Coherent oscillation is achieved in the geometry of a double-phase conjugate mirror in nominally undoped ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ with $647\mathrm{nm}$ radiation of a ${\mathrm{Kr}}^{+}$ laser. The specific temporal dynamics of oscillation with modulated intensity and periodic $0\text{–}\pi \text{–}0\text{–}\pi$ variations of the oscillation wave phase is similar to that observed earlier using a semilinear oscillator geometry. The described oscillator ensures submillisecond delay time in the appearance of the phase conjugate wave.