Introduction Optical advanced modulation formats combined with coherent reception provide a reduced symbol rate and spectral width along with digital compensation of linear impairments for fiber optic transmission. The combination allows carriers to upgrade 10-Gb/s networks to 100-Gb/s networks with minimal change to the fiber plant. Coherent technology has been mainly deployed in long-haul networks, and it is now starting to be deployed in metro networks. The main impediments have been the cost and footprint of the transceivers, which consist of a tunable laser, an advanced format modulator, a coherent .