The influence of directly modulated DFB LD sub-mode oscillation on long-span transmission system

Error-rate floors have been observed in several long-span transmission experiments at 500 Mb/s, using 1.5- mu m distributed-feedback laser diodes (DFB LDs) and 1.3- mu m zero dispersion optical fibers. It is proposed that for the threshold gain difference between main and submode (for DFB LDs), Delta alpha is a good parameter to specify the submode oscillation characteristics. It is experimentally and theoretically confirmed that the threshold gain difference Delta alpha must be greater than 5-6 cm/sup -1/, to avoid the error rate floor at 500 Mb/s. It was also confirmed that lambda /4 phase-shifted DFB LDs can easily satisfy this condition.<
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