In a double two-wave mixing arrangement a signal wave couples with two different pump waves in a photorefractive Bi_(12)TiO_(20) crystal with applied electric DC field. Both pump waves fall into the crystal at different locations (or at different times at the same location). A phase shift from steady state in the signal wave leads in both cases to different mixed signal intensities. Based on this, phase shifts of the incident signal wave can be measured with a resolution down to #lambda#/50 (wavelength #lambda#=663 nm) and the sign of the phase shift can be determined.
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