An optical frequency comb can be generated by modulating the amplitude and/or phase of a continuous-wave laser with an external modulator driven by a radio-frequency source. In this manner, the frequency comb is centered around the optical frequency provided by the continuous-wave laser and the modulation frequency or repetition rate is given by the external radio-frequency source. The advantage of this method is that it can reach much higher repetition rates (>10 GHz) than with mode-locked lasers and the two degrees of freedom of the comb can be set independently. The number of lines is lower than with a mode-locked laser (typically a few tens), but the bandwidth can be significantly broadened with nonlinear fibers. This type of optical frequency comb is usually called electrooptic frequency comb. The first schemes used a phase modulator inside an integrated Fabry–Perot cavity, but with advances in electro-optic modulators new arrangements are possible.