Spectral shift and broadening of DFB lasers under direct modulation

The spectrum of DFB lasers, directly modulated at 144 and 622 Mb/s, is found to be considerably wider than the modulating random sequence rate. The broadening, which may be as high as ten times the data bandwidth, is accounted for by frequency changes (chirp) along each bit, incorporating a recently observed short time constant on the order of 10-20 ns. These changes will also introduce a few gigahertz spectral shifts as a function of the bit rate.