Abstract:
A module that includes both an “add-in”/“drop-out” pair of ports and a “drop-in”/“add-out” pair of ports comprises an arrangement of elements that combines an optical signal having a chosen wavelength with an optical signal applied at the “add-in” port, and outputs the combined signal at the “add-out port.” Concurrently, the module extracts an optical signal with the same wavelength from an optical signal applied at the “drop-in” port signal, yielding an optical signal at the “drop-out” port that is missing that same wavelength. When the amount of information that needs to be sent from a first network node to a second, remote, node, is greater than that which a single wavelength can handle, a plurality of the above-described modules are interconnected within the first node by optically coupling the “add” ports in a “daisy chain” fashion and the “drop” ports in a “daisy chain” fashion, with each module operating at a different wavelength.

Description:
RELATED APPLICATION 
     This application relates to provisional application filed Dec. 7, 2000, which bears the Application No. 60/251,771, which is incorporated herein in its entirety. 
    
    
     BACKGROUND 
     This invention relates to fiber optic networks, and particularly to WDM optical networks. 
     The public&#39;s increasing demand for bandwidth has accelerated the interest in wavelength division multiplexing (WDM) technology. Using WDM, data can be transmitted at high rates on each of several wavelengths of light, sharing a single optical fiber. Currently, systems exist in which a fiber carries over 100 Gb/s of data using 40 or more wavelengths. Conventionally, individual optical channels are “dropped” by inserting a filter in the main fiber path. The filter diverts, and thus effectively extracts, a given wavelength to a separate port that, often, is connected to equipment that demodulates the diverted optical signal to recover data that had previously modulated an optical signal at that given wavelength. Similarly, the addition of an optical channel is typically achieved by the insertion of a filter in the main fiber path, which filter injects light arriving at the node at a desired wavelength into the main optical path. When multiple wavelengths are to be dropped or added, either multiple optical filters must be inserted in the main optical path at the location of the node, or a multi-wavelength multiplexer/demultiplexer is used. 
     An effective design approach is needed for efficient utilization of optical fiber capacity. 
     SUMMARY 
     An advance in the art is achieved with an arrangement that interconnects optical modules that comprise both an “add-in”/“drop-out” pair of ports and a “drop-in”/“add-out” pair of ports. The module includes a conventional arrangement of elements that combines an optical signal having a chosen wavelength with an optical signal applied at the “add-in” port, and outputs the combined signal at the “add-out port.” Concurrently, the module extracts an optical signal with the same wavelength from an optical signal applied at the “drop-in” port signal, yielding an optical signal at the “drop-out” port that is missing that same wavelength. 
     When the amount of information that needs to be sent from a first network node to a second, remote, node, is greater than that which a single wavelength can handle, a plurality of the above-described modules are interconnected within the first node by optically coupling the “add” ports in a “daisy chain” fashion and the “drop” ports in a “daisy chain” fashion, with each module operating at a different wavelength. A similar arrangement is effected at the remote node, and two-way communication between the two nodes is realized with two-fiber optical link that couples the two nodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  depicts the prior art arrangement of equipment in a WDM system; 
         FIG. 2  presents a block diagram of a single add/drop communication module; 
         FIG. 3  depicts a First-In-First-Out (FIFO) arrangement of nodes A and B; 
         FIG. 4  depicts a First-In-Last-Out (FILO) arrangement of nodes A and B; 
         FIG. 5  depicts a FILO arrangement of nodes A and B, with amplifiers for boosting signal power; 
         FIGS. 6 and 7  show different arrangements with a mid-span node; 
         FIG. 8  presents a protected ring arrangement in conformance with the principles of this invention; 
         FIGS. 9A and 9B  show one-level and two-level multiplexers, respectively, that can be employed in an embodiment where a group of wavelengths is extracted from, or injected into an optical signal; and 
         FIG. 10  shows one physical layout of add-in, add-out, drop-in and drop-out ports on an equipment module, and the interconnection of a stack of modules; 
         FIG. 11  depicts an arrangement where the wavelength extracting and wavelength injecting filters in a node are combined within a single piece of equipment; 
         FIG. 12  shows a WDM module arrangement, in accord with the filter notion presented in  FIG. 11 , for the first node in  FIG. 7 ; and 
         FIG. 13  shows a WDM module arrangement, in accord with the filter notion presented in  FIG. 11 , for the mid-span node in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a conventional WDM arrangement that includes equipment modules  30 ,  41 ,  42 ,  43 , and  44 . These modules are typically installed in an equipment frame, or rack, often as depicted in  FIG. 1 . Module  30  is an “add/drop” module that basically extracts specific, different, wavelengths that are present on an incoming fiber of a duplex fiber cable  39  connected to input port  31 , and injects the very same wavelengths to result in an output WDM optical signal at output port  32  that is coupled to the second fiber in duplex fiber cable  39 . More specifically, module  30  has a filter element  45  that receives an optical signal from input port  31 , and develops an output. Filter element  46  receives an optical signal from the output of filter element  45 , and develops an output. Filter element  47  receives an optical signal from the output of filter element  46 , and develops an output. Lastly, filter element  48  receives an optical signal from the output of filter element  47 , and develops an output that is applied to output port  32 . Filter elements  45 - 48  are basically identical except that each extracts and injects an optical signal of a different wavelength. More specifically, filter element  45  effectively comprises two serially connected components, where the first component extracts an optical signal of a specified wavelength (λ 1 ), and the second component inject an optical signal of the same specified wavelength. Accordingly, filter element  45  outputs an optical signal of wavelength λ 1  at port  33  and receives an optical signal at port  33 . Similarly, filter element  46  outputs an optical signal of wavelength λ 2  at port  34  and receives an optical signal at port  34 , filter element  47  outputs an optical signal of wavelength λ 3  at port  35  and receives an optical signal at port  35 , and filter element  48  outputs an optical signal of wavelength λ 4  at port  36  and receives an optical signal at port  36 . Ports  33 - 36  are duplex ports, and they are coupled with duplex “leads” to optical-to-electrical converter modules  41 - 44 , respectively. Each such converter accepts a modulated optical signal of a specified wavelength and converts the information contained therein to electronic form, as well as, conversely, modulated information unto an optical signal of the specified wavelength. 
       FIG. 2  presents a block diagram of an add/drop communication module  10  that is employed in some of the arrangements disclosed herein. It comprises a conventional “add” submodule  20  that is interposed between “add-in” port  11  and “add-out” port  12 , and a “drop” submodule  25  that is interposed between “drop-in” port  14  and “drop-out” port  13 . Specifically, submodule  20  comprises a laser  21 , a modulator  22  that is responsive to an electronic signal arriving from input data port  15  and to the output of laser  21 , developing an optical signal that is modulated by the input data signal, and a filter  23  that combines the optical energy developed by element  22  with the optical energy arriving at port  11  (much like the second component within filter  45 ). The output of filter  23  forms the output of submodule  20  that is applied to port  12 . Submodule  25  comprises a filter  26  (much like the first component within filter  45 ) that accepts optical energy from port  14  in a range of wavelengths and separates that energy into two optical signals: one signal that is applied to converter  24 , which includes signals within a narrow band of a preselected wavelength, and one signal that is applied to port  13 , which includes signals with the remaining wavelengths. Converter  24  extracts electronic signal that modulates the optical signal that is applied thereto, and applies the extracted electronic signal to port  16 . There is no inherent requirement (as in the filter of element  45 ) that the wavelength extracted by filter  26  be identical to the wavelength of the signal injected into filter  23 , although in the context of this disclosure that is the practice. 
       FIG. 3  presents an arrangement in accordance with the principles disclosed herein. It comprises modules  100 ,  101 ,  102 , and  103  in a network node A, and modules  110 ,  111 ,  112 , and  113  in a second, remote, network node B. The interconnections within node A are such that the “add-out” port of one module (e.g. module  100 ) connects to the “add-in” port of the next module in the direction of signal flow (i.e., module  101 ). Herein, this interconnection pattern is referred to as a “daisy chain” interconnection pattern. The signal flow through the “drop” ports is in the opposite direction but the notion is the same; to wit, the “drop-out” port of one module (e.g. module  103 ) connects to the “drop-in” port of the next module (i.e., module  102 ). The arrangement at node B is identical to that at node A with respect to the interconnection of the various modules. 
     Nodes A and B are interconnected by a pair of optical paths (e.g., fibers  120  and  130 ) that form a duplex, bi-directional, optical cable. Fiber  120  passes signals from node A to node B by connecting the “add-out” output of module  103  to the “drop-in” input of module  110 . Fiber  130  passes signals from node B to node A by connecting the “add-out” output port of module  110  to the “drop-in” input of module  103 . The arrangement of elements disclosed in  FIG. 3  permits placing the “add-out” port of module  103  (to which fiber  120  is connected) to be in close physical proximity (i.e., less than 2 inches apart) to the “drop-in” input port of module  103  (to which fiber  130  is connected). This is a significant advantage because fibers  120  and  130  are sheathed within a single cable, and the separated fiber ends that extend from the cable (often called “pig tails”) can be short and of roughly equal lengths, and be easily installed. 
     The set of wavelengths that is used for the various interconnected modules in node B ought to be identical to the set of wavelengths used in the modules of node A (unless these modules interact with other (not shown) module), if it is desired to have each module in node A exchange information with some module in node B. There is a one-to-one relationship between the modules of node A and node B. The sequence of wavelengths that is used in node B does not have to be related to the sequence of wavelengths used in node A (the depicted sequence in modules  100 ,  101 ,  102 , and  103  of node A being λ 1 , λ 2 , λ 3 , λ 4 , respectively). Advantageously, however, some order is beneficial; and the most beneficial order is one where the sequence of wavelengths used in node B is identical to the order used in node A. This order, which results in the same loss at all wavelengths (because all wavelengths go through the same number of modules between the point where the wavelength is added and the point where the wavelength is dropped), yields what is known as a FIFO (First In-First Out) arrangement. This is the arrangement shown in  FIG. 3 . 
     Another arrangement, which is beneficial simply because it is orderly and simple to provision, is a FILO arrangement. In a FILO arrangement the wavelength that was added first is dropped last. Such an arrangement is shown in  FIG. 4 . A disadvantage of the FILO arrangement over the FIFO arrangement is that each wavelength is subjected to a different amount of attenuation, ranging from one wavelength that is attenuated by 2N modules (where N is the number of modules in nodes A) to another wavelength that is attenuated by no modules. One solution to the attenuation issue is the insertion of amplifiers as shown, for example, in  FIG. 5 . 
       FIG. 6  presents an arrangement that involves nodes A, B, and C. Nodes A and C are terminal nodes, and node B is a mid-span node. In  FIG. 6 , node A employs wavelengths λ 1 , and λ 2 , where λ 1  is used to communicate with a module in node B and λ 2  is used to communicate with a module in node C. The second module in node B communicates with the second module in node C. In accord with the principles disclosed herein, the module in node B that communicates in some wavelength with a module in node A (e.g., λ 1  in  FIG. 6 ) precedes—in the signal path where signals flow from node A to node B, then to node C—the module in node B that communicates in the same wavelength with a module in node C. That means that module  131 , which extracts a signal of wavelength λ 1  from the upper optical signal path where signals flow from left to right, precedes module  132 . Having first extracted the signal at wavelength λ 1 , module  132  can inject a signal of the same wavelength λ 1 . In the opposite direction, module  132  extracts the λ 1  signal from the lower optical path where signals flow from right to left (injected by module  141 ) and once that wavelength is extracted, module  131  can inject a signal of the same wavelength λ 1 . Thus, module  121  communicates with module  131 , module  122  communicates with module  142 , and module  132  communicates with module  141 . The important point to note is that in an arrangement where there is a mid-span node, such as node B in  FIG. 6 , a wavelength can be reused. However, it is necessary to drop, or extract, that wavelength in the mid-span node before injecting data at that wavelength reuses it. 
       FIG. 7  shows an arrangement where node B has four modules, and two wavelengths (λ 1 , and λ 2 ) are dropped in modules  211  and  212  of node B before they are reinserted in module  213  and  214  of node B (in the optical path where signals flow in node B from left to right). In  FIG. 7 , the λ 1  wavelength is extracted in module  211  and reinserted in module  213 , and thereafter, wavelength λ 2  is extracted in module  212  and reinserted in module  214 . One can almost think of module B as two successive modules B′ and B″; each of which, in accordance with the principles of this invention comprises two separate and distinct paths, with one path comprising a serial connection of solely “drop-out” submodules, and the one path comprising a serial connection of solely “add-in” submodules. 
       FIG. 8  presents a block diagram of an arrangement that is suitable for insertion in a protected ring—which is a ring that employs two independents closed paths where signals flow in opposite directions. The arrangement illustrated provides for the dropping and adding of groups of up to four wavelengths when four modules are used. 
     It may be noted that the arrangements disclosed so far effectively present a serial approach to the dropping and adding of wavelengths, and since each add/drop module imposes a loss to the wavelengths that merely pass through the module, it follows that the number of modules that may be used between a point where a wavelength is injected and the point where the same wavelength is extracted ought to be limited. This limitation is overcome by handling a group of wavelengths at each module, constituting an optical signal of a preselected band of wavelengths κ i , where the optical signal of band Λ i  is constructed with a multiplexer that contains one or more levels, and the group is dispersed with a corresponding demultiplexer. This is illustrated in  FIG. 9A  with a single-level multiplexer that has 16 ports, and in  FIG. 9B  with a two-level multiplexer arrangement that employs multiplexer elements that have 4 ports each. Advantageously, the subgroups of wavelengths in  FIG. 9B , Λ i1 , Λ i2 , Λ i3 , . . . are adjacent to each other, and disjoint. For example wavelengths λ 1  through λ 4  are all shorter than wavelengths λ 5  through λ 8 , etc. The multiplexing of the various wavelengths in a group into a single signal, and the corresponding demultiplexing of a group signal into the individual wavelengths that form the group (which is not explicitly shown) may be accomplished with a conventional manner. 
     The physical arrangement of the ports on a module affects the practical process of interconnecting the  FIG. 1  modules. A good arrangement results in a simple, uniform, pattern of interconnections that, in turn, leads to a simple interconnections task. The structure disclosed herein enables the equipment designer to effect a physical arrangement where the “add-out” and the “drop-in” ports are in close physical proximity to each other and, similarly, the “add-in” and the “drop-out” port are in close physical proximity to each other. This is illustrated in  FIG. 10 , where the interconnection of the four  FIG. 3  modules ( 100 - 103 ) is shown, and where each module has the “add-out” port  11  in close proximity to the “drop-in” port  13  and the “add-in” port  12  in close proximity to the “drop-out” port  14 . This arrangement results is a very regular interconnection pattern for the node, which is very important because it permits installation and maintenance personnel to quickly effect correct interconnections. 
     It may be noted that the  FIG. 1  module, which results in the  FIG. 10  interconnection when four modules are desired to be interconnected, yields an arrangement where a disruption occurs if a module needs to be replaced. Although optical modules are typically very reliable, and the replacement of a module is a task that is measured in seconds (or at least it ought to be, in light of the interconnection simplicity of modules constructed in accord with the principles disclosed herein), it must be pointed out that replacing a module does disrupt the optical signal and, consequently, for a short duration all wavelengths are lost. By separating the filters that extract (or inject) a wavelength from the optical-to-electrical conversion means (or electrical-to-optical conversion means) and placing all of the filters of a node in a separate equipment module yields an arrangement where the conversion modules can be removed and replaced with no disruption to the system&#39;s operation except for the communication channel where the conversion module is replaced. This is illustrated in  FIG. 11 , where all of the filters of node A in the  FIG. 3  arrangement are combined in equipment module  30 , leaving four conversion modules  71 - 74  that, in a sense, correspond to modules  100 - 103  of  FIG. 3 . 
       FIG. 12  shows a WDM arrangement in accordance with the principles disclosed herein, for node A of  FIG. 7  where module  50  comprises a series of filter pairs  61 - 62 ,  63 - 64 , and  65 - 66 , operating at respective wavelengths λ 1 , λ 2 , and λ 3 . Filter pair  61 - 62  interacts with duplex optical port  53  that is coupled to optical/electrical converter module  42 . Similarly, filter pair  63 - 64  interacts with duplex optical port  54  that is coupled to optical/electrical converter module  43 , and filter pair  65 - 66  interacts with duplex optical port  55  that is coupled to optical/electrical conversion module  44 . Node A interacts with node B through ports  57  and  58  that are coupled to filters  65  and  66 , respectively. Artisans should quickly realize that the  FIG. 12  arrangement could be used for node C of  FIG. 7 . 
       FIG. 13  shows a WDM arrangement in accordance with the principles disclosed herein, for mid-span node B of  FIG. 7 . It is very similar to the  FIG. 12  arrangement, except for the specific wavelengths employed, and the user of four filter pairs within module  50 ′ (and four corresponding optical/electrical conversion modules). It may be noted that module  50 ′ can be replaced with two, serially connected, modules (x) each of which handles wavelengths λ 1 , and λ 2 . A module x is, thence, a module that comprises serially connected filter pairs, each operating at a different wavelength. 
     The disclosure above presents the principles of this invention by way of illustrative embodiments, but it should be understood that various modifications and variations could be implemented without departing from the scope and spirit of this invention, as defined in the following claims. To give just one example, the implementations depicted in the drawings contemplate using fibers to interconnect nodes. Free space, however, can also be used in some applications.