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.

2 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. 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.

3 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.

4 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.

5 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. When the bit rate increases or when the format changes, the electronics need to be upgraded.

6 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.

7 * 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. 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).

8 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.

9 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. 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.

10 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.