Abstract:
A new architecture is proposed for an optical node in a wavelength division multiplexed network. The optical node may be an optical add/drop node. Conventional add/drop nodes utilize one of two architectures—broadcast and blocking. The broadcast architecture is an architecture in which a copy of an optical signal is dropped to a drop path of a node while another copy continues on a through path. Thus, channels that occupy a specific portion of wavelengths (or spectrum) prior to the node are not available for use subsequent to the add/drop connectivity. In a blocking architecture, at least the through path (and often the drop path) is spectrally filtered. This permits wavelength reuse for add/drops in subsequent portions of the network. This disclosure proposes an optical node architecture that enables starting out with a low cost approach, such as broadcast, but includes connections to permit ‘in-service’ upgrade to more capable architecture. Increasing spectral reuse is enabled through the architecture.

Description:
PROVISIONAL APPLICATION  
       [0001]     The present application claims priority under 35 U.S.C. § 120 of a provisional application 60/410,996 filed on Sep. 16, 2002, the entirety of which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The field of the invention generally relates to optical nodes, for example optical add/drop multiplexers (OADM). More particularly, the invention relates to an upgradeable/reconfigurable optical node architecture and method for upgrading an optical node and for providing distributed spectral filtering.  
       BACKGROUND OF THE INVENTION  
       [0003]     Conventional add/drop nodes utilize one of two architectures—broadcast and blocking architectures. The broadcast architecture is an architecture in which a copy of an optical signal is dropped to a drop path of a node while another copy continues on a through path. Multiple channels are not allowed to occupy same portions of the spectrum inside the transmission medium due to interference. Thus, channels that occupy a specific portion of wavelengths (or spectrum) prior to the node are not available for use subsequent to the add/drop connectivity. However, other unused portions of the spectrum are available for use subsequent to the node. The broadcast architecture may be sufficient in wavelength division multiplexing (WDM) systems with large aggregate channel capacities.  
         [0004]     In a blocking architecture, at least the through path (and often the drop path) is spectrally filtered. This permits wavelength reuse for add/drops in subsequent portions of the network. The reuse of wavelengths provides advantages by making additional channels available for connecivity, thereby increasing the total capacity available on the communications network.  
         [0005]     Optical networks provide a tremendous capacity advantage. Entities wishing to take advantage of the advantages that optical networks offer, must usually make a decision based on their current needs (which may be modest and predictable) and their future needs (which are typically unpredictable). An entity may decide to acquire a network to meet its short-term needs because of it&#39;s present financial constraints.  
         [0006]     However, this approach carries a risk that the network will be insufficient and may cost more in the long run because the entire network has to be replaced due to inadequacies of the network. Also, any upgrades may require the network to be shut down prior to the upgrade. Such a shut down is costly since no service can be provided, which in turn shuts down a revenue stream. In an industry such as telecommunications, shut down can be extremely costly indeed.  
         [0007]     Another approach is to project a long-term need and acquire a network with capabilities to meet the long-term need. This approach also carries inherent risks as well. In the short run, the investment in the network will be wasted to the extent that there will be excess capacity. In the long run, the needs of the entity may change in a different direction and the acquired network will not be able to handle the changed needs efficiently.  
         [0008]     An approach is desired whereby an initial network deployment may be achieved at relatively low cost while allowing for sufficient capacity. As the capacity demand grows, it is desired to provide an easily upgradeable path as well as a strategy for recovering previously inaccessible capacity, all without service disruptions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     Features of the present invention will become more fully understood to those skilled in the art from the detailed description given hereinbelow with reference to the drawings, which are given by way of illustrations only and thus are not limitative of the invention, wherein:  
         [0010]      FIGS. 1   a - 1   c  are block diagrams illustrating varying embodiments of an optical add/drop node architecture according to the present invention;  
         [0011]      FIG. 2  is a block diagram illustrating a bi-directional variation of an optical add/drop node architecture according to an embodiment of the present invention;  
         [0012]      FIG. 3  is a block diagram illustrating a wavelength division multiplexed optical communication system including optical add/drop nodes;  
         [0013]      FIG. 4  is a block diagram illustrating a wavelength division multiplexed optical communication system including optical add/drop nodes further illustrating wavelength domains and spectral reuse concept according to an embodiment of the present invention;  
         [0014]      FIG. 5  is a block diagram illustrating a wavelength division multiplexed optical communication system including optical add/drop nodes and optical spectral equalizers further illustrating wavelength domains and spectral reuse concept according to an embodiment of the present invention;  
         [0015]      FIGS. 6   a - c  are block diagrams illustrating embodiments of various types of spectral blocking filter according to the present invention;  
         [0016]      FIGS. 7   a - b  are block diagrams illustrating embodiments of various types of optical circulators according to the present invention; and  
         [0017]      FIG. 8  is a block diagram illustrating a particular blocking filter according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]     For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. The same reference numbers and symbols in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. The scope of the invention is defined by the claims and equivalents thereof.  
         [0019]     The expression “optically communicates” as used herein refers to any connection, coupling, link or the like by which optical signals carried by one optical 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 and/or devices. Likewise, the expresssions “connection”, “operative connection”, and “optically placed” as used herein are relative terms and do not necessarily require a direct physical connection.  
         [0020]     In an aspect of the present invention, an optical node architecture is proposed that enables starting out with a low cost approach, such as broadcast. The architecture includes connections to permit ‘in-service’ upgrade to a more capable spectrally blocking system. In other words, the upgrade may take place without any substantial disruption in service. Such ‘in-service’ upgrade capability can save significant costs by reducing or eliminating all together any shut down time.  
         [0021]     It is to be understood that upgrade is not the only benefits of the architecture. For example, the connection can also allow for in-service maintenance to take place as well.  
         [0022]      FIGS. 1   a  and  1   b  are block diagrams illustrating an embodiment of an optical node architecture according to the present invention. In  FIG. 1   a , the optical node  100  includes a through path coupler  108  receiving input optical signals from the input port  102 . The through path coupler  108  is configured to provide the received input optical signals and provide the same through its outputs. In this instance, the through path coupler  108  is shown as outputting first and second through optical signals to first and second outputs  110   a  and  110   b , respectively. However, it is to be understood that there can be more that two outputs. The through path coupler  108  may not spectrally filter and thus the first and second through optical signals carry the same data.  
         [0023]     The optical node  100  also includes a first optical filter  112 , which receives the first through optical signal via the first output port  110   a . The first optical filter may be a basic initial patch cord which allows for direct optical pass through of the first through optical signal, or it may be a simple optical filter to perform rudimentary filtering.  
         [0024]     The optical node  100  further includes a selective connector, which is represented as an optical switch  116  in  FIG. 1   a . The selective connector  116  is initially optically connected to the first optical filter  112  as shown, and directs optical transmission signals to the output port  118 . As the name implies, the selective connector  116  is able to switch connection to direct optical optical transmission signals from the other optical filters.  
         [0025]     As noted above, the through pass coupler  108  provides the second through optical signal to the second output  110   b . Initially this path may be left unconnected as shown in  FIG. 1   b . More specifically, in  FIG. 1   b , the dotted lines indicate that a second optical filter  114  is not present in the initial version of the optical node  100 .  
         [0026]     However, it is this availability of the second path that enables an upgrade to take place without disruption in service. To accomplish the upgrade, the second optical filter  114 , which may be an upgraded filter relative to the first optical filter  112 , may be placed and connected to the second output  110   b  of the through path coupler  108 . When connected, the second optical filter  114  receives and processes the second through optical signal. Recall that the second through optical signal may be idential to the first through optical signal. To complete the upgrade, the optical switch serving as the selective connector  116  is controlled as to switch the connection as to direct the output from the second optical filter  114  to the output port  118 . The optical switch may switch connections in the order of milliseconds and perhaps even as short as on the order of nanoseconds. From a user perspective, there is little to no perceived service disruption.  
         [0027]     The optical node  100  may include drop and add couplers  104  and  120 , respectively, to enable drop/add capability. As shown in  FIG. 1   a - 1   c , the drop coupler  104  is optically connected to the input port  102  and provides the input optical signal to the through path coupler  108  via a through path  106   a . The drop coupler  104  may also provide the input optical signal via a drop path  106   b . Portions of the input optical signal may be processed through apparatus and/or systems connected to the drop path  106   b . Typically, the drop coupler  104  is spectrally unfiltered and thus the same input optical signal is provided on both the through and drop paths  106   a  and  106   b , respectively.  
         [0028]     The add coupler  120  receives the optical signal selected by the selective connector  116  and may receive optical transmission from the add path  126  and outputs the received signals as output optical signals to the output port  118 .  
         [0029]     Optionally, variable optical attenuators (VOAs) may be connected to the optical through paths to equalize optical signal amplitudes such as the amplitudes of the optical signals on the through and the add paths. For example, as shown in  FIGS. 1   a - 1   c , a first VOA  122  may be optically placed between the first optical filter  112  and the selective connector  116  and a second VOA may be placed between the second optical filter  114  and the selective connector  116 .  
         [0030]     The VOAs themselves may serve as a replacement of or as a complementary addition to the optical switch  116 . In other words, the VOAs may also be used as the selective connector  116 . If VOAs are used for switching purposes, then a combining coupler  128  may be used as shown in  FIG. 1   c.    
         [0031]     A controller (not shown) may be used to control the VOAs. A conventional service channel, overlay IP network, DCC (or other overhead channel), or other communication methods may be used to communicate administrative, maintenance, and control information to enable intelligent control of the selective connector  116 —whether it be made of an optical switch, VOAs, or a combination of both.  
         [0032]     Typically, optical communications systems transport signals uni-directionally on a fiber. Bi-directionality may be achieved through using a pair of uni-directional systems. In such instances, pairs of optical nodes of the embodiments described in  FIGS. 1   a - 1   c  may be required. If reliability is to be enhanced, additional nodes may be provided to provide redundancy.  
         [0033]     It is also the case that optical components are often reciprocal, i.e., they may be operated bi-directionally with same properties. Further, optical add/drop nodes frequently have identical configuration of optical filters in both directions.  FIG. 2  is a block diagram illustrating a bi-directional variation of an optical node architecture according to an embodiment of the present invention.  
         [0034]     In the west to east direction (indicated by a dashed line), the optical node  200  of  FIG. 2  is much like the optical node  100  of  FIGS. 1   a - 1   c . A first drop coupler  204   a  can be considered to be equivalent to the drop coupler  104  of the optical node  100 . Similarly, a first add coupler  220   a  can be considered to be equivalent to the add coupler  120 . The east to west signal transmission is accomplished by the addition of a second drop coupler  204   b  and a second add coupler  220   b . The bi-directionality is enabled by first and second optical circulators  230   a  and  230   b.    
         [0035]     Circulators may be described as non-reciprocal devices that redirect light from port-to-port sequentially in one direction only and are generally known in the art.  FIG. 7   a  illustrates a counter-clockwise circulator  700   a  and  FIG. 7   b  illustrates a clockwise circulator  700   b . With the counter-clockwise circulator  700   a , light entering port  702   a  is directed counter-clockwise to exit the next port in sequence which is port  704   a . Likewise, light entering the port  704   a  is directed to port  706   a . The function of the clock-wise circulator  700   b  is similar except that the direction is clock-wise as indicated.  
         [0036]     As noted above, with reference to  FIG. 2 , the bi-directionality of the optical node  200  may be achieved through the use of the circulators  230   a  and  230   b . For example, the circulator  230   a  may be a counter-clockwise circulator and the circulator  230   b  may be a clockwise circulator. In other words, the first circulator  230   a  may direct optical signal traffic from the first drop coupler  204   a  to the through path coupler  108  and direct optical signal traffic from the through path coupler  108  to the second add coupler  220   b . The second circulator  230   b  may direct optical signal traffic from the selective connector  116  to the first add coupler  220   a  and direct optical signal traffic from the second drop coupler  204   b  to said selective connector  116 .  
         [0037]      FIG. 3  is a block diagram illustrating a WDM optical communication incorporating several optical nodes  312  (denoted as OADMs) into a fiber optic transmission system.  
         [0038]      FIG. 4  is a block diagram illustrating a wavelength division multiplexed optical communication system including OADM nodes further illustrating wavelength domains and spectral reuse concept according to an embodiment of the present invention. While the system  400  is bi-directional, for illustration purposes, it suffices to describe the lower portion of the system showing transmission of signals in a left to right direction.  
         [0039]     As shown, the optical communication system  400  includes a plurality of transmitters  402  transmitting input signals, a multiplexer  404  multiplexing the signals from the plurality of transmitters  402  to a fiber optic line  406 , a demultiplexer  408  demultiplexing signals from the fiber optic line  406 , and a plurality of receivers  410  receiving the demultiplexed signals from the demultiplexer  408 .  
         [0040]     Between the multiplexer  404  and the demultiplexer  408 , one or more optical nodes  412  may be optically placed. In this instance, three optical nodes  412 - 1 ,  412 - 2 , and  412 - 3  are illustrated. However, it is to be understood that the embodiment is not limited to only three optical nodes  412 . The optical nodes  412  may be spectrally unfiltered OADMs that prohibits subsequent spectral reuse.  
         [0041]     However, other OADMs may include spectral filters  600   a ,  600   b , and  600   c  such as illustrated in  FIGS. 6   a - 6   c .  FIG. 6   a  illustrates a generalized blocking filter (BF)  600   a  which receives as optical input a spectrum of wavelengths represented by the symbol A. The BF  600   a  blocks a subset of the spectrum and outputs the remainder (or complementary set) of the wavelengths of the spectrum. Whereas the BF  600   a  blocks an arbitrary portions of the spectrum,  FIG. 6   b  illustrates a BF  600   b  which only allows a contiguous portion of the spectrum to pass through. The BF  600   b  may be viewed as a particular case of the general BF  600   a .  FIG. 6   c  illustrates a reconfigurable blocking filter (RBF)  600   c  wherein the particular wavelengths blocked may be chosen through control signals. A description of a particular type of RBF may be found in a provisional application ______, Attorney Docket 4450-0403P, filed on Aug. 4, 2003 by the common Assignee of the present application and is hereby incorporated by reference in its entirety.  
         [0042]     A blocking OADM  412  may block not only the channels that are dropped at the node itself, but may also block channels that have been broadcast from precedings nodes that lack a blocking filter. This allows significant portions of the spectrum to be recovered for reuse.  
         [0043]     An example of spectrum reuse, according to an embodiment of the present invention, is described with reference to  FIG. 4 . In this instance, it is assumed that all optical nodes  412  are OADMs. As shown, in domain 1 (between the plurality of receivers  410  and the optical node  412 - 2 ) supports wavelength λ1. It is assumed that the OADM  412 - 1  is unfiltered. As such, the wavelength λ1 from a first leg of domain 1 (between the plurality of transmitters  402  and OADM  412 - 1 ) is transmitted through the OADM  412 - 1  into a second leg (between OADMs  412 - 1  and  412 - 1 ). As a result, connectivity on the second leg cannot use λ1 and chooses λ2 instead. However, it is assumed that OADM  412 - 2  is spectrally blocks both wavelengths λ1 and chooses λ2 permitting reuse of both wavelengths in domain 2.  
         [0044]     The presence of blocking filters at selected OADM sites permit a network to be segmented into “broadcast” domains.  
         [0045]     It is to be noted that use of blocking OADMs is not the only way to achieve spectral reuse.  FIG. 5  is a block diagram illustrating a wavelength division multiplexed optical communication system including optical add/drop nodes and optical spectral equalizers further illustrating wavelength domains and spectral reuse concept according to an embodiment of the present invention. Again, only the lower portion will be described.  
         [0046]     As shown, the optical communication system  500  includes the plurality of transmitters  402  transmitting input signals, the multiplexer  404  multiplexing the signals from the plurality of transmitters  402  to the fiber optic line  406 , the demultiplexer  408  demultiplexing signals from the fiber optic line  406 , and the plurality of receivers  410  receiving the demultiplexed signals from the demultiplexer  408 .  
         [0047]     Between the multiplexer  404  and the demultiplexer  408 , one or more optical nodes  512  may be optically placed. While only two optical nodes  512 - 1  and  512 - 2  are shown, it is understood that the number is not so limited.  
         [0048]     The system  500  may also include optical spectrum equalizers (OSE)  514  optically placed between the optical nodes  512 - 1  and  512 - 2 . Again, while only two OSEs  514 - 1  and  514 - 2  are shown, the number of OSEs is not so limited and may be as few as one depending on signal attenuation capacities, as explained herein below.  
         [0049]     In this instance, both optical nodes  512 - 1  and  512 - 2  are assumed to be spectrally unfiltered OADMs. This would normally prohibit spectral reuse. OSEs are typically used to reshape channel powers to achieve power flatness across the channels.  
         [0050]     Another use for OSEs is to reduce noise in the unused regions of the spectrum. A noise suppression of more than 20 dB in selected spectrum regions can provide enough isolation to allow spectral reuse. It may be that a single OSE may not be capable of providing sufficient suppression. Typical OSE module may provide a dynamic range for noise suppression of ˜10 to 20 dB, which is generally not sufficient. However, cascading multiple OSE modules after the node where the origina spectrum is dropped may provide sufficient isolation in selected spectrum regions. The embodiment as illustrated in  FIG. 5  is based on this concept.  
         [0051]     As shown, wavelength λ1 is supported in domain 1 (between the plurality of transmitters  402  and OADM  512 - 1 ). The same wavelength λ1 is transmitted through the OADM  512 - 1  but the OSEs  514 - 1  and  514 - 2  progressively attenuate the wavelength λ1. After passing through the OSEs  514 - 1  and  514 - 2 , the the wavelength λ1 is sufficiently attentenuated to allow its reuse in domain 3. The connectivity in domain 2 (center segment) is assumed to be established with wavelength λ 2  and this wavelength may not be reused in domains 1 and 3. Again, the number of OSE is not limited to two, but it should be enough sufficiently attenuate the chosen wavelength(s) to allow them to be reused in subsequent segments of the system  500 .  
         [0052]     The embodiments shown in  FIGS. 4 and 5  may be viewed as distributed spectral filtering systems In the case of  FIG. 4 , filtering is provided by elements, namely the OADMs  412 , dedicated to channel suppression. In case of  FIG. 5 , filtering is provided in a more distributed sense by elements, namely the OSEs  514 , whose primary task is to flatten the power spectrum and reduce noise accumulation. In this sense, the “broadcast” channels are viewed as noises to be gradually removed from appropriate segments of the network.  
         [0053]     As indicated above, a single OSE is typically not sufficient for spectral reuse, but reuse can be enabled by cascading of multiple OSEs. Thus in an embodiment of the present invention, a blocking filter such filters  600   a ,  600   b , and  600   c  may be a combination of a series of two or more OSEs. For example, as shown in  FIG. 8 , the optical filter  800  may include OSEs  804 - 1  and  804 - 2 . The filtering is provided by the OSEs  804 - 1  and  804 - 2 . Note that the optical filter  800  may be utilized as either the first or second optical filter  112 ,  114 .  
         [0054]     It is noted that in most long-haul systems, OSEs are already included for other reasons. In this instance, the optical filter  800  having only one OSE  804 - 1  may be sufficient.  
         [0055]     With the various embodiments of the present invention, a dynamically upgradeable optical network architecture is provided. Initially, the network may start with low cost approach but include connections to permit in-service upgrades to more capable architecture.  
         [0056]     While the invention has been described with reference to the exemplary embodiments thereof, it is to be understood that various modifications may be made to the described embodiments without departing from the spirit and scope of the invention thereof. The terms as descriptions used herein are set forth by way of illustration only and are not intended as limitations.