Generation of multi-octave-spanning laser harmonics by cascaded quasi-phase matching in a monolithic ferroelectric crystal

Abstract

We report the generation of the second to the sixth harmonics of a fundamental frequency covering two and a half octaves in a multiply-periodically poled lithium tantalate crystal by cascaded quasi-phase-matched frequency mixing. A frequency comb that is composed of these harmonics will permit the synthesis of a train of periodic subcycle subfemtosecond pulses in a compact setting. , by mixing pulses from a mode-locked laser with pulses from a synchronously pumped optical parametric oscillator (OPO) where m is the comb frequency spacing, 0 is a static offset phase, and m is the phase difference between adjacent comb components; q is an integer with value from a few for the Raman approach to tens of thousands for the mode-locked lasers approaches. Equation A common approach to generating laser harmonics is phase-matched harmonic generation and mixing using birefringent crystals. Owing to the nature of phase matching the harmonics are phase coherent relative to each other. However, birefringent phase matching has beam walk-offs, and polarization correction is required to generate many harmonics so that the process is quite complicated. There would be phase fluctuations when the harmonics are split and then recombined during generation. Thus using birefringent phase matching will require elaborate phase locking and stabilization to maintain phase coherence among the harmonics [1]. Quasi-phase-matched (QPM) nonlinear mixing is another efficient approach to generating laser harmonics. Two-stage cascaded parametric generation and generation to the third harmonic in a single QPM crystal have been demonstrate