Metro access Long- haul Metro regional < 1,000- km fiber Up to 16 ROADMs Metro core < 500- km fiber Up to 24 ROADMs < 100- km fiber FIGURE 1. A typical optical transport network, with the three types of metro networks. NOV/ DE C 20 12 14 ® BENNY MIKKELSEN is co- founder and CTO of Acacia Communications Inc. FEATURE By BENNY MIKKELSEN Coherent 100G transceivers for metro applications would have to be smaller and more power- efficient than current longhaul modules. Fortunately, paths toward this goal have emerged. Challenges and key technologies for coherent metro 100G transceivers S ERVICE PROVIDERS are currently adopting coherent transceivers for their 100G DWDM backbone applications to achieve the higher capacity and longer fiber reach that coherent technology offers. Coherent transceivers also provide cost- effective electronic equalization of fiber impairments – such as chromatic dispersion and polarizationmode dispersion – as well as extensive performance monitoring capabilities that enable easy installation and network management. These benefits help service providers meet bandwidth demand growth while reducing the total cost of ownership. With their superior optical performance and ease of use, coherent transceivers are also very attractive for the emerging 100G metro market. The metro- market segment typically develops two to three years after the long- haul market but is two to three times larger in size. Following this timeline, the 100G metro market is expected to take off in 2014 and attracting significant attention from equipment manufacturers and transceiver suppliers. The size, power dissipation, and cost structure of today’s coherent transceivers, which were developed for long- haul applications, limit these modules’ application in the more cost- sensitive metro networks where space comes at a high premium, making line- card density very important. Consequently, a path to a lower- cost coherent module in a smaller footprint has become increasingly important. DWDM metro- network requirements There is no universal definition of a metro network, but the segment is generally divided into three subcategories: metro regional, metro core, and metro access ( Figure 1). The metro regional and metro core cover distances of 500– 1,000 km and 100– 500 km, respectively, and typically include a large number of reconfigurableoptical- add/ drop- multiplexer ( ROADM)