Single-contact DFB lasers were frequency modulated by pseudorandom data at rates up to 8 Gbit/s by direct current modulation with no pre-equalisation of the modulation signal. The signals were optically demodulated by Fabry-Perot interferometers for conversion to ASK signals which were detected by a direct-detection receiver. At 8 Gbit/s the receiver sensitivity was − 22dBm.