Parameters affecting mode partition noise in laser transmission systems

An analysis is carried out to determine the parameters affecting mode partition noise in a laser transmission system operating at a gigabits per second data rate using long dispersive fibers. The analysis is based on a Monte Carlo simulation technique of mode partition noise in systems using single-mode or multimode lasers. Some interesting results were found when the fiber dispersion and/or bit rate were high and raised cosine shaping of the data was used. The results show that, as the dispersion is increased, the bit error rate floor, due to the mode partition noise, increases to a maximum value and then decreases substantially to a minimum and oscillates periodically between those two extremes. The period between maximums is determined by the bit rate, fiber dispersion, and mode spacing. Experimental results substantiating the theoretical predictions are given for a single-mode laser. For the case of a multimode laser, it was found that because of pulse shaping there are certain modes which contribute most to mode partition noise. Specifically, the modes contributing most to mode partition noise in typical 560 Mb systems are typically the fifth to seventh modes, which are displaced from the central mode on the short wavelength side. This result suggests a new criterion for choosing multimode lasers with low mode partition noise based on the intensity of those modes