Oblique-cut longitudinal electrooptic modulators

The minimum longitudinal half-wave voltages are calculated for five oxygen-octahedral crystals with large linear electrooptic coefficients. The predicted values range from 11 kV (LiTaO3) to 75 V (BaTiO3). Experimental results have been obtained for LiNbO3and LiTaO3. The minima occur in directions with finite birefringence; the resulting limitations in angular aperture and temperture sensitivity are considered, and a compensation scheme to off-set them is discussed. The limiting operating frequency for uncompensated modulators can stem from joule heating in the transparent electrodes. The effect is evaluated and a figure of merit is derived. The values are surprisingly similar for the five crystals considered.