As a means of modulating lasers, acousto-optical cells are a relatively simple and generally satisfactory solution. When used in the zero-order mode to modulate the undiffracted light, they do suffer from the disadvantage that the maximum contrast between the on and off states is typically 10 to 1. This cannot be improved by increasing cell length because it orginates from rediffraction of once-diffracted light. By spatial filtering the light between passes through multiple cells, or alternatively, multiple passes through a single cell, this contrast can be improved by removing the once-diffracted orders before they can be rediffracted into the zero-order beam. Experimentally we have demonstrated an improvement of a factor of 10 in contrast by using two cells, each of which by itself produces only a 10 to 1 contrast. In addition to using filters consisting of stops and apertures, it is possible to achieve similar results with polarization filters. Multiple applications of this filtering improve contrast as the power of the contrast of a given step, thus contrast of modulation in zero order can be improved to any desired degree.
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