A dual-comb spectrometer 5 with two lasers 10, 12 serving as a local oscillator and an interrogator. The lasers output light beams with respective frequency combs C1, C2 of defined free spectral range, FSR. A detector 30 can detect heterodyne mixing of the combined beams to detect an RF frequency comb C3. Respective control signals are supplied to the lasers which have functional forms configured to cause the frequencies of the lasers' frequency combs C1, C2 to tune over a defined fraction of their FSR. This enables a reduction of the effective spectral sampling period by a factor equal to the ratio of the FSR to the spectral resolution of the spectrometer, which will typically be several orders of magnitude, so that the spectral sampling period can be reduced from the GHz to the MHz range, which in turn enables a gapless spectrum to be obtained in a short time.