Bi-directional optical transmission system with coherent detection

An optical device includes a first optical port connected to a first optical fiber, and a second optical port connected to a second optical fiber. The optical device further includes first optical components that switch first optical traffic carried via a first set of optical channels from the first optical port to the second optical port, and second optical components that switch second optical traffic carried via a second set of optical channels from the second optical port to the first optical port. The second set of optical channels includes different optical channels than the first set of optical channels. The optical device also includes a receiver that coherently detects portions of the first optical traffic and the second optical traffic, and converts the detected portions of the first and second optical traffic to electrical signals for delivery to a node or network external to the optical device.

BACKGROUND

Optical networks employing 10 gigabit Ethernet (10 GE) transport Ethernet frames at a rate of 10 gigabits per second. A router in such an optical network typically includes multiple client interfaces, each of which uses a single optical carrier (e.g., light of a single wavelength) for receiving and/or transmitting data. Transport equipment connects to the router via multiple client interfaces, which each use the single optical carrier, to receive data transmitted from the client interfaces of the router. The transport equipment may further include multiple transport cards, each of which transmits outgoing data over a single optical carrier. The transport equipment sends the data via the single optical carriers to destination transport nodes in the optical network.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A bi-directional optical transmission system, described herein, employs multiple optical channels that are used to transmit data in opposite directions over a same strand of optical fiber. The optical nodes of the bi-directional optical transmission system use multiple wavelength selective switches, such as, for example, “gridless” wavelength selective switches, in conjunction with optical circulators for switching and directing optical traffic heading in two different directions through the optical nodes. Each of the optical nodes further includes a transponder having coherent receivers that coherently detect optical signals to provide better filtering of the received optical signals so as to distinguish between the optical signals and back reflection noise.

FIG. 1is a diagram that depicts an exemplary optical network100that deploys optical nodes for carrying, amplifying, switching, and adding/dropping bi-directional, multi-wavelength optical traffic transiting in two directions across optical network100. Optical network100includes optical nodes110-1and110-2(hereinafter generically and individually referred to as “optical node110”) interconnected by a span of optical fiber120-1. As further shown inFIG. 1, optical node110-1may connect to another optical node (not shown) via a span of optical fiber120-2, and optical node110-2may connect to another optical node (not shown) via a span of optical fiber120-3. Optical nodes110-1and110-2may include, as described in further detail below, optical devices having components for amplifying optical traffic transiting optical network100through optical nodes110-1and110-2, and for adding new optical traffic for transit via optical network100and for dropping optical traffic off of optical network100(i.e., switching optical traffic to a destination accessible via optical node110-1or optical node110-2). The left side of optical network100depicted inFIG. 1is arbitrarily designated as “west” and the right side of optical network100is arbitrarily designated as “east.” Therefore, optical traffic being carried by optical network100in a direction from optical node110-2to optical node110-1is designated as “east to west” traffic of the bi-directional optical traffic. Additionally, optical traffic being carried by optical network100in a direction from optical node110-1to optical node110-2is designated as “west to east” traffic of the bi-directional optical traffic. The bi-directional optical traffic may include optical signals carried via multiple different optical channels, with each optical channel including light of a different wavelength (λ). As described further below, the “west-to-east” traffic of the bi-directional optical traffic comprises a first set of optical channels, and the “east-to-west” traffic of the bi-directional optical traffic comprises a second set of optical channels that includes different optical channels than the first set of optical channels.

Optical nodes110-1and110-2may each further connect to additional nodes, networks or systems not shown inFIG. 1. For example, optical nodes110-1and110-2may each connect, via a transponder (not shown), to a local area network (LAN) or a wide area network (WAN), and may switch incoming optical signals, and convert the optical signals to electrical signals (e.g., packet data) for transmission over the LAN or WAN. The configuration of components of optical network100illustrated inFIG. 1is for illustrative purposes. Other configurations may be implemented. Therefore, optical network100may include additional, fewer and/or different components that may be configured in a different arrangement from that depicted inFIG. 1. For example, though two optical nodes110-1and110-2with an interconnecting span120-1of optical fiber is depicted inFIG. 1, optical network100may include additional optical nodes110interconnected directly, or via intervening optical fibers, to optical nodes110-1and/or110-2. Optical network100may also include additional optical nodes110not connected with optical nodes110-1or110-2either directly or indirectly.

FIG. 2Adepicts exemplary components of optical node110. As shown, optical node110may include a Reconfigurable Optical Add/Drop Multiplexer (ROADM)200interconnected between two optical amplifiers210-1and210-2that further connect to optical node ports220-1and220-2, respectively. In the case of optical node110-1, optical node port220-1connects to optical fiber span120-2, and optical node port220-2connects to optical fiber span120-1. In the case of optical node110-2, optical node port220-1connects to optical fiber span120-1and optical node port220-2connects to optical fiber span120-3. As further shown inFIG. 2A, ROADM200connects to a transponder230. Optical amplifiers210-1and210-2amplify multi-wavelength optical signals transmitted from optical node110. ROADM200includes a wavelength division multiplexing system that switches optical traffic transiting optical node110from optical node port220-1to optical node port220-2, and transiting optical node110from optical node port220-2to optical node port220-1. ROADM200further includes “add” multiplexing functionality for receiving and adding optical signals, via transponder230, to outgoing optical signals transmitted via optical node port220-1or optical node port220-2. ROADM200also includes “drop” multiplexing functionality for receiving and “dropping” optical signals, via transponder230, to another node or system connected to optical node110.

As described further with respect toFIG. 4below, ROADM200multiplexes and demultiplexes data traffic carried via multiple optical channels (e.g., multiple optical wavelengths) with a first set of optical channels carrying optical traffic in an east-to-west direction, and a second set of optical channels carrying optical traffic in a west-to-east direction, and with the channels of the first set of optical channels being offset by a certain channel spacing from the channels of the second set of optical channels.

Referring toFIG. 2B, ROADM200includes components for switching east-to-west optical traffic received at optical node port220-2through to optical node port220-1, for switching a portion of the east-to-west traffic to be “dropped” (shown as “E-to-W drop”) to transponder230, and for switching new traffic “added” via transponder230into the east-to-west traffic (shown as “E-to-W add”). ROADM200further includes components for switching west-to-east traffic received at optical node port220-1through to optical node port220-2, for switching a portion of the west-to-east traffic to be “dropped” (shown as “W-to-E drop”) to transponder230, and for switching new traffic “added” via transponder230into the west-to-east traffic (shown as “W-to-E add”).

Transponder230includes components for receiving outgoing electrical signals (e.g., encapsulated packet data), converting the outgoing electrical signals to outgoing optical signals, and supplying the outgoing optical signals to ROADM200for switching in an appropriate west-to-east or east-to-west direction for delivery to a destination optical node110in optical network100. Transponder230further includes components for receiving incoming optical signals from ROADM200, converting the incoming optical signals to corresponding electrical signals (e.g., encapsulated packet data), and supplying the converted electrical signals to one or more nodes or networks connected to transponder230(not shown inFIG. 2B). The one or more networks may include one or more of a Public Switched Telephone Network (PSTN), a wireless network, a local area network (LAN), a wide area network (WAN), a metropolitan area network (MAN), an intranet, or the Internet. The wireless network may include a satellite network, a Public Land Mobile Network (PLMN), or a wireless LAN or WAN (e.g., Wi-Fi).

MCS330-1and MCS330-2may each include any type of multicast switch operable with WSSs320-1through320-4for switching “dropped” optical signals to transponder230, and for switching “added” optical signals from transponder230towards an appropriate optical port220of optical node110that corresponds to the destination of the optical signals. WSSs320-1through320-4may, in one implementation, each include a “gridless” WSS. A “gridless” WSS may perform switching between WSS ports without having to lock the spectrum of the switching to any predetermined channel plan. A “gridless” WSS, therefore, enables flexibility in channel allocation in optical network100. Gridless WSSs320may include, for example, a liquid crystal on silicon (LCOS) switching element or a micro-electromechanical system (MEMs) switching element employed in a variable phase spatial light modulator (SLM). Other types of gridless WSSs320that enable the implementation of a flexible grid may be used in ROADM200. The use of “gridless” WSSs further avoids self-lasing within optical nodes110.

Optical circulators310-1and310-2each may include a 3-port optical circulator that can be used to separate optical signals that travel in opposite directions over an optical fiber, where light entering any optical port exits from the next optical port. For example, light from optical signals received at optical node port220-2enters port2of circulator310-2and exits port3for receipt at WSS320-1, and light from optical signals received from WSS320-4enters port1and exits port2towards optical node port220-2. As another example, light from optical signals received at optical node port220-1enters port2of circulator310-1and exits port3for receipt at WSS320-3, and light from optical signals received from WSS320-2enters port1and exits port2towards optical node port220-1.

Port2of circulator310-2connects to optical node port220-2via optical amplifier210-2(not shown), port3of circulator310-2connects to WSS320-1, and port1of circulator310-2connects to WSS320-4. Optical signals transiting in an east-to-west direction across optical network100, from optical node port220-2into port2of circulator310-2of optical node110, will exit port3of circulator310-2for input into WSS320-1. WSS320-1switches optical signals of the input optical signals, destined for a node or network connected to optical node110, via MCS330-1of add/drop module305to transponder230. WSS320-1additionally switches optical signals of the input optical signals destined for another node in optical network100to an input of WSS320-2. WSS320-2receives input optical signals from WSS320-1, and input optical signals from MCS330-2of add/drop module305“added” by transponder230(described below), and switches those optical signals to port1of circulator310-1. Optical signals switched by WSS320-2to port1of circulator310-1exit port2of circulator310-1for transit in the east-to-west direction via optical node port220-1of optical node110.

Port2of circulator310-1connects to optical node port220-1via optical amplifier210-1(not shown), port3of circulator310-1connects to WSS320-3, and port1of circulator310-1connects to WSS320-2. Optical signals transiting in a west-to-east direction across optical network100, from optical node port220-1into port2of circulator310-1of optical node110, will exit port3of circulator310-1for input into WSS320-3. WSS320-3switches optical signals of the input optical signals, destined for a node or network connected to optical node110, via MCS330-1of add/drop module305to transponder230. WSS320-1additionally switches optical signals of the input optical signals destined for another node in optical network100to an input of WSS320-4. WSS320-4receives input optical signals from WSS320-3, and input optical signals from MCS330-2of add/drop module305“added” by transponder230(described below), and switches those optical signals to port1of circulator310-2. Optical signals switched by WSS320-4to port1of circulator310-2exit port2of circulator310-2for transit in the west-to-east direction via optical node port220-2of optical node110.

MCS330-1receives optical signals, that are destined for another node or network (not shown) connected to transponder230, for “dropping” by add/drop module305to a receiver (Rx)340of transponder230. MCS330-1receives optical signals from WSS320-1received at optical node110via optical node port220-2, and receives optical signals from WSS320-3further received at optical node110via optical node port220-1. MCS330-1switches the received optical signals to Rx340of transponder230. Rx340of transponder230detects the optical signals, converts the optical signals to corresponding electrical signals (e.g., packet data), and forwards the electrical signals towards the destination node or network connected to transponder230. Rx340of transponder230may include a coherent detection system, including use of a local oscillator (LO), which removes back reflection noise due to phase detection. Rx340of transponder230may further include components for forward error correction and digital signal processing.

MCS330-2receives optical signals from transmitter (Tx)350of transponder230, that are destined for delivery to another node in optical network either in an east-to-west direction out optical node port220-1of optical node110or in a west-to-east direction out optical node port220-2of optical node110for “adding” via add/drop module305.

Tx350receives electrical signals from another node or network (not shown) connected to transponder230, converts the electrical signals to corresponding output optical signals, and transmits the optical signals to MCS330-2for switching in an east-to-west or west-to-east direction out of optical node110that corresponds to the destination of the optical signals.

MCS330-2switches the received optical signals from Tx350of transponder230, intended for a destination reachable via optical node port220-1of optical node110, to WSS320-2. WSS320-2, in turn, switches the received optical signals from MCS330-2to port1of circulator310-1for exit from port2of circulator310-1. MCS330-2further switches the received optical signals from Tx350of transponder230, intended for a destination reachable via optical node port220-2of optical node110, to WSS320-4. WSS320-4, in turn, switches the received optical signals from MCS330-2to port1of circulator310-2for exit from port2of circulator310-2.

FIGS. 4A-4Cdepict directional channel spectrum allocation for optical traffic carried in an east-to-west and a west-to-east direction via optical network100. As shown inFIGS. 4A-4C, west-to-east channels400(shown via dashed lines) may be offset from east-to-west channels410(shown via solid lines) such that directional cross-talk is minimized. In the channel spectrum allocation shown inFIG. 4A, the channels for the west-to-east direction400are assigned and spaced with a channel spacing (Δλspacing). In the channel spectrum allocation shown inFIG. 4B, the channels for the east-to-west direction410are also assigned and spaced with the channel spacing (Δλspacing). The optical wavelengths of each channel of west-to-east channels400may be evenly spaced relative to one another, and the optical wavelengths of each channel of east-to-west channels410may also be evenly spaced relative to one another but at an offset relative to the channels of west-to-east channels400.FIG. 4Cdepicts the channel spectrum allocation of channels in both the west-to-east direction400and the east-to-west direction410, on a single wavelength plot, with a channel spacing offset (λoffset) instituted between the channels in the two opposition directions, where the channels in both directions are carried by a single optical fiber. In the example ofFIG. 4C, the west-to-east channels400are offset (λoffset) by one half of a channel spacing with east-to-west channels410. For example, if the channel spacing between each of the channels of west-to-east channels400is a wavelength interval of Δλspacing, then the channels of the east-to-west channels410may be offset from the west-to-east channels400by a wavelength offset of ½ of Δλspacing(λoffset=½*Δλspacing). The directional channel spectrum allocation for optical traffic which enables bi-directional optical traffic in optical network100via a single optical fiber, as depicted inFIG. 4C, effectively doubles the fiber capacity of the optical fibers carrying optical traffic within optical network100.

FIG. 5depicts one exemplary implementation of optical amplification within optical nodes110of optical network100where each optical amplifier210of optical node110includes a Raman amplifier. As shown inFIG. 5, optical amplifier210-2of optical node110-1includes an optical coupler500-1and a Raman pump510-1. Optical coupler500-1couples the light from Raman pump510-1to optical fiber span120-1such that non-linear interaction between the optical signals, transiting out of optical node110-1in a west-to-east direction over optical network100, and the pump light causes amplification of the optical signals. As further shown inFIG. 5, optical amplifier210-1of optical node110-2includes an optical coupler500-2and a Raman pump510-2. Optical coupler500-2couples the light from Raman pump510-2to optical fiber span120-1such that non-linear interaction between the optical signals, transiting out of optical node110-2in an east-to-west direction over optical network100, and the pump light causes amplification of the optical signals.

FIG. 6depicts one exemplary implementation of optical amplification within optical nodes110of optical network100where optical amplifier210-2of optical node110-1includes a Raman amplifier and an Erbium Doped Fiber Amplifier (EDFA) connected in series for optical amplification. In the exemplary implementation shown inFIG. 6, optical amplifier210-1of optical node110-2includes a Raman amplifier as described above with respect toFIG. 5. The EDFA of optical amplifier210-2includes optical circulators600-1and605-1and EDFAs610-1and615-1. Optical signals transiting out of optical node110-1in a west-to-east direction are directed by circulator605-1to EDFA615-1for amplification of the optical signals. The amplified optical signals from EDFA615-1are directed by circulator600-1to coupler500-1for further amplification by Raman pump510-1. Optical signals transiting optical fiber span120-1into optical node110-1in an east-to-west direction are directed by optical circulator600-1to EDFA610-1for amplification of the optical signals. The amplified optical signals from EDFA610-1are output to circulator605-1which directs the signals towards ROADM200(not shown inFIG. 6). Though optical amplifier210-1of optical node110-2is shown inFIG. 6as including only a Raman amplifier, optical amplifier210-1may, alternatively, include the EDFA and Raman amplifier of optical amplifier210-2of optical node110-1.

FIG. 7depicts another exemplary implementation of optical amplification within optical nodes110of optical network100where optical amplifier210-2of optical node110-1and optical amplifier210-1of optical node110-2each include only an EDFA. The EDFA of optical amplifier210-2includes optical circulators600-1and605-1and EDFAs610-1and615-1. Optical signals transiting out of optical node110-1in a west-to-east direction are directed by circulator605-1to EDFA615-1for amplification of the optical signals. The amplified optical signals output from EDFA615-1are directed by circulator600-1for transmission over optical fiber span120-1. Optical signals transiting optical fiber span120-1into optical node110-1in an east-to-west direction are directed by optical circulator600-1to EDFA610-1for amplification of the optical signals. The amplified optical signals from EDFA610-1are output to circulator605-1which directs the signals towards ROADM200(not shown inFIG. 6).

Optical signals transiting optical network100in a west-to-east direction and received over optical fiber span120-1at optical node110-2are directed via circulator600-2to EDFA610-2for optical amplification. The amplified optical signals output from EDFA610-2are directed by circulator605-2to ROADM200(not shown) of optical node110-2for switching the optical signals towards their destinations in optical network100.

FIG. 8is a flow diagram that illustrates an exemplary process for switching optical traffic, received at optical node110via west optical device port220-1, towards destinations reachable via optical device port220-2or towards destinations reachable via transponder230. The exemplary process ofFIG. 8may be implemented by ROADM core300of optical node110.

The exemplary process may include west optical circulator310-1receiving incoming optical traffic, via west optical device port220-1of optical node110, at port2of circulator310-1(block800). Optical traffic that includes optical signals carried via multiple different optical channels (e.g., multiple optical wavelengths) may be received at optical device port220-1of optical node110, and may transit to port2of west optical circulator310-1of ROADM core300(seeFIG. 3). As described with respect toFIG. 4, the optical channels of the optical traffic traveling in a “west-to-east” direction may be different channels than the optical channels of optical traffic traveling in a “east-to-west” direction (described below with respect toFIG. 9). West optical circulator310-1directs the optical traffic received at port2for output at port3of the circulator to WSS3320-3(block810). The received optical traffic may be directed by west optical circulator310-1through port2and out port3of circulator310-1to an input of WSS3320-3.

WSS3320-3switches a first portion of the optical traffic, intended for a destination reachable via east optical device port220-2, to WSS4320-4(block820). Based on a destination of the optical signals of the optical traffic, WSS3320-3may switch optical signals, of the “west-to-east” optical traffic, being sent to a destination reachable via optical device port220-2of optical node110to WSS320-4for further switching towards port220-2. WSS3320-3switches a second portion of the optical traffic, intended for a destination node or network reachable via transponder230, to MCS1330-1of add/drop module305(block830). Transponder230may connect to another node(s) or network(s) that serves as a destination(s) for the second portion of the “west-to-east” optical traffic. Based on the destination of the optical signals of the optical traffic being the other node(s) or network(s) connected to transponder230, WSS3320-3may switch the optical signals to add/drop module305for switching by MCS1330-1to receiver340of transponder230.

WSS4320-4receives “added” optical signals, switched by MCS2330-2from the transmitter350of transponder230and intended for a destination reachable via east device optical port220-2(block840). Another node(s) or network(s) connected to transponder230may transmit electrical signals (e.g., packet data) for delivery via optical network100using optical node110as an entry point to optical network100. Upon conversion of the electrical signals to optical signals, transponder230transmits the optical signals to MCS2330-2, which switches the optical signals to WSS4320-4for further switching towards optical device port220-2. WSS4320-4switches the “added” optical signals, and the first portion of the optical traffic, intended for a destination reachable via east optical device port220-2, to port1of east circulator310-2(block850). East circulator310-2directs optical signals from WSS4320-4, received at port1, to port2for output towards towards east optical device port220-2(block860). Optical signals, switched from WSS3320-3and from MCS2330-2, are switched by WSS4320-4to port1of east optical circulator310-2and directed out port2of circulator310-2for transmission via the optical fiber connected to optical device port220-2. The exemplary process ofFIG. 8may be repeated for all optical traffic received at optical device port220-1of optical node110.

FIG. 9is a flow diagram that illustrates an exemplary process for switching optical traffic, received at optical node110via east optical device port220-2, towards destinations reachable via optical device port220-1or towards destinations reachable via transponder230. The exemplary process ofFIG. 9may be implemented by ROADM core300of optical node110.

The exemplary process may include east optical circulator310-2receiving incoming optical traffic, via east optical device port220-2of optical node110, at port2of circulator310-2(block900). Optical traffic that includes optical signals carried via multiple different optical channels (e.g., multiple optical wavelengths) may be received at optical device port220-2of optical node110, and may transit to port2of east optical circulator310-2of ROADM core300(seeFIG. 3). As described with respect toFIG. 4, the optical channels of the optical traffic traveling in an “east-to-west” direction may be different channels than the optical channels of optical traffic traveling in a “west-to-east” direction (described above with respect toFIG. 8).

East optical circulator310-2directs the optical traffic received at port2to port3of the circulator for output to WSS1320-1(block910). The received optical traffic may be directed by east optical circulator310-2through port2and out port3of circulator310-2to an input of WSS1320-1. WSS1320-1switches a first portion of the optical traffic, intended for a destination reachable via east optical device port220-1, to WSS2320-2(block920). Based on a destination of the optical signals of the optical traffic, WSS1320-1may switch optical signals, of the “east-to-west” optical traffic, being sent to a destination reachable via optical device port220-1of optical node110to WSS2320-2for further switching towards port220-1.

WSS1320-1switches a second portion of the optical traffic, intended for a destination node or network reachable via transponder230, to MCS1330-1of add/drop module305(block930). Transponder230may connect to another node(s) or network(s) that serves as a destination(s) for the second portion of the “west-to-east” optical traffic. Based on the destination of the optical signals of the optical traffic being the other node(s) or network(s) connected to transponder230, WSS1320-1may switch the optical signals to add/drop module305for switching by MCS1330-1to receiver340of transponder230.

WSS2320-2receives “added” optical signals, switched by MCS2330-2from the transmitter350of transponder230and intended for a destination reachable via west device optical port220-1(block940). Another node(s) or network(s) connected to transponder230may transmit electrical signals (e.g., packet data) for delivery via optical network100using optical node110as an entry point to optical network100. Upon conversion of the electrical signals to optical signals, transponder230transmits the optical signals to MCS2330-2, which switches the optical signals to WSS2320-2for further switching towards optical device port220-1.

WSS2320-2switches the “added” optical signals, and the first portion of the optical traffic, intended for a destination reachable via west optical device port220-1, to port1of west optical circulator310-1(block950). West circulator310-1directs optical signals from WSS2320-2, received at port1, out port2towards west optical device port220-1(block960). Optical signals, switched from WSS1320-1and from MCS2330-2, are switched by WSS2320-2to port1of west optical circulator310-1and directed out port2of circulator310-1for transmission via the optical fiber connected to optical device port220-1. The exemplary process ofFIG. 9may be repeated for all optical traffic received at optical device port220-2of optical node110.

FIG. 10is a flow diagram that illustrates an exemplary process for adding optical signals to bi-directional optical traffic transiting optical node110or dropping optical signals from the bi-directional optical traffic received at optical device ports220-1and220-2of optical node110. The exemplary process ofFIG. 10may be implemented by add/drop module305and transponder230of optical node110.

The exemplary process may include MCS1330-1of add/drop module305receiving optical signals from WSS3320-3and WSS1320-1and switching the optical signals to receiver340of transponder230(block1000). As described with respect to block830ofFIG. 8, and block930ofFIG. 9, portions of the optical traffic received at either optical device port220-1or optical device port220-2may be destined for a node(s) or network(s) connected to transponder230. WSS3320-3and WSS1320-1may switch the portions of the optical traffic to MCS1330-1which, in turn, switches the optical signals to receiver340of transponder230. Receiver340of transponder230coherently detects optical signals received from MCS1330-1and converts the detected optical signals to corresponding electrical signals (block1010). Receiver340of transponder230includes a coherent optical detector that detects the optical signals switched as an output from MCS1330-1and converts those optical signals to electrical signals for delivery to the destination node(s) or network(s) connected to transponder230. Transponder230forwards the electrical signals towards an appropriate destination node(s) or network(s) (block1020). The electrical signals (e.g., packet data) may be forwarded, for example, to a destination node connected to a LAN or WAN that is connected to transponder230.

Transponder230receives electrical signals destined for a node reachable via either of the optical device ports220-1or220-2of optical node110(block1030). A node connected to, for example, a network (e.g., LAN or WAN) that is further connected to transponder230may send a series of packets, destined for another node reachable via optical device port220-1, to transponder230of optical node110. The series of packets may include encapsulated digital data (i.e., electrical signals). Transmitter350of transponder230may convert the electrical signals to corresponding optical signals and transmit the optical signals to MCS2330-2of add/drop module305(block1040). Transmitter350may include an optical transmitter that converts the digital data of the received electrical signals into optical signals (e.g., optical pulses on certain optical channels) for “adding” to optical traffic transiting optical node110, and transmits the optical signals into an input of MCS2330-2. MCS2330-2of add/drop module305receives the “added” optical signals and switches the optical signals to WSS2320-2or WSS4320-4based on a destination of the optical signals (block1050). If the “added” optical signals have a destination reachable via optical device port220-1, then MCS2330-2switches the optical signals to WSS2320-2for switching to west circulator310-1. If the “added” optical signals have a destination reachable via optical device port220-2, then MCS2330-2switches the optical signals to WSS4320-4for switching to east circulator310-2.

The foregoing description of implementations provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. For example, while a series of blocks has been described with respect toFIGS. 8-10, the order of the blocks may be varied in other implementations. Moreover, non-dependent blocks may be performed in parallel.