The combination of phase, intensity, and contrast effects during electroopticmodulation is theoretically and exper- imentally investigated. One consequenceof this combination is the modification of the amplitude of thesingle-frequency signals which are commonly used as working points forelectrooptic mod- ulators and for the measurements of the electroopticcoefficients. Another consequence of direct intensity modulation is to shiftthe double-frequency points of the transfer function from the positions theynormally occupy at the intensity extrema. They can even make them disappear ifthe direct intensity modulation is stronger than the phase modulation. Suchphenomena are expected with any ferroelectric material in which a significantpart of the incident light is deflected or scattered by domain walls or grainboundaries. They can lead to considerable mistakes in the determination of theelectrooptic coefficients. Appropriate procedures to extract the differentcontributions are explained. Experimental results in rubidium hydrogen selenateare given, and consequences of the working of electrooptic modulators arediscussed.
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