30-THz upconversion of an AlGaAs diode laser with AgGaS₂: Bridging the several-terahertz frequency gap in the near infrared

We report as much as 6-μW upconverted cw radiation at λ = 778 nm from the sum-frequency mixing of a 50-mW diode laser at λ = 842 nm and a 200-mW CO₂ laser at λ = 10.2 μm by use of a 15-mm-long silver thiogallate crystal. This nonlinear material provides a convenient connection between IR and visible/near-IR optical standards. We describe simple frequency chains based on this upconversion process that permit the absolute frequency measurement of many visible or near-IR possible standards at the 10^-12 accuracy level. Transposition of terahertz-scale frequency-difference measurements from the near IR to the 10-μm domain is also proposed.