Quantum spatial dynamics of high-gain parametric down-conversion accompanied by cascaded up-conversion
Quantum cascaded up-conversion (CUpC) of parametric down conversion (PDC) in a finite nonlinear $χ^{(2)}$-crystal is studied theoretically within parametric approximation. The exact solution for creation and annihilation operators presented in the form of Bogoliubov transformation is valid for the high-gain regime and explicitly includes the non-zero wavevector-mismatch both for the PDC and CUpC. With the use of characteristic equation parametric amplification and oscillating regimes are analysed for degenerate, three- and four-mode cases. We show that the parametric amplification exists under the fulfilment of the cascaded phase-matching conditions while both the PDC and CUpC processes are separately non-phase-matched. The influence of CUpC on quadrature squeezing of degenerate PDC is estimated.