The frequency response of a single-mode fibre recirculating delay-line filter has been measured from 0 to 18 GHz using a broadband measurement system that includes an inteferometric waveguide modulator and a high-speed photodetector. An optical-fibre notch filter having a fundamental passband at 740 MHz exhibited 23 overtones that were uniform to within ±1.5 dB over the entire measurement range.