Abstract:
A method and apparatus for assigning channel bands to a wavelength division multiplexed (WDM) system is disclosed. The WDM system includes an arrangement of optical amplifiers, banded optical add-drop multiplexers (OADMs) having blocking filters, and a tunable edge filter. The channel band assignment method suppresses amplified stimulated emissions (ASE) in a partially populated system in which at least one of the blocking filters is not in the transmission path. Channel bands are assigned from one edge of an amplified transmission window to the other in a sequential order and the tunable edge filter is adjusted to attenuate ASE wavelengths not blocked by the blocking filters as channels are assigned. A channel decommissioning method is also disclosed which employs similar techniques and the tunable edge filter to suppress ASE as blocking filters are removed or otherwise deselected from the transmission path.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention generally relates to a method and system for improving an optical communications network. More particularly, the invention relates to a method and system that suppresses ring lasing or ASE (amplified stimulated emission) peaking in a wavelength division multiplexed (WDM) optical communications network. 
     2. Description of Related Art 
     Wavelength Division Multiplexing (WDM) is used to transmit a plurality of optical channels via an optical wave guide medium where each channel carries information signals within a network or system. Each channel within the WDM signal is associated with a particular wavelength (actually, each channel includes a range of wavelengths but is commonly referred to using its center wavelength), thereby increasing the information capacity of fiber optic systems. Such optical systems include, for example, local area networks (LANs), telecommunication systems and cable television systems. Depending on the optical system configuration and the transmission path, various optical and/or electrical amplifiers are required to provide the necessary optical power to efficiently transmit and amplify the WDM signal to each component within the network. 
     WDM optical networks have traditionally been used for long haul point to point (linear) networks. However, with the increasing demands on communication systems, WDM optical networks can also be used in smaller system configurations, such as local or regional telephone or data networks. In these systems, communication signals are usually transmitted over a limited geographic area to various nodes within a network. A particular node can be configured to drop one or more information bearing or payload channels from the WDM signal, process the information contained in the dropped channels and add the channels containing new information to the WDM signal for transmission to other nodes in the network. 
     An optical add/drop multiplexer (OADM) is typically be used at a node to drop one or more channels from the WDM signal while permitting the remaining channels (express channels) to continue on the optical transmission system. Channels may be added to the WDM signal downstream of the dropped (blocked) channels. 
     To drop a channel, an optical blocking filter is used to filter out the channel(s) being dropped from the WDM signal. Various forms and types of such blocking filters are used such as a fiber Bragg gratings (FBG), thin film filters, etc. Such blocking filters are typically transmissive with respect to the express channels (those channels passing through the OADM) and reflective with respect to the dropped channels. U.S. Pat. No. 5,748,349 illustrates one example of a known OADM that utilizes a series of Bragg gratings to drop channels. 
     In metropolitan or regional areas, WDM systems having a ring configuration can be used to provide high capacity data links between several nodes. In other words, WDM networks may be configured in a ring topology in which the channel wavelengths may circulate around the ring. Such ring topologies are typically utilized in metropolitan areas. WDM rings carry multiple optical signals (channels) on a looped optical communication path. 
     It is also customary for wavelengths to be dropped (“blocked”) from the ring by placing an optical filter on the ring. Such a blocking filter presents a large loss in the transmission path over its wavelength interval of interest (stop band), over and above the ring&#39;s fiber loss. Each unique wavelength placed on a ring will have an associated filter to drop it from the ring. 
     Unused wavelengths, however, may not have associated blocking filters deployed on the ring and so the ring&#39;s transmission loss at these wavelengths is the same as the fiber loss. 
     Fiber optic ring networks may employing optical amplifiers (OAs) to overcome the transmission losses associated with the fiber and optical elements situated on the ring. While OAs compensate for transmission loss by amplifying the signals present on the ring, they also generate noise (ASE) over the entire spectrum of the OAs bandwidth. ASE power lying within the blocking filter&#39;s stop band is attenuated. If portions of the optical spectrum are not blocked, then ASE at those unblocked wavelengths will re-circulate around the ring. If the round-trip gain of the ring exceeds unity, then the ASE power at unblocked wavelengths will increase due to positive feedback. The ASE power that grows under such conditions may actually cause lasing, but more commonly, it simply creates power fluctuations for the signals carried on the ring. These power instabilities may disrupt the useful signals&#39; quality of transmission and are undesirable. 
     SUMMARY OF THE INVENTION 
     An embodiment of the invention includes an arrangement of optical amplifiers, banded optical add-drop multiplexers (OADMs), a tunable edge filter, a fiber-optic ring network and a wavelength assignment method that suppresses ASE on a WDM system, thereby eliminating signal amplitude instabilities. The WDM system may be a ring-based architecture having a recirculation path or a linear network architecture. 
     Another embodiment of the invention includes a wavelength division multiplexed optical communication system having a transmission path carrying a WDM signal with a plurality of channel bands, comprising an optical amplifier optically coupled to the transmission path of the system and having an associated amplified transmission window; a plurality of optical add-drop multiplexers each including a drop filter with an associated drop band capable of dropping at least one channel band from the WDM signal; said optical add-drop multiplexers injecting the channel bands into said fiber optic network such that the injected channel bands occupy a first edge portion of the amplified transmission window; a tunable edge filter optically coupled to the transmission path, said tunable edge filter having a pass band and a stop band; and a network manager operatively connected to said tunable edge filter, said network manager adjusting said tunable edge filter such that the stop band of the tunable edge filter substantially blocks ASE wavelengths of unoccupied channel bands. 
     The WDM system may be a ring-based system and the transmission path is recirculation path. The WDM system may also be a linear system having a linear architecture. 
     Another embodiment of the invention includes a wavelength assignment method that suppresses ASE on a WDM system having a transmission path with an amplified transmission window, comprising: assigning a first group of channels to a first channel band occupying a first edge portion of the amplified transmission window; assigning a second group of channels to a second channel band substantially adjacent to the first channel band in a channel adding direction, wherein the channel adding direction is away from the first edge portion of the amplified transmission window; and adjusting a tunable edge filter in the transmission path such that a stop band of the tunable edge filter substantially blocks ASE wavelengths of unassigned channel bands. 
     Yet another embodiment of the invention includes a wavelength decommissioning method that suppresses ASE on a WDM system having a transmission path with an amplified transmission window transmitting a plurality of channels bands assigned to a first edge portion of the amplified transmission window, the method comprising: decommissioning a first channel band from among the plurality of channel bands, the first channel band being substantially adjacent to a second channel band in a channel deleting direction, wherein the channel deleting direction is towards the first edge portion of the amplified transmission window; and adjusting a tunable edge filter in the transmission path such to expand a stop band of the tunable edge filter to substantially block ASE wavelengths of the decommissioned first channel band. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a block diagram of a conventional WDM ring architecture; 
         FIG. 2  is a block diagram of a WDM system according to the invention having a ring architecture; 
         FIG. 3  is a graph of wavelength versus power illustrating the wavelength assignment method and edge filter tuning concepts of the invention; 
         FIG. 4  is a graph of wavelength versus power illustrating the wavelength assignment method and edge filter tuning concepts of the invention; 
         FIG. 5  is a graph of wavelength versus power further illustrating the wavelength assignment method and edge filter tuning concepts of the invention; 
         FIG. 6  is a graph of wavelength versus power still further illustrating the wavelength assignment method and edge filter tuning concepts of the invention; 
         FIG. 7  is a high level flowchart implementing the inventive wavelength assignment method and edge filter tuning for the addition of a new channel band; 
         FIG. 8  is a high level flowchart implementing the inventive wavelength assignment method and edge filter tuning for the decommissioning of an existing channel band; 
         FIG. 9  is a block diagram of another WDM system according to the invention having a ring architecture; 
         FIG. 10  is a high level flowchart implementing the inventive wavelength assignment method and edge filter tuning for the addition of a new channel band; 
         FIG. 11  is a high level flowchart implementing the inventive wavelength assignment method and edge filter tuning for the decommissioning of an existing channel band; 
         FIG. 12  is a block diagram of a tunable edge filter that may be utilized in the invention; and 
         FIG. 13  is a block diagram of a linear WDM system according the invention and showing further applicability of the inventive concepts. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof. 
     The expression “optically communicates” as used herein refers to any connection, coupling, link or the like by which optical signals carried by one optical system element are imparted to the “communicating” element. Such “optically communicating” devices are not necessarily directly connected to one another and may be separated by intermediate optical components or devices. Likewise, the expressions “connection” and “operative connection” as used herein are relative terms and do not require a direct physical connection. 
       FIG. 1  illustrates a typical and conventional ring based system  1 . Nodes A,B,C,D,E,F,G and H are add/drop locations, where channels are added and dropped on the system  1 . Lengths of optical fiber  10  connect the various nodes A–H in a ring-shaped topology. The arrows indicate different communication links from node to node and between nodes and different systems and may be implemented by various conventional optical connections and switching devices to route the channels to the proper destination. As mentioned above, such a conventional ring system  1  may carry a WDM signal. 
       FIG. 2  illustrates the inventive WDM transmission system  2  including nodes A, B, C, and D ( 100 A,  100 B,  100 C, and  100 D) in which lengths of optical fiber  10  connect the nodes in a ring-based architecture. Although not shown the system  2  may also include conventional optical communication links from node to node as in  FIG. 1 . The nodes  100  include any known or future developed optical add/drop multiplexer (OADM) and any known or future developed optical device capable of at least dropping one or more WDM channels. A non-limiting sampling of such known OADMs may be found in U.S. Pat. Nos. 5,748,349; 5,778,118; 6,002,503; 6,069,719; and 6,243,179. 
     The only requirement of an OADM and node  100  for inclusion in the inventive systems is a blocking filter  30  as further shown in  FIG. 2 . The blocking filter  30  may be constructed, as also known in the art, using any device capable of filtering a channel band (e.g. fiber Bragg gratings, dielectric thin film filters, interference filters, arrayed waveguide gratings (AWGs), wavelength selective switches, etc). Such blocking filters typically reflect certain wavelengths (e.g. the wavelengths being dropped) while permitting the remainder to be transmitted. In this way, a channel band may be blocked from the WDM signal and routed to another optical path. 
     Preferably, the blocking filters  30  drop a band of channels. As illustrated using double headed arrows for the nodes  100 , the WDM ring system  2  is generally capable of dropping bands of channels at various points. Such a channel-band architecture is employed to minimize the express loss on the system. Express loss is the loss experienced by channels passing through an optical network element as opposed to the loss experienced by channels being dropped or added to the WDM signal path. Channel band filters which drop multiple wavelengths, as opposed to individual channel filters, are fewer in number and generally result in lower express losses. Generally speaking, however, the band size of the blocking filters  30  can correspond from 1 to N wavelengths depending on the system. 
     At least one of the nodes  100  must also be capable of adding channels to the WDM signal on the network  10  otherwise no traffic would be carried on the system  2 . Although channels are typically added at the same node  100  in which they are dropped this is not a requirement: once a channel is dropped the wavelength range occupied by that channel is generally available such that any downstream node  100  may add channel(s) to that range. 
     Typical OADMs generally have the ability to both add and drop channels. Because the invention focuses on the blocking filters  30  or lack thereof in a partially populated system, a detailed description of channel adding devices and specific architecture of the OADM nodes  100  is not necessary. For the purposes of this invention a “partially populated system” is one in which less that the total number of channel bands and corresponding blocking filters  30  are present. This is illustrated in  FIG. 2  where the flocking filter  30 D is shown in dotted lines indicating that it has not been installed or otherwise switched into the transmission path such that it is capable of dropping its associated channel band (e.g. λ 25 →λ 32 ). 
     Continuing with the inventive architecture shown in  FIG. 2 , a tunable edge filter  50  is disposed on the transmission path of the system  2 . The tunable edge filter  50  receives the WDM signal on transmission path of system  2  and filters the WDM signal according to an edge filter profile as shown in  FIGS. 4–6  and further discussed below. The tunable edge filter  50  may be placed anywhere on the transmission path of system  2 . In more complex system architectures having multiple signal transmission paths it is generally sufficient to place the tunable edge filter  50  on a recirculation path which is defined as a segment of the network in which a signal may recirculate. If more than one recirculation path is present then a tunable edge filter  50  may be disposed on each such recirculation path. 
     The tunable edge filter  50  may be realized as illustrated in  FIG. 12 . The input fiber  601  and output fiber  616  may be spliced into the fiber  10  or connected using a variety of optical connectors as is known in the art. As further shown in  FIG. 12 , the WDM signal carried in the input fiber  601  of tunable edge filter  50  is composed of multiple wavelengths  602 , each wavelength carrying an associated data signal. Light emerges from the input fiber  601  in a cone-shaped beam  603  which is subsequently collimated  605  by a lens  604 . An aperture  606  may be used to limit the width of the beam incident upon the diffraction grating  608 . Each wavelength within the incident beam  607  is dispersed at a unique angle. 
     A mirror  610  which is disposed at an appropriate angle collects the dispersed wavelengths  609  and reflects them  611  towards a focusing lens  614 . A beam stopper  612  is placed before the focusing lens  614  and may partially block a continuous portion of the beam  611  incident upon the focusing lens  614 . The position of beam stopper  612  effectively determines which wavelengths are incident upon lens  614 . The beam stopper&#39;s position may be adjusted along the linear direction  613  by a stepper motor  618  or other equivalent device to vary the transition wavelength of the device  50 . Those wavelengths which are not blocked by beam stopper  612  are focused by lens  614  into a beam  615  which is incident upon the output fiber  616 . An illustrative output signal spectrum is shown  617  which also shows the stop band and pass band are also illustrated in  FIG. 12 . 
     The tunable edge filter  50  includes a control input  55  that accepts a control signal. The control signal commands the stepper motor  618  which, in turn, moves the beam stopper  612  along direction  613  to thereby adjust the stop band and pass band of the tunable edge filter  50 . 
     The invention is not limited to the specific design of the tunable edge filter  50  shown in  FIG. 12 . Instead, the invention is intended to utilize any known or future developed tunable edge filter capable of accepting a control signal via a control signal input  55  and, in response, adjusting a stop band and pass band in the wavelength ranges of interest. The control signal may be supplied by a manual adjustment mechanism accessible by a technician or from a control device such as a conventional network management system adapted to generate such a control signal. 
     The WDM ring  10  of inventive system  2  also includes an optical amplifier  20 . Although two such amplifiers  20  are shown only one is necessary. As mentioned above, it is the amplification along a recirculation path of a partially populated system that leads to a problem solved by the invention. Thus, at least one segment of the system  2  should have an optical amplifier  20 . Optical amplifier  20  is also a conventional element and includes any device that has the ability to increase the signal strength of plural WDM channels without requiring an optical to electrical conversion. Typical non-limiting examples include the EDFA (erbium doped fiber amplifier), EDWA (erbium doped waveguide amplifier), other rare earth or otherwise optically active material doped amplifiers, Raman amps, etc. 
       FIG. 3  further illustrates the concepts of the invention by showing a graph of power versus wavelength for the WDM channel bands. The exemplary system  2  corresponding to the graph of  FIG. 3 , includes blocking filters  30  each of which drops a band of 8 contiguous channels.  FIG. 3  illustrates this channel band drop architecture by showing bands A, B, C and D each of which includes 8 channels as indicated by the arrows and wavelength numbers (λ 1 →λ 24 ). The bands A, B, C and D correspond to channel bands of the WDM signal traveling on the WDM system  2 . 
     Preferably, there is a guard band separating adjacent channel bands (e.g. the space between Band A and Band B). In a fully populated system, the bands (A, B, C, . . . ) substantially cover the entire usable bandwidth of the amplifier  20  and hence ring lasing or ASE peaking is not an issue. However, in partially populated systems where one or more of the bands and the associated blocking filter  30  is not present, a part of amplifier ASE is not blocked and depending on such factors as amplifier ripple and tilt there is a possibility of ASE peaking or ring lasing occurring. The invention operates as follows in order to block any unused channel band wavelengths in the system and thereby prevent ASE peaking and ring lasing. 
     In general, the invention utilizes the technique of populating the blocking filters  30  on the ring  10  from one edge of the amplifier bandwidth to the other edge. This could be from the short wavelength edge to the long wavelength edge or vice versa. This is diagrammatically indicated in  FIGS. 3 ,  4  and  6  with an arrow labeled as the “channel adding direction.” This channel band assignment method works as long as the blocking filters  30  are deployed in a sequential manner in the ring  10 . 
     The physical placement of blocking filters  30  with respect to the ring architecture  2  is not dispositive. Indeed, the order shown in  FIG. 2  (A then B then C then D in a clockwise direction around the ring) is purely for ease of illustration. It is the order of channel assignment and deployment that is a dispositive and inventive factor that is utilized to advantage. 
     When decommissioning a channel from the network, the blocking filters  30  are depopulated in the reverse order as diagrammatically indicated in  FIGS. 5 and 6  with an arrow labeled as the “channel deleting direction.” Further details of these figures will be discussed below in relation to the flowcharts. 
     The amplified transmission window, as further illustrated in  FIG. 3 , may be utilized as the reference to assign channel bands. The amplified transmission window may simply be the amplifier  20  bandwidth. The amplified transmission window may be further refined to account for the portion of the spectrum that incurs amplified transmission in a recirculation path and accounts for the transmission window(s) of the optical transmission media (fiber, connectors, nodes) carrying the WDM signal in addition to the bandwidth(s) of one or more amplifiers  20  on the network. 
     As further shown in  FIG. 3 , the tunable edge filter  50  has an associated edge filter profile composed of an edge filter stop band and an edge filter pass band. In general, the tunable edge filter  50  limits the bandwidth of amplified transmission window of the ring to just the part of the bands that are being used and blocks the bands that are unused. 
     The inventive process generally operates by assigning a first group of channels to a first channel band occupying a first edge portion of the amplified transmission window. For example, channel band A is assigned to the right edge (longest wavelength portion) of the amplified transmission window as shown in  FIG. 3 . A second group of channels may be assigned to a second channel band B substantially adjacent (a guard band may be present as mentioned above) to the first channel band A in a channel adding direction. The channel adding direction is quite clear in  FIG. 3  and subsequent channel assignments will occur in sequence along this direction as shown and as further illustrated in  FIG. 4 . 
     The assignment of a second or subsequent channel band calls for adjusting the tunable edge filter  50  such that a stop band of the tunable edge filter substantially blocks ASE wavelengths of unassigned channel bands. As shown in  FIG. 3 , the spectrum of the amplified transmission window includes unassigned channel bands D, E, and F. The tunable edge filter  50  is preferably adjusted so that the wavelengths of the unassigned channel bands are blocked such that any ASE power in the unassigned bands is also blocked. 
       FIG. 7  illustrates a formal process for assigning a channel band to the system  2 . Starting ( 200 ) the process of assigning a channel band may occur whenever a network engineer or other person desires to add a channel band to a partially populated system  2 . The channel band assigning process includes the addition of a blocking filter  30  to the network for dropping the channel band being assigned and continues by identifying ( 230 ) the frequency of the leading edge. The leading edge is shown in  FIG. 3  and may include the wavelength of the channel at the outer limit of the assigned channel bands (e.g. in  FIG. 3  the frequency of λ 4 ) and the bandwidth of that channel (WDM channels are often referred to in terms of the center wavelength and have an associated bandwidth albeit relatively narrow). The leading edge may also account for the shape of the edge filter profile so as to not attenuate the assigned channel band with any portion of the tunable edge filter stop band. The leading edge may also account for any guard band that may be present. The leading edge may be identified ( 230 ) manually by a technician surveying the system or by referring to a channel information database  80  as discussed in more detail below in relation to  FIGS. 9 and 10 . 
     The inventive process may then adjust ( 240 ) the tunable edge filter such that the edge of the stop band substantially matches the leading edge. This is illustrated by comparing  FIGS. 3 and 4 .  FIG. 4  adds two channel bands (D and E) to the existing channel bands (A, B and C) illustrated in  FIG. 4 . As can be seen the stop band of the edge filter is adjusted such that it substantially matches the leading edge of the newest channel band (E). The adjusting ( 240 ) may be performed manually by a technician supplying the control signal but it is preferred that the control signal be fed to the control input  55  of the tunable edge filter  50  from a node or network manager  70  as further discussed in relation to  FIGS. 9 and 10 . The process is then finished ( 290 ) the channel band assignment process. 
       FIGS. 5 and 8  illustrates a channel band decommissioning process. Starting ( 300 ) the decommissioning process may occur whenever a network engineer or other person desires to decommission a channel band from system  2 . The process may also start ( 300 ) at the initiation of a network manager  70  as further discussed below in relation to  FIGS. 9 and 11 . Decommissioning is a distinct concept from dropping channels. The decommissioning process involves adjustments to the network such that a particular channel band is no longer present on the system  2 . This process starts ( 300 ) by removing (depopulating) one of the blocking filters  30  (the one responsible for dropping the channel being decommissioned) and continues by identifying ( 230 ) the frequency of the trailing edge. 
     The trailing edge is shown in  FIG. 5  is generally the shortest wavelength of the channel band(s) remaining after decommissioning. In the illustrations of  FIGS. 4 and 5  channel band E is being decommissioned and the trailing edge of the remaining channels is λ 32 . Like the leading edge, the trailing edge may include the wavelength of the channel at the outer limit of the assigned channel bands (after decommissioning) and the bandwidth of that channel (WDM channels are often referred to in terms of the center wavelength and have an associated bandwidth albeit relatively narrow). The trailing edge may also account for the shape of the edge filter profile so as to not attenuate the remaining, assigned channel band(s) with any portion of the tunable edge filter stop band. The trailing edge may also account for any guard band that may be present. The trailing edge may be identified ( 330 ) manually by a technician surveying the system or by a network manager  70  referring to a channel information database  80  as discussed in more detail below in relation to  FIGS. 9 and 10 . 
     The inventive process may then adjust ( 350 ) the tunable edge filter such that the edge of the stop band substantially matches the trailing edge. This is illustrated by comparing  FIGS. 4 and 5 .  FIG. 5  decommissions channel band E with respect to  FIG. 4 . As can be seen the stop band of the tunable edge filter  50  is adjusted such that it substantially matches the trailing edge of the remaining channel band (D). The adjusting ( 350 ) may be performed as described above in relation to step  240 . The process is then finished ( 390 ) the channel band decommissioning process. 
       FIG. 6  illustrates an alternative to the invention. Namely, the channel adding direction of the previous figures is from long to short wavelengths.  FIG. 6  reverses this direction and adds channels from the short to the long wavelengths. The important point is that the channels are assigned beginning from one edge portion (short or long) of the amplified transmission window and moving towards the other edge portion (long or short) in a sequential manner. Likewise, the channel bands are decommissioned from one end of the amplified transmission window to the other as indicated by the channel deleting direction shown in  FIGS. 6 and 5 . The choice between the options may be affected by the use of a service channel. 
     As discussed below in more detail, service channels are conventionally used to transmit housekeeping data around the network. Typical wavelengths for service channels include 1625 nm and 1310 nm. The amplified transmission window may center around, for example, 1550 nm. If a 1625 nm service channel is used then the tunable edge filter  50  should have a pass band that encompasses this wavelength. The edge filter profile shown in  FIG. 6  may not be appropriate for a 1625 nm service channel in this example since the stop band overlaps the service channel wavelength. In this example, an edge filter profile like the one shown in  FIGS. 4–6  is preferred. 
       FIG. 9  illustrates a second embodiment of the inventive WDM transmission system  3  including nodes A, B, C, and D ( 100 A,  100 B,  100 C, and  100 D) in which lengths of optical fiber  10  connect the nodes in a ring-based architecture. Many of the components are similar or identical to those shown in system  2  illustrated in  FIG. 2  and described above in detail. Thus, only the differences will be explained further. 
     One such difference is the addition of a network manager  70 . The network manager  70  may be based on conventional network management software applications that monitor and control WDM optical networks. An example of a system having such a network manager is shown in U.S. Pat. No. 6,163,392. In essential part, the network manager  70  is a software application adapted to the functionality of the invention, running on a conventional microprocessor based computer architecture and having the ability to communicate with the system  3 . 
     Specifically, the network manager  70  may interface with a service channel modem (SCM)  90  which provides an optical communication link with the nodes  100  over a service channel. A service channel, usually outside the information bearing or payload channel bandwidth, is typically included in optical systems for carrying system housekeeping information, for example network monitoring and control signals as well as telemetry information, to and/or from nodes in the network. Because the service channel wavelength is outside the payload channel bandwidth, separate components configured to receive and process the service channel are typically required. Examples of known service channels and related components for handling them are described in U.S. Pat. Nos. 5,532,864; 5,798,855; and 6,141,125. 
     Alternatively or in conjunction therewith, the network manager  70  may communicate with the nodes (e.g. node  110 B) using an electrical signal carried over, for example, an overlay IP-based network  95 . With one or more of these electrical and communication pathways, the network manager  70  may gather information about the network  3  such as which channel bands have been assigned, which are being decommissioned, and the frequencies of at least the leading and trailing edges of the channel bands. This information may be stored in channel information database  80  which is essentially a memory device operatively connected to the network manager  70 . 
     The network manager  70  also includes an edge filter tuning application  75  which may access the channel information database  80  and adjust the tunable edge filter  50  via control input  55 . The specific functions performed by the edge filter tuning application  75  are described in relation to the flowcharts of  FIGS. 10 and 11 . 
     As further shown in  FIG. 9 , some of the nodes may include more than one blocking filter. For example, node D ( 100 D) includes blocking filters  30 D,  30 E,  30 F, and  30 G. More accurately, node D includes the ability to add blocking filters  30 D,  30 E,  30 F, and  30 G as indicated by the dotted lines used to illustrate these filters. Thus, the system  3  is a partially populated system. Moreover, it is generally preferable from a system control standpoint to have the ability to switch blocking filters into and out of the WDM signal path under the control of network manager  70  and via the service channel and/or the electrical network  95 . 
       FIG. 10  illustrates the processes performed by the edge filter tuning application  75  of the network manager  70  when assigning a channel band. Starting ( 400 ) the process of assigning a channel band may at the initiation of a network manager  70  in response to a request from an operator or automatically. The network manager  70  then determines ( 410 ) the existing channel bands. This may be accomplished by referred to the channel information database  80  which is a repository of channel information. The channel information may be gathered by the network manager  70  by communicating with the nodes  100  via the service channel. Once the existing channel bands are determined the network manager selects ( 420 ) the next blocking filter  300  in the channel adding direction. This may be accomplished by sending a control signal over the service channel to the appropriate node  100  having the next selectable blocking filter  30  in the sequence (channel adding direction). 
     The network manager  70  also identifies ( 430 ) the frequency of the leading edge by referring to the channel information database  80  which stores the leading and trailing frequencies of channel bands or at least keeps track of a current leading frequency. The network manager may then adjust ( 440 ) the tunable edge filter  50  such that the edge of the stop band substantially matches the leading edge by sending a control signal to tunable edge filter  50  via control signal input  55  (e.g. optically via the SCM  90  or electrically using a direction connection as shown or via an electrical network  95 ). 
     Optionally, the network manager  70  may also enable the launch of channel(s) in the channel band of the selected blocking filter  30 . An enabling/disabling mechanism may be useful to prevent channel launch until the system is properly tuned and stabilized. The process is then finished ( 490 ) an iteration the channel band assignment process. Of course, the process may be repeated to assign additional channel band(s). 
       FIG. 11  illustrates a channel band decommissioning process performed by the network manager  70 . Starting ( 500 ) the decommissioning process may at the initiation of a network manager  70  in response to a request from a user or automatically. After determining ( 510 ) which channel bands currently exist in the system  3 , the network manager deselects ( 520 ) an appropriate one of the blocking filters  30  (the one responsible for dropping the channel being decommissioned) and continues by identifying ( 530 ) the frequency of the trailing edge. The determining ( 510 ) and identifying ( 530 ) steps are largely the same as steps  410  and  440  with the exception being the identification of a trailing edge rather than a leading edge. The deselecting ( 520 ) is generally the reverse process of selecting ( 420 ). As mentioned above, it is generally preferable from a system control standpoint to have the ability to switch blocking filters into and out of the WDM signal path under the control of network manager  70  and via the service channel and/or the electrical network  95  and the deselecting ( 520 ) may utilize such a technique. 
     The network manager  70  may then adjust ( 550 ) the tunable edge filter  50  such that the edge of the stop band substantially matches the trailing edge. The control signal input  55  is used for this purpose and is fed a control signal by the network manager  70  as shown. The process is then finished ( 590 ) the channel band decommissioning process. 
       FIG. 13  illustrates a third embodiment of the inventive WDM transmission system  4  including nodes A, B, C, and D ( 100 A,  100 B,  100 C, and  100 D) in which lengths of optical fiber  10  connect the nodes in a linear architecture. Many of the components are similar or identical to those shown in system  2  illustrated in  FIG. 2  and described above in detail. Thus, only the differences will be explained further. 
     The linear WDM transmission system  4  shown in  FIG. 13  may be part of a much larger network as indicated by the connection points  4  and  5 . Like the ring systems  2  and  3 , linear system  4  may be a “partially populated system” in which less that the total number of channel bands and corresponding blocking filters  30  are present. This is illustrated in  FIG. 13  where the flocking filters  30 C and  30 D are shown in dotted lines indicating that it has not been installed or otherwise switched into the transmission path. Thus, the corresponding channel bands (e.g. λ 27 →λ 24  and λ 25 →λ 32 ) are not blocked and ASE from the amplifiers  20  may experience peaking and interfere with system  4  performance. The invention solves this problem by limiting the ASE bandwidth using the same techniques of channel assignment, adjusting the tunable edge filter  50  and channel decommissioning that are described above in detail. 
     The invention has been described with reference to particular examples but is not limited thereto. For example, the number of channels per channel band is not limited to eight per band and can include any number from 1 to N. Generally, however, plural channels per band are preferred to minimize express loss. Furthermore, the channel bands need not have the same number of channels per band such that one channel band may have eight channels while another may have 16 channels. Still further the number of nodes  100  may be varied and is not limited to the illustrative examples shown in the figures. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.