Triangular spectral phase tailoring for the generation of high-quality picosecond pulse trains

The generation of high quality pulse trains at repetition rates of several tens of GHz remains a crucial step for optical telecommunications, optical sampling or component testing applications. Unfortunately, the current bandwidth limitations of optoelectronic devices do not allow the direct generation of well-defined optical pulse trains with low duty cycles. A linear solution is based on a direct temporal phase modulation that is then converted into an intensity modulation thanks to a dispersive element. However, this approach suffers from a limited extinction ratio or from the presence of detrimental temporal sidelobes. We introduce here theoretically and experimentally an alternative scheme where the quadratic spectral phase is replaced by a triangular one. With such a specific phase processing, Fourier-transform limited structures are obtained, with properties that may present some similarities with Akhmediev breathers at the point of maximum focusing. Experimental validation carried at repetition rates between 10 and 40 GHz confirm that high-quality close-to-Gaussian pulse trains can be achieved with an excellent extinction ratio and with a duty cycle below 1/4, in full agreement with our numerical simulations and analytical predictions. The resulting pulse train exhibits a remarkable stability. The proposed approach can be extended to process several wavelengths simultaneously as demonstrated by the experimental generation of 4 interleaved pulse trains in the conventional C-band. The versatility of the proposed scheme also enables the generation of pulse trains with varying pulse-to-pulse delays or durations.