A SUBCARRIER-MULTIPLEXED COHERENT FSK SYSTEM USING A MACH-ZEHNDER MODULATOR WITH AUTOMATIC BIAS CONTROL

We demonstrate a subcarrier-multiplexed coherent optical transmission system using a push-pull Mach-Zehnder intensity modulator. Six subcarrier channels cover a six-octave bandwidth and are frequency-shift keying (FSK) modulated at 45 Mb/s. With an optical phase modulation index of 0.38, a receiver sensitivity of -36 dBm is obtained. Second-order intermodulation is canceled by more than 20 dB due to the balanced structure of the modulator. This results in an 8-dB improvement in the receiver sensitivity compared to the system using an optical phase modulator. We also demonstrate a simple automatic bias-control scheme for the Mach-Zehnder modulator. It utilizes the optical signal from the complementary output port of the dual-output modulator.