplexer  51  and its associated narrowband mux/demux  39 . 
     In operation, the wideband demultiplexer  51  receives the WDM signal and functions to isolate the desired first band from the WDM signal spectrum which contains the K interleaved channels. At the narrowband mux/demux  39 , the first band is separated into its constituent receive channels ch 1 , ch 3  and transmit channels ch 2 , ch 4 , which are then respectively coupled to the optical transmitter unit  46  and the optical receiver unit  45  for bidirectional communication over with other nodes  34 ,  35  of the bidirectional WDM ring network over the first optical band of the WDM signal spectrum via the transmit and receive channel signals ch 1 , ch 2 , ch 3 , ch 4 . 
     The through band of the WDM signal spectrum is passed through the wideband demultiplexer  51  to the wideband multiplexer  52  where it is combined with the second band. As a result, the N–K channels are passed through the wideband demultiplexer  51  to the wideband multiplexer  52  to be combined therein with the new set of interleaved channels ch 1 ′, ch 2 ′, ch 3 ′ and ch 4 ′ to form the new WDM signal for bidirectionally communicating with other nodes  2 ,  3  along the fiber  32 . 
     In order to isolate ch 1 , ch 2 , ch 3  and ch 4  with adequate channel isolation, node  33  performs two filtering steps. The first is accomplished by the wideband demultiplexer  51  by isolating from the WDM signal spectrum a band large enough to contain ch 1 , ch 2 , ch 3  and ch 4 . As noted above, to isolate a portion of the WDM signal spectrum containing multiple channels, the wideband demultiplexer  51  may, for example, consist of a wideband optical filter such as that illustrated in  FIG. 3A  with a passband corresponding to the desired band to be isolated.  FIG. 3B  further describes the selection of the first band. Illustrated therein is an example of a transfer function which can be used by the wideband demultiplexer  51  for isolating the first band from the WDM signal spectrum which in  FIG. 3B  is shown as a normalized WDM signal having an optical spectrum extending from λ1 to λ9. For clarity, each of the four channels referred to in  FIG. 2  namely ch 1 , ch 2 , ch 3  and ch 4  has been assigned to a particular wavelength labelled λ1, λ2, λ3, λ4. From this plot, it can be observed that a single channel guard band (λ5) is set aside to reduce channel mixing between adjacent bands (further details below). It can also be observed that the shape of the band OADM  51  filter is such that the isolation provided to the first band from the through band linearly increases moving away from the first band. For example, the isolation provided to the first band from the through channels closest to the outside channels of the first band is shown to be 13 dB while the isolation from the second closest through channels increases to 20 dB. 
     Referring now back to  FIG. 2 , the second filtering step occurs where the first band isolated by the wideband demultiplexer  51  is separated by the narrowband mux/demux  39  into its constituent K interleaved channels. In order to achieve this, the narrowband mux/demux  39  may comprise a chain of optical filters with each filter associated to a specific wavelength so as to separate a particular transmit or receive channel from the first band.  FIG. 4A  illustrates an example of such a scheme. The narrowband mux/demux  39  depicted therein is comprised of four narrowband optical filters  56 ,  57 ,  58 ,  59  with filter  56  coupled to receive the first band from the wideband demultiplexer  51  of  FIG. 2 . The filters  56 ,  57 ,  58 ,  59  are arranged in concatenation and respectively connected to ch 1 , ch 3 , ch 2 , ch 4 . The receive channels, ch 1  and ch 3 , are generated by the optical transmitter unit  46  of  FIG. 2  while the transmit channels, ch 2  and ch 4 , are directed to the optical receiver unit  45  also of  FIG. 2 . 
     In operation, the receive channels, ch 1  and ch 3 , generated by the optical transmitter unit  46  are respectively coupled into the first band by filters  56 ,  57  while the transmit channels, ch 2  and ch 4 , are respectively extracted from the first band by operation of filters  58 ,  5 . 9  and directed to the optical receiver unit  45  for further processing and rerouting. 
     To further describe the operation of the narrowband mux/demux  39  as a whole,  FIG. 4B  shows an example of a transfer function which can be implemented by the narrowband mux/demux  39  for separating the first band signal spectrum into its constituent 4 channels. For clarity,  FIG. 4B  shows the first band signal spectrum normalized and the wavelength-channel assignment used identical to that used in  FIG. 3B  (i.e., λ1, λ2, λ3 and λ4 being respectively assigned to ch 1 , ch 2 , ch 3  and ch 4 ). As noted before, ch 1  and ch 3  are propagating in one direction, and ch 2  and ch 4  propagating in the opposite direction. From this plot, it can be observed that according to the present invention, the isolation provided between co-propagating channels within a band is higher than the isolation provided between adjacent counter-propagating channels within that band. For example, the isolation between co-propagating channels ch 2  and ch 4  is shown to be 30 dB while the isolation between adjacent transmit and receive channels, ch 2  and ch 3  is 15 dB. 
     This double filtering also helps reducing the possibility of channel mixing as between adjacent bands of the WDM signal spectrum. As a result, the width of the guard band necessary can also be reduced. In this particular example, the use of the wideband demultiplexer  51  together with the narrowband mux/demux  39  designed with the above-described filtering isolation requirements only requires a single channel guard band. 
     The presence of the wideband demultiplexer  51  to isolate the first band from the WDM signal spectrum before its constituent transmit and receive channels are actually separated and the wavelength interleaving of the transmit and receive channels within that first band makes it possible to lower the filter isolation requirements of the narrowband mux/demux  39 . 
     In particular, because adjacent channels within a band are counter-propagating, the effective channel spacing between co-propagating transmit or receive channels within that band is twice that featured by an arrangement where adjacent channels are co-propagating. Thus, for a given narrowband mux/demux  39  filter shape, co-propagating channels are spaced further apart from one another (twice the separation) which results in isolation between co-propagating channels higher than necessary. According to the present invention, the effective channel spacing between adjacent channels within a band can therefore be reduced while maintaining the crosstalk figure between adjacent channels to an acceptable level (further details below). As the effective channel spacing between adjacent channels is reduced, a corresponding reduction in the figure of merit (FOM) of the narrowband mux/demux is achieved which results in lower filtering costs. 
     As noted above, the wider separation provided between counter-propagating channels permits a reduction in the effective spacing between adjacent channels with very little effect on the associated crosstalk figure. This is because adjacent channels are counter-propagating and therefore require less isolation from one another as the directionality of the narrowband optical filters  56 ,  57 ,  58 ,  59  used in the narrowband mux/demux  39  of  FIG. 2  is selected to be high enough to maintain the crosstalk between adjacent channels to an acceptable level. Reflections must also be filtered out by the narrowband optical filters  56 ,  57 ,  58 ,  59  but as the optical return loss is typically greater than 20 dB for currently available narrowband optical filters such as those used in the narrowband mux/demux  39  of  FIG. 2 , the crosstalk component caused by reflections initially attenuated by the optical return loss is effectively reduced by the narrowband optical filters  56 ,  57 ,  57 ,  59  to a acceptable level. 
     To further illustrate this, there is provided below an example of a crosstalk calculation for adjacent channels counter-propagating within a band. This will be followed a another crosstalk calculation example for channels counter-propagating in adjacent bands. 
     In a bidirectional WDM ring network designed in accordance with the present invention. The power of a transmit channel such as ch 2  or ch 4  measured at the input of the mux/demux  39  can be as much as 30 dB below that of an adjacent transmit channel. However, the channel isolation between adjacent transmit and receive channels together with losses (transmission and reflection) incurred by the channels as they propagate through the filters  56 ,  57 ,  58 ,  59  ( FIG. 4A ) result in an acceptable crosstalk figure. For the bidirectional WDM ring network of  FIG. 2 , the crosstalk power between a transmit channel and an adjacent receive channel measured at the input of the narrowband mux/demux  39  can be calculated as follows:
 
Crosstalk power=( Ptx−Prx )−2 *F _loss− R _loss− Ch   —   I  
 
where
         Ptx=Power of a transmit channel;   Prx=Power of a receive channel;   F_loss=Filter transmission loss;   R_loss=Optical return loss; and   Ch_I=Channel isolation.       

     Assuming for the particular mux/demux filter arrangement shown in  FIG. 4A  a 30 dB power delta between adjacent transmit and receive channels with the filters  56 ,  57 ,  58 ,  59  selected to provide 15 dB of channel isolation and exhibit a 4 dB transmission loss and a 24 dB reflection loss, the crosstalk power of adjacent transmit and receive channels contained within a band such as, for example, ch 2  and ch 4  or ch 1  and ch 3  is carried out as follows:
 
Crosstalk power=(30 dB)−2*4 dB−24 dB−15 dB
 
     According to the above calculation, the crosstalk figure for adjacent transmit and receive channels within a band is −17 dBc. 
     The crosstalk power between a transmit channel of a band and a receive channel of an adjacent band such as, for example, ch 4  and ch 6  (see  FIG. 3B ) can also be calculated:
 
Crosstalk power=( Ptx−Prx )− Tr _loss ( mux+span+demux )− I (band+channel)
 
where
         Ptx=Power of a transmit channel within a band;   Prx=Power of a receive channel within an adjacent band;   Tr_loss (mux)=Mux filter transmission loss;   Tr_loss (span)=band OADM filter transmission loss;   Tr_loss (demux) Demux filter transmission loss;   I (band)=Band isolation; and   I (channel)=Channel isolation.       

     Assuming for the particular band OADM  36  and mux/demux filter arrangement shown in  FIG. 2  a 30 dB power delta between ch 4  and ch 6 , a 4 dB span loss and the band OADM filter  54  (see  FIG. 3A ) designed to provide 13 dB of band isolation and exhibit a 4 dB band filter and channel filter transmission loss, the crosstalk power of adjacent transmit and receive channels respectively contained within adjacent bands is calculated as follows:
 
Crosstalk power=(30 dB)−(4 dB+4 dB+4 dB)−(13 dB+30 dB)
 
     According to the above calculation, the crosstalk figure for ch 4  and ch 6  is −25 dBc. 
     While the invention has been described above with reference to a particular network topology, further modifications and improvements to support other network configurations 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. 
     In particular, the invention has been described above with respect to a bidirectional WDM ring network. It is understood that the invention could also be applied to other topologies such as linear networks. Further, the invention is not restricted to single fiber network schemes and could also be used in WDM systems with multiple fiber links including networks with dedicated and shared protection as well as unprotected circuits. 
     The WDM signal described above in accordance with the invention has been defined as having N interleaved optical channels where the even numbered channels propagating in one direction, and the odd numbered channels propagating in the opposite direction. It is to be understood that the N channels of the WDM signal could alternatively be wavelength interleaved with the odd numbered channels propagating in one direction, and the even numbered channels propagating in the opposite direction. 
     It is also to be understood that while the WDM signal has been described as having all of its N channels interleaved, other channel arrangements of the WDM signal may be used without departing from the scope of the invention in its broader aspect. In particular, the WDM signal used in accordance with this invention may have the channels K contained in the first band interleaved while the channels N–K contained in the remainder portion of the WDM signal spectrum all propagate in the same direction. Alternatively, the WDM signal used may have the channels K′ contained in the second band interleaved while the channels N–K contained in the remainder portion of the WDM signal spectrum all propagate in the same direction. 
     The first and the second band of the WDM signal have been described above as each having interleaved channels where the number of channels propagating in a particular direction is equal to the number of channels propagating in the opposite direction. It is to be understood that other channel arrangements may be used in each of the first and second band where the number of channels propagating in each direction is different. Further, it is also to be understood that the channels propagating in opposite directions within the first and/or the second band may be arranged not to be interleaved. 
     The bidirectional WDM ring network described above in accordance with the invention operates with a four channel first band and a four channel second band at each node. It is understood that the number of channels in each of the first and second band used at each node can be decreased to a minimum of two channels (one transmit channel and one receive channel) per band or alternatively increased in accordance with the principles enunciated therein to suit the more demanding channel capacity requirements of emerging network applications. 
     At any one node of the bidirectional WDM ring network, the first band may be selected to have the same spectral width than that of the second band. In this situation, it is also understood that the second band may be located within the spectral range vacated by the first band as a result of the first band being isolated from the WDM signal spectrum. Alternatively, the second band may be located in a different spectral range, as for example one which would have been vacated as a result of a band being isolated earlier in the WDM signal transmission path by another node of the bidirectional WDM ring network. 
     Further, if the second band of a node is to be located within the spectral range vacated by the first band isolated at the same node, the new set of interleaved channels K′ contained in the second band may use the same wavelengths as those previously used by the K channels isolated. Alternatively, the channels K′ of the second band may use different wavelengths (albeit within the same spectral range) provided these wavelengths are suited for WDM communications. 
     The nodes of the bidirectional WDM ring network have been described with an first and second band. It is understood that the nodes may alternatively be designed in accordance with the present invention with only a first band or in the further alternative, with only a second band. Still in the further alternative, the nodes may be designed to isolate or combine multiple bands. Accordingly, it is understood that the number of band OADMs and narrowband mux/demux used at each node would have to be adjusted for servicing the additional bands isolated or combined from or to the WDM signal spectrum.