Chapter 2 - ROADM-Based Networks

Chapter 2 - ROADM-Based NetworksBrandon C. Collings* and Peter Roorda**
* 2 Applegate Drive, Robbinsville, NJ, ** 61 Bill Leathem Drive, Ottawa, Ontario
CHAPTER OUTLINE HEAD
2.2.1. Wavelength blocker ......................................................................... 27
2.2.3. Wavelength selective switch............................................................. 30
2.4. Increased density and functional integration of ROADM technology ......................37
2.5. Emerging applications and uses of ROADM networks ...........................................39
2.5.1. Colorless add/drop architectures....................................................... 41
2.6. Summary...........................................................................................................43
Acronyms .................................................................................................................44
References ...............................................................................................................45
2.1 INTRODUCTION Early optical networks capitalized on the high bandwidth capacity and transmission reach offered by the combination of an optical frequency carrier and very low loss optical telecommunications fiber. As the need for bandwidth capacity continued to increase, the combination of multiple optical signals at distinct wavelengths onto a single transmission fiber, each carrying independent information streams, provided an economically attractive method to further increase the total information carried by a single fiber. Known as wavelength division multiplexing (WDM) and later dense wavelength division multiplexing (DWDM), this technique has driven the total bandwidth capacity of a single fiber from a relatively meager 155 Mb/s well into the multiple Tb/s capacities of today’s commercial systems, an increase of over four orders of magnitude.
Given that DWDM technology transports independent information on separate wavelength channels, the use of wavelength-specific filtering to physically separate and route traffic within a network has proven to be a very cost effective approach to
Optically Amplified WDM Networks. DOI: 10.1016/B978-0-12-374965-9.10002-0 Copyright 2011 Elsevier Inc. All rights reserved.
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24 CHAPTER 2 ROADM-based networks
managing the overall traffic topology within an optical network. It is preferable to using exclusively electrical switching fabrics for two general reasons: The first is that the costs of the optical filtering elements to separate and insert a single DWDM wavelength channel are typically significantly lower than the corresponding costs of terminating all the DWDM channels into the electrical domain, electrically grooming that traffic, and then regenerating all traffic bypassing the node. The second reason is that, unlike typical electronic receiving, switching, and regenerating equipment, optical filtering is agnostic to the protocol and line rate of the traffic carried by a particular wavelength. This results in simpler network operation and upgrade as new and different traffic protocols and line rates can be introduced without replacing any intermediate elements of the network infrastructure.
Prior to the introduction of reconfigurable optical network technology, networks generally consisted of point-to-point optical transmission systems interconnecting electrical switching fabrics or ring networks with optical channel filters permanently deployed at each node of the ring to extract and insert wavelengths from and onto the ring. Figure 2.1 illustrates a transmission network with fixed wavelength multiplexing and demultiplexing terminal nodes with electronic based regeneration and bandwidth management situated at the center of each node. In early DWDM deployments in the mid-to-late 1990s, point-to-point topologies were the norm. Optical reach limitations typically resulted in the need to terminate all wavelengths for regeneration purposes. In shorter reach networks where reach was not a constraint, electrical access to the payload to allow sub-wavelength granularity bandwidth management also necessitated the termination of wavelengths into electrical cross-connects or synchronous optical networks (SONET) or synchronous digital hierarchy (SDH) add-drop multiplexers (ADMs).
As the optical reach of systems was extended and the number of wavelengths supported per fiber increased, it became both possible and desirable for some wavelength channels to optically bypass some nodes to both simplify the node and eliminate opto-electronic termination equipment previously required to allow traffic to bypass the node. To enable this, fixed wavelength optical add-drop multiplexers (OADMs), shown in Figure 2.2, were introduced in rings or linear chains. These
FIGURE 2.1
Diagram of a typical point-to-point DWDM transmission system employing fixed wavelength
multiplexers and demultiplexers, electronic regeneration, and sub-wavelength bandwidth
management
Diagram of a typical point-to-point DWDM transmission system employing fixed wavelength
OADM multiplexers and demultiplexers to add and drop optical wavelengths
2.1 Introduction 25
OADMs were constructed using fixed filters that essentially determined at the time of the network commissioning which wavelengths would locally drop and which would pass through at each node. In addition to being fixed, wavelength banding could be used to reduce loss and filter-based spectral degradation.
The introduction of fixed OADMs provided the opportunity to save network cost by eliminating unnecessary optical to electrical to optical conversion, but carried with it a number of key limitations that ultimately limited their application. Network operators were required to carefully plan the network topology at the time of deployment based on how they expected the network traffic to evolve. If these predictions were not accurate and barriers to new bandwidth service deployments surfaced, inefficient workarounds or new system deployments became necessary, both costly consequences considering the initial network may have unused, but inaccessible, bandwidth elsewhere. This effectively reintroduced much of the cost savings that had been achieved by optical bypass. Furthermore, when channels were added into fixed DWDM networks, significant care was needed to control the power level and the manner by which the channel was introduced to optimize performance and ensure existing channels were not impacted. While this was quite manageable in point-to-point configurations, the complexity of the power engineering quickly increased as fixed OADMs were introduced, essentially limiting fixed OADMs to tightly constrained applications (specifically linear OADM chains or hubbed rings). The combination of forecast-intolerant initial filter deployments with complex and manual wavelength deployment procedures conspired to slow network capacity deployment.
Prior to the age of consumer driven internet applications and the concurrent explosion of user bandwidth consumption [1], reasonably accurate network planning was possible. Today however, network operators are simultaneously faced with escalating bandwidth requirements, less predictable and more dynamic traffic topologies, and an increasingly competitive marketplace. Therefore, many have turned to reconfigurable optical add/drop multiplexer (ROADM) technology to provide an optical network infrastructure over which they can flexibly deploy
wavelengths generally between any pair of nodes with relative independence from how other active wavelengths in the network are already provisioned. Furthermore, this flexibility is available to the operator without requiring any interruption of existing traffic, a critical requirement of network operation. This flexibility allows the network operators to grow and adapt the traffic topologies of their networks as traffic demands emerge in a cost effective and operationally efficient manner. It also extends the operational life of a deployed network infrastructure as capacity increases in regions with unanticipated higher demands can be accommodated. In comparison, in fixed-wavelength topology systems, the number of wavelengths serving a given region is determined during the initial network deployment, and areas with higher than anticipated growth can become capacity bottlenecks, resulting in the need for inefficient solutions. Also, as ROADM networks provide an infrastructure that is effectively optically transparent to the line rate and protocol of the traffic passing through it, newly provisioned wavelengths can use higher bit rates, seamlessly support traffic of differing protocols, and take advantage of recent transmission technology advancements such as advanced modulation formats [2] and impairment mitigation strategies [3,4].
Historically, optical networks have often been constructed using ring topologies with electronic switch fabrics interconnecting traffic between optical ring networks as needed at junction points. Another attractive feature of modern ROADM networks is the ability to replace these electronic interconnection facilities with the capability of directly routing optical wavelength channels between optical rings without translating the signals through the electrical domain. More specifically, ROADM networks can optically interconnect wavelengths between different node degrees (fiber trunk pairs entering a network node), essentially creating an all- optical wavelength cross-connect capable of routing wavelengths between any pair of degrees (provided no two like wavelength signals are attempted to be routed to the same degree). Given that equivalent electrical switch fabrics and associated optical transponders are more expensive to deploy, power, spare, and maintain, avoiding the translation through the electronic domain frequently yields significant network cost savings. This multidegree optical interconnection capability enables more general mesh network topologies and a flatter network structure that is generally easier to manage and expand. However, capitalizing on the ability to optically route wavelength channels within the mesh network requires that the optical transmitted wavelengths both have sufficient reach and do not suffer significant impairments from propagating through the ROADM nodes [5].
Finally, ROADM networks generally offer an increased level of embedded monitoring of the optical transmission layer as well as the automation of optical power control and wavelength routing. Many ROADM networks incorporate optical channel power monitors which, along with the ROADM components’ ability to independently control each channel’s optical power, enable the ROADM network to continuously monitor and automatically optimize every channel’s power level. This results in a network that is simpler and more reliable to deploy as fewer manual adjustments are required; more robust over time as power levels are
2.2 Evolution of the ROADM component and network 27
continuously adjusted toward an optimum operating point; and easier to monitor and detect problems before they become critical. These features translate into decreased operational expenditures and the reduction of new bandwidth service deployment intervals from multiple months down to, potentially, days or less, allowing for more rapid customer capture and less time to revenue from new services.
2.2 EVOLUTION OF THE ROADM COMPONENT AND NETWORK 2.2.1 Wavelength blocker Optical reconfigurability with wavelength granularity was initially introduced into two-degree nodes (nodes with two bi-directional pairs of transmission fibers) using a typical node architecture as shown in Figure 2.3. A broadband optical power coupler on the inbound transmission fiber provides a copy of all inbound wavelengths to a demultiplexing filter structure, allowing any independent combination of wavelengths to be received at this node. A similar structure, used in the reverse direction and connected to a coupler on the outbound transmission fiber, enables the injection of any combination of wavelengths. The critical optical component in this design is the wavelength blocker (WB), a two-port device that can independently attenuate (block) each and any combination of wavelength channels. The WB is placed between the drop coupler of one degree of the node and the add coupler of the other degree with the responsibility of blocking any wavelength signal terminating at this node. By blocking a wavelength channel that has been dropped, a new signal at the same wavelength can be inserted in the outbound direction of the opposite side of the node, allowing wavelength channels to be reused on either side of the node. This overall node architecture allows the network operator to independently select which channels entering/leaving a node are to be dropped/added by directing the WB to block those wavelength channels and which are to be routed through the node by directing the WB to be transparent to those channels.
FIGURE 2.3
In general, WB components are composed of three functional stages as shown in Figure 2.4. In the first stage, the wavelength channels are spatially separated by a dispersive element such as a diffraction grating. Each of the wavelength signals then encounters a spatial array of optically attenuating elements with one or more independently controllable element(s) per wavelength channel. Finally, all the spatially dispersed wavelength signals are recombined and launched into a single output fiber. Thus, channels can be independently attenuated or blocked through adjustment of the respective attenuation elements. The ability to independently attenuate unblocked channels enables equalization of the optical power levels of channels expressed through a node. Channel power equalization provides greater reach of the optical signals as the power levels of weaker channels (caused by uneven optical amplifier gain, spectrally dependent fiber attenuation, and Raman- induced inter-channel energy exchange) are renormalized. As the equalization is typically automated, continuous and driven by closed-loop feedback from direct channel power level measurements, this also provides greater system stability and reliability.
The WB solves the two key problems associated with fixed OADMs. First, decisions on which wavelengths are add/dropped can be flexibly determined as the network grows, avoiding stranding of unused wavelengths inherent in banded and fixed-wavelength OADMs. Secondly, the means to automate the power control function on a per-wavelength basis allows more controlled introduction of new wavelengths into the system and vastly reduces the complexity of provisioning new wavelengths. The high-capacity long-haul and regional portions of the core network were the initial segments in which systems incorporating WBs were deployed. Given the significant bandwidth these networks carried and the growth they experienced, the ability to flexibly add new wavelength channels reduced the operational expense of supporting that growth and extended the systems’ ability to accommodate that growth. Also, as these systems typically propagated over long distances, the ability to accurately and dynamically maintain the channels’ optical power levels at the desired levels increased the performance and reliability of the transmission which results in lower operational costs.
FIGURE 2.4
2.2.2 Planar light wave circuitry (PLC)-ROADM An early alternative approach to ROADM implementation was to use planar light wave circuitry (PLC) technology. PLC has been extensively used for arrayed waveguide gratings (AWG) for multiplexing and demultiplexing elements. As active optical functions, such as space switches and attenuators, can also be constructed in solid state PLC technology using Mach-Zehnder interferometers (MZI), the natural evolution of ROADM component technology was to integrate per-wavelength optical routing and power control capabilities with the channel multiplexer and demultiplexer. A functional diagram of a typical PLC-ROADM component and node is shown in Figure 2.5. Similar to the WB architecture, an optical copy of the channels entering a node is directed to a channel demultiplexing structure so that any combination of channels may be locally received. The other copy is directed to the opposite degree of the node and separated according to wavelength using an AWG demultiplexer. Each channel is then directed to a dedicated 2x1 optical space switch with the other input to the switch constituting the respective add port for that channel. The position of each 2x1 switch determines if the locally added channel or the channel from the opposite side of the node will be directed outbound from the node. The output of each switch is passed through an independent optical attenuator followed by all channels being recombined into a single output by an AWG channel multiplexer. The MZIs typically allow relatively fast (w1ms) switching enabling optical protection switching applications.
The channel demultiplexer for channels propagating in one direction is integrated with the demultiplexer-switch/attenuator array-multiplexer structure for the channel propagating in the opposite direction (see Figure 2.5) to form a single component that provides the combined functionality for one fiber degree as depicted in the figure. An identical component is used for the second fiber degree in order to maintain equipment separability between opposite degrees of the node, and therefore diverse risk groups for protection purposes.
Integration and co-packaging of these elements enable reduced overall costs relative toWB architectures. In addition, optical power monitors can be readily integrated
FIGURE 2.5
into the PLC component allowing very high-speed and accurate channel power level monitoring at add, express, and drop locations with low incremental costs. However, similar to theWB, node architectures built from PLC-ROADMs are practically limited to two-degree implementations. Another critical characteristic of PLC-ROADM technology (not necessarily present inWB components) is the limited ability to design AWG multiplexers with wide and predominantly flat channel passbands. For metro networks where a given optical channel may need to propagate through 16 or more nodes, and thus PLC-ROADMs, the repeated filtering by the AWG multiplexers and demultiplexers progressively reduces the bandwidth available to the channel.While the cascadability performance is acceptable for 100 GHz channel spaced applications with 10 Gb/s wavelengths where less than 30 GHz of net bandwidth is required (primarily due to the unlocked wavelength drift of the transmitting laser), PLC-ROADMs with a 50 GHz channel spacing cannot support that necessary amount of bandwidth. As the industry looked toward higher channel line rates, the need to provide as much channel bandwidth as possible, through more than 16 nodes and with 50GHz channel spacing, became a severe limitation for PLC-ROADM devices and consideration of these devices for new network designs declined.
2.2.3 Wavelength selective switch The third generation of ROADM technology and the one that is dominant in current systems is the wavelength selectable switch (WSS). Figure 2.6 shows the basic functional diagram of a 1xN port WSS which is capable of independently routing each channel injected into the common port to any one of the N output ports. Typically, WSS components are optically bidirectional such that the channels from the N ports are selectively multiplexed according to origin port onto the single common (output) port. In addition, the WSS can impart a provisioned amount of attenuation independently to each wavelength channel or may block any channel. Note that when N¼1, the resulting WSS has the same functionality as a WB.
The common node architecture construction using WSS elements is shown in Figure 2.7. Similar to the WB node architecture (see Figure 2.3), a copy of the
FIGURE 2.6
FIGURE 2.7
Diagram of a WSS-based 4-degree ROADM node using a multiplexing WSS and colored
add/drop
incoming channels is demultiplexed allowing any combination of channels to be locally received. The WSS is positioned on the outbound side of each degree. The channels coming into each degree are further split into multiple fibers with a copy of each set of channels directed to one of the N ports of each WSS of every other degree (except the degree in which the channels entered the node). Channels that are to be locally added are multiplexed together and injected into one of the N ports of the WSS. The common port of the WSS is connected to the output transmission fiber of the respective degree. Thus, across the N ports of every WSS, a copy of every channel entering the entire node is present. For each degree and for each wavelength, the WSS is then provisioned to route wavelength signals from one of the N ports to its respective common port. Therefore, with this architecture, any wavelength entering a node can be routed to the output of any one or more other degrees, limited by the condition that only a single channel for each wavelength may leave via each degree. Channel power level equalization is implemented by each WSS, independently imparting the proper amount of attenuation to each channel routed through the WSS.
As can be seen in Figure 2.7, the value of N for the WSS determines the maximum number of degrees the node configuration can support. For core metro and long-haul networks, values of N as high as 8 are currently of interest. However, for networks residing closer to the end consumer with less need for multidegree optical mesh connectivity, there is strong interest in WSSs with N¼2 simply for the purpose of reducing the cost of the WSS component yet maintaining the full channel granularity flexibility of the higher-degree configurations.
Figure 2.8 shows an alternative node architecture where a WSS is installed into each degree with the common port of each WSS connected to the inbound transmission fiber of respective degree. Some of the N ports are then connected to an
FIGURE 2.8
Diagram of a WSS-based 4-degree ROADM node using a demultiplexing WSS and colorless
add/drop
input port of N:1 power splitters/combiners (PS) in each of the other degrees. For channels that are to be locally received, some of the N WSS ports are designated as drop ports and the WSS is instructed to route only one wavelength channel to each of these ports. EachWSS is then configured to direct each channel to the desired degree or drop port, or to block it completely. Channels to be locally added are combined using an M:1 optical power combiner and the output connected to one of the N:1 power combiner inputs. Note that the provisioning of the WSSs and each add multiplexing stage must be coordinated such that only one signal per wavelength is directed to each of the N:1 couplers to avoid a collision of same-wavelength signals. Channel power level equalization is also executed by introducing the proper attenuation to each channel routed through each WSS.
In this architecture, each of the drop ports is not permanently assigned a specific wavelength, but rather the wavelength to be dropped from each of these ports is determined by which wavelength is routed to that port by the WSS. Similarly, the add ports have no permanent wavelength assignment. Add/drop ports with this characteristic are know as “colorless” ports and are attractive as they increase the level of flexibility as the operating wavelength of a given colorless port can be assigned and changed by the network operating system at any time. Also, as typically only a subset of the full complement of channels entering a node are needed to be locally added/dropped, colorless ports allow for fewer add/drop ports to be physically presented (relative to AWG demultiplexers which typically possess ports for all channels) thereby saving valuable faceplate space and cost.
Figure 2.9 shows the general functional construction of a WSS component which is typically similar to that of a WB. The wavelength signals inserted into the common port of the WSS are spatially separated by a wavelength dispersive device and directed to an array of actuators capable of deflecting each wavelength beam in a controllable angle orthogonal to the axis of wavelength dispersion. The deflected
FIGURE 2.9
Diagram of a general example of the internal design of a WSS
signals are then recombined in the wavelength dispersion direction resulting in N beams of channel groups with each beam comprised of wavelengths that received the same amount of angular deflection. Each beam is coupled into one of the N output fibers. Thus, each channel can be independently routed by adjusting the deflection angle of each actuator for each respective wavelength to the deflection angle corresponding to the desired output port. To operate the device in reverse, the same process is followed with the light propagating in the reverse direction. Per channel attenuation and blocking is generally obtained by adjusting the deflection angle of each channel actuator to detune the coupling efficiency with the output fiber.
WSS components can use a variety of deflection angle actuator array technologies [6] including micro electro-mechanical systems (MEMS) mirror arrays [7], arrays of liquid crystal on silicon (LCoS) phase modulators [8], liquid crystal (LC) polarization based switches [9], digital light processing (DLP) mirror arrays [10], and combinations of multiple technologies [11].
For MEMS actuators, an array of tiltable mirrors is fabricated, each with a highly reflective coating. By tilting each mirror (typically one mirror per channel), the beam for each channel can be independently steered among the output fibers allowing the desired output fiber to be selected and coupling efficiency to that fiber to be controlled.
LCoS actuators are composed of a large two-dimensional array of optical phase modulators. Angular deflection is achieved by establishing a linearly varying phase retardation profile in one direction of the two-dimensional array. This manipulation of the phase front of the optical beam causes the beam to be deflected by an angle proportional to the rate of change of the phase retardation across the array. The other axis of the array is aligned with the wavelength dispersion direction.
Actuators using LC technology use the polarization rotation capability of a layer of LC material to variably adjust the orientation of the polarization state of the passing light, followed by a birefringent wedge, to spatially separate the incoming beam into two separate beams with a splitting ratio dependent on the alignment of the polarization state of the light with the axis of the wedge. Therefore, by controlling the polarization state with the LC layer, the beam can be deflected into one of two directions, or split into both directions with a variable ratio. By cascading x sets of these beam steering arrangements with decreasing (or increasing) deflection magnitudes, the ability to deflect the initial beam into 2x discrete output ports is achieved. Similar to the other technologies, an array of such deflection actuators is aligned to the wavelength dispersion direction. Attenuation and blocking can be achieved by adding a stage to partially or fully deflect the beam in an unused direction.
The DLP mirror array actuator is composed of a two-dimensional array of binary tiltable mirrors. One axis of the array is aligned in the dispersion direction. In the orthogonal direction, all mirrors are actuated to the same binary state deflecting the entire beam. As the dimension of the individual mirrors is smaller than the practical optical beam size; the beam covers a large number of individual mirrors. Given the binary nature of the DLP mirror tilt, N values greater than 2 may require multiple bounces off of different sections of the DLP array. Similarly, attenuation can be achieved by tilting a fraction of the mirrors covered by the beam; however, as the discarded power is reflected in the other binary direction, attenuation functionality requires an additional bounce off an independent section of DLP mirrors.
The most critical performance parameters for the WSS are generally the spectral bandwidth and shape of each wavelength channel and the suppression of wavelength channels from being emitted from unintended ports. To support higher line rate wavelengths, a wide and square channel bandwidth is necessary to avoid bandwidth narrowing from the cumulative filtering resulting from multiple passes through WSSs within the network. Secondly, sufficient suppression of wavelength channels from passing to unintended ports is critically necessary as insufficient suppression can result in an accumulation of optical crosstalk and ultimately signal performance degradation [12]. When used to build a WSS, each switch engine technology has both strengths and weaknesses. MEMS mirror arrays provide channel bandwidths in excess of 40GHz (for a 50GHz channel spacing) and port isolation performance typically better than 45dB, and allow for short optical path lengths that translate into compact physical component volumes maintained as the number of ports is increased. However, as the channel wavelength filtering is defined by the MEMS mirror geometry, variable wavelength channel passbands is not practical and wavelength channels cannot be broadcast to multiple output ports. Also, as channel attenuation is implemented through slight misalignment of the coupling to the output fiber, the open-loop attenuation accuracy depends on the stability of the MEMS mirror. LCoS and LC based actuators can support both variable wavelength channel passbands, accurate open-loop attenuation, and channel broadcast to multiple output ports to some extent; however, port isolation is typically around or
2.3 Impact on optical amplifiers requirements 35
below 40dB and wide channel passband widths can require a more challenging optical system design. Also, longer optical path lengths are generally needed which results in larger component volumes as the port count increases or the channel spacing decreases.
2.3 IMPACT ON OPTICAL AMPLIFIERS REQUIREMENTS Reconfigurable optical networks require optical amplification to compensate the attenuation of the transmission fiber in generally the same manner as needed by earlier fixed wavelength networks. However, due to the significant differences in both the network and node architectures along with the increasingly dynamic utilization of the networks’ flexibility, the requirements placed on the optical amplifiers differ significantly.
To implement the flexibility and photonic cross-connect capabilities that define a reconfigurable optical network, the typical ROADM node architecture requires two optical amplifiers (as shown in the figures in the preceding section). In general, one amplifier on the inbound fiber to the node is positioned and designed to compensate the loss of the preceding fiber span, whereas the second amplifier is typically positioned adjacent to the outbound fiber to compensate the splitting and component losses within the node and prepare the channel optical powers for launch into the transmission fiber. Given this reference node configuration and the loss characteristics of modern ROADM components, the gain required by the outbound amplifier is generally around 19 dB (9dB for 8-degree splitting loss, 6 dB for theWSS component and 3 dB for channel power equalization). As the loss within the node is reasonably constant, this outbound optical amplifier (OA) is designed to have a constant amount of gain or a reasonably narrow variable gain range. In contrast, the inbound amplifier often requires larger amounts of gain, often 25 dB or more, to compensate for fiber spans of 80 km or more. Furthermore, as the fiber span loss varies from span to span (and it is operationally desirable to design as few amplifier variants as practical within a given network system product offering), the inbound amplifier is typically designed to have variable gain to allow a single amplifier variant to support the corresponding range of span losses. Also, as most networks incorporate chromatic dispersion compensation on a per span basis, the losses of these compensation elements are typically offset by inserting the dispersion compensation element between two gain stages of the inbound amplifier. Because the dispersion compensation elements have significant insertion loss, adding them between the input amplifier and output amplifier together would significantly degrade the performance of the ROADM node for two reasons: first, because the loss between the amplifiers would become significantly larger and comparable to or even greater than the span loss, and second, because typical dispersion compensation elements are composed of dispersion compensating fibers in which significant nonlinear penalties are generated at low powers, the power launched from the input amplifier would need to be reduced, which would degrade the ROADM node noise performance.
For many network applications where fiber spans are longer, the gain required by the inbound amplifier is often larger than that required of the outbound amplifier, and as the nominal output power levels of both inbound and outbound amplifiers is often similar (for economic considerations), the degradation of a given channel’s optical signal-to-noise ratio (OSNR) is dominated by the noise performance of the inbound amplifiers. As ROADMs enable all-optical mesh networking, the need is growing for optical links to propagate through larger numbers of nodes and fiber spans before requiring expensive conversion into the electrical domain, thereby applying pressure to reduce the OSNR degradation a link experiences as much as possible.
Historically, one key limitation of optically amplified transmission systems has been the flatness of the amplification gain spectrum. As the gain spectrum of erbium- doped fiber optical amplifiers is naturally far from constant versus wavelength and changes depending upon the erbium ion inversion operating point of the amplifier, many techniques have been employed to mitigate this characteristic and thereby provide as consistently spectrally constant gain as possible. However, despite many amplifier improvements, in non-ROADM systems, non-flat gain remains a considerable source of system reach limitations. Several spectral equalization approaches have been developed to compensate the unequal spectra gain and thereby extend the link reach by periodically resetting those spectral areas or channels that have received too little or too much gain back to their desired nominal power levels. As current ROADM components have the ability to manipulate each channel independently, they generally have the ability to also independently impart a controllable amount of attenuation to each channel, thereby providing channel power level equalization at each ROADM node. This naturally incorporated channel equalization allows for frequent channel power level rebalancing and minimizes the excursion magnitude each channel may experience before being rebalanced and consequently minimizes the potential subsequent OSNR degradation and nonlinear penalties. In order to accomplish this channel power equalization, some means by which to measure the individual channel powers is necessary in order to assign the proper amount of equalization.
This ability to measure and control individual channel power levels at each network node generally allows ROADM networks to incorporate highly automated channel power level management. This provides the network operator a number of advantages including increased and accurate visibility into the network’s health and current operating points, continuous and accurate channel power level optimization, and the elimination of manual power level adjustments during installation (which requires time and expertise, is not typically high resolution, is not dynamic, and is prone to errors). In addition, this automated power control functionality can also be harnessed to provide the power level controlled introduction and removal of individual channels and nodes within an operating network. By implementing these controls, events that trigger rapid changes to the optical power levels in the network resulting in detrimental and transient gain deviations within the amplifiers can be limited. Specifically, the power levels of newly provisioned channels can be slowly increased such that the gain control algorithms of the amplifiers (and the channel
2.4 Increased density and functional integration of ROADM technology 37
power equalizers) can adiabatically adapt resulting in very limited power excursions of pre-existing channels. Similarly, when adding new segments to an operating network, the gain of the newly commissioned line amplifiers can be slowly increased so that no rapid power fluctuations are experienced. Conversely, during the decommissioning of a channel or network segment, the automated power control management system can slowly reduce the corresponding power appropriately until its level is insufficient to perturb the remaining portions of the network when the transponder or amplifier is finally disabled or physically disconnected. Therefore, within a properly operating system, power transients are quite significantly miti- gated for a large number of the events within the normal scope of network operations. However, these control systems cannot mitigate all power level transient- causing events that may occur, namely fiber or hardware equipment failure and improper fiber cable disconnection. But, in each of these cases, the consequence is a rapid decrease in optical power within the network. As it is reasonable to architect and design ROADM network systems and their optical power management systems such that events resulting in a rapid increase in optical power should be rare under proper operating conditions, power level transient mitigation within the optical amplifiers may limit their focus on controlling gain transients resulting from a rapid decrease in input optical power.
The inclusion of per-channel power level measurement also provides the opportunity to employ more sophisticated optical amplifier gain control algorithms that use this knowledge of the current present wavelength channel population as well as their relative power levels to provide more accurate and spectrally constant gain. However, there are some considerable practical limitations to this capability as the components employed to measure the channel power levels (optical channel monitors) typically make periodic measurements that are slow relative to the desired transient control time scales. Therefore, this information can be used to fine-tune the amplifier gain control during periods where the power levels are effectively stable; however, as real-time information of the current situation is not available during the short time scale while a transient event is transpiring, the gain control improvements are somewhat limited.
2.4 INCREASED DENSITY AND FUNCTIONAL INTEGRATION OF ROADM TECHNOLOGY Earlier sections in this chapter have highlighted the evolution of ROADM networks as enabled by the expanding performance and capabilities or ROADM technology. An important additional trend is the move toward higher ROADM component densities and the functional integration of ROADM sub-elements.
A ROADM node is built up of a number of functional modules (often referred to as “line cards” or “circuit packs”): amplifier modules act as pre-amplifiers for signals entering the node and booster amplifiers for signals launching onto the fiber, ROADM modules provide the key optical switching and power attenuation
functions, multiplexer and demultiplexer modules provide channel separation for add/drop, and optical channel monitor modules provide per channel power monitoring at select points in the node. These modules are deployed onto a telecom shelf that provides communications among the modules and from a shelf controller that manages the coordinated operation of the elements.
Typically, the key modules required for a two-degree ROADM node can occupy a large portion of an equipment chassis. The space consumed is largely dictated by the physical size of the underlying optical components in the node. Amplifier and ROADM components use up a significant portion of the space on the motherboard of shelf modules. Specifically, for ROADM modules, the height of the components often results in shelf modules that consume multiple “slots” in the shelfdtypically two or three slots per ROADM module in a 20-slot shelf. For this reason, recent component development activities have focused on miniaturization of ROADM components with particular emphasis on new low-profile wavelength selective switches with component height to enable single-slot implementations.
As enabling optical components shrink, combining multiple architecturally adjacent components into a single module is required to further improve density. While collapsing the entire ROADM node into a single module is problematic due to the resulting “single point of failure” for terminating traffic, the goal of combining the amplifiers, ROADM components, and optical channel monitor (OCM) for one degree of a ROADM node into a single module is attracting considerable attention within the industry. This goal requires not only miniaturization of the various components, but also requires that the packaging for each sub- element be optimized.
Figure 2.10 a) indicates the key optical functional blocks of one direction of a WSS-based ROADM node. Today this configuration is often separated into multiple modules which may include a two-slot WSS module, a pre-amplifier module, a booster amplifier module, an OCM module, and a shelf processor to coordinate control and management of the node. Figure 2.10 b) is a photograph of an example module with all these functions integrated. Achieving this density requires not only miniaturization of the optical sub-elements, but also novel packaging approaches and advanced mechanical and thermal design techniques. This type of higher level integration will allow not only density improvements but the cost reductions associated with elimination of the electronics and mechanical costs of housing each element in individual modules. Once the traditional demarcation points between optical functions have been blurred through integration, there is potential to optimize the combined erbium-doped fiber amplifier (EDFA)-WSS- OCM control to improve responsiveness and accuracy versus a node where real-time control is limited to individual modules. Also, this integrated approach will reduce the operational complexities associated with multiple different cards including inventory management, node commissioning, and fault isolation. The integration conceived of in Figure 2.10 is likely only an initial step toward more fundamental integration that may be achievable with such approaches as monolithic planar waveguide circuits.
(a)
(b)
FIGURE 2.10
a) Functional diagram and b) photograph of a highly integrated ROADM circuit pack incor-
porating two optical amplifiers, a WSS, a channel monitor, and associated passive optical
components and control electronics
2.5 EMERGING APPLICATIONS AND USES OF ROADM NETWORKS For most operators, the initial ROADM network deployments were motivated by wavelength topology flexibility and the multidegree cross-connect (ring interconnect) capabilities offered by ROADM network systems. These features use the dynamic switching capabilities of the ROADM systems during the wavelength provisioning
process. However, in general, once a wavelength is deployed within the network, it remains unchanged relatively indefinitely.
Network operators are now looking at leveraging the dynamic flexibility inherent within ROADM networks to capture more operational advantages by using that network flexibility to better and more efficiently respond to transient events within the network such as fiber breaks and equipment failures. In an operation physically similar to the configuration of a new wavelength, a wavelength channel that has encountered a failed segment in the network can be re-provisioned along a new physical route avoiding the failed segment and its connectivity restored. In many cases, this “re-provisioning,” or photonic restoration, can be accomplished exclusively through the network management system without the requirement of any physical intervention into the network and can typically be accomplished in an interval of seconds to minutes. Typically, this photonic restoration would operate as a secondary mechanism to a primary service protection switch (such as SONET/ SDH or L2/L3). Therefore, the photonic restoration capabilities enabled by the ROADM network’s flexibility allows the network operator to increase the overall service availability by quickly and efficiently restoring connections with minimal field operations. Furthermore, with an automated control plane (such as generalized multi-protocol label switching [GMPLS]), this restoration capability can be integrated into the network operating system such that restoration activities are automated and occur without any operator intervention.
Beyond restoration, the ability to re-route in-service wavelengths enables operators to balance traffic loads within their networks and relieve emerging congestion points by redirecting wavelengths away from the congestion and through areas of lower traffic. It also lets operators proactively shift critical traffic away from areas with planned potentially invasive maintenance activities.
If photonic layer restoration is used simply to increase network availability by restoring a secondary path of a protected service, the desire to achieve wavelength reroute intervals in a minimal amount of time may not be a high priority assuming the interval is naturally on the order of seconds to minutes. Therefore, the speed in which wavelengths need to be turned up within the network (as in the case of a re- routed wavelength) may be sufficiently slow with respect to the transient suppression capabilities of the modern optical amplifiers. However, if interest develops in photonic layer restoration as a primary mechanism for service recovery (potentially for lower cost, best effort service classes), then one would expect the rate in which a wavelength would need to be turned up within a network to be minimized and perhaps begin to approach speeds where the capabilities of modern optical amplifier transient suppression capabilities for rapid optical power increase and decrease become critically necessary.
Common ROADM architectures deployed today as illustrated in Figures 2.3, 2.5, and 2.7 impose some wavelength routing limitations that effectively prevent the full flexibility required for wavelength recovery and dynamic reconfigurability. Specifically, the wavelength and the ingress/egress direction of the add/drop channels are permanently assigned. This is a direct result of the fact that the transmit/
2.5 Emerging applications and uses of ROADM networks 41
receive signal is connected to a wavelength-specific port on a multiplexer/demultiplexer that is connected to WSS and couplers assigned to a specific direction.
2.5.1 Colorless add/drop architectures Currently, ROADM networks are transitioning to support “colorless” add/drop ports which, unlike colored add/drop ports, do not have a permanently assigned wavelength channel but rather are provisioned as to which wavelength channel will be added/dropped. Architectures to achieve colorless add/drop switching were dis- cussed in an earlier section and illustrated in Figure 2.8. Colorless ports are attractive as generally fewer total add/drop ports are needed, resulting in simpler operations and potentially more compact physical interfaces. Also, coupled with wavelength tunable transmitters, colorless ports allow the wavelength to be selected and provisioned remotely, further simplifying deployment and enabling the remote and rapid modification of a channel’s wavelength needed to support ROADM-based wavelength connection recovery and rerouting.
Colorless ports are generally created by replacing the fixed wavelength demultiplexing and multiplexing elements with either a WSS (only a single channel provisioned per port) as shown in Figure 2.8, or a power splitter and tunable filter array. In some cases, a power combiner is used as the multiplexing element.
To support the growing interest in systems with a greater number of colorless ports, 1xNWSSs where N is greater than 8 are required. For the WSS, this generally means either a greater range of angular deflection is needed from the switch engine in order to address more fiber ports, a greater density of ports per deflection angle unit (together with some optical system modifications to pack coverage of more ports within the same deflection angle range), or a combination of both, are needed. With a MEMS switch engine, either can be achieved without requiring a change of the spot size in the diffraction direction on the mirror itself, thereby preserving isolation and bandwidth performance as well as physical size. LCoS based engines have some limitations on the maximum deflection angle from the pixelated 0-to-2 dimensional phase modulator array but can achieve greater deflection accuracy by illuminating a larger number of pixels in the deflection axis of the array. However, this may require modifications to the optical system in order to accommodate the larger coverage. LC stack-based engines require additional stacks for additional deflection stages, requiring a longer beam focus through the stacks and consequently a wider spot size, which likely translates in a larger optical system length and generally WSS device.
2.5.2 Directionless add/drop architectures In conventional ROADM nodes, each degree contains a separate group of channel add/drop ports which are multiplexed/demultiplexed within that degree and permanently leave/enter the node through their respective degree (see Figure 2.7). This permanent association of an add/drop port with a particular transmission
direction limits the dynamic flexibility of the network as the route of a signal from an add/drop port cannot be remotely altered until that signal reaches an adjacent ROADM mesh node.
Thus, a second currently emerging trend is the development of architectures that support “directionless” add/drop ports in which the degree, and thereby transmission fiber pair in which the signals enter/leave the node, is fully flexible and is provisioned through the operation system. Therefore, with directionless ports, the network operator is able to select, provision, and alter a signal pair’s route at truly any point within the network giving greater and more thorough flexibility [13].
An example of a node with directionless add/drop ports is shown in Figure 2.11. A copy of the incoming channels from each degree is injected into the ports of a 1xN WSS which selects which wavelengths from which degrees are routed to its common port. This port is connected to the common port of a separate 1xM WSS which demultiplexes these wavelengths intoM respective colorless and directionless ports. Given the single fiber connection between the two WSSs, only one instance of each wavelength channel may be dropped per bank of ports. For multiplexing, the same demultiplexing assembly can be used in the reverse direction or a power coupling structure may be used.
To relieve the potential application limitations presented by the wavelength blocking characteristic within each the add/drop banks of the architecture illustrated in Figure 2.11, architectures based upon a new type of WSS are being considered. An example is shown in Figure 2.12. This architecture incorporates a new type of
FIGURE 2.11
2.6 Summary 43
WSS that has Nmulti-wavelength ports on one side andM ports on the other and has the functionality that wavelengths can be independently routed between any port from the group of N ports and any port from the group of M ports, provided no two signals of the same wavelength are routed to the same port. However, signals within the same wavelength channel, but entering through different ports from the group of N ports, can independently be routed to distinct ports from the group of M ports. Therefore, the add/drop bank created using this wavelength non-blocking NxMWSS can support multiple add/drop ports being provisioned to the same wavelength channel but associated with different node degrees, removing the wavelength blocking limitation and further increasing the flexibility and capability of the network.
The ideal architectures and supporting technologies for colorless and directionless ROADM nodes remain a subject of ongoing development and research, but a consensus is emerging that these general approaches will yield the required improvements in network flexibility and responsiveness.
2.6 SUMMARY The inclusion of optical reconfigurability into optical networks has had a profound impact on the application, construction, and operation of optical transmission systems and has placed some additional requirements as well as removed some traditional burdens on the optical amplification. All-optical mesh networking enabled by the WSS has resulted in the need for optical wavelengths to propagate further through the network, yet overcoming greater total loss given the additional
loss introduced by the ROADM node elements, placing greater burden on the quality of the optical amplification. However, through automated and more accurate channel power level control, some stringent gain and transient flatness requirements can be relaxed.
As the capabilities, functionalities, and complexities of optically reconfigurable networks continue to evolve, the need for optical amplification and the requirements placed on that amplification are likely to increase. The introduction of colorless and directionless add/drop ports is expected to facilitate a more dynamic use of optical networks. This evolution will likely result in greater emphasis on network stability and improved transient behavior, in turn requiring higher performance optical amplification capabilities.
ACRONYMS
ADM
References 45
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Chapter 2- ROADM-Based Networks
Wavelength blocker
Wavelength selective switch
Increased density and functional integration of ROADM technology
Emerging applications and uses of ROADM networks
Colorless add/drop architectures
Directionless add/drop architectures

Chapter 2 - ROADM-Based Networks

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Transcript of Chapter 2 - ROADM-Based Networks