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The relations are established which allow the calculation of double-refraction walk-off angle for type I and type II processes of second harmonic generation. As an example these angles are calculated numerically in the case of second harmonic generation of 1064 nm from a Nd:YAG laser under phase-matched conditions for eight orthorhombic non-linear crystals: iodic acid, ammonium oxalate, lithium formate, potassium titanyl phosphate, calcium tartrate tetrahydrate, lithium gallium oxide, barium sodium niobate and potassium niobate.

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The infrared reflectivity spectra polarized along the c-axis of potassium dihydrogen phosphate are studied for both, the ferro-and paraelectric phase, over a wide temperature range: 10 t o 420 K.The reflectivity spectra are fitted to the factorized form of the dielectric function. The temperature dependence of the optical mode parameters: frequencies, damping, and oscillator strengths are reported for both the phases. The analysis shows that besides the soft mode, a relaxational process of the very low-frequency region plays an important role near the phase-transition temperature.Nous avons 6tudiB les spectres de reflexion dans I'infrarouge lointain polarises suivant l'axe c du phosphate diacide de potassium, dansles phases ferro-et paraelectrique, sur un vaste domaine de temphrature: 10 B 420 K. Les spectres de reflexion sont ajustes B I'aide de la forme factorisbe de la fonction diklectrique. La variation des p3ramhtres des modes optiques avec la temperature: frkquences, amortissements et forces d'oscillateur, est Btudiee dsns les deux phases. L'analyse que nous prbsentons ici montre qu'en plus du mode mou, u n processus de type relaxation, joue un rale important au voisinage de la temperature de transition de phase.

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