Important technologies for highly stable coherent system operation

Essential technologies for stabilizing the operation of coherent optical-fiber communication systems, such as packaging, several automatic control technologies, and an automatic start-up procedure are described. In particular, polarization fluctuation compensation is shown to be indispensable for coherent system operation. Both a polarization control system and a polarization diversity receiver were developed, and stable operation was achieved in each system, which greatly improved the practicability of the coherent systems. Using several technologies, 10-h error-free operation was achieved in a 400-Mb/s-162-km FSK (frequency shift keying) heterodyne single filter detection system. this 10-h error-free operation, which corresponds to less than a 7*10/sup -14/ error rate, greatly advanced the feasibility of achieving an actually operating coherent system. >