Cavity-enhanced polarization-independent frequency conversion for vector beams

Vector beams with spatially varying polarization, represented on the higher-order Poincaré sphere, are indispensable in optical communications, imaging, and quantum information. However, nonlinear frequency conversion of vector beams remains challenging due to polarization sensitivity and limited conversion efficiency. Here, we report the first demonstration of an external-cavity-enhanced, polarization-independent frequency converter that performs efficient difference-frequency generation of vector beams. Benefiting from cavity enhancement within the Sagnac configuration, our converter achieves external quantum efficiencies of 43.85% for l=1 and 16.83% for l=2, confirming efficient continuous-wave vector beam frequency conversion. Spatial-Stokes measurements verify that the converted beams preserve their vectorial polarization structure. This work provides an efficient method for wavelength manipulation of vector beams, with broad applicability in structured-light control and nonlinear photonic interfaces.