The plane-wave dynamics of 3*omega => (2*omega, omega) subharmonic optical parametric oscillators containing a second harmonic generator of the idler wave omega is analyzed analytically by using the meanfield approximation and numerically by taking into account the field propagation inside the media. The resonant Chi(2):Chi(2) cascaded second-order nonlinearities induce a mutual injection-locking of the signal and idler waves that leads to coherent self phase-locking of the pump and subharmonic

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... waves, freezing the phase diffusion noise. In case of signal-and-idler resonant devices, largely detuned sub-threshold states occur due to a subcritical bifurcation, broadening out the self-locking frequency range to a few cavity linewidths.