Automatic bias control of Mach-Zehnder modulators for QPSK and QAM systems

A close-loop control technique for automatic bias control of inner and outer Mach–Zehnder modulators in a dual parallel configuration is presented. The Mach–Zehnders are configured as dual parallel optical modulators. The technique is based on applying low amplitude pilot tones at three different direct current biasing ports of the modulators. By carefully monitoring the amplitude of the beating between these pilot tones, a close-loop control technique can be developed that will automatically monitor and follow the drift of the direct current biasing voltage from its optimum. Hence the technique will enable the modulators to exhibit long term stability for the quadrature phase shift keying or M-quadrature amplitude modulation systems.