Optical pulse source for generating optical supercontinuum pulses, comprising an optical pump laser operable to generate optical pump pulses at a pump pulse repetition rate Rf; a nonlinear optical element comprising a microstructured optical fiber arranged to receive the optical pump pulses and configured to spectrally broaden the optical pump pulses to generate optical supercontinuum pulses; an optical modulator provided between the optical pump laser and the microstructured optical fiber and operable to selectively control the launch of optical pump pulses into the microstructured optical fiber at a variable, reduced repetition rate Rr=Rf/N, wherein N is a positive integer, to thereby control the repetition rate of optical supercontinuum pulses generated within the nonlinear optical element; and wherein the optical pulse source is configured to provide a plurality of different repetition rates and nominally identical spectral broadening for the different repetition rates.