Abstract

We propose a novel predistortion technique for multidimensional quadrature amplitude modulation and polarized intensity rotational frequency shift keying based on M-ary differential phase shift keying (QAM-PIRFSK-MDPSK) optical modulation with a direct detection system. While the QAM-PIRFSK-MDPSK optical modulation is one of the most spectral efficient modulation techniques, inter-dimensional interference (IDI) limits the modulation order so that it is difficult to further increase the spectral efficiency. Therefore, it is very important to mitigate the IDI. Because most of the IDI in QAM-PIRFSK-MDPSK is caused by the intensity fluctuation due to the quadrature component of QAM, the quadrature component of QAM is suppressed by the bipolar non-return-to-zero signal addition. We verified the performance enhancement with the symbol error rate curve through simulations and experiments. With the proposed technique, a significant improvement in signal-to-noise ratio gain in QAM-PIRFSK-MDPSK was experimentally demonstrated.