We outline a novel method performing all-optical envelope detection of radio-frequency signals for radio-over-fiber links. A high frequency modulated signal with a slower-varying envelope is injected into a DFB laser which, due to gain suppression effects, recovers only the envelope of the optical signal. We characterize the DFB gain suppression effect in terms of injected signal wavelength and power level requirements. System performance is assessed, including experimental bit-error rate results; these illustrate successful envelope detection for a 20 GHz carrier with ASK modulation operating at 2.5 Gbit/second. Preliminary results at 5.5 Gbit/s show significant potential for application in hybrid optical-wireless communications networks.