Noise Immunity of Multilevel Pulse-Modulated Signals Propagating along a Dispersive Optical Path

A fiber-optic transmission system for multi-level pulse amplitude modulated signals with direct photodetection on the receiving side is considered. A mathematical model for the transmission of M-level PAM signals through the dispersion optical path based on the Gaussian approximation of the transmitted pulses has been developed. The criterion of the noise immunity evaluation is the narrowing of the eye closure in the receiver. Formulas that allow estimating the power penalty due to fiber dispersion, depending on the levels, duration and duty cycle of optical pulses, as well as the parameters of the optical fiber are derived. Comparative estimates of the noise immunity of transmission PAM −4 and PAM-2 signals with bit rates of 12.5 Gbps, 25 Gbps, 50 Gbps, 100 Gbps, 200 Gbps, 400 Gbps over a standard optical fiber at a wavelength of 1550 nm are given. In comparison with the transmission of PAM-2 signals, the use of PAM-4 signals is preferable at a signal transmission rate of more than 25 Gbit/s and power penalties of more than 2.5 dB, provided that the bit error correction technology is used in the transceiver equipment, which halves the bandwidth requirements for optoelectronic components.