INTERFEROMETRIC NOISE-REDUCTION THROUGH INTRABIT FREQUENCY EVOLUTION OF DIRECTLY MODULATED DFB LASERS

A new technique is demonstrated that improves the performance of optic al networks subject to interferometric noise, Every transmitter compri ses a selected DFB laser that is NRZ ASK directly modulated with a lar ge modulation depth, Under modulation, the center frequency of the DFB laser is found to vary over the duration of each bit, falling during mark bits and rising during space bits with an exponential-like depend ency of characteristic time constant similar to 20 ns, The optical fre quency at the midpoint of each bit interval is not constant but depend s upon the sequence of the preceding bits, Therefore, on the interfere nce of a delayed-replica parasitic crosstalk waveform the interferomet ric noise generated on detection may be reduced by RF filtering accord ing to the difference in center frequency of the interfering bits, Thi s noise suppression which requires no additional hardware has been suc cessfully modeled and experiment demonstrates little performance gain for a single interferer, as predicted. However, in the presence of mul tiple interferers significant improvement is predicted and observed at sub-Gb/s rates.