ROADM Architectures and Technologies for Agile Optical Networks

ROADM Architectures and Technologies for Agile Optical Networks Louay Eldada* DuPont Photonics Technologies, 100 Fordham Road, Wilmington, MA 01887, USA ABSTRACT We review the different optoelectronic component and module technologies that have been developed for use in ROADM subsystems, and describe their principles of operation, designs, features, advantages, and challenges We also describe the various needs for reconfigurable optical add/drop switching in agile optical networks For each network need, we present the different ROADM subsystem architecture options with their pros and cons, and describe the optoelectronic technologies supporting each architecture Keywords: reconfigurable optical add/drop multiplexer, agile optical network, wavelength blocker, planar lightwave circuit, wavelength selective switch, wavelength cross-connect, silica, polymer, MEMS, liquid crystal 1 INTRODUCTION Large amounts of information traveling on multiple wavelengths around an optical network need to be switched at the network nodes formation arriving at a node is forwarded to its final destination via the best possible path, which is determined by such factors as distance, cost, and the reliability of specific routes The conventional way to switch the information is to convert the input fiber optical signal to an electrical signal, perform the switching in the electrical domain, then convert the electrical signal back to an optical signal that goes down the desired output fiber This optical-electrical-optical (O-E-O) conversion uses systems that are expensive, bulky, and are bit-rate/protocol dependent Reconfigurable optical add/drop multiplexers (ROADMs) allow avoiding the unnecessary O-E-O conversion, enabling O-O-O systems that use optical switching, which has significant advantages for carriers and service providers Optical switching involves lower capital expenditures (capex), as there is no need for a large amount of expensive high-speed electronics Furthermore, operational expenditures (opex) are decreased and reliability is increased because fewer network elements such as back-to-back terminals are required Reducing the complexity also makes for physically smaller switches Additionally, optical switches are relatively future-proof An electrical switch has electronics designed to detect incoming optical signals of specific bit rates and formats When the bit rate increases or when the format changes, the electronics need to be upgraded ROADMs route the optical signals directly, and are bitrate/protocol transparent, therefore future upgrades of bit-rate or protocol can be accommodated without the need to upgrade the switch A ROADM network element typically includes: Transponders ROADM Subsystem Optical Service Channel Optical Power Monitoring Amplifiers (Pre-Amp & Post-Amp) Dispersion Compensation Module We focus in this manuscript on ROADM subsystems, describing the various technologies used to build them, and the different subsystem architectures enabled by each technology * louayeldada@usadupontcom; phone 9782031300; fax 9782031299; wwwphotonicsdupontcom

2 ROADM SUBSYSTEM TECHNOLOGIES Until recently, ROADM systems did not exist, their components were unselected, and their market was unclear Today, every major system vendor has a ROADM offering, and a large number of component vendors have announced ROADM products based on a variety of technologies, some more mature than others We review the different optical component technologies that have been developed for use in ROADM subsystems, and describe their principles of operation, designs, advantages, and challenges The technology platforms that we cover include MEMS, liquid crystals (liquid crystal device (LCD) and liquid crystal on silicon (LCoS) technologies), and monolithic and hybrid planar lightwave circuits (PLC) based on silica on silicon and polymer on silicon platforms For each technology, we describe the corresponding ROADM subsystem architectures in terms of functionality, features, size, cost, and maturity Reconfigurable optical networks have needs for various types of ROADM Figure 1 shows some of the connectivity functions needed at nodes in ring and mesh networks Fig 1 Types of ROADM needed at optical network nodes Table 1 lists the functionalities of the four main types of ROADM subsystems, where ROADM is used in the broadest sense to include Wavelength Blockers (WB), Planar Lightwave Circuit (PLC) ROADM, Wavelength Selective Switches (), and Wavelength Cross-Connects (WXC) Table 2 lists the different subsystems developed in each ROADM type, the configurations implemented, the Telcordia qualification status, and the deployment waves in which they were utilized Table 3 summarizes the pros and cons of each ROADM type

TABLE 1 Functionalities of the main ROADM subsystem types DWDM port: multi-channel port, λ port: single-channel port Wavelength Blocker (WB) Planar Lightwave Circuit (PLC) ROADM Wavelength Selective Switch () Wavelength Crossconnect (WXC) WB PLC 1xN Deg N WXC 2 DWDM ports (1, 1 ) Dynamic Channel Equalizer (DCE) with blocking capability Blocks or attenuates λ s No built-in Add/Drop 2 DWDM ports (1, 1 ) + 2N single-λ ports (N Add, N Drop) One 2x2 or two 1x2 switches per λ Switches individual λ s from ( or Add) to ( or Drop) N+1 DWDM ports (1, 1, N-1 Service) Service: Add or Drop or in-service expansion 1xN: switches λ s from to ( or Drop) Nx1: switches λ s from ( or Add) to 2N DWDM ports (N-1, N-1, 1 Add, 1 Drop) Built with 1xN s or Demuxes / NxN switches / Muxes Switches λ s from ( or Add) to ( or Drop) TABLE 2 Specific subsystems implemented for each ROADM type, including configurations, Telcordia qualification status, and deployment waves ROADM Subsystem Technology Configurations Telcordia Deployment WB PLC WXC LCD WB PLC Small Switch Array & AWGs 1 N MEMS 1 N LCD 1 N LCoS PLC Matrix Switches & AWGs 1x2 Up to 80 50-GHz channels 1x2 Up to 40 100-GHz channels Up to 1 9 Up to 90 50-GHz channels Up to 1 4 Up to 80 50-GHz channels Up to 1 11 Up to 100 50-GHz channels Up to 16 16 Yes Yes No Yes No Yes Wave 1 Wave 2 Wave 3A Wave 3B (Trials) Wave 3B (Trials) Wave 4 (Dvpt) TABLE 3 Pros and cons of the main ROADM types ROADM WB PLC WXC Configuration Splitter Wavelength Blocker Wavelength Selective Switch Combiner Combiner Pros First to be ready # A/D ports = N (all λ s) Lowest cost Degree 2 ROADM solution 100% (N) Add/Drop/OCM on Day 1 Dynamic Thru & Add channel balancing 10G 24 node, 40G 8 node cascadability Small size, low power consumption Scalable, volume manufacturability proven Most popular technology in service Solid state, no free-space optics, no moving parts, no vibration sensitivity, Telcordia qual Can be colorless (any multiple λ s from any port to any port) Can be mesh upgradeable Any multiple λ s from any port to any port Upgradeable to built-in max degree Lowest cost for Degree 5-8 λ upgradeable Simple software & hardware Cons High first cost DCE function only, taps & splitters / combiners needed for A/D Fixed λ/port, or tunable filters/lasers Not mesh upgradeable Not easily upgradeable to Degree>2 Not easily colorless (requires large switches, eg 40x8) High first cost, not cost-effective for Degree 2, for high-degree nodes only Has either Add or Drop, & no OCM 3 modules per degree if colorless & mesh upgradeable (Deg 4: 12 mod) # A/D ports < N, not λ upgradeable Different suppliers different technologies Free-space optics, vibration sensitive Max degree built in hardware Next generation for some carriers Cost if non-plc demux/switches/mux

3 ROADM SUBSYSTEMS 31 Wavelength Blocker The WB was the first commercially available channel-switching module for ROADM, thus it was used in the earliest deployed ROADM-enabled systems The bare WB module is essentially a dynamic channel equalizer (DCE) where maximum attenuation provides a shut-off (or ON/OFF switch) function A WB-based ROADM subsystem has a Broadcast and Select architecture (Fig 2) and utilizes free-space optics based on either MEMS or LCD actuation It is mostly used in long-haul networks, and typically has 80 channels with a channel spacing of 50 GHz The ports are typically colored They can be made colorless through the use of tunable filters at the Drop ports and tunable lasers at the Add ports, but that approach is prohibitively expensive The relatively high price of the WB module for the limited functionality it provides makes it less competitive in today s market, with its use being now limited to older systems, designed before the availability of more competitive solutions 1 Splitter DROP 1xN Splitter x u m e D 2 N M ux Combiner OPM ADD Nx1 Combiner Filters Receivers Transmitters Fig 2 Architecture of a WB ROADM subsystem 32 Planar Lightwave Circuit ROADM The PLC ROADM is the most cost-effective ROADM solution It offers on day one Add/Drop ports for 100% of the channels, which typically are 40 C-band channels with 100-GHz spacing The PLC ROADM consists of a pair of modules (one Add module, one Drop module), and two such module pairs are typically used at a network node, as is common in degree-2 nodes in metro ring networks With 80% of metro nodes being degree-2 nodes, the PLC ROADM is today the most widely deployed ROADM solution Figure 3 depicts the architecture of a typical degree-2 node, showing two module pairs, East and West, with the Express put of each Drop module feeding the Express put of the corresponding Add module East Drop East Add Drop Module East Express Add Module Line Add Module West Express Drop Module Line West Add West Drop Fig 3 Architecture of a PLC-ROADM-based degree-2 node

A functional schematic of DuPont s PLC ROADM is shown in Fig 4 It performs channel demultiplexing/ multiplexing, add/drop switching, and optical power monitoring/load balancing (shared by the Express and Add signals) Add Module 1 2 Express 40 Line Control Electronics Express Line Power, Data Add1 Add40 Tap Drop Module Tap Control Electronics Drop1 Drop40 Power, Data Fig 4 Configuration of DuPont s PLC ROADM solution DuPont s PLC ROADM, individually packaged chips are spliced together, namely silica-on-silicon arrayed waveguide grating (AWG) chips and polymer-on-silicon chips that include switches and VOAs 1-3 The total number of channels is 40 The Drop module includes an AWG, where the ouptuts can be monitored The Add module includes 48 optical components: 2 40-channel AWGs, 2 20-channel switch/voa arrays, one tap coupler, and 43 tap monitors The optical components are mounted on both sides of two PCBs that contain the control electronics The Add and Drop modules are shown in Fig 5 (a) (b) Fig 5 PLC ROADM (a) Add and (b) Drop modules with optical and electrical connectors The fiber-to-fiber insertion loss for an Express signal going through a PLC-based node is 9 db between 1528 and 1565 nm wavelength (Fig 6), including the 20% tap/2% tap/demux/switch/voa/2% tap/mux/2% tap The VOAs have a dynamic range of 20 db, the PDL is 01 db at minimum insertion loss and 03 db at maximum attenuation, the channel-to-channel crosstalk is 50 db, the switch isolation (extinction) is 50 db, the polarization mode dispersion (PMD) is 001 ps, and the chromatic dispersion (CD) is 5 ps/nm

0-10 sertion Loss (db) -20-30 -40-50 -60-70 -80 1528 1532 1536 1540 1544 1548 1552 1556 1560 Wavelength (nm) Fig 6 Transmission spectrum of the Express aggregate signal in a polymer-based 40-channel ROADM The flat-top AWGs used in these modules are custom-designed for ROADM applications, and exhibit low loss, wide bandwidth, small ripple, and low CD (Fig 7) The worst-case insertion loss is 25dB, the 05 db bandwidth (BW) is 045 nm (56 GHz), the 1 db BW is 052 nm (65 GHz), the 3 db BW is 068 nm (85 GHz), the passband ripple is 02 db, and the CD is within ±2 ps/nm These properties, combined with the low CD of the switches/voas/tap monitors, allow the ROADM subsystem to be operated at high bit rates with minimal penalty The cascadability for this subsystem in ring networks, as measured experimentally with non-return-to-zero on-off keying (NRZ-OOK) modulated signals, is at least 24 nodes at 10 Gbps (OC-192), and at least 8 nodes at 40 Gpbs (OC-768) 0-5 sertion Loss (db) -10-15 -20-25 -30-35 25 GHz 50 GHz ITU -40 1940 1941 1942 Frequency (THz) Fig 7 Spectrum of a channel in a flat-top AWG custom-designed for ROADM subsystems A key property of the polymer-based ROADM is its low electrical power consumption The steady-state power consumption is below 10 W, a remarkably low number that was achieved because of the low switching and attenuation power, owing to the large thermo-optic coefficient of the polymeric material This unique advantage of polymers, especially in DuPont s polymer-based devices, is well recognized in the industry 4

DuPont s PLC components were qualified to Telcordia protocols GR-1209-CORE and GR-1221-CORE, 5 and DuPont s 40-channel PLC ROADM modules passed Telcordia GR-1312-CORE/GR-63-CORE qualification, as summarized in Tables 4 and 5 TABLE 4 Pass criteria for the Telcordia GR-1312-CORE / GR-63-CORE tests performed on DuPont s 40-channel PLC ROADM modules Parameter Unit Spec Pass Criteria Thru IL @ 0 db Attenuation db 12 Shift ±05 Thru IL @ 15 db Attenuation db 27 Shift ±05 Thru PDL @ 0~15dB Attenuation db 09 PDL 09 Wavelength Accuracy (Center λ -ITU) nm ±004 CW-ITU ±004 VOA Set Accuracy @ 15 db Attenuation db ±05 (IL 15dB -IL 0dB )-15 ±05 Switches Operational VOAs Operational Tap Monitors Operational All ROADM functions Operational TABLE 5 Test conditions and final status of the Telcordia GR-1312-CORE / GR-63-CORE qualification of DuPont s 40-channel PLC ROADM Test High Temperature Shock Low Temperature Shock Mechanical Vibration Mechanical Shock Damp Heat Storage Temperature Cycling ESD Test Conditions 25 C to -40 C in 130 minutes, dwell at -40 for 72 hours, remove from chamber cold 25 C to 70 C in 80 minutes, dwell at 70 C for 72 hours, remove from chamber hot 5-50Hz, 01 oct/min, 15G, 50-500Hz, 025 oct/min, 3G, 3 axes 4-inch drop on resting side, edge, and corner (>20G) 85 C/85% RH for 1000 hours -40 C to 85 C, 2 C/min, 30 minute dwell at extremes, 100 cycles, 500 cycles for info 5000V applied to case, 1500V applied to each pin, 3 times Status A second generation of the PLC ROADM of Fig 5 is being developed at DuPont, with the implementation involving further integration through chip-to-chip attachment The advantages of chip-to-chip integration include: Elimination of fiber arrays between chips, resulting in cost reduction Elimination of space needed for fiber ribbons and splices Elimination of excess loss by replacing two fiber array pigtails with a single chip-to-chip coupling Improvement in reliability due to the reduction in the number of interfaces The two types of chips being attached are silica-on-silicon AWG chips and polymer-on-silicon switch/voa array chips Figure 8a shows a sub-assembly of this hybrid 40-channel ROADM 3 Figure 8b shows the output when channel 10 is dropped The chip-to-chip alignment and attachment is performed in a manner similar to that of pigtailing a chip with a fiber array

0 channel 10 dropped -10 Silica AWG -20 ] B d [ L I -30 Polymer PLC -40-50 1529 1534 1539 1544 1549 1554 1559 Wavelength [nm] (a) (b) Fig 8 (a) Subassembly of a 40-channel PLC ROADM consisting of a pigtailed chip-to-chip assembly of a silica AWG chip and a polymer switch/voa array chip, and (b) output of this assembly when channel 10 is dropped Next-generation AWGs will be made in high-index-contrast silicon oxynitride and will be more compact The AWGs will also be passively compensated to achieve virtually athermal behavior with zero power consumption and zero turn-on time delay The athermal AWGs have a wavelength temperature stability that is better than ±03 pm/ C from -30 to +70 C, compared to more than 10 pm/ C wavelength shift for a standard non-compensated AWG 6 33 Wavelength Selective Switch A 1 N can be used either for degree-n connectivity or for adding/dropping channels A has colorless ports, allowing any subset of the total number of channels to exit any port When used to support mesh connectivity (Fig 9a), a 1 5 can support a degree-4 node, including supporting a ring-to-ring interconnection this application, when a single 1 N is used for each degree, fixed multiplexing is used for adding and dropping signals, making the Add and Drop ports colored order to support both mesh connectivity and colorless Add/Drop ports (Fig 9b), three s are needed for each degree The significant cost premium for colorless functionality makes deployment economically unpractical 7 Coupler Coupler Drop DeMux Mux Add Drop Add Transponders Transponders (a) (b) Fig 9 Architectures for each degree in a -based degree-4 node with (a) colored and (b) colorless Add/Drop ports Figure 10 shows a degree-4 node where, in addition to supporting mesh connectivity, the architecture provides colorless Add/Drop ports As shown, a total of 12 1 5 s are required to provide the functionality, a proposition that makes this architecture prohibitively costly

Transponders Add Drop Transponders Drop Add W S W S North Coupler W S South West West Drop Coupler Coupler Add East East North S W Coupler South W S Drop Add S W Transponders Transponders Fig 10 Architecture of a -based degree-4 node where both mesh connectivity and colorless Add/Drop ports are supported A total of 12 1x5 s are required, making this technically attractive solution economically unpractical n 1xN switches λ1, λ 2,, λ n DE λ 1 λ 2 D1 D2 D3 λ n Any number ofλ s D4 D5 D6 D7 D8 Any number ofλ s Fig 11 Functional diagram of an n-channel 1 9 used at an Add/Drop node, providing one Express port and 8 Drop ports

Figure 11 shows schematically the functionality of a 1 9 that can support a degree-8 node The degree-8 node would require 8 1 9 s for mesh mesh connectivity alone, and 24 1 9 s to additionally support colorless Add/Drop ports The number of elements shown in the functional diagram of Fig 11 is the actual number that would be needed if a is implemented in a guided-wave platform However, most implementations use free-space optics, where typically a single bulk diffraction grating is shared for all the demultiplexing and multiplexing functions, as shown schematically in Fig 12 8 The generic of Figure 12 represents the basic design used with all free-space actuation mechanisms, including MEMS, LCD, and LCoS Diffraction Grating MEMS/LC Switch Array Front End Spherical Mirror Fig 12 Schematic diagram of a free-space 34 Wavelength Cross-Connect WXCs provide complete N N connectivity solutions for mesh networks The mesh topology offers increased network capacity, efficiency, and reliability through an increased number of connections and a higher level of redundancy This topology is highly desirable from an operational point of view, however its chief drawback is capex, because of the volume of hardware required Although the opex savings increase the return on investment and quickly outweigh the capex spent, the upfront capex investment has been a significant barrier to the broad deployment of mesh networks PLC-based solutions address the capex concern because they take advantage of integration, which inherently delivers significant cost reduction for complex optical circuitry Some of the most important criteria in a WXC are: Non-blocking reconfigurable node Reliable configuration (several medium size switch matrices) Optical properties (IL, XT, etc) No regeneration, no wavelength conversion For a degree N node (N directions, with N input fibers and N output fibers) and M wavelengths per fiber, a WXC needs N demuxes, N muxes, and M N N switches Figure 13 shows a degree 8 node in a mesh network with 40 channels The WXC subsystem includes 8 40-channel mux/demux pairs and 40 8 8 switches Each 8 8 switch operates on a single wavelength and switches it from any input fiber to any output fiber, thus providing truly colorless ports

IN 1 8x8 λ 1 OUT 1 IN 2 8x8 λ 2 OUT 2 IN 3 8x8 λ 3 OUT 3 IN 4 8x8 λ 4 OUT 4 IN 5 8x8 λ 5 OUT 5 IN 6 8x8 λ 6 OUT 6 IN 7 IN 8 8x8 λ 7 8x8 λ 40 OUT 7 OUT 8 Fig 13 Architecture of a WXC subsystem for a degree 8 node with 40 channels per fiber Future-proof mesh systems are already being built with degree 8 node capability DuPont produced for this application an 8-channel intelligent matrix switch that performs 8 8 switching with power monitoring and power balancing on a single polymer-on-silicon PLC This chip (architecture in Fig 14, packaged chip in Fig 15) includes 112 1 2 switches that make up a strictly non-blocking 8 8 switch matrix, 8 optical power tap monitors, and 8 VOAs for power level control The photocurrents generated by the photodiodes are used by a feedback electronic circuit to control the VOAs, thereby achieving closed-loop automatic power balancing on all the channels INPUT 1 INPUT 2 INPUT 3 INPUT 4 INPUT 5 INPUT 6 INPUT 7 INPUT 8 OUTPUT 1 OUTPUT 2 OUTPUT 3 OUTPUT 4 OUTPUT 5 OUTPUT 6 OUTPUT 7 OUTPUT 8 1x2 Switch VOA Power Tap Photodiode Fig 14 Chip architecture of an 8 8 matrix switch with power monitoring and automatic power balancing The fiber-to-fiber insertion loss of the component shown in Fig 15, between 1528 and 1610 nm wavelength, is 4 db (including 5% tapped power) The VOAs have a dynamic range of 20 db The PDL is 01 db at minimum insertion loss and 03 db at maximum attenuation The PMD is 001 ps, and the CD is 01 ps/nm The switch isolation (extinction) is 50 db and the crosstalk from any port to any port is 50 db 9

Fig 15 8 8 matrix switch with power monitoring and automatic power balancing DuPont s polymer-on-silicon PLC solution takes advantage of the high level of integration to deliver low cost matrix switches, thereby reducing the capex barrier for WXC deployment A degree-8 WXC where the switching core is based on DuPont s PLC matrix switches (40 8 8 switches between 8 40-channel AWG pairs) provides full mesh connectivity and true colorless ports at a fraction of the cost of 1xN -based WXCs Furthermore, the PLC WXC solution provides a compact footprint, high optical performance, low electrical power consumption, solid state reliability, and high scalability 4 CONCLUSION We reviewed the different optoelectronic technologies developed for use in ROADM subsystems, and described their principles of operation, designs, advantages, and challenges The technology platforms that we covered include PLC, MEMS, LCD, and LCoS We described the use of these technologies in the four main ROADM subsystem types: WB, PLC,, and WXC For each ROADM type, we described the functionality, features, size, cost, maturity, as well as Telcordia qualification and deployment status We contrasted the different types, showing the pros and cons of each approach Except for the WB, whose use today is limited to legacy systems, each ROADM type has its optimal place in the network: small-switch-array-based PLC ROADMs are the optimal solution for degree 2 nodes, cross-connects based on free-space 1xN s are optimal for degree 3-4 nodes, and PLC matrix-switch-based WXCs are the most cost-effective solution for nodes of degree 5 and higher Since 80% of nodes in optical networks are degree 2, the small-switcharray-based PLC ROADM will continue to be most widely used ROADM solution for the foreseeable future REFERENCES 1 L Eldada, Photonic integrated circuits, in Encyclopedia of Optical Engineering, Ed R Driggers, Marcel Dekker, New York (2003) 2 L Eldada, Organic photonics, chapter in Microphotonics: Hardware for the formation Age, Ed L Kimerling, MIT, Cambridge (2005) 3 L Eldada et al, Hybrid Organic-organic Optoelectronic Subsystems on a Chip, Proc SPIE 5729, 200 (2005) 4 D J Dougherty, Advances in planar lightwave circuits, Proc OFC/NFOEC (2005) 5 L Eldada, Telcordia Qualification and Beyond: Reliability of Today s Polymer Photonic Components, Proc SPIE 5724 (2005) 6 L Eldada et al, 40-Channel Ultra-Low-Power Compact PLC-Based ROADM Subsystem, Proc OFC/NFOEC (2006) 7 B Basch et al, DWDM System Architecture and Design Trade-Offs, Proc OFC/NFOEC (2006) 8 B P Keyworth, ROADM Subsystems & Technologies, Proc OFC/NFOEC (2005) 9 L Eldada et al, telligent optical cross-connect subsystem on a chip, Proc OFC/NFOEC (2005)