We propose a spatially chirped quasi-phase-matching (QPM) scheme that enables ultrabroadband second-harmonic-generation (SHG) by using a fan-out QPM grating to frequency-convert a spatially chirped fundamental wave. A “zero-dispersion” 4f system maps the spectral contents of ultrabroadband fundamental onto different spatial coordinates in the Fourier plane, where the fundamental is quasi-monochromatic locally in picosecond duration, fundamentally canceling high-order phase mismatch. A fan-out QPM grating characterized by a linear variation of the poling period along the transverse direction exactly supports the QPM of the spatially chirped beam. We theoretically demonstrate the SHG of an 810-nm, 12.1-fs pulse into a 405-nm, 10.2-fs pulse with a conversion efficiency of 77%.

We report an experimental generation of few-cycle pulses at 53 MHz repetition rate. Femtosecond pulses with pulse duration of 181 fs are firstly generated from an optical parametric oscillator (OPO). Then, the pulses are compressed to sub-three-cycle with a hybrid compressor composed of a commercial single-mode fiber and a pair of prisms, taking advantage of the tunability of the OPO and the numerical simulating of the nonlinear compression system. Our compressed optical pulses possess an ultrabroadband spectrum covering over 470 nm bandwidth (at -10dB), and the output intensity fluctuation of our system is less than 0.8%. These results show that our system can effectively generate few-cycle pulses at a repetition rate of tens of megahertz with excellent long-term stability, which could benefit future possible applications.