Demonstrating the Pockels effect in a conoscopic beam path

The experiment P5.4.5.1 demonstrates the Pockels effect in a lithium niobate crystal placed in a conoscopic beam path. The crystal is illuminated with a divergent, linearly polarized light beam, and the transmitted light is viewed behind a perpendicular analyzer. The optical axis of the crystal, which is birefringent even when no electric field is applied, is parallel to the incident and exit surfaces; as a result, the interference pattern consists of two sets of hyperbolas which are rotated 90° with respect to each other. The bright lines of the interference pattern are due to light rays for which the difference Δ between the optical paths of the extraordinary and ordinary rays is an integral multiple of the wavelength λ. The Pockels effect alters the difference of the main refractive indices, n_o - n_e, and consequently the position of the interference lines. When the so-called half-wave voltage U_λ is applied, Δ changes by one half wavelength. The dark interference lines move to the position of the bright lines, and vice versa. The process is repeated each time the voltage is increased by U_λ.