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 approximately 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.