A photonic digital-to-analogue converter (DAC), and its method of operation are disclosed. The photonic DAC comprises an array of semiconductor laser diodes (408, 410, 412), each of which is modulated with different relatively low frequency (<2GHz), low power (<-10 dBm), Large bit count (>14) source digital signals at least partially representative of an original RF signal. The optical outputs of the laser diodes have varying wavelengths and are fed into a wavelength division multiplexer 414 which combines them into ideally one multiplexed optical output which is then pulse modulated by a Mach-Zehnder modulator 416 with a pulse train 418 having a sample rate relatively much higher than the frequency of the source modulating digital signal to provide pulsed optical waveforms. The pulsed optical waveforms are then passed through a dispersive medium 422, such as an optical fibre, which stretches the pulses 424 before they fall on a photodetector 426 which provides a corresponding electrical analogue output 428.
