WDM ring transmission system having two hubs

[0002] The present invention is directed toward a wavelength division multiplexed (WDM) ring transmission system having two hubs for providing redundant connections to the WDM ring transmission system for improved protection.

[0003] Wavelength division multiplexing (WDM) is being explored as an approach for economically increasing the capacity of existing fiber optic networks. WDM systems typically include a plurality of transmitters, each respectively transmitting optical signals on a designated wavelength or channel. As a result, fiber capacity can be increased by a multiple equal to the number of channels.

[0004] WDM systems have been deployed in long distance networks in a point-to-point configuration consisting of end terminals spaced from each other by one or more segments of optical fiber. In metropolitan areas, however, WDM systems having a ring or loop configuration are currently being developed. Such systems typically include a plurality of nodes located along the ring, each node providing a connection to an office building, for example. Each optical signal carried by the ring has a unique wavelength corresponding to a particular destination. Thus, for example, one node may add and drop optical signals having a wavelength of 1552.5 nm, while another node may add and drop optical signals at 1553.3 nm. An optical hub is often positioned at a single location along the ring, providing an interface between the WDM ring and an external network. The hub receives either optical or electrical signals from the (external) network which are destined for one of the nodes coupled to the WDM ring. The hub then emits an optical signal modulated in accordance with the signals received from the network at a wavelength associated with the recipient node. The modulated signal is next sensed at the destination node. This node, in turn, can emit optical signals at the associated wavelength, which are detected by the hub and converted to either optical or electrical signals to be transmitted to the external network.

[0005] If the connection between the network and the hub fails, or a fault occurs within the hub itself, the entire WDM ring and all the nodes connected thereto are isolated from the network. Accordingly, there is a need for a fault tolerant WDM ring transmission system which can maintain a reliable connection to an external network through a hub.

[0006] Consistent with the present invention, a WDM looped or ringed transmission system is provided including first and second optical hubs coupled along the ring. The first optical hub has first and second input ports, as well as first and second output ports. The first input port is configured to receive a first plurality of optical signals, each at a respective one of a first plurality of wavelengths, and the first output port is configured to output the first plurality of optical signals to a looped optical communication path. The second input port is configured to receive the first plurality of optical signals and a second plurality of optical signals, each at a respective one of a second plurality of wavelengths. In addition, the second output port of the first optical hub is configured to output the second plurality of optical signals.

[0007] The second optical hub comprises first and second input ports and first and second output ports. The first input port is configured to receive the second plurality of optical signals from the looped optical communication path, and the second input port is configured to receive the first plurality of optical signals. The first output port is configured to output the second plurality of optical signals, and the second output port is configured to output the first and second pluralities of optical signals to the second input port of the first optical hub.

[0008] Advantages of the present invention will be apparent from the following detailed description of the presently preferred embodiments thereof, which description should be considered in conjunction with the accompanying drawings in which:

[0009] FIG. 1 illustrates a WDM ring transmission system in accordance with an embodiment of the present invention;

[0010] FIG. 2 illustrates a detailed schematic of first and second optical hubs in accordance with a first embodiment of the present invention;

[0011] FIG. 3 illustrates an optical add/drop multiplexer;

[0012] FIG. 4 illustrates a transceiver;

[0013] FIG. 5 illustrates a detailed schematic of the first and second hubs in accordance with a second embodiment of the present invention;

[0014] FIG. 6 illustrates a WDM ring transmission system in accordance with a further embodiment of the present invention;

[0015] FIG. 7 illustrate a detailed schematic of a first hub shown in FIG. 6; and

[0016] FIG. 8 illustrates a detailed schematic of a second hub shown in FIG. 6.

[0017] In accordance with the present invention, a WDM ring transmission system is provided having two hubs. Optical signals are paired and each signal within a given pair is supplied to a respective hub, but one of the hubs includes optical switches for choosing one optical signal within each pair for transmission to the nodes of the WDM ring. Accordingly, if a fault occurs in one of the network-hub connections or one of the hubs becomes defective, the switches couple optical signals from the operational hub to the WDM ring. In addition, optical signals emitted by nodes coupled to the WDM ring are output from both hubs, thereby insuring that signals can be output from the ring even if one of the hubs or one of the hub-network connections fails.

[0018] Turning to the drawings in which like reference characters indicate the same or similar elements in each of the several views, FIG. 1 illustrates a WDM ring or looped transmission system 100 in accordance with an embodiment of the present invention. Data from external network 110 is supplied to WDM ring transmission system 100 via connections such as network interface circuits 112 and 114. Optional network interface circuits 112 and 114 include optical transmitters (e.g., directly or externally modulated lasers and associated circuits) that emit optical signals at a respective one of wavelengths λ₁to λ₄to hubs 116 and 118. The optical signals input to hub 118 propagate, for example, in clockwise direction (in FIG. 1) to hub 116. Hub 116, in turn, selectively emits one or more of the λ₁to λ₄optical signals supplied from network interface circuit 112 or one or more of the λ₁to λ₄optical signals input from hub 118.

[0019] The λ₁to λ₄optical signals then propagate along looped optical communication path 120, e.g., a single optical fiber, to nodes 122, 124, 126 and 128, which respectively extract one of the optical signals at a respective one of wavelengths λ₁to λ₄. Each of nodes 122, 124, 126 and 128 further emit optical signals at a respective one of wavelengths λ₅to λ₈carrying data for transmission to network 110. The λ₅to λ₈optical signals propagate in a clockwise direction shown in FIG. 1 to hubs 116 and 118. The λ₅to λ₈optical signals are next passively split in node 118, and then demultiplexed in node 116. Accordingly, the λ₅to λ₈optical signals are output from both hubs to respective network interface circuits 112 and 114. The network interface circuits may further include photodetectors and associated electrical and/or optical components for supplying electrical and/or optical signals to network 110 corresponding to the optical signals emitted by nodes 122, 124, 126 and 128.

[0020] FIG. 2 illustrates hubs 116 and 118 associated with optical communication path 120 in greater detail. Optical signals at wavelengths λ₁to λ₄are supplied to both hubs 116 and 118 through inputs 202-1 to 202-4 and 208-1 to 208-4 of ports 202 and 208, respectively. The optical signals input through port 208 are typically supplied to optical multiplexer 234 having a plurality of inputs, each of which receiving a respective one of optical signals at wavelengths λ₁to λ₄. These optical signals are combined and passed through output 216 onto a single fiber 218. Optical combiner 220, coupled to fiber 218, combines the optical signals at wavelengths λ₁to λ₄with optical signals at wavelengths λ₅to λ₈received through port 212 of hub 118 for output through port 214.

[0021] The combined optical signals at wavelengths λ₁to λ₈are next fed to optical demultiplexer 222 through port 206. Optical demultiplexer 222, including, for example, an arrayed waveguide router, separates according to wavelength and supplies each optical signal at a respective output. As further shown in FIG. 2, optical signals at wavelengths λ₅to λ₈are directed to network interface circuit 112 via port outputs 222-5 to 222-8 and through corresponding sub-port outputs 204-1 to 204-4 of port 204. Optical signals at wavelengths λ₁to λ₄, however, are supplied via sub-port outputs 222-1 to 222-4 and fibers 224-1 to 224-4 to respective optical switches 226-1 to 226-4. Optical switches 226-1 to 226-4 are also configured to receive optical signals at wavelengths λ₁to λ₄from network interface 112 via port 202.

[0022] Switches 226-1 to 226-4 selectively couple one of outputs 222-1 to 222-4 or sub-port inputs 202-1 to 202-4 to output port 262, via multiplexer 234, in response to control signals generated by detect and compare circuit 228. The control signals are generated in response to light sampled at each of the optical signal wavelengths λ₁to λ₄. For example, optical couplers 230 and 232 supply sampled light at a particular wavelength, e.g., λ₄to detect and compare circuit 228, which, in turn, compares these signals and adjusts switch 226-4 so that the best 4 optical signal is output through port 262. Additional couplers (not shown) sample light at the remaining wavelengths λ₁to λ₃so that detect and compare circuit 228 can compare optical signals output from ports 222-1 to 222-4 with optical signals from sub-ports 202-1 to 202-4. The comparison criteria used by circuit 228 may be as simple as the presence or absence of light, or may be as complex as examining diagnostic overhead bits in data traffic. If the light supplied from output 222-4 of demultiplexer 222, for example, fails, a control signal is output from detect and compare circuit 228 such that switch 226-4 directs the optical signal at wavelength λ₄from network interface 112 for output through port 262. On the other hand, loss of signal detection of the λ₄optical signal input through sub-port input 202-4 would cause optical switch 226-4 to output the optical signal at wavelength λ₄received through port 206 to port 262. In a similar fashion, detect and compare circuit 228 compares the sensed light from ports 222-1 to 222-3 with light from port inputs 202-1 to 202-3, and generates control signals to individually adjust switches 226-1 to 226-4 so that the best signal at each wavelength is output through port 262.

[0023] Typically, nodes 122, 124, 126 and 128 have a similar construction and comprise an optical add/drop multiplexer and transceiver, which will now be described with reference to FIGS. 3 and 4, respectively. FIG. 3 illustrates optical add/drop multiplexer 300 provided in node 122, for example. This add/drop multiplexer is also described, for example, in U.S. patent application Ser. No. 08/956,807, filed Oct. 23, 1997 incorporated by reference herein. Optical signals at wavelengths λ_1-4are fed via optional connector 311 to dielectric filter 313 in a direction indicated by arrow 312. Typically, dielectric filter 313 is configured to drop or select one of wavelengths λ_1-4, for example λ₁, while reflecting the remaining wavelengths, λ_2-4.

[0024] After passing through filter 313, the optical signal at wavelength λ₁is input to transceiver 400, to be discussed in greater detail below, through port 314. Optical signals emitted by transceiver 400 at wavelength λ₅are passed through port 317 to an additional dielectric filter 318. Remaining optical signals at wavelengths λ_2-4, reflected from filter 313, propagate along optical line 330 in a direction indicated by arrow 319 to filter 318. Filter 318 is configured to pass wavelength λ₅, for example, and reflect remaining wavelengths λ_2-4to fiber 350. Accordingly, optical signals at wavelength λ₅are combined with the remaining optical signals at wavelengths λ_2-4on to fiber 350 such that these channels propagate in a common direction indicated by arrow 320 through optional connector 319. Connectors 311 and 319 are typically selected from the group of commercially available FC/PC, FC/APC, SC/PC, SC/APC, biconic, ST, and Diamond E2000 connectors. Alternatively, connectors 311 and 319 can be omitted and optical connections to the add/drop element can be made with fusion splices, for example.

[0025] Transceiver 400 is shown in FIG. 4, and described in U.S. patent application Ser. No. 60/073,750, incorporated by reference herein. Transceiver 400 receives λ₁optical signals output from add/drop multiplexer 300 via input port 401. Receiver 404 typically includes a photodetector for sensing these optical signals and outputs electrical signals in response thereto, and clock and data recovery circuitry. Optionally, the electrical signals output from receiver 414 are next decoded by FEC decoder circuit 410, as described, for example, in a U.S. patent application Ser. No. 08/955,058 entitled “Remodulating Channel Selectors For WDM Optical Communication Systems” by S. B. Alexander et al., filed Oct. 21, 1997 and incorporated by reference herein, which decodes and corrects errors present in the electrical signals output from receiver 404. The decoded electrical signals are next supplied to optical transmitter 412 which, in turn, outputs corresponding optical signals through port 431 to user equipment external to WDM ring transmission system 100.

[0026] As further shown in FIG. 4, transceiver 400 includes receiver 414, including a photodetector, for example, that receives optical signals through input port 432, and converts the received optical signals into electrical signals. Receiver 414 also includes circuitry that performs clock and data recovery from these electrical signals. The output of receiver 414 is optionally coupled to forward error correction (FEC) encoder circuit 416 described, for example, in the patent application by S. B. Alexander et al. supra. The encoded output from FEC circuit 416 is coupled to laser driver 418, which supplies appropriate current to laser diode 420. Laser diode 420 is thus modulated by the output of laser driver 418 in accordance with the encoded electrical signals. Alternatively, laser diode 420 can be operated in a continuous wave (CW) mode and the output modulated with a Mach-Zehnder external modulator, as described, for example, in U.S. Pat. No. 5,504,609, incorporated herein by reference. Typically, a coupler 424 supplies a relatively small fraction of light output from the laser diode 420 to wavelength control circuit 422 for adjusting the temperature, and thus the wavelength of light output from laser diode 420 is tuned to be substantially equal to wavelength λ₅. The remaining light output from laser diode 420 is supplied to optical add/drop multiplexer 300. As a result, optical signals at wavelength λ₅containing data associated with node 122 is supplied to WDM ring transmission system 100.

[0027] It should be noted that in the absence of FEC circuits 410 and 416 electrical signals generated by receivers 404 and 414 are typically supplied directly to transmitter 412 and laser driver circuit 418, respectively.

[0028] Nodes 124, 126 and 128 have a similar construction as node 122 described above. However, the add/drop multiplexers and transceivers cooperate to extract optical signals at wavelengths λ₂, λ₃, and λ₄, respectively, while adding optical signals at wavelengths λ₆λ₇, and λ₈, respectively. Returning to FIG. 2, the λ₆optical signals, as well as the optical signals at wavelength λ₅, are output from node 128 and fed through port 212 to splitter 240. The optical signals are then supplied to combiner 220, as discussed above, and to optical demultiplexer 250, which separates and outputs the optical signals at wavelengths λ₅to λ₈to network interface circuit 114 via corresponding outputs 210-1 to 210-4 of port 210. As further discussed above, the λ₅to λ₈optical signals are passed to hub 116 along with optical signals at wavelengths λ₁to λ₄input from network interface circuit 114. Typically, the wavelengths of the λ₅to λ₈optical signals are different than the λ₁to λ₄optical signals so that both groups of optical signals can propagate along the same fiber with little interference.

[0029] Thus, the optical signals at wavelengths λ₅to λ₈are output from WDM ring transmission system 100 at two locations, thereby improving reliability. Moreover, as discussed above, optical signals at wavelengths λ₁to λ₄are input at two locations, thereby insuring a continuous flow of data to and from WDM ring transmission system 100.

[0030] In accordance with an additional embodiment of the present invention, optical switches 226-1 to 226-4 are replaced by regenerative circuits comprising photodectors, comparator or comparison circuits, and laser transmitters. For example, as shown in FIG. 5, optical signals at wavelengths λ₁to λ₄input through port 202 are respectively supplied to photodetectors 510-1 to 510-4, which, in turn, supply corresponding electrical signals to comparator circuits 514-1 to 514-4, respectively. Optical signals output from demultiplexer 222 are also input to photodetectors 512-1 to 512-4, which output corresponding electrical signals to a respective one of comparator circuits 514-1 to 514-4. Comparator circuits 514-1 to 514-4 effectively pass electrical signals from either photodetectors 510-1 to 510-4 or 512-1 to 512-4 to optical transmitters 516-1 to 516-4, which supply appropriate optical signals at wavelengths λ₁to λ₄to multiplexer 234.

[0031] Comparator circuits 514-1 to 514-4 may simply sense the presence or absence of electrical signals generated by photodetectors 510-1 to 510-4 and 512-1 to 512-4, or may use diagnostic overhead bits in the data traffic as decision criteria. In any event, the comparator circuit passes electrical signals from the photodetector sensing the more healthy data stream. For example, if a portion of network interface circuit 112 supplying optical signals at wavelength λ₂is disabled, photodetector 510-2 ceases to supply electrical signals to comparator circuit 514-2. Accordingly, comparator circuit 514-2 detects the absence of the output from photodetector 510-2, and passes electrical signals output from photodetector 512-3 to λ₂transmitter 516-2. Thus, transmitter 516-2 retransmits the λ₂optical signals in accordance with the electrical signals generated by photodetector 512-3.

[0032] In the embodiment shown in FIG. 5, photodetectors 510-1 to 510-4 can be located outside hub 116, for example in network interface circuit 112. In which case, electrical signals can be supplied directly from network interface circuit 112, for example.

[0033] In order to provide additional protection, a second WDM ring transmission system can be provided in conjunction with WDM ring transmission system 100. Such an additional WDM ring transmission system could maintain communication in the event of a fault, such as a break within WDM ring transmission system 100. The additional WDM ring transmission system, however, would carry optical signals propagating in a direction opposite the optical signals in WDM ring transmission 100.

[0034] An alternative to such a second counter-propagation WDM ring transmission system is to incorporate the above-described functionality of the second WDM ring on the same fiber. In which case, optical signals having respective first wavelengths propagate in one direction while other optical signals at respective second wavelengths propagate in a second direction along the same fiber. In accordance with a further embodiment of the present invention, two hubs provide protection in such a transmission system. In particular, as seen in FIG. 6, optical signals at wavelengths λ₉to λ₁₂are input at hubs 650 and 750 and propagate in a counter-clockwise direction in the figure. Optical signals at wavelengths λ₁to λ₄are also input at hubs 650 and 750, but propagate in a clockwise direction. Node 880, for example, drops optical signals at wavelength λ₉and adds optical signals at wavelength λ₁₃, which continue propagating in the same direction as the λ₉signals. In a similar fashion, optical signals at wavelength λ₁, propagating in the clockwise direction, are dropped at node 820, while optical signals at wavelength λ₅are added at this node. Other nodes, e.g., nodes 840 and 860, similarly add and drop one or more selected pairs of wavelengths propogating in a common direction. For simplicity, only nodes 820 and 880 are illustrated adding and dropping wavlengths. It is understood, however, that other nodes can be provided to add and drop the remaining wavelengths shown in FIG. 6. Wavelengths λ₅to λ₈and λ₁₃to λ₁₆added by nodes coupled to optical communication path 120 are output at both hub 650 and hub 750.

[0035] FIGS. 7 and 8 respectively illustrate hubs 650 and 750 in greater detail. Hubs 650 and 750 comprise input and output ports (not shown) as well as detect and compare circuits and associated couplers (not shown) similar to those described above with reference to FIG. 2. Moreover, hubs 650 and 750 also comprise switches 624, 626, 712 and 714; multiplexers 612, 622, 717 and 722; demultiplexers 614, 620, 716 and 724; optical combiners 618 and 718; optical splitters 616 and 720, which operate in a similar fashion as corresponding components described above with respect to hubs 116 and 118. Further discussion of the operation of these components shown in FIGS. 7 and 8 will thus be omitted. In addition, it is noted that the regenerative circuits discussed above with reference to FIG. 5 can replace the switches shown in FIGS. 7 and 8.

[0036] Unlike hubs 116 and 118, however, hubs 650 and 750 further typically comprise conventional coarse mux/demuxes having optical filters, for branching and combining groups of optical signals at particular wavelengths so that optical signals propagating in a specific direction are supplied to the appropriate combination of switches, multiplexers, demultiplexers, combiners and splitters. Thus, in FIG. 7 for example, coarse mux/demux 630 branches a group of optical signals at wavelengths λ₁₃to λ₁₆and supplies these signals to splitter 616. Coarse mux/demux 630 also directs or combines a group of optical signals at wavelengths λ₁to λ₄onto optical communication path 120. Coarse mux/demux 610, however, branches optical signals at wavlengths λ₁to λ₈and supplies these optical signals to demultiplexer 620, while outputting optical signals at wavelengths λ₉to λ₁₆to course mux/demux 726 via optical path portion 690.

[0037] In FIG. 8, filters 726 and 710 operate in a similar manner to appropriately branch and direct the optical signals at wavelengths λ₁to λ₁₆. Accordingly, due to the presence of course mux/demxes 610, 630, 710 and 726, hubs 650 and 750 can effectively add and drop optical signals propagating in different directions.

[0038] While the foregoing invention has been described in terms of the embodiments discussed above, numerous variations are possible. Accordingly, modifications and changes such as those suggested above, but not limited thereto, are considered to be within the scope of the following claims. For example, the present invention is not limited to the number of wavelengths discussed above. Rather, the present invention encompasses any suitable number of wavelengths. Moreover, one or more additional nodes can be provided between hubs 116 and 118 shown in FIG. 1, as well as between hubs 650 and 750 in FIG. 6.