High-contrast coherent upconversion of focused twin beams

Sum-frequency generation (SFG) driven by bright twin beams yields a coherent contribution from correlated signal–idler mode pairs together with an incoherent background. To explore conditions for high-contrast upconversion against the background, we systematically measure SFG from focused twin beams by scanning the SFG crystal position z and signal–idler delay τ. For a nearly single-Gaussian spatial mode, the incoherent background forms a single symmetric peak along the z-axis, whereas the coherent component peaks at a displaced position, enabling contrast enhancement by offsetting the SFG crystal from the incoherent peak position. With multiple spatial modes, the reduction in the coherent component at the incoherent peak position becomes more pronounced. We attribute this behavior mainly to Gouy-phase evolution during propagation. These results highlight the longitudinal position z as a key parameter for achieving high-contrast coherent upconversion of focused twin beams.