The temporal evolution of the diffraction efficiency of the photorefractive grating written by two coherent light beams in the two-wave mixing configuration in a nominally pure KNbO3 single crystal was measured at low and high modulation depths, without any applied electric field. It has been obtained from measurements performed in this work, a faster development of the diffraction efficiency at low modulation depth than the obtained at high modulation depth. A strong non-linear dependence of the diffraction efficiency of the photorefractive grating on the modulation depth in the steady state was obtained. The dependence of the diffraction efficiency on the modulation depth measured in this work was described correctly by theoretical relations from the band transport model assuming a correction factor that reduces the fringe contrast.
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