Filter topologies for optical add-drop multiplexers

A filter topography is possible that can reduce the overall cost of a single Optical Add-Drop Multiplexer (OADM) or a series of OADMs that are within a network. The key is to have one of the wavelengths of a WDM signal being input to a filtering apparatus empty. The filtering apparatus can then with a reduced cost filter the received WDM signal such that a channel at a wavelength adjacent to the empty wavelength is removed and a channel is inserted at the previously empty wavelength. In one implementation of this topology within an OADM, a single filter can be used to perform both filtering operations due to the empty wavelength ensuring no corruption of the removed channel at the adjacent wavelength. In another implementation, the filtering operations are performed by two separate filters that each have asymmetrically reduced tolerances. In either case in which there is a reduced number of filters or reduced tolerances for those filters, a reduction in cost will be found.

FIELD OF THE INVENTION
 This invention relates generally to filter designs and more specifically to
 both apparatus and network level filter topographies for Optical Add-Drop
 Multiplexers (OADMs).
 BACKGROUND OF THE INVENTION
 The amount of information communicated over an optical fiber communication
 system is increased with the use of optical wavelength division
 multiplexing. Wavelength Division Multiplexed (WDM) systems employ WDM
 signals consisting of a number of optical signals at different
 wavelengths, hereinafter referred to as channels or information carrier
 signals, to transmit information on optical fiber cables. Each channel is
 modulated by one or more information signal, resulting in the capability
 to transmit a significant number of information signals over a single
 optical fiber cable. It is recognized that although a WDM signal comprises
 a plurality of wavelengths capable of carrying channels, not all the
 wavelength must contain a channel.
 To facilitate the subtraction and/or addition of particular channels to
 and/or from the WDM signal at different points within a network, OADMs are
 employed that consist of a plurality of optical filters. These OADMs are
 used to selectively extract Channels, hereinafter referred to as drop
 channels, from a WDM signal while the remaining channels, hereinafter
 referred to as through channels, travel through. The OADMs can also be
 used to add channels, hereinafter referred to as add channels, to a WDM
 signal, using wavelengths that have been vacated as a result of channels
 being dropped at the OADM in question or at an OADM earlier in the
 transmission path. Since more than one channel usually needs to be
 accessed at a network node, multi-channel OADMs are used such that a
 plurality of channels can be dropped and/or added from and/or to a
 received WDM signal.
 There are a number of different implementations for a multi-channel OADM.
 One key factor that must be considered when considering different
 implementations is the cost of the filters that are utilized. Filters
 increase in cost as their Figure of Merit (FOM) increases, their FOM being
 a measure of the complexity of the filter. One skilled in the art would
 understand that the FOM increases as the tolerance of the filter that is
 used increases. One factor that can cause an increase in the required
 tolerance is an increase in the ratio of the passband to dead band,
 described herein below. Hence, when considering the cost of any particular
 OADM, one must consider the number of filters required and the overall
 tolerance of those filters.
 With reference to FIGS. 1 and 2, well-known implementations for OADMs are
 now described. Firstly, FIG. 1 depicts an OADM that comprises a separate
 channel demultiplexer 102 and channel multiplexer 104. For this OADM
 implementation, the demultiplexer 102 extracts channels from a WDM signal
 while the multiplexer 104 inserts channels within the WDM signal output
 from the demultiplexer 102.
 The channel demultiplexer 102, in this case with four channels to be
 dropped, comprises a first alignment block 106, first and second
 columnating lenses (CL) 108,110, first, second, third and fourth drop
 filters (DF) 112a,112b,112c,112d and first, second, third and fourth
 clean-up filters (CF) 114a,114b,114c,114d. The alignment block 106 is
 utilized to ensure that beams of light being transmitted between the
 filters and columnating lenses are aligned properly for optimal
 performance. The first columnating lens 108 receives an input WDM signal
 S.sub.IN (t) in a form capable of being transmitted on a fiber optic
 cable, transforms the signal into an extended beam signal, and transmits
 the extended beam WDM signal in the direction of the first drop filter
 112a. In the example being shown in FIG. 1, the drop filter 112a receives
 the extended beam WDM signal, filters out a channel at wavelength
 .lambda.1 with the use of a single wavelength filter, and forwards the
 remainder of the extended beam WDM signal onto the next drop filter 112b.
 The isolation of the channel at wavelength .lambda.1 is not perfect and so
 an additional filter may be required to ensure that only the required
 channel is sent on for further processing, in this case this is done with
 the first clean-up filter 114a. Additional pairings of drop and clean-up
 filters proceed within the channel demultiplexer 102 of the OADM, each
 operating similar to that of the first drop and clean-up filters 112a,114a
 but for different wavelengths (.lambda.2,.lambda.3,.lambda.4). In the case
 depicted in FIG. 1, after four drop filters the resulting extended beam
 WDM signal is received by the second columnating lens 110 which converts
 the signal to a form transmittable over fiber optic cable. The signal
 output from the second columnating lens 110, although not carrying the
 channels that were dropped within the demultiplexer 102 of the OADM, still
 may contain channels at other wavelengths.
 The channel multiplexer 104 of the OADM of FIG. 1 comprises a second
 alignment block 116, third and fourth columnating lenses 118,120 and
 first, second, third and fourth add filters 122a,122b,122c,122d. The
 second alignment block 116 operates in a similar manner to the first
 alignment block 106, as do the third and fourth columnating lenses 118,120
 with respect to the first and second columnating lenses 108,110
 respectively. Each of the add filters 122a,122b,122c,122d are single
 wavelength filters that insert channels at the wavelengths .lambda.1,
 .lambda.2, .lambda.3, and .lambda.4 respectively. An output WDM signal
 S.sub.OUT (t) that is similar to the input WDM signal S.sub.IN (t) but
 with different channels at wavelengths .lambda.1 through .lambda.4 is
 transmitted from the fourth columnating lens 120.
 With the separate channel demultiplexer and multiplexer 102,104, a large
 number of filters are required. For each of the drop channels, two single
 wavelength filters are required, while for each of the add channels, one
 single wavelength filter is needed. For the case shown in FIG. 1 with four
 channels being dropped and then subsequently added, twelve single
 wavelength filters are used. In general, it can be seen that for N
 channels being dropped and added, 3N single wavelength filters are
 required. In addition, if higher through isolation is required, an
 additional single wavelength filter per wavelength would be required. This
 would bring the total to 4N single wavelength filters required for this
 design.
 There are other well-known implementations for the OADM depicted within
 FIG. 1 that would result in a similar number of filters being required.
 One such implementation does not utilize the alignment blocks 106,116, but
 rather has columnating lenses on either side of each filter element.
 Instead of the WDM signal being transported from filter to filter within
 extended beam format, fiber optic cable is used.
 Another well-known implementation for an OADM is a band OADM as depicted
 within FIG. 2. In this implementation, an input WDM signal S.sub.IN (t) is
 received at a first band filter 202 which extracts a band of wavelengths,
 in this case wavelengths .lambda.1 through .lambda.4, and passes a WDM
 signal comprising the remaining channels to an isolation filter 204. The
 isolation filter 204 ensures that no channels are at wavelengths .lambda.1
 through .lambda.4 without extracting or inserting any channels at other
 wavelengths. Subsequently, the isolation filter 204 outputs the resulting
 WDM signal to a second band filter 206 at which point a band of
 wavelengths, in this case .lambda.1 through .lambda.4, are inserted,
 generating an output WDM signal S.sub.OUT (t)
 The channels that are extracted at the first band filter 202 are separated
 by a series of single wavelength drop filters 208a,208b,208c,208d. These
 individual channels are then further filtered with respective clean-up
 filters 212a,212b,212c,212d and output for further processing. As depicted
 in FIG. 2, the channels inserted at the second band filter 206 are
 combined prior to the insertion with the use of a series of single
 wavelength add filters 212a,212b,212c,212d.
 Similar to the OADM of FIG. 1, the band OADM of FIG. 2 has a large number
 of required filters. As can be seen for the case within FIG. 2, twelve
 single wavelength filters are required for the drop, clean-up, and add
 filters along with three four wavelength filters for the first and second
 band filters 202,206 and the isolation filter 204. In general, it can be
 seen for the case of a band of N wavelengths being dropped and added,
 there would be 3N single wavelength filters and three N wavelength filters
 required.
 There are some network situations in which only a small number of filters
 are required in any one OADM. One such well-known network is depicted in
 FIG. 3. This network comprises a hub 302 and first, second, and third
 network nodes 304,306,308. As depicted in FIG. 3, the hub 302 communicates
 independently with each of the nodes 304,306,308 through the fiber optic
 cables 310,312,314,316. These cables are connected up within a ring
 configuration that has all transmissions in one direction. In this case,
 the hub 302 comprises a multi-channel OADM capable of dropping and adding
 the channels at wavelengths .lambda.1, .lambda.2, and .lambda.3. The nodes
 304,306,308 each comprise a single wavelength OADM for dropping and adding
 channels at respective wavelengths .lambda.1, .lambda.2, and .lambda.3.
 Hence, it can be seen that the hub 302 communicates with the first node
 304 via the channel at wavelength .lambda.1, the second node 306 via the
 channel at wavelength .lambda.2, and the third node 308 via the channel at
 wavelength .lambda.3.
 The key difficulty with the network configuration as depicted within FIG. 3
 is the cost of the OADM within each of the network nodes. Since the same
 wavelength that is being dropped is also being added at the same node, the
 tolerance of the single wavelength filters within the OADMs must be
 extremely high to prevent cross-talk problems. Alternatively, isolation
 filters are required.
 Since the cross-talk possibility is extremely high in this configuration, a
 typical solution is to use a first set of channels for transmission from
 the hub 302 to the nodes 304,306,308 and another set of channels for
 transmission from the nodes to the hub. For instance, this could be done
 by using wavelengths .lambda.1, .lambda.2, and .lambda.3 for transmitting
 data to the respective network nodes 304,306,308, similar to that shown in
 FIG. 3, and using wavelengths .lambda.4, .lambda.5, and .lambda.6 for
 transmitting data to the hub 302 from the respective nodes 304,306,308.
 The key problem with this is that the bandwidth efficiency of the network
 becomes only 50% as at any one time only half the channels are being used.
 A further solution to improve the cost of OADMs is not to insert channels
 within adjacent wavelengths. Such unused wavelengths, commonly called dead
 bands, allow a relaxation of the tolerances required for the filters used
 by decreasing the ratio of passband to dead band. Unfortunately, at the
 same time as reducing the cost of the filters used by reducing the
 tolerances needed, the bandwidth efficiency of the overall network is
 significantly reduced. For every used wavelength there is an unused
 wavelength, making the efficiency 50%. In situations where the bandwidth
 of the network is critical such a low bandwidth efficiency is not
 acceptable.
 OADM designs and network configurations of OADMs are required that reduce
 the overall cost of the network while not limiting the bandwidth
 efficiency. To accomplish this, the number of filters and the tolerance of
 the filters must be reduced without significantly sacrificing the limited
 bandwidth of the network.
 SUMMARY OF THE INVENTION
 The present invention has specific filter topologies at the apparatus
 and/or network level of a WDM network to mitigate one or more of the
 disadvantages with regard to the present invention. The present invention
 takes advantage of wavelengths within a WDM signal that do not have
 channels. The key to the present invention is to organize the filters
 within an apparatus or a network so that a filtering apparatus receives a
 WDM signal with an empty wavelength. In a first possibility, this allows
 multiple tasks to be possible during a single filtering operation, such as
 inserting a channel and removing another channel with a single filter. In
 a second possibility, the tolerance of filters within the filtering
 apparatus can be reduced. In either case, the result should be a lower
 cost due to either a reduced number of filters or reduced tolerances on
 those filters used.
 The present invention, in a first broad aspect, is a method of inserting a
 channel into a WDM signal. The first step of the method is to receive a
 first WDM signal that has no channel at a first wavelength. Next, the
 first WDM signal is filtered so that a channel at a second wavelength
 adjacent to the first wavelength is removed from the WDM signal and a
 channel is inserted at the first wavelength. This results in the
 generation of a second WDM signal. In one preferable embodiment, the
 filtering of the first WDM signal is done with use of a single filter
 which has a passband comprising both the first and second wavelengths. In
 another preferable embodiment, the filtering is done with two filters with
 passbands comprising the first and second wavelengths respectively that
 have asymmetrically reduced tolerances.
 In a second broad aspect, the present invention is a filtering apparatus
 that is arranged to implement the method of the first broad aspect. In
 this case, the filtering apparatus filters the first WDM signal so that a
 channel at a second wavelength adjacent to the first wavelength is removed
 from the WDM signal and a channel is inserted at the first wavelength.
 Further, the filtering apparatus outputs a second WDM signal generated
 from the filtering of the first WDM signal that has no channel at the
 second wavelength. Once again, in one embodiment this filtering is done
 with a single filter while in another embodiment it is done with two
 filters with lower tolerances.
 In other aspects, the present invention is an OADM or a network node that
 incorporates one or more of the filtering apparatuses of the second broad
 aspect. In one preferred embodiment, the present invention is an OADM
 which includes an initial filter, a number of intermediate filters that
 are consistent with the filtering apparatus of the second broad aspect,
 and a final filter, the OADM performing both demultiplexing and
 multiplexing operations on a WDM signal. In another preferred embodiment,
 the present invention is a network that includes a hub and a number of
 network nodes that further include a filtering apparatus according to the
 second broad aspect.
 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 To reduce the costs of OADMs, either the number of filters required and/or
 the tolerances needed must be reduced. The key to one preferred embodiment
 of the present invention is to configure filters within an OADM so that
 the number of filters can be reduced compared to previous implementations
 with similar functionality. The key to another of the preferred
 embodiments of the present invention is to configure the OADMs within a
 network so as to reduce the required tolerances for the filters within the
 OADMs of the network nodes.
 The configurations described herein below for the preferred embodiments of
 the present invention utilize the lack of a channel at a particular
 wavelength, hereinafter referred to as an empty wavelength, as an
 advantage during a filtering stage for a WDM signal. In a first preferred
 embodiment, the lack of a channel at a particular wavelength within the
 received WDM signal allows the filtering stage to remove a channel from a
 wavelength adjacent to the empty wavelength while inserting a new channel
 at the previously empty wavelength with the use of only a single filter.
 In a second preferred embodiment, the existence of an empty wavelength
 within the received WDM signal allows the next filtering stage to perform
 the functions of the first preferred embodiment but with two filters of a
 lower tolerance than previous implementations and hence a lower cost.
 An OADM according to the first preferred embodiment described briefly above
 is now described in detail with reference to FIG. 4. This OADM comprises
 an alignment block 402, first and second columnating lenses (CL) 404,406,
 first and second single wavelength filters (1.lambda. F) 408a,408b, first,
 second, and third two wavelength filters (2.lambda. F) 410a,410b,410c, and
 first, second, third, and fourth clean-up filters (CF)
 412a,412b,412c,412d. The alignment block 402 is used to ensure that the
 filters and columnating lenses remain on two planes that are perfectly
 parallel, thus allowing for light beams to be properly angled.
 The first columnating lens 404 receives an input WDM signal S.sub.IN (t)
 which comprises a plurality of channels at different wavelengths. The
 columnating lens 404 converts the WDM signal S.sub.IN (t) from a form
 transmitted on a fiber optic cable to an extended beam WDM signal and
 transmits the extended beam WDM signal to the first single wavelength
 filter 408a via the alignment block 402.
 The single wavelength filter 408a is a three port filter in which a
 particular channel, in this case at wavelength .lambda.1, is extracted
 from the extended beam WDM signal and forwarded to the first clean-up
 filter 412a. The clean-up filter 412a ensures that only the channel at
 wavelength .lambda.1 is sent for processing while any other wavelengths
 extracted unintentionally at the first single wavelength filter 408a are
 removed. The resulting extended beam WDM signal with the channel at
 wavelength .lambda.1 removed is reflected at the first single wavelength
 filter 408a and transmitted via the alignment block 402 to the first two
 wavelength filter 410a. This resulting WDM signal, although comprising the
 same wavelengths as the WDM signal received at the first single wavelength
 filter 408a, is in fact a different WDM signal since it contains different
 channels. For simplicity, the WDM signals generated between filters in the
 OADM of FIG. 4 are herein below referred to as intermediate WDM signals.
 The first two wavelength filter 410a is a four port filter that receives
 the intermediate signal output from the filter 408a and filters it such
 that a particular channel, at wavelength .lambda.2 in FIG. 4, is extracted
 while a channel is inserted at the wavelength removed at the previous
 filter 408a, in this case wavelength .lambda.1. Similar to the operation
 of the first clean-up filter 412a, the second clean-up filter 412b is used
 to further filter the extracted channel at wavelength .lambda.2. The
 insertion of the channel at wavelength .lambda.1 is possible with the same
 filter as the extraction of the channel at wavelength .lambda.2 since the
 wavelengths are adjacent and the channel at wavelength .lambda.1 has
 previously been removed and so will not contaminate the extraction of the
 channel at wavelength .lambda.2. With a passband that comprises both
 wavelength .lambda.1 and .lambda.2, the first two wavelength filter 410a
 can filter off the channels at these wavelengths from the received
 intermediate WDM signal, in this case only the channel at wavelength
 .lambda.2 remains, and can insert channels at these wavelengths, in this
 case a channel at wavelength .lambda.1. If a channel was also inserted for
 wavelength .lambda.2, cross-talk problems could result with the inserted
 channel at wavelength .lambda.2 also being extracted. The end result of
 these operations is an outputting of a further intermediate WDM signal.
 The second and third two wavelength filters 410b,410c and third and fourth
 clean-up filters 412c,412d operate in a similar manner as the first two
 wavelength filter 410a and the second clean-up filter 412b respectively.
 The second two wavelength filter 410b and third clean-up filter 412c
 operate together to extract the channel at wavelength .lambda.3 and insert
 a channel at wavelength .lambda.2. The third two wavelength filter 410c
 and fourth clean-up filter 412d operate together to extract the channel at
 wavelength .lambda.4 and insert a channel at wavelength .lambda.3. After
 these filters, the second single wavelength filter 408b receives the
 intermediate WDM signal output from the third two wavelength filter 410c
 and inserts a channel at wavelength .lambda.4, the wavelength that
 corresponds to the channel removed at the filter 410c. The resulting WDM
 signal is then reflected to the second columnating lens 406 which converts
 the signal into a form that can be transmitted along a fiber optic cable
 as output WDM signal S.sub.OUT (t).
 The key to the preferred embodiment OADM of FIG. 4 is the removal of a
 channel at a wavelength directly prior to the insertion of a channel at
 the same wavelength. The same filter (a four port two wavelength filter)
 may be used for the insertion of a channel at the previously extracted
 wavelength and the extraction of a channel at an adjacent wavelength.
 There are a number of problems with this embodiment, but the advantages as
 will be described herein below in most cases out way these problems. One
 problem with the embodiment of FIG. 4 is the difficulty of aligning the
 four port filters required to be used for the two wavelength filters.
 Another problem is the increased cost required with the use of these two
 wavelength filters due to the increased passband needed, that being a
 passband wide enough for two wavelengths.
 There are a number of important advantages of the OADM as depicted in FIG.
 4 over the previous implementations. For one, the number of filters is
 significantly reduced. It can be seen in FIG. 4 that only nine filters
 were required to operate an OADM for four wavelengths while in the OADM
 implementations of FIGS. 1 and 2 twelve and fifteen filters were needed
 respectively. In general, with N wavelengths being extracted and inserted
 within the embodiment of FIG. 4, it can be seen that 2N+1 filters would be
 required. Therefore, as the number of wavelengths requiring extraction and
 insertion increases, the advantages of the preferred embodiment of FIG. 4
 increase since the OADM implementations of FIGS. 1 and 2 require 3N and
 3N+3 filters respectively. A further advantage of the preferred embodiment
 of FIG. 4 is that no dead bands are required. This results in a higher
 bandwidth efficiency compared to network topologies that use dead bands.
 Yet further, the preferred embodiment of FIG. 4 allows for lower through
 loss when compared to a single channel ADM cascade as depicted within FIG.
 1. In FIG. 4, only five filter bounces are required for a four channel
 add/drop functionality while in FIG. 1, eight filter bounces are needed
 for similar functionality.
 There are numerous alternative embodiments possible for the preferred
 embodiment depicted in FIG. 4. For one, although the preferred embodiment
 of FIG. 4 is depicted using an alignment block 402 and an extended beam
 signal, the present invention could be implemented in a 3 and 4 port
 design in which fiber optic cable connects the individual filters of the
 OADM with additional columnating lenses included on either side of each
 filter. Further, although the embodiment depicted in FIG. 4 utilizes
 clean-up filters, these filters are not required in embodiments in which
 the filters extracting the channels are of sufficiently high tolerance.
 Yet further, although the OADM of FIG. 4 operates to extract and insert
 four wavelengths, this is not meant to limit the scope of the present
 invention. More or less wavelengths can be extracted and/or inserted while
 still implementing an OADM of the preferred embodiment of FIG. 4. The key
 to this embodiment is that a WDM signal is received at a filter with an
 empty wavelength, and the single filter subsequently can extract a channel
 at a wavelength adjacent to the empty wavelength at the same time as
 inserting a channel at the previously empty wavelength.
 The second preferred embodiment of the present invention is now described
 with reference to FIGS. 5 and 6. In this embodiment, the principles of the
 present invention are applied on a network level as will be described
 herein below. The network depicted within FIG. 5 comprises a communication
 hub 502 and first, second, and third network nodes 504,506,508. The hub
 502 and the network nodes 504,506,508 together make a ring configuration
 with a first fiber optic cable 510 coupled between the hub 502 and the
 first node 504, a second fiber optic cable 512 coupled between the first
 and second nodes 504,506, a third fiber optic cable 514 coupled between
 the second and third nodes 506,508, and a fourth fiber optic cable 516
 coupled between the third node 508 and the hub 502. In operation, a WDM
 signal with at least four channels (at wavelengths
 .lambda.1,.lambda.2,.lambda.3,.lambda.4) is transmitted through the ring
 created with the hub 502 and the nodes 504,506,508.
 In the configuration of FIG. 5, the hub 502 transmits data to the first,
 second and third nodes 504,506,508 within channels at respective
 wavelengths .lambda.2, .lambda.3 and .lambda.4, and receives data from the
 first, second and third nodes 504,506,508 within channels at respective
 wavelengths .lambda.1, .lambda.2 and .lambda.3. Hence, the first, second,
 third and fourth fiber optic cables 510,512,514,516 have WDM signals that
 comprise channels at wavelengths .lambda.2,3,4, .lambda.1,3,4,
 .lambda.1,2,4 and .lambda.1,2,3 respectively transmitted through them. In
 operation, a channel at wavelength .lambda.2 is inserted within a WDM
 signal by the hub 502 and extracted by the first network node 504. Also, a
 channel at wavelength .lambda.1 is inserted to the received WDM signal at
 the first node 504 and extracted at the hub 502. Similar operations allow
 the remaining nodes 506,508 to communicate with the hub 502.
 The key to this preferred embodiment of the present invention is the
 extraction of a channel on a first wavelength by a first network device
 and then not inserting a new channel on the first wavelength until at a
 further network device. This results in a distance in which no channel is
 on a particular wavelength, hence an empty wavelength. In the example
 shown in FIG. 5, there is no channels at wavelength .lambda.1 within the
 first fiber optic cable 510, no channel at wavelength .lambda.2 within the
 second fiber optic cable 512, no channel at wavelength .lambda.3 within
 the third fiber optic cable 514, and no channel at wavelength .lambda.4
 within the fourth fiber optic cable 516. These empty wavelengths being
 transmitted between network devices within the ring configuration allow
 other wavelengths to be extracted with filters of reduced cost as will be
 described in detail with reference to FIG. 6.
 A block diagram of a preferred embodiment of the network configuration of
 FIG. 5 is depicted in FIG. 6. In this embodiment, the first, second and
 third network nodes 504,506,508 each comprise an extraction filter
 602a,602b,602c and an insertion filter 604a,604b,604c. Within the first
 node 504, the extraction filter 602a removes the channel at wavelength
 .lambda.2 from the WDM signal received at the first node 504 and the
 insertion filter 604a adds a channel at wavelength .lambda.1 to the WDM
 signal output from the extraction filter 602a. Similarly, within the
 second and third nodes 506,508, channels at wavelengths .lambda.3 and
 .lambda.4 are removed by the extraction filters 602b,602c respectively and
 channels at wavelengths .lambda.2 and .lambda.3 are added by the insertion
 filters 604b,604c respectively.
 The configuration of FIGS. 5 and 6 allows for the extraction filters
 602a,602b,602c and the insertion filters 604a,604b,604c to have
 asymmetrically lower tolerances than would be required in the well-known
 embodiment depicted in FIG. 3. For example, since the wavelength .lambda.1
 is empty when the first node 504 receives a WDM signal, the extraction
 filter 602a for the channel at wavelength .lambda.2 can be made with lower
 tolerances on the .lambda.1 side of the filter. A filter that has a
 passband comprising wavelength .lambda.2 and has a tolerance that could
 pass a portion of a channel at wavelength .lambda.1 may be used without
 significant corruption of the extracted channel since at this point no
 channel is at wavelength .lambda.1. Similarly, the other extraction
 filters 602b,602c can also have asymmetrically lower tolerances than
 previous designs would require. The extraction filter 602b can be designed
 to have a passband comprising wavelength .lambda.3 while also having a
 tolerance that allows any channel at wavelength .lambda.2 to be removed as
 well since, in this case, no channels are at wavelength .lambda.2 at this
 point. For similar reasons, the extraction filter 602c can be designed to
 have a passband comprising wavelength .lambda.4 without fully preventing a
 channel at wavelength .lambda.3 from being removed.
 The insertion filters 604a,604b,604c can also have asymmetrically reduced
 tolerances. The insertion filter 604a for instance can allow a channel at
 wavelength .lambda.1 to be inserted by having a passband comprising
 wavelength .lambda.1 while not fully preventing the removal of a channel
 on wavelength .lambda.2 due to a low tolerance. Since, in the case
 depicted in FIG. 6, no channel is at wavelength .lambda.2 when inserting a
 channel at wavelength .lambda.1, there is no need to ensure that filter
 604a doesn't affect signals at wavelength .lambda.2. This low tolerance on
 one side of the insertion filter 604a could actually be used to ensure the
 remaining channel at wavelength .lambda.2 has been removed from the WDM
 signal traversing it. Similar asymmetrical tolerance relaxations can be
 used with the insertion filters 604b,604c for wavelength .lambda.3 and
 .lambda.4 respectively when inserting a channel at wavelengths .lambda.2
 and .lambda.3 respectively.
 One key advantage of the embodiment depicted in FIGS. 5 and 6 is the
 reduced costs for the filters as a result of the decrease in the tolerance
 requirement of the extraction and insertion filters within the network
 nodes. This embodiment does not affect tolerance requirements of the
 filters within the hub 502, though an OADM according to the preferred
 embodiment of the present invention depicted within FIG. 4 could be used
 within the hub 502 to reduce the number of filters required.
 Another advantage of the embodiment of the present invention depicted in
 FIGS. 5 and 6 is a reduction in the cross-talk within the network,
 cross-talk being the accidental extraction of a channel at a wavelength
 other than the wavelength attempting to be extracted from. A sufficiently
 large reduction in cross-talk can result in the elimination of the need
 for clean-up filters, hence reducing the cost of the OADMs within the
 nodes 504,506,508. As well, the reduction in cross-talk can further reduce
 the through loss.
 Similar to the embodiment of the present invention of FIG. 4, the
 embodiment of FIGS. 5 and 6 further do not require any dead bands. Because
 of the lack of dead bands, the bandwidth efficiency of the embodiment
 depicted in FIGS. 5 and 6 is N-1/N where N is the number of wavelengths
 (one more than the number of network nodes). Within any one fiber optic
 cable, all but one wavelength is being used for transmission of channels.
 In the case shown in FIGS. 5 and 6, N equals four and so the efficiency is
 75%. Other OADM schemes have bandwidth efficiencies between 50% to 75% and
 so the network configuration of FIGS. 5 and 6 can result in an increased
 efficiency, especially as N increases.
 The main downside of the network configuration of FIGS. 5 and 6 is the
 increased complexity caused by planning wavelength allocation. It can be
 seen that this disadvantage is not as significant as the decreases in cost
 resulting from this implementation of the present invention.
 Although the embodiment of the present invention depicted in FIGS. 5 and 6
 is preferable, this is not meant to limit the scope of the present
 invention. For instance, in one embodiment more than one wavelength is
 extracted and inserted at one or more of the network nodes. In this case,
 either more than one extraction and insertion filters are used or, in the
 case of adjacent wavelengths requiring extraction, an extraction filter is
 used for a band of wavelengths. In this case, more than one wavelength
 would be empty during the transmission on the fiber optic cables, hence
 adjusting the calculation with respect to bandwidth efficiency.
 Persons skilled in the art will appreciate that there are yet more
 alternative implementations and modifications possible for implementing
 the present invention, and that the above implementation is only an
 illustration of this embodiment of the invention. The scope of the
 invention, therefore, is only to be limited by the claims appended hereto.