Q. Jiang et M. Kavehrad, A SUBCARRIER-MULTIPLEXED COHERENT FSK SYSTEM USING A MACH-ZEHNDER MODULATOR WITH AUTOMATIC BIAS CONTROL, IEEE photonics technology letters, 5(8), 1993, pp. 941-943
We demonstrate a subcarrier-multiplexed coherent optical transmission
system using a push-pull Mach-Zehnder intensity modulator. Six subcarr
ier channels cover a six-octave bandwidth and are frequency-shift keyi
ng (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 i
ntermodulation is canceled by more than 20 dB due to the balanced stru
cture of the modulator. This results in an 8-dB improvement in the rec
eiver sensitivity compared to the system using an optical phase modula
tor. We also demonstrate a simple automatic bias-control scheme for th
e Mach-Zehnder modulator. It utilizes the optical signal from the comp
lementary output port of the dual-output modulator.