Bias Control for Optical OFDM Transmitters

The optical orthogonal frequency-division multiplexing transmitter typically employs a quadrature modulator (QM) to achieve up-conversion for high-capacity long-haul transmission. Bias tracking of the QM is essential to minimize transmitter impairments and to maintain long-term stable operation. A dither-free bias control circuit is proposed to maintain optimum bias points of the QM. By monitoring the QM output optical power Po, optimum bias points can be acquired and tracked via minimization of the statistics (mean and variance) of Po. Numerical studies of the bias control circuit reveal a 3-σ residual bias error within 0.075-7π incurring a bias error induced Q2penalty of ~0.6 dB.