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Fig. 1. Typical ADP crystal.
using cells of this type. One important difficulty deals with liquid purity and manifests itself because many of the materials which exhibit a high Kerr effect are deteriorated by the electric field required to produce the effect. It therefore becomes necessary to constantly pump new liquid through the cell to keep it pure, an obvious disadvantage.
Continuing with materials which exhibit optical retardation when placed in an electric field, we come to a group of solids studied for the most part by Pockels, and the effect is known as the Pockels effect.2 Examples of some of these materials are quartz, sodium chloride and zinc sulfide crystals. These materials are stable solids and form a suitable group on which light modulation studies can be built. Almost every early experimenter who has worked with the Pockels effect cites the zinc sulfide crystal as the one with the ideal property for light modulation. To the present time, unfortunately, no zinc sulfide crystals have been synthesized, and pure crystals of usable size are rarely found in nature. So, while the zinc sulfide crystal is the most desirable, it nonetheless becomes necessary to study other crystals, particularly the types which can be easily synthesized in the laboratory. Two such crystals are ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (KDP). The ADP crystal has many commercial uses
Fig. 2. Application of voltage to crystal structure.
and is comparatively easy to grow. KDP crystals take longer to grow and do not have as many commercial applications, although it will be shown that they do have certain advantages over the ADP for sound modulator applications.
Having stated an intention to use a crystal to produce a desired effect, it is necessary to state which section of the complete crystal will be used, since crystals may have different optical properties for different sections through them. The section of the ADP crystal which displays the property we wish to study is a basal section, or a "Z" cut as it is termed. Figure 1 shows a complete crystal of ADP, and the dotted portion shows the basal section. Another consideration necessary to produce the desired effect is the application of the electric field to the crystal section chosen, or how the crystal shall be submerged in the electric field. For zinc sulfide, the electric field is applied perpendicular to the light beam passing through the crystals.
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March 1953 Journal of the SMPTE Vol. 60