At present, compared to the complex modulation - coherent detection system, the intensity modulation - direct detection system occupy the dominant position in the metro network and the access network for its low cost and easy implementation. However, with the increasing demand for network capacity, the spectrum of traditional OOK-based modulation technology is difficult to meet the requirements, so the higher spectral efficiency modulation format is need. IEEE has already put PAM4 as the transmission standard of the next generation Ethernet, and has formulated the standard of different transmission speed and different transmission distance. However, in the current standard, the transmission distance is only limited to 20 km. It is difficult to meet the distance requirements for the metro network and relay less access network. Therefore, in order to meet the long-distance optical fiber transmission, in the case of high launch power, signals will induce serious nonlinear effects, which greatly limits the transmission distance. Therefore, solving the nonlinear effect of the intensity modulation-direct detection system is a problem worthy of study.

Prof. Songnian Fu and Master student Shiwei Zhou of the NGIA, Huazhong University of Science and Technology, China, collaborated with the State Key Laboratory of Optical Communication Technologies and Networks, WRI to make use of the Volterra series expansion method to compensate the signal transmission through optical fiber transmission. It is found that the higher the number of taps, the better the recovered signal quality, but the higher the computational complexity. Therefore, in order to achieve a balance between the computational complexity and the recovered signal quality, a filter with variable number of taps is designed. In the face of long-distance, high-speed transmission system, the use of a larger number of taps; in the face of short-range, low-rate transmission system, using smaller tap number.

Fig. 1 System setup


Fig. 2 (left) system performance after 100 km transmission (right) achieve setup for different system setup

This research proposes and applies the nonlinear compensation algorithm to extend the transmission distance of the intensity modulation-direct detection system, and provides a flexible choice of complexity for different system configurations. This work for the metro network and access network has practical significance.

The paper was published at Optics Letters (Vol. 41, No. 8, pp. 1805–1808, 2016) and this work was supported in part by the 863 High Technology Plan (2015AA015502), the National Natural Science Foundation of China (61275069, 61331010, and 61322507), and State Key Laboratory of Optical Communication Technologies and Networks, Wuhan Research Institute of Posts & Telecommunications Open Fund (2016OCTN‐01).