Sub-THz Optical Frequency Comb Generation by Efficient Repetition-Rate Multiplication of a 250-MHz Mode-locked Laser

We demonstrate pulse repetition-rate multiplication (RRM) of a low-rate (sub-GHz) commercial mode-locked laser (MLL) source, and show the potential of the method for optical frequency comb (OFC) generation with ultra-broad frequency spacings. Experimentally, MLL pulses at a 250-MHz repetition rate are sent to a dispersion compensating fiber where pulse RRM is produced through temporal self-imaging, achieving record-high multiplication factors (up to 1540). The rate multiplied pulses are used as a pump signal to modulate the phase of a continuous-wave light through cross-phase modulation (XPM) in a highly nonlinear fiber, leading to the generation of OFCs with ultra-broad frequency spacings (up to 385 GHz). Furthermore, a detailed numerical analysis of the demonstrated scheme is also carried out, involving simulations of the RRM and XPM processes. Through this analysis, the effects of residual dispersions and pump power are investigated towards optimization of the performance of the OFC generation mechanism. The demonstrated approach is efficient in that it fundamentally preserves the input pulses' energy, it offers an important degree of versatility, e.g., in regards to the central optical wavelength and frequency spacing of the produced combs, and it requires no high-speed electronics.