ADVANCES IN ROADM TECHNOLOGIES AND …

1 ADVANCES IN ROADM TECHNOLOGIES AND subsystems September, 2010 2 | Page ABSTRACT Unti l recentl y, reconfigurable opti cal add/drop multi plexer ( 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 la rge number of component vendors have announced ROADM products based on a variety of TECHNOLOGIES , some more mature than others. We review the different opti cal component TECHNOLOGIES that have been developed for use in ROADM subsystems , and describe their principles of operation, designs, advanta ges, and challenges. The technology platforms that we cover include MEMS, li quid crystals (li quid crysta l devices (LCD) and li quid crysta l on sili con (LCoS) TECHNOLOGIES ), and monolit hic and hybrid pla nar li ghtwave circuit s (PLC) based on sili ca on sili con and polymer on sili con platforms.

2 For each technology, we describe the corresponding ROADM subsystem archit ectures in terms of functi onality, features, siz e, cost, and maturit y. Keywords: reconfigurable opti cal add/drop multi plexer, wavelength blocker, wavelength selective swit ch, opti cal cross-connect, MEMS, li quid crystal, pla nar li ghtwave circuit , sili ca, polymer, Telcordia qualification 1. INTRODUCTION Large amounts of information traveli ng on multi ple wavelengths around an opti cal network need to be swit ched at the network nodes. Information arri ving at a node is forwarded to it s final desti nation via the best possible path, which is determined by such factors as dista nce, cost, and the reliability of specific routes. The conventi onal way to swit ch the information is to convert the input fiber opti cal signal to an electrical signal, perform the swit ching in the electrical domain, then convert the electrical signal back to an opti cal signal that goes down the desired output fiber.

3 This opti cal-electrical-opti cal (O-E-O) conversion uses systems that are expensive, bulky, and are bit -rate/protocol dependent. ROADMs allow avoiding the unnecessary O-E-O conversion, enabli ng O-O-O systems that use opti cal swit ching, which has significant advanta ges for carriers and service providers. Opti cal swit ching involves lower capital expendit ures (capex), as there is no need for a la rge amount of expensive high-speed electronics. Furthermore, operational expendit ures (opex) are decreased and reliability is increased because fewer network elements such as back- to-back terminals are required. Reducing the complexit y also makes for physically smaller swit ches. Additi onall y, opti cal swit ches are relatively future-proof. An electrical swit ch has electronics designed to detect incoming opti cal signals of specific bit rates and formats.

4 When the bit rate increases or when the format changes, the electronics need to be upgraded. ROADMs route the opti cal signals directly, and are bit -rate/protocol transparent, so future upgrades of bit -rate or protocol can be accommodated wit hout the need to upgrade the swit ch. A ROADM network element typically includes: Transponders ROADM Subsystem Opti cal Service Channel Opti cal Power Monit oring Ampli fiers (Pre-Amp & Post-Amp) Dispersion Compensation Module We focus in this manuscript on ROADM subsystems , describing the various TECHNOLOGIES used to buil d them, and the different subsystem archit ecture enabled by each technology. * phone ; fax ; 3 | Page 2. ROADM SUBSYSTEM TECHNOLOGIES Reconfigurable opti cal networks have needs for various types of ROADM . Figure 1 shows some of the dynamic functi ons needed at nodes in ring and mesh networks.

5 Fi g. 1. Types of ROADM needed at optic al network nodes. Table 1 defines the four main types of ROADM , where ROADM is used in the broadest sense to include Type I/II ROADM , Wavelength Selecti ve Swit ches (WSS), and Opti cal Cross-Connects (OXC). Table 2 li sts for the four main ROADM types the key justi fications for their deployment, their compatibility wit h prior generations, the opti cal components used for each type, and the TECHNOLOGIES used for the components. TABLE 1. Definition of ROADM types. ROADM Type Node Degree subsystems per Node Add/Drop Channels Colorless Multi ple s per Port Type I ROADM 2 2 subsystems per Degree 2 Node N * No No Type II ROADM 2 2 subsystems per Degree 2 Node M ** Yes No Wavelength Selectiv e Switch (WSS) N N subsystems per Degree N Node** M-1 (1 M W SS) ** Yes Yes Optical Cross-Connect (OXC) N 1 Subsystem per Degree N Node N/A (Mesh Connectivity only) Yes Yes * N: number of channels ** M N ** , 4 1 4 W SS subsystems are needed for the interconnection of 2 fiber-pair rings ** , 1 5 W SS provides 1 express port and 4 Add/Drop ports TABLE 2.

6 ROADM types, the main justif ications for their deployment, their compatibility with prior generations, and the optical components used in each ROADM type. ROADM Type Justification Compatibility Optical Components (Technology) Type I ROADM Fixed ports Stranded capacity reduction Dual-use as DGE, DCE W avelength Blocker (LCD or M EMS) + Fi xed Filters (TFF) Demux + Small Switch Array + Mux (PLC) Type II ROADM Any to any po rt No manual intervention, monitor & contr ol Retain blocker, add tunable filt ers and tunable lasers, no impact to thru path ; or all PLC solution, more cost-effective W avelength Blocker (LCD or M EMS) + Tunable Filters/Lasers Demux + Small Switch Array + Mux + M N Switches (PLC) Wavelength Selectiv e Switch (WSS) Any multiple s to any port Ring interconnect with out OEO Select locations only; interoperability with oth er n odes, same lasers 1 N W avelength Selective Switch (LCD or LCoS or MEMS) Optical Cross-Connect (OXC) Any multiple s from any port to any port Mesh interconnect, mesh protection Select locations only N N Matrix Swit ch (PLC) N N W avelength Selective Switch (LCD or LCoS or MEMS) 4 | Page Split ter TABLE 3.

7 Specific ROADM subsystems , status of their implementation and Telc ordia qualif ication, and identif ication of the subsystems used in the main deployment waves. ROADM Subsystem Implementation Status Telcordia Qualified Main Deployment Waves Type I ROADM b ased on LCD WB Up to 80 channels Yes W ave 1 Type I ROADM b ased on MEMS WB Up to 80 channels No Type I ROADM b ased on PLC WB Up to 40 channels Yes Type I ROADM b ased on PLC SSA Up to 40 channels Yes W ave 2 Type II ROADM b ased on LCD WB Up to 80 channels Yes Type II ROADM b ased on MEMS Up to 80 channels No Type II ROADM b ased on PLC SSA In Development No WSS based on MEMS 1 N WSS Up to 1 9 No W ave 3A (Add/Drop) WSS based on LCD 1 N WSS Up to 1 4 No W ave 3B (Ring-to -Ring Interconnect) WSS based on LCoS 1 N WSS In Development No OXC based on PLC Matrix Switch Up to 8 8 Yes W ave 4 -- Preparation Sta ge OXC based on MEMS N N WSS Development Not Starte d No OXC based on LCD N N WSS Development Not Starte d No OXC based on LCoS N N WSS Development Not Starte d No From Tables 2 and 3, the main approaches in ROADM subsystems being deployed by carriers can be summariz ed as: Wavelength Blocker (WB) used in deployment wave 1 Small Swit ch Array (SSA) used in deployment wave 2 Wavelength Selective Swit ch (WSS) used in deployment wave 3 Opti cal Cross-Connect (OXC) to be used in deployment wave 4 Table 4 summariz es the pros and cons of each of these approaches.

8 Table 5 li sts ROADM use by market and approach, some component and system vendors, and some carriers who deployed the systems. OXCs are not deployed yet. TABLE 4. Pros and cons of the main ROADM approaches being deployed by carriers. ROADM Configuration Pros Cons W B SSA Wavelength Blocker Combi ner D e M m u u x x Fi rst to be ready # A/D ports = N (all s) Low cost Small size Simple softw are & hardware # A/D ports = N (all s) Large size Expensive Fi xed /port Not degree upgradeable Fixed /port Not degree upgradeable W SS OXC Wavelength Selective Switch Combiner Any multiple s to any port Degree upgradeable Any multiple s from any port to any port Degree upgradeable, upgradeable Simple softw are & hardware Small size Expensive, extra $ for Add Complex software & hardware # A/D ports M < N, not upgradeable Large size Next generation for some carriers TABLE 5.

9 ROADM use by market and approach, some component and syste m vendors, and some carriers who deployed the systems. Market (Approach) Component Vendors System Vendors Carriers (System Suppliers) Long-Haul (W B) JDSU, Avanex, DuPont, CoAdna Lucent, Ciena, Huawei, Marconi, Siemens Qwest (Lucent), Verizon (Lucent), GigBE project (Ciena), MCI (Ciena), BT (Marconi), MCI (Siemens), AT&T (Siemens) Metro (W B) JDSU, Avanex, DuPont, CoAdna, LightConnect, Polychromix, Xte llus Alcatel/Tropic, Mahi, Lucent/Movaz Verizon, MCI, SBC (Alcatel/Tropic), BellSouth (Tella bs), NTT, Comcast (Lucent/Movaz, Cisco, Fujitsu, Nortel), Cox (Fujitsu), Bright House (Fuji tsu), Shaw (Fuji tsu) Metro (SSA) JDSU, DuPont, OpTun, Chromux, Neophotonics, NEL Cisco, Tella bs, Hitachi Metro (W SS) JDSU, DuPont, CoAdna, Engana, Metconnex, Capella Fujitsu, Merito n, Nortel 5 | Page D E M U X M U X D E M U X.

10 3. ROADM subsystems Type I ROADM A Type I ROADM is the simplest type of ROADM . It has fixed (colored) ports, and it is offered at the lowest price of any ROADM subsystem. Type I ROADM Generation 1 One type of subsystem implemented in DuPont s PLC platform is an SSA-based Type I ROADM that performs channel demulti plexing/multi plexing, add/drop swit ching, and opti cal power monit oring/l oad balancing (shared by the Express and Add signals). The subsystem configuration is shown in Fig. 2. 1 .2 . 3 2 IN 1 3 2 5 % T a p .. O U T O U T 15% T a p C on t rol E le c tro nic s 1 .2 . 3 2 IN 1 DRO P 3 2 1 3 2 A D D P o w e r, D a t a Fi g. 2. Configuration of an SSA-based Gen 1 Type I ROADM subsystem implemente d in DuPont s PLC platform. In this subsystem, individually packaged chips are spli ced together, namely sili ca-on-sili con arrayed waveguide grating (AWG) chips and polymer-on-sili con chips that include swit ches and The tota l number of channels N is 32.