The nonlinear distortion caused by the Mach-Zehnder modulator (MZM) is one of the main limiting factors for the improvement of the transmission performance of the intensity modulation and direct detection (IM/DD) optical communication system. In order to solve the problem, an improved Volterra nonlinear equalization (VNLE) method is proposed. Compared with the traditional VNLE that uses the least mean square (LMS) to calculate the tap coefficients, the improved VNLE uses the least square (LS) method to obtain more stable convergence. The simulation results show that the VNLE based on LS has better performance when solving complex nonlinear damage. For 25 Gbaud 4-level pulse amplitude modulation (PAM4) signals, the improved VNLE can reduce the bit error rate (BER) to below 10-4 in a 7-km-long single-mode optical fiber transmission system. In addition, in order to make the BER below 10-3, the transmission distance that the improved VNLE can withstand is about 1.5 km longer than that of the traditional VNLE.

In this paper, a distributed Bragg reflection (DBR) type Er^(3+)/Yb^(3+) co-doped fiber laser of high output power and high slope efficiency was developed. Its gain medium was a 4.45-m-long Er^(3+)/Yb^(3+) co-doped fiber. When it was pumped by a 1064-nm Nd:YAG, the linewidth of output laser was measured as 0.072 nm by 3 dB and 0.192 nm by 25 dB at 1552.08 nm. The maximum output power was measured as 69 mW. Its power stability was < 5%, side mode suppression ratio was 59 dB, and the output wavelength stability was ±0.01 nm. The laser had a threshold of 12 mW and a slope efficiency of 22%.