Nearly-degenerate twin signals generated by optical parametric oscillator in inverted KTA twin crystals

Consider an optical parametric oscillator (OPO) in a single (KTiOPO4) KTP crystal where type-II non-critical phase-matching condition based on birefringence is satisfied. A pair of the signal and idler is generated. It was demonstrated previously [1] that each signal is split into the twin signals whereas each idler is split into the twin idlers in periodically-inverted KTP stack composite. The wavelengths of the twin signals and twin idlers are dictated by the quasi-phase-matching (QPM) condition: Δkl = ±π. The twin signals or twin idlers were used for the efficient THz generation [2], with the THz wave exhibiting the significantly-reduced quantum noise [3]. Moreover, they can be used to restore the blurred images caused by the phase distortion in the beam path [4]. Furthermore, through periodic inversion of the crystal plates, the spatial walk-off effect for extraordinary waves can be mitigated, resulting in the significant reduction of OPO threshold and significant increase in the OPO conversion efficiency [1].