Oxide crystals for electro-optic Q-switching of lasers

A new generation of oxide crystals is emerging for electro-optic Q-switching or control of high-power pulsed lasers. Unlike the acousto-optic Q-switches in which the total turn-off time is limited by the duration of sound wave propagation (110-220 ns/mm) across the beam diameter, the electro-optic devices provide a short (< 10 ns) response needed for minimum losses. Extinction ratios of better than 100 : I for electro-optic crystals ensure their reliable hold-off. By contrast, acousto-optic devices are characterized by single-pass dynamic losses of approximately 40%, which hinders their use in high-gain lasers. The basic principles of electro-optic Pockels cells are discussed. The performance characteristics of Q-switching for traditional electro-optic materials [deuterated potassium dihydrogen phosphate (DKDP), lithium niobate (LNB)] and other new electro-optic crystals, such as barium metaborate (BBO) and crystals belonging to the langasite (LGS) and potassium titanyl phosphate (KTP) families, are reviewed comparatively. Particular emphasis is placed on KTP-type electro-optic crystals, primarily on rubidium titanyl phosphate RbTiOPO4 (RTP), which stand out in their ability to provide Q-switching at extremely high frequencies or repetition rates up to 200 kHz. The use of both X- and Y-oriented double crystals as Q-switches in order to combine large electro-optic coefficients and low quarter-wave hold-off voltages with excellent thermal stability of the device is considered.