Static and dynamic optical spectra of 1.55 Â¿m DCPBH-DFB lasers are analysed. These lasers were packaged in hermetic sealed enclosures and directly modulated with transmitter circuitry. We have found that their static spectra observed under CW operation have fine structure induced by optical feedback from the far end of the package fibre pigtail. The dynamic spectra taken under pulsed modulation differ significantly from the static spectra in two ways. The dynamic spectra are narrower than the structured static spectra when a fixed data pattern is applied to the lasers and broader when a random data pattern is used. This difference is attributed to the finite build-up time required to produce a sustained amplitude oscillation at the relaxation oscillation frequency, within the external optical cavity, after the lasers are turned on. The structured CW and the altered dynamic spectra are found not to degrade the transmission through 50 km of fibre at bit rates as high as 650 Mbit/s.