Mode power partition events in nearly single-frequency lasers

Frequent power dropouts are observed in the single dominant line of a number of dc-biased 1.3-μm injection lasers, each of which exhibits a nearly single-longitudinal-mode CW spectrum. During the dropout events, some of which essentially extinguish the dominant line for 1 to 8 ns, total laser output power is observed to remain constant: the power lost from the dominant mode appears in a side mode of very low average power. These events are capable of causing bit errors in a communications system. It was found that the frequency of events greater than a given threshold value falls exponentially with an increasing ratio of main-to-side-mode powers and that this ratio must exceed 100:1 in order to avoid a significant contribution to system error rates when the laser is operated continuously and an external modulator is used. These observations are in agreement with recently published calculations. The requirements on mode power ratio for the case of directly modulated lasers are much more severe. We present a calculation of the optical power penalty resulting from these mode power fluctuations.