Sub-100-fs pulses at 1030 nm with THz repetition rate using dual-frequency beat compression in photonic crystal fiber

We demonstrate the generation of femtosecond pulse trains at 1030 nm with repetition rates tunable from 330 GHz to 1 THz in a fully polarization-maintaining (PM) fiber system. Two detuned single-frequency lasers create a dual-frequency optical beat signal that undergoes strong nonlinear evolution in a PM photonic crystal fiber (PM-PCF) pumped in the anomalous dispersion regime, producing pulses with durations ranging from about 120 fs down to 60 fs without external post-compression. To deliver the required peak power, the dual-frequency signal is carved into 1-ns-long bursts at a repetition rate of 1 MHz. The nonlinear propagation is well described by breather theory. At higher input powers, the breathers evolve into Raman-shifted fundamental solitons, whose self-frequency shift and noise-driven dynamics are confirmed experimentally and numerically. These results highlight the versatility of the dual-pump scheme and the rich nonlinear dynamics of PM-PCFs, establishing a route toward compact sources of tunable, ultra-high-repetition-rate short pulses.