Sum-frequency generation as a detector of high-power two-mode squeezing

We introduce sum-frequency generation (SFG) as a physical detector for high-power two-mode squeezed coherent states with arbitrary frequency separation, as produced by parametric oscillators above threshold. Using a formalism of collective modes, we describe both two-mode squeezing and degenerate squeezing on equal footing and derive simple relations between the input degree of squeezing and the measured SFG quadrature noise. We show that the SFG quadratures provide a direct measure of the input squeezing and discuss the sensitivity limits of the proposed method.