Spectrum regulation for mid-infrared ultrafast pulses via a time-synchronization aperiodically poled LiNbO₃.

Restricted to temporal separation during the coupled-waves interaction, aperiodically quasi-phase-matching (QPM) nonlinear crystals are primarily implemented for prechirped pulses, showing limited applications in ultrafast temporal scale. Under the proposed time-synchronization framework, pump and signal waves travel with identical group-velocity, which permits sustaining energy transfer in long aperiodically poled LiNbO₃ crystals (APPLN) even with ultrafast pulse duration. With the help of this structure, adiabatic frequency conversion shows extra advantages compared with the common cases, which enables lower stretching ratio and smoother gain spectrum. Focusing on the typical mid-infrared wavelength of ~3 μm, we numerically study the potential performance of APPLN with chirp-free ultrabroad interacting waves. In contrast to the spectral shift and conversion efficiency degradation presented by its traditional Type-0 QPM counterpart, the proposed design demonstrated impressive ability to obtain arbitrary spectrum via a simple femtosecond OPA/OPO. Peculiarly, the QPM chirp rate sign plays a significant role to the output spectrum, and a positive chirp rate is preferential in delivering a bandwidth-controllable spectrum. The proposed design provides a promising technical route to achieve spectrum manipulation in ultrafast temporal scale.