Discretely tunable comb spacing of a frequency comb by multilevel phase modulation of a periodic pulse train

We experimentally demonstrate tunable comb spacing of an original 10-GHz periodic frequency comb by spectral Talbot effect over an unprecedented range of even and odd comb spacing division factors, from 2 to 9. The implementation has been achieved by periodic electro-optic (EO) temporal phase modulation of the original comb (conventional mode-locked optical pulse train) with multilevel modulation functions, produced by an arbitrary waveform generator (AWG). These comb spacing division processes have been observed through the use of a high-resolution (20-MHz) optical spectrum analyzer. Comb spacing tuning is achieved without essentially affecting the spectral bandwidth and total energy of the original comb signal. Our results also confirm that the spectral Talbot method does not require carrier-envelope phase stabilization in the input frequency comb. Numerical studies on the impact of deviations in the applied phase modulation functions confirm the robustness of the technique, in agreement with the experimental results.