A recirculating optical delay line 30 has a laser 32 controlled by wavelength control 33 so as to vary the wavelength of radiation over time to provide a first input 34 to an optical modulator 35. The modulator 35 modulates the intensity of the first input 34 with a pulsed electromagnetic frequency signal 36 to produce a pulsed modulated optical signal 39. The signal 39 passes through an optical coupler 40 into a delay loop 41 having a delay fibre 44 arranged to delay the signal 39 for a predetermined duration. The wavelength control 33 is arranged to vary the first input 35 so as to ensure that overlapping pulses of the signal 39 in the delay loop 41 are at different wavelengths, thereby inhibiting optical mixing effects between the overlapping pulses.