Nonlinear Longitudinal KTN Modulator.

The longitudinal quadratic electrooptic effect in the solid solution material KTa(x)Nb(1-x)O(3) (KTN) has been investigated. It is shown that the effect is optimum for the [110] direction. For this direction the effective longitudinal electrooptic coefficient for a stress-free crystal is given by g' = |g(12) - g(11) + g(44)|. With this effect half-wave voltages as low as 100 V can be achieved in thin plates (<200 microm). It is shown that the paraelectric and ferroelectric electrooptic effects in these plates can be described by using the Devonshire thermodynamic formalism. In particular the temperature variation of the half-wave voltage is derived and comparison with the experimentally observed variation shows this to be anomalously large. Expressions are also derived for the transfer function of a quadratic modulator in both the biased and unbiased states. These expressions are in good agreement with the experimental results. Finally, the limitations and applications of the longitudinal KTN modulator are described. It is concluded that power dissipation problems limit the operation of longitudinal KTN devices to frequencies less than 100 kHz.