OPTICAL COHERENT BROAD-BAND TRANSMISSION FOR LONG-HAUL AND DISTRIBUTION-SYSTEMS USING SUBCARRIER MULTIPLEXING

The system configuration and future possibility of a coherent optical transmission system using subcarrier multiplexing (SCM) techniques are described. First, signal multiplexing techniques for coherent optical transmission are compared, and appropriate application for coherent S CM system is discussed. By comparing optical modulation methods, optic al frequency modulation (FM) using direct modulation of a DFB-LD and a heterodyne detection is shown to be feasible. Transmission system usi ng a configuration with a local laser in the transmitter is unaffected by polarization and is cost-effective. Phase noise can be suppressed by a phase-noise-canceling circuit (PNC) in a heterodyne receiver. Thi s circuit is also effective for compensating for the frequency instabi lity of light sources. A theoretical simulation of a coherent SCM syst em showed that a 100 channel of 30-MHz FM signal or a 15 channel of 15 5-Mb/s signal can be distributed to 10 000 subscribers using single st age or double stage optical amplifiers. To confirm the feasibility of coherent SCM transmission, preliminary experiments have been performed with optical FM by 2-channels 560-Mb/s ASK-SCM signal and a heterodyn e detection using 3-electrode DFB-LD's. With a local LD setting at the transmitter, a span-loss margin of 40.7 dB was obtained using single stage Er3+-doped fiber amplifier (EDFA), and the system was insensitiv e to the state of polarization in the transmission fiber. In a subcarr ier transmission of 560 Mb/s DPSK signal, the PNC effectively suppress ed phase noise and the influence of IF frequency deviation. Finally, a pplication of coherent SCM combined with optical frequency division mu ltiplexing (OFDM) is discussed. A model allocation of optical carriers is proposed and possible receiving channel number is estimated.