Frequency Chirp Suppression of Directly Modulated Lasers Based on Look-Up Table Nonlinear Predistortion Combining Equivalent Circuit Model

The nonlinear distortion from directly modulated lasers (DMLs) is a major challenge to meet the explosive data traffic for improving optical communication systems performance. To practically and cost-effectively preview the compensation scheme, a look-up table (LUT) nonlinear predistortion combined with an equivalent circuit model method is proposed to suppress the frequency chirp. The ideal injection current is theoretically obtained from the large signal rate equation and utilized to construct 3, 5, 7, and 9-bit LUTs for non-return-to-zero (NRZ) signals to find the optimally simple and viable way. Based on the normalized root-mean-square error (NRMSE) and bit error rate (BER) assessment, the 5-bit LUT is identified as the most appropriate for eliminating the transient chirp. Considering the growing demand for high capacity, the four-level pulse amplitude modulation (PAM4) and PAM8 signals are also investigated to examine the commonality of the LUT. Furthermore, an intensity modulation direct detection (IM/DD) system utilizing a laser model as a light source is implemented to further verify the effectiveness of the solution. The results show that the improved LUT technique can enhance the performance of the communication system and provide guidance for the nonlinear suppression of actual lasers.