Dependence of the Contrast Ratio on Crystal Thickness in an Electroabsorptive Spatial Light Modulator That Uses GaAs

The contrast ratio of an optically addressed spatial light modulator that uses electroabsorption in a GaAs single crystal is discussed experimentally and theoretically. The modulator has the same structure as a Pockels readout optical modulator. The contrast ratio depends strongly on the change in the absorption coefficient and on the thickness of the GaAs crystal. From the experimental results and from theoretical investigations of the Franz-Keldysh effect, the change in the absorption coefficient is estimated by use of the quadratic equation of an applied electric field that is not excessively strong. Under this condition, an optimum thickness of the GaAs crystal plate that will yield the maximum contrast ratio can be determined.