Key technologies for a coherent optical trunk line system

A high capacity continuous phase frequency shift keying (CPFSK) heterodyne transmission system is proposed from the components point of view. In the coherent trunk line system, reliability, compactness and high sensitivity are important characteristics to achieve a practical system. To realize such characteristics, the authors developed several key components, such as a compact frequency distributed-feedback laser diode package, a reliable filter squeezer polarization controller, a novel dual-PIN high electron mobility transistor receiver, an efficient chromatic dispersion equalizer, and an intelligent frequency controller. By applying these components to a 2.5 Gb/s CPFSK transmission system, -47 dBm receiver sensitivity and stable operation in 764 km normal dispersion fiber transmission were achieved. This high sensitivity value and a long span transmission capability will be effective for both transmission capacity upgrades with frequency division multiplexing (FDM) and also for transmission span expansion with optical amplifier repeaters