Combining and distributing amplifiers for optical network and method Number:7,075,712 from the United States Patent and Trademark Office (PTO) owispatent A system for distributing optical signals comprises a plurality of optical splitter stages. The optical splitter stages are each operable to passively split each of one or more optical signals provided to the stage into a plurality of optical signals. At least one of the optional splitter stages comprises an amplification stage, with the amplification stage including a plurality of gain media. The gain media are each operable to amplify an optical signal with pump power.<H distributing, amplifiers, Combining, and, d, 7075712.html, Patent, pto, 7075712

Title: Combining and distributing amplifiers for optical network and method

Abstract: A system for distributing optical signals comprises a plurality of optical splitter stages. The optical splitter stages are each operable to passively split each of one or more optical signals provided to the stage into a plurality of optical signals. At least one of the optional splitter stages comprises an amplification stage, with the amplification stage including a plurality of gain media. The gain media are each operable to amplify an optical signal with pump power.

Patent Number: 7,075,712 Issued on 07/11/2006 to Kinoshita, et al.

Inventors: Kinoshita; Susumu (Plano, TX); Pecqueur; Remi (Garland, TX); Tian; Cechan (Plano, TX)
Assignee: Fujitsu Limited (Kawasaki, JP)

Appl. No.: 159499

Filed: May 30, 2002

Current U.S. Class: 359/349 ; 398/66

Current International Class: H01S 3/00 (20060101); H04J 14/00 (20060101)

Field of Search: 359/349 398/66

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Primary Examiner: Hughes; Deandra M.
Attorney, Agent or Firm: Baker Botts L.L.P.

Claims

What is claimed is:

1. An optical node for distributing optical signals, comprising: a plurality of optical splitter stages within the optical node, the optical splitter stages each operable to passively split each of one or more optical signals provided to the stage into a plurality of optical signals having substantially identical content, the plurality of optical stages comprising: a first optical splitter stage including an optical splitter operable to split a first stage ingress signal into a plurality of first stage egress signals; and a second optical splitter stage including a different optical splitter for each of the first stage egress signals, each second-stage optical splitter operable to split the associated first stage egress signal into a plurality of second stage egress signals; the first and second optical splitter stages each comprising an amplification stage, the amplification stage including a plurality of gain media; the gain media each operable to amplify an associated egress signal from the associated optical splitter stage with pump power; and wherein the optical splitter stages comprise a plurality of branches, and further comprising couplers connecting at least two of the branches to reduce an imbalance of signal power of the branches.

2. The system of claim 1, wherein the gain media comprise erbium-doped optical fibers.

3. The system of claim 1, wherein the gain media comprise doped optical fibers.

4. The system of claim 1, wherein the gain media comprise erbium-doped waveguides.

5. The system of claim 1, wherein the amplification stages includes a pump splitter operable to feed pump power into the amplification stage.

6. The system of claim 5, wherein the amplification stage is downstream of a stage including a pump splitter operable to feed pump power into the amplification stage.

7. The system of claim 1, the plurality of optical splitter stages each comprising one or more optical splitters, the optical splitters operable to passively split a provided optical signal into a plurality of optical signals.

8. The system of claim 7, further comprising: a plurality of the optical splitters comprising pump combiners; and the pump combiners each operable to feed pump power into an optical splitter stage, including the pump combiner.

9. The system of claim 7, wherein the optical splitters each comprise a 2:2 coupler.

10. The system of claim 7, wherein the optical splitters each comprise a 3:3 coupler.

11. The system of claim 1, further comprising a preamplifier, the preamplifier including a pump coupler operable to feed pump power into the preamplifier, a gain medium operable to amplify a preamplifier ingress signal with the pump power and a gain flattener operable to limit gain in the preamplifier.

12. The system of claim 11, further comprising an isolator coupled between a transmission link and the preamplifier.

13. The system of claim 1, further comprising a controller operable to control gain of optical signals in a distribution system.

14. The system of claim 13, the controller operable to limit a power of optical signals at specified points in the distribution system to less than about 17 dBm.

15. The system of claim 13, wherein the controller comprises an automatic gain controller (AGC), the AGC operable to monitor a power of ingress and egress optical signals of the distribution system and to control pump power based on the power of the ingress and egress optical signals.

16. The system of claim 14, wherein the specified points comprise expansion ports of the distribution system.

17. The system of claim 1, wherein the amplification stage is operable to substantially eliminate a power loss of the signal being passively split.

18. A method for distributing an optical signal at an optical node, comprising: passively splitting, at the optical node, an ingress optical signal comprising one or more channels into a plurality of egress optical signals each having substantially identical content as the ingress optical signal and each comprising the one or more channels, wherein passively splitting the ingress optical signal comprises: using a first optical splitter stage including an optical splitter to split the ingress signal into a plurality of first stage egress signals; and using a second optical splitter stage, including a different optical splitter for each of the first stage egress signals, to split each of the first stage egress signals into a plurality of second stage egress signals; amplifying, at the optical node, each of the first stage and second stage egress optical signals with pump power; and reducing an imbalance of signal power resulting from the optical signal being passively split.

19. The method of claim 18, wherein the egress optical signals are each amplified in a gain medium comprising a doped fiber.

20. The method of claim 18, wherein the egress optical signals are each amplified in a gain medium comprising an erbium-doped fiber.

21. The method of claim 18, wherein the egress optical signals are each amplified in a gain medium comprising an erbium-doped waveguide.

22. The method of claim 18, wherein the optical signal comprises a single channel, further comprising providing each of the egress optical signals to a subscriber of the channel.

23. The method of claim 18, further comprising preamplifying the ingress optical signal prior to passively splitting the ingress optical signal.

24. The method of claim 18, further comprising controlling amplification gain.

25. The method of claim 18, further comprising: monitoring a power level of the ingress optical signal and an egress optical signal; determining a gain based on the power levels of the ingress and egress optical signals; and controlling pump power based on the determined gain.

26. The method of claim 18, further comprising limiting the power of the egress optical signals to an eye-safe level.

27. The method of claim 18, wherein amplifying each of the egress optical signals substantially eliminates a power loss of the egress signals relative to the ingress signal.

28. An optical node for distributing an optical signal, comprising: means for passively splitting, at the optical node, an ingress optical signal comprising one or more channels into a plurality of egress optical signals each having substantially identical content as the ingress optical signal and each comprising the one or more channels, wherein the means for passively splitting the ingress optical signal comprises: a first optical splitter stage means operable to split the ingress signal into a plurality of first stage egress signals; and a second optical splitter stage means operable to split each of the first stage egress signals into a plurality of second stage egress signals; means for amplifying, at the optical node, each of the first stage and second stage egress optical signals with pump; and means for reducing an imbalance of signal power resulting from the optical signal being passively split.

29. The system of claim 28, wherein the egress optical signals are each amplified in a gain medium comprising a doped fiber.

30. The system of claim 28, wherein the egress optical signals are each amplified in a gain medium comprising an erbium-doped fiber.

31. The system of claim 28, wherein the egress optical signals are each amplified in a gain medium comprising an erbium-doped waveguide.

32. The system of claim 28, wherein the optical signal comprises a single channel, further comprising providing each of the egress optical signals to a subscriber of the channel.

33. The system of claim 28, further comprising means for preamplifying the ingress optical signal prior to passively splitting the ingress optical signal.

34. The system of claim 28, further comprising means for controlling amplification gain.

35. The system of claim 28, further comprising: means for monitoring a power level of the ingress optical signal and an egress optical signal; means for determining a gain based on the power levels of the ingress and egress optical signals; and means for controlling pump power based on the determined gain.

36. The system of claim 28, further comprising means for limiting the power of the egress optical signals to an eye-safe level.

37. The system of claim 28, wherein the means for amplifying each of the egress optical signals is operable to substantially eliminate a power loss of the egress signals relative to the ingress signal.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to optical transport systems, and more particularly to a combining and distributing amplifier for an optical network.

BACKGROUND OF THE INVENTION

Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers are thin strands of glass capable of transmitting the signals over long distances with very low loss.

Optical networks often employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity. In WDM and DWDM networks, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is based on the number of wavelengths, or channels, in each fiber and the bandwidth, or size of the channels. In WDM, DWDM and other optical networks, microelectro-mechanical switches (MEMS), arrayed waveguide gratings (AWGs), interleavers, and/or fiber gratings (FGs) are typically used to add and drop traffic at network nodes and to multiplex and demultiplex traffic at network nodes.

SUMMARY OF THE INVENTION

The present invention provides a combining and distributing amplifier for an optical network and a corresponding method that eliminate or reduce problems and disadvantages associated with previous systems and methods. In a particular embodiment, one or more traffic channels are amplified in line while being combined and/or distributed.

In accordance with one embodiment of the present invention, a system for distributing optical signals includes a plurality of optical splitter stages. The optical splitter stages are each operable to passively split each of one or more optical signals provided to the stage into a plurality of optical signals. At least one of the optional splitter stages comprises an amplification stage, with the amplification stage including a plurality of gain media. The gain media are each operable to amplify an optical signal with pump power.

Technical advantages of the present invention include providing a combining and/or distributing amplifier and method. In one embodiment, optical signals are distributed or combined using optical couplers in stages and the coupler loss reduced and/or cancelled out with erbium doped fiber (EDF), erbium doped waveguide (EDW), or other suitable in-line amplifiers.

Another technical advantage of the present invention includes providing combining and/or distributing amplifiers with flexible channel spacing. In one embodiment, distributors/combiners include couplers, doped in-line amplifiers, tunable filters, wide-band receivers, and tunable transponders. The tunable transponders can transmit any wavelength in a pre-determined determined signal band. The wide-band receivers can receive any wavelength signal in the signal band. The tunable filter in front of the receiver can select any wavelength in the signal band. Therefore, any signal signed on a certain wavelength may be transmitted to any receiver. This configuration,supports various data-rate services, such as for example, 150 megabits per second (Mb/s) 600 Mb/s, 2.4 gigabits per second (Gb/s), 10 Gb/s and 40 Gb/s, and various modulations schemes, such as for example, direct modulation and external modulation.

Still another technical advantage of the present system includes providing a modular combiner/distributor architecture. In one embodiment, a primary combiner/distributor board may provide ingress signals to a plurality of extension ports each configured to receive an upgrade board with further combiner/distributor stages and functionality. In addition, a modular pump array board may be provide to feed pump energy to the in-line amplifiers in the primary and upgrade combiner/distributor boards. As a result, users may "pay as they grow" and only pay for the number of components needed at a particular time.

Still another technical advantage of the present invention includes providing an automatic gain control (AGC) or other suitable controller for in-line amplifiers of a combiner/distributor unit. In one embodiment, the controller measures energy of ingress and egress signals and adjusts pump power provided to in-line amplifiers based on the measured energy of the signals and gain determined based on the measured energy. Signal power at the expansion ports may be limited to an eye-safe or other suitable level to, among other things, prevent injuries to operators servicing the boards in hazardous conditions.

Still another technical advantage of the present invention includes providing a low cost unit wherein multiplexers and demultiplexers may be omitted in a node. This allows for less-expensive and more reliable photonic nodes and networks. Whereas traditional optic networks have fixed channel spacing, large losses in the optical nodes, and high cost due to complicated configurations, the present invention provides low cost, simple network nodes with no channel-spacing restrictions, no band-pass narrowing, and low loss. These features are well suited to future metro networks, access networks, and other networks wherein the cost must be low with a high implementation and network flexibility, and high compatibility between different vendors and new technologies.

Still another technical advantage of the present invention includes providing a passive, low cost, multicast function for optical signals. In one embodiment, optical signals may be naturally multicast by splitting the optical signal into multiple signals via multioptical coupler stages. The split signals may be amplified in-line during the splitting process to cancel or minimize the splitting loss effect and/or enhance signal strength in the resulting signals multicast to a plurality of users. In this way, cable TV and other similar services may be efficiently supported by an optical network in a cost-effective manner.

Still another technical advantage of the present invention is an increase in pump efficiency through the use of pump couplers connecting the branches of the distributor and/or combiners. In addition, the couplers may reduce signal power imbalance of the split signals. In a particular embodiment, pump efficiency may be increased by 30% or more and outputs equalized to within 1 dB.

It will be understood that the various embodiments of the present invention may include some, all, or none of the enumerated technical advantages. In addition, other technical advantages of the present invention may be readily apparent to one skilled in the art from the following figures, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like numerals represent like parts, in which:

FIG. 1 is a block diagram illustrating an optical network in accordance with one embodiment of the present invention;

FIG. 2 is a block diagram illustrating details of the node of FIG. 1 in accordance with one embodiment of the present invention;

FIG. 3 is a block diagram illustrating details of an optical coupler of the node of FIG. 2 in accordance with one embodiment of the present invention;

FIG. 4 is a block diagram illustrating the open ring configuration and light path flow of the optical network of FIG. 1 in accordance with one embodiment of the present invention;

FIG. 5 is a block diagram illustrating the optical supervisory channel (OSC) flow in the optical network of FIG. 1 in accordance with one embodiment of the present invention;

FIG. 6 is a block diagram illustrating protection switching and light path protection in the optical network of FIG. 1 in accordance with one embodiment of the present invention;

FIG. 7 is a flow diagram illustrating a method for protection switching for the optical network of FIG. 1 in accordance with one embodiment of the present invention;

FIG. 8 is a block diagram illustrating OSC protection in the optical network of FIG. 1 in response to a line cut in accordance with one embodiment of the present invention;

FIG. 9 is a flow diagram illustrating a method for OSC protection switching in the optical network of FIG. 1 in accordance with one embodiment of the present invention;

FIG. 10 is a block diagram illustrating OSC protection in the optical network of FIG. 1 in response to an OSC equipment failure in accordance with one embodiment of the present invention;

FIG. 11 is a flow diagram illustrating a method for inserting a node into the optical network of FIG. 1 in accordance with one embodiment of the present invention;

FIG. 12A is a block diagram illustrating details of a distributing amplifier of the node in FIG. 2 in accordance with one embodiment of the present invention;

FIG. 12B is a block diagram illustrating details of a distributing combiner of the node in FIG. 2 in accordance with one embodiment of the present invention;

FIG. 13 is a block diagram illustrating details of the pre-amplification module of FIGS. 12A and 12B in accordance with one embodiment of the present invention;

FIG. 14 is a block diagram illustrating details of the amplified splitter stages module of FIGS. 12A and 12B in accordance with one embodiment of the present invention;

FIGS. 15A 15D are block diagrams illustrating configurations of the amplified splitter stages module of FIGS. 12A and 12B in accordance with various other embodiments of the present invention;

FIGS. 16A and 16B are block diagrams illustrating details of the pre-amplification system and the amplified splitter stages modules of FIGS. 12A and 12B in accordance with other embodiments of the present invention;

FIG. 17 is a flow diagram illustrating a method for distributing an optical signal in an amplified splitter stages module in accordance with one embodiment of the present invention;

FIG. 18 is a flow diagram illustrating a method of amplifying and combining a plurality of signals into a single signal in accordance with one embodiment of the present invention;

FIG. 19 is a block diagram illustrating an optical network in accordance with another embodiment of the present invention;

FIG. 20 is a block diagram illustrating details of an add/drop node of the network of FIG. 19 in accordance with one embodiment of the present invention;

FIG. 21 is a flow diagram illustrating a method of adding and dropping signals to and from the node of FIG. 20 in accordance with one embodiment of the present invention;

FIG. 22 is a block diagram illustrating an add/drop node for the network of FIG. 19 in accordance with another embodiment of the present invention;

FIG. 23 is a block diagram representing an inter-ring module connecting two networks in accordance with an embodiment of the present invention;

FIG. 24 is a block diagram representing an inter-ring module connecting three networks in accordance with one embodiment of the present invention; and

FIG. 25 is a flow diagram illustrating a method of communicating a signal between ring networks in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an optical network 200 in accordance with one embodiment of the present invention. In this embodiment, the optical network 200 is a flexible open ring network.

Referring to FIG. 1, the network 200 includes a first fiber optic ring 202 and a second fiber optic ring 204 connecting nodes 206, 208, 210, and 212. As with network 10, network 200 is an optical network in which a number of optical channels are carried over a common path at disparate wavelengths. The network 200 may be a wavelength division multiplexing (WDM), dense wavelength division multiplexing (DWDM), or other suitable multi-channel network. The network 200 may be used in a short-haul metropolitan network, and long-haul inter-city network or any other suitable network or combination of networks.

In network 200, optical information signals are transmitted in different directions on the rings 202 and 204 to provide fault tolerance. The optical signals have at least one characteristic modulated to encode audio, video, textual, real-time, non-real-time and/or other suitable data. Modulation may be based on phase shift keying (PSK), intensity modulation (IM) and other suitable methodologies.

In the illustrated embodiment, the first ring 202 is a clockwise ring in which traffic is transmitted in a clockwise direction. The second ring 204 is a counterclockwise ring in which traffic is transmitted in a counterclockwise direction. The nodes 201 are each operable to add and drop traffic to and from the rings 202 and 204. As used herein, the term "each" means every one of at least a subset of the identified items. In particular, each node 201 receives traffic from local clients and adds that traffic to the rings 202 and 204. At the same time, each node 201 receives traffic from the rings 202 and 204 and drops traffic destined for the local clients. In adding and dropping traffic, the nodes 201 may multiplex data from clients for transmittal in the rings 202 and 204 and may demultiplex channels of data from the rings 202 and 204 for clients.

Traffic may be added to the rings 202 and 204 by inserting the traffic channels or otherwise combining signals of the channels into a transport signal of which at least a portion is transmitted on a ring. Traffic may be dropped by making the traffic available for transmission to the local clients. Thus, traffic may be dropped and yet continue to circulate on a ring. In a particular embodiment, traffic is passively added to and dropped from the rings 202 and 204. "Passive" in this context means the adding or dropping of channels without power, electricity, and/or moving parts. An active device would thus use power, electricity or moving parts to perform work. In a particular embodiment, traffic may be passively added to and/or dropped from the rings 202 and 206 by splitting/combining, which is without multiplexing/demultiplexing, in the transport rings and/or separating parts of a signal in the ring.

In a particular embodiment, traffic is passively added to and passively dropped from the rings 202 and 204. In this embodiment, channel spacing may be flexible in the rings 202 and 204 and the node elements on the rings 202 and 204 need not be configured with channel spacing. Thus, channel spacing may be set by and/or at the add/drop receivers and senders of the nodes 201 coupled to the client. The transport elements of the nodes 201 communicate the received traffic on the rings 202 and 204 regardless of the channel spacing of the traffic.

Each ring 202 and 204 has a terminating point such that the rings 202 and 204 are "open" rings. The opening in the rings 202 and 204 may be a physical opening, an open, crossed, or other non-closed switch, a deactivated transmission device or other obstruction operable to completely or effectively terminate, and thus remove channels from the rings 202 and 204 at the terminal points such that interference of each channel with itself due to recirculation is prevented or minimized such that the channels may be received and decoded within normal operating limits.

In one embodiment, the rings 202 and 204 are open, and thus terminate, in the nodes 201. In a particular embodiment, the rings 202 and 204 may terminate in neighboring nodes 201 at corresponding points along the rings 202 and 204. Terminal points in the rings 202 and 204 may be corresponding when, for example, they are between add and/or drop devices of two neighboring nodes or when similarly positioned within a same node. Further details regarding the open ring configuration are described below in reference to FIG. 4.

FIG. 2 illustrates details of the node 201 in accordance with one embodiment of the present invention. In this embodiment, optical supervisory channel (OSC) traffic is transmitted in an external band separate from the revenue-generating traffic. In a particular embodiment, the OSC signal is transmitted at a wavelength of 1510 nanometers (nm).

Referring to FIG. 2, the node 201 comprises counterclockwise transport element 220, clockwise transport element 222, distributing element 224, combining element 226, and managing element 228. In one embodiment, the elements 220, 222, 224, 226 and 228 as well as components within the elements may be interconnected with optical fiber links. In other embodiments, the components of this and other modes may be implemented in part or otherwise with planar waveguide circuits and/or free space optics. In addition, as described in connection with nodes 12, the elements of node 201 may each be implemented as one or more discrete cards within a card shelf of the node 201. Exemplary connectors 230 for a card shelf embodiment are illustrated by FIGS. 12A and 12B. The connectors 230 may allow efficient and cost effective replacement of failed components. It will be understood that additional, different and/or other connectors may be provided as part of the node 201.

Transport elements 220 and 222 may each comprise passive couplers or other suitable optical splitters/couplers 330, ring switch 214, amplifier 215, and OSC filters 216. Optical splitters/couplers 330 may comprise splitters/couplers 330 or other suitable passive device. Ring switch 214 may be a 2.times.2 or other switch operable to selectively open the connected ring 202 or 204. In the 2.times.2 embodiment, the switch 214 includes a "cross" or open position and a "through" or closed position. The cross position may allow for loopback, localized and other signal testing. The open position allows the ring openings in the nodes 201 to be selectively reconfigured to provide protection switching.

Amplifier 215 may comprise an erbium-doped fiber amplifier (EDFA) or other suitable amplifier. In one embodiment, the amplifier is a preamplifier and may be selectively deactivated to open a connected ring 202 or 204 to provide protection switching in the event of failure of the adjacent switch 214. Because the span loss of clockwise ring 202 may differ from the span loss of counterclockwise ring 204, the amplifier 215 may use automatic gain control (AGC) to realize gain-flatness against input power variation. The preamplifier 215 and the switch 214 are disposed in the transport elements 220 and 222 inside of the OSC filters 216 and between the ingress OSC filter 216 and the add/drop splitters/couplers 330. Thus, the OSC signal may be recovered regardless of the position of switch 214 or operation of preamplifier 215. In another embodiment, OSC signals may be transmitted in-band with revenue-generating traffic by placing an OSC filter between the couplers 232 and 234 and between the couplers 236 and 238. OSC filters 216 may comprise thin film type, fiber grating or other suitable filters.

In the specific embodiment of FIG. 2, counterclockwise transport element 220 includes a passive optical splitter set having a counterclockwise drop coupler 232 and a counterclockwise add coupler 234. The counterclockwise transport element 220 further includes OSC filters 294 and 298 at the ingress and egress edges, counterclockwise amplifier 240 between the ingress OSC filter 294 and drop coupler 232 and counterclockwise ring switch 244 between amplifier 240 and drop coupler 232. Thus, the switch 244 in this embodiment is on the ingress side of the transport element and/or drop coupler. The counterclockwise transport element 220 may also include a dispersion compensation fiber (DCF) segment 235 to provide dispersion control. In one embodiment, DCF segment 235 may be included where the network 200 operates at rates at or above 2.5 G and/or the previous node is greater than a short hop on the connected ring.

Clockwise transport element 222 includes a passive optical splitter set including clockwise add coupler 236 and clockwise drop coupler 238. Clockwise transport element 222 further includes OSC filters 296 and 300, clockwise amplifier 242, and clockwise ring switch 246. OSC filters 296 and 300 are disposed at the ingress and egress edges of the clockwise transport element 222. The clockwise amplifier 242 is disposed between the ingress OSC filter 300 and the drop coupler 238 while the clockwise ring switch 246 is disposed between the amplifier 242 and the drop coupler 238. Thus, the switch 246 in this embodiment is on the ingress side of the transport element and/or drop coupler. The clockwise transport element 222 may also include a DCF segment 245 to provide dispersion compensation depending, as previously discussed, on the data transport rate and/or the length of the span to the previous node.

Distributing element 224 may comprise a drop coupler 310 feeding into the distributing module (DM) 316. DM 316 is operable to split the signal from coupler 310 into a plurality of signals, amplify the signals, and forward the signals to the drop leads 314. Further details regarding DM 316 are described in reference to FIG. 12A. The drop leads 314 may be connected to one or more tunable filters 266 which in turn may be connected to one or more broadband optical receivers 268.

Combining element 226 may be a combining amplifier and may comprise a combining module (CM) 328 which may be connected to one or more add optical senders 270 via add leads 312. CM 328 is operable to combine the signal from leads 312 into a single signal, which is forwarded via fiber 326 into splitter 324. Further details regarding CM 318 are described in reference to FIG. 12B. Splitter 324 further comprises two optical fiber egress leads which feed into clockwise add segment 306 and counterclockwise add segment 302.

Managing element 228 may comprise OSC senders 272 and 281, OSC interfaces 274 and 280, OSC receivers 276 and 278, and an element management system (EMS) 290. Each OSC sender, OSC interface and OSC receiver set forms an OSC unit for one of the rings 202 or 204 in the node 201. The OSC units receive and transmit OSC signals for the EMS 290. The EMS 290 may be communicably connected to a network management system (NMS) 292. NMS may reside within node 201, in a different node, or external to all of the nodes 201.

EMS 290, NMS 292 and/or other elements or parts of the described nodes or networks may comprise logic encoded in media for performing network and/or node monitoring, failure detection, protection switching and loopback or localized testing functionality of the network 200. Logic may comprise software encoded in a disk or other computer-readable medium and/or instructions encoded in an application specific integrated circuit (ASIC), field programmable gate array (FPGA), or other processor or hardware. It will be understood that functionality of EMS 290 and/or NMS 292 may be performed by other components of the network 200 and/or be otherwise distributed or centralized. For example, operation of NMS 292 may be distributed to the EMS of nodes 201 and the NMS omitted. Similarly, the OSC units may communicate directly with NMS 292 and EMS 290 omitted.

The node 201 further comprises counterclockwise add fiber segment 302, counterclockwise drop fiber segment 304, clockwise add fiber segment 306, clockwise drop fiber segment 308, OSC fiber segments 282, 284, 286, and 288, and optical spectrum analyzer (OSA) connectors 250, 254, 256, and 258. The OSA connectors may be angled connectors to avoid reflection. Test signal may sometimes be fed into the network from connectors 248 and 252. As previously described, a plurality of passive physical contact connectors 230 may be included where appropriate so as to communicably connect the various elements of node 201.

In operation, the transport elements 220 and 222 are operable to passively add local traffic to the rings 202 and 204 and to passively drop at least local traffic from the rings 202 and 204. The transport elements 220 and 222 may further be operable to passively add and drop the OSC signal to and from the rings 202 and 204. More specifically, in the counterclockwise direction, OSC filter 294 processes an ingress optical signal from counterclockwise ring 204. OSC filter 294 filters OSC signal from the optical signal and forwards the OSC signal to the OSC interface 274 via fiber segment 282 and OSC receiver 276. OSC filter 294 also forwards or lets pass the remaining transport optical signal to amplifier 240. By placing the OSC filter 294 outside of the ring switch 244, the node 201 is able to recover the OSC signal regardless of the position of the ring switch 244.

Amplifier 240 amplifies the signal and forwards the signal to ring switch 244. Ring switch 244 is selectively operable to transmit the optical signal to coupler 232 when the ring switch 244 is set to the through (closed) setting, or to transmit the optical signal to OSA connector 250 when the ring switch 244 is set to the cross (open) setting. Further details regarding the OSA connectors are described below.

If ring switch 244 is set in the cross position, the optical signal is not transmitted to couplers 232 and 234, the ring 204 is open at the node 201, and dropping of traffic from the ring 204 does not occur at node 201. However, adding of traffic at node 201 occurs and the added traffic flows to the next node in the ring 204. If the ring switch 244 is set in the through position, the optical signal is forwarded to couplers 232 and 234 and adding and dropping of traffic to and from the ring 204 may occur at node 201.

Coupler 232 passively splits the signal from switch 244 into two generally identical signals. A passthrough signal is forwarded to coupler 234 while a drop signal is forwarded to distributing element 224 via segment 304. The signals may be substantially identical in content and/or energy. Coupler 234 passively combines the passthrough signal from coupler 232 and an add signal comprising local add traffic from combining element 226 via fiber segment 302. The combined signal is passed to OSC filter 298.

OSC filter 298 adds an OSC signal from the OSC interface 274, via the OSC sender 272 and fiber segment 284, to the combined optical signal and forward the combined signal as an egress transport signal to ring 204. The added OSC signal may be locally generated data or may be received OSC data passed through the EMS 290.

In the clockwise direction, OSC filter 300 receives an ingress optical signal from clockwise ring 202. OSC filter 300 filters the OSC signal from the optical signal and forwards the OSC signal to the OSC interface 280 via fiber segment 286 and OSC receiver 278. OSC filter 300 also forwards the remaining transport optical signal to amplifier 242.

Amplifier 242 amplifies the signal and forwards the signal to ring switch 246. Ring switch 246 is selectively operable to transmit the optical signal to coupler 238 when the ring switch 246 is set to the through setting, or to transmit the optical signal to OSA connector 254 when the ring switch 246 is set to the cross setting.

If the ring switch 246 is set in the cross position, the optical signal is not transmitted to couplers 238 and 236, the ring 204 is open at the node 201, and dropping of traffic from the ring 202 does not occur at node 201. However, adding of traffic to the ring 202 occurs at node 201. If the ring switch 246 is set in the through position, the optical signal is forwarded to couplers 238 and 236 and adding and dropping of traffic to and from the ring 202 may occur at node 201.

Coupler 238 passively splits the signal from switch 246 into generally identical signals. A passthrough signal is forwarded to coupler 236 while a drop signal is forwarded to distributing unit 224 via segment 308. The signals may be substantially identical in content and/or energy. Coupler 236 passively combines the passthrough signal from coupler 238 and an add signal comprising local add traffic from combining element 226 via fiber segment 306. The combined signal is passed to OSC filter 296.

OSC filter 296 adds an OSC signal from the OSC interface 280, via the OSC sender 281 and fiber segment 288, to the combined optical signal and forwards the combined signal as an egress transport signal to ring 202. As previously described, the OSC signal may be locally generated data or data passed through by EMS 290.

Prior to addition to the rings 202 and 204, locally-derived traffic is transmitted by a plurality of add optical senders 270 to combining element 226 of the node 201 where the signals are combined, amplified, and forwarded to the transport elements 220 and 222, as described above, via counterclockwise add segment 302 and clockwise add segment 306. The locally derived signals may be combined by the optical coupler 324, by a multiplexer or other suitable device.

Locally-destined traffic is dropped to distributing element 224 from counterclockwise drop segment 304 and clockwise drop segment 308. Distributing element 224 splits the drop signal comprising the locally-destined traffic into multiple generally identical signals and forwards each signal to an optical receiver 268 via a drop lead 314. The signal received by optical receivers 268 may first be filtered by filters 266. Filters 266 may be tunable filters or other suitable filters and receivers 268 may be broadband or other suitable receivers.

EMS 290 monitors and/or controls all elements in the node 201. In particular, EMS 290 receives an OSC signal in an electrical format via OSC filters 294, 296, 298 and 300, OSC receivers 276 and 278, OSC senders 272 and 281, and OSC interfaces 274 and 280. EMS 290 may process the signal, forward the signal and/or loopback the signal. Thus, for example, the EMS 290 is operable to receive the electrical signal and resend the OSC signal to the next node, adding, if appropriate, node-specific error information or other suitable information to the OSC.

In one embodiment each element in a node 201 monitors itself and generates an alarm signal to the EMS 290 when a failure or other problem occurs. For example, EMS 290 in node 201 may receive one or more of va