Elusive bit-error-rate floors resulting from transient partitioning in 1.5-μm DFB lasers

Some 1.5-μm distributed feedback (DFB) lasers modulated at 1.7 Gb/s exhibit bit-error rate (BER) floors for certain fiber lengths, and hence these floors are difficult to detect. For example, a laser that does not exhibit a BER floor for 43 km of fiber may exhibit a BER floor after only 27 km. It is shown that the source of such BER floors is a narrow pulse resulting from transient partitioning between the DFB TE and TM modes. The narrow TM pulses cause false bits if the fiber delay between the modes positions the TM energy within a zero time slot. It is shown that for the 11 lasers studied, TM partitioning floors rarely occur in lasers with adequate threshold gain difference. When the duration of the TM partitioning is only a small fraction of a bit period, the BER floor is effectively removed using a polarized film in the transmission path to suppress the TM mode. Similarly, increased suppression of the TM mode has been achieved by changing the laser structure to reduce the TM mode gain