Optically programable quasi phase matching in four-wave mixing.

Quasi-phase matching (QPM) enhances nonlinear optical processes by compensating for phase mismatch, but traditional methods require permanent material modifications, limiting applicability in centrosymmetric media like standard optical fibers. We introduce the first efficient, optically controlled QPM in perturbative nonlinear optics, achieved through temporal modulation of counter-propagating pump waves. This induces a dynamic spatial modulation of nonlinear polarization in a polarization-maintaining fiber, enabling spatiotemporal QPM for four-wave mixing without altering the medium. We demonstrate broadband wavelength conversion across 298 nm-including the C- and L-bands of optical telecommunications-with a conversion efficiency of 5.4%. Our results also show tunable spectral shaping and wavelength agility through simple control of the pump waves. This reconfigurable, all-optical technique not only overcomes limitations of conventional QPM but also opens new possibilities for adaptable nonlinear optics. Potential applications span classical data processing, fiber sensing, quantum state control, and robust frequency conversion in dynamically programmable photonic systems.