An optical device is disclosed wherein a Fabry-Perot etalon (12) is used as a spectral mode selector externally coupled to a source of modelocked optical pulses (10). By adjusting the cavity length of the etalon (12), a plurality of equally spaced predetermined modes output from the source are transmitted by the etalon. These modes which are transmitted combine coherently to create a stream of optical pulses (16) having a higher repetition rate than the repetition rate of the source.