Coherent optical oscillator with periodic zero-πphase modulation

The four-wave-mixing coherent optical oscillator is described, which is emitting light wave with regular alternation of its phase between two discrete values zero and $\pi$. Such a periodic self-modulation is a consequence of competition of photorefractive gratings formed by movable charge species of two different signs. The period of modulation depends on overthreshold coupling strength and on decay time of slow photorefractive grating. It falls to few seconds range for ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ crystals at ambient temperature but it can go down to microsecond range for photorefractive semiconductors.