Wavelength-tunable and 180 nm-bandwidth second-order nonlinear frequency conversions in all-fiber system

Achieving broadband second-order nonlinear processes in a fully integrated fiber platform has long been a challenge, as silica fibers intrinsically lack second-order nonlinear susceptibility due to their centrosymmetric and amorphous structure. Here, we introduce an all-fiber strategy that overcomes this limitation by integrating microfibers with few-layer gallium selenide crystals, enabling controlled and broadband optical frequency mixing. We reveal the critical role of time-domain synchronization in sum-frequency generation (SFG) and demonstrate multi-frequency mixing from four continuous-wave sources, producing ten converted wavelengths through simultaneous second-harmonic generation (SHG) and SFG. Remarkably, the system operates at low excitation thresholds, supporting broadband SFG with two superluminescent diode sources and yielding an unprecedented SHG continuum extending to ~180 nm with a supercontinuum source. Furthermore, the broadband SFG spectrum can be tuned over 70 nm by coupling with a quasi-monochromatic laser. This approach establishes a scalable and versatile platform for regulating the wavelength and bandwidth of nonlinear processes in optical fibers, opening pathways toward tunable broadband light sources and advanced all-fiber photonic technologies.