Direct detection lightwave systems: why pay more?

Requirements on detection technology are examined from the system and economic perspectives. The two possible detection approaches, direct and coherent, are described, and their theoretical detectability limits are considered. Examples of both types of networks that have been demonstrated are presented. Direct detection networks are then considered, and examples of demonstrated networks are presented. The same is then done for coherent networks. The relative advantages and disadvantages of direct and coherent networks are discussed, and it is concluded that for some time to come, direct detection will be the solution of preference, particularly for networks.<>