Low loss polycrystalline SiGe core fibers for nonlinear photonics

Polycrystalline silicon-germanium (SiGe) core fibers offer great potential as flexible platforms for microscale optoelectronic and nonlinear optical devices. Compared to silicon (Si) core fibers, the SiGe material provides the potential for higher nonlinear coefficients, extended mid-infrared wavelength coverage, and a means to tune the bandgap and index of refraction by varying the Ge composition. Here, SiGe core fibers (10 at% Ge) were fabricated using the molten core drawing method, followed by CO₂ laser irradiation to improve the homogeneity of the core. The transmission properties of the fibers were further optimized using a fiber tapering method to tailor the core diameter and re-grow the crystal grains. The resulting tapered SiGe fiber exhibited an average linear loss of ∼3 dB cm^-1 across the wavelength range 1.5 - 2.5 µm, allowing for nonlinear optical characterization of this new fiber type. Measurements of the nonlinear figure of merit demonstrate the potential for higher nonlinear performance compared to the pure Si core fibers, particularly for wavelengths >2 µm, indicating that the SiGe fiber platform could open up new opportunities for mid-infrared nonlinear photonics.