Optical coherent broad-band transmission for long-haul and distribution systems using subcarrier multiplexing

Abstract

The system configuration and future possibility of a coherent optical transmission system using subcarrier multiplex- ing (SCM) techniques are described. First, signal multiplexing techniques for coherent optical trans- mission are compared, and appropriate application for coherent SCM system is discussed. By comparing optical modulation meth- ods, optical frequency modulation (FM) using direct modulation of a DFB-LD and a heterodyne detection is shown to be feasible. lkansmission system using 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. This circuit is also effective for compensating for the frequency instability of light sources. A theoretical simulation of a coherent SCM system showed that a 100 channel of 30-MHz FM signal or a 15 channel of 155-Mb/s signal can be distributed to 10 000 subscribers using single stage or double stage optical amplifiers. To confirm the feasibility of coherent SCM transmission, pre- liminary experiments have been performed with optical FM by 2-channels 560-Mb/s ASK-SCM signal and a heterodyne 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 insensitive to the state of polarization in the transmission fiber. In a subcarrier transmission of 560 Mb/s DPSK signal, the PNC effectively suppressed phase noise and the influence of IF frequency deviation. Finally, application of coherent SCM combined with optical frequency division multiplexing (OFDM) is discussed. A model allocation of optical carriers is proposed and possible receiving channel number is estimated.