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 P_o, optimum bias points can be acquired and tracked via minimization of the statistics (mean and variance) of P_o. Numerical studies of the bias control circuit reveal a 3-σ residual bias error within 0.075-7π incurring a bias error induced Q² penalty of ~0.6 dB.