System impact and calibration method of power imbalances for a dual-polarization in-phase/quadrature optical transmitter.

For the latest 400-Gb/s or upcoming 1-Tb/s single-carrier optical fiber communications systems, dual-polarization quadrature amplitude modulation (DP-QAM) based on in-phase (I) and quadrature (Q) optical transmitter is the only possible solution. In a coherent DP-IQ transmitter, the power difference between the I and Q branches or orthogonally polarized (X and Y) channels are known as the IQ or XY power imbalance, respectively. Uncompensated IQ and XY power imbalances are always troublesome and can significantly limit the performance in long-haul transmission. In this work, we first experimentally investigate the impact of IQ and XY power imbalances on various QAM formats and baud rates in high-speed transmission. A DP-16-QAM up to 86 GBd is investigated, aiming for beyond 400 Gb/s applications. Then, a convenient pre-calibration method based on optical interference is proposed and experimentally demonstrated to detect and compensate both the IQ and XY power imbalances. This method can use the existing modulator structure and low-speed photodiode in the DP-IQ transmitter to obtain the optical interference power by injecting a specific coded electrical signal. After adjusting the time skew of each channel, the power imbalance between two channels can be calibrated by minimizing the optical interference power. The results fit well with the given theoretical analysis, and the proposed technique can be adopted to the DP-IQ transmitter with different QAM formats.