Guiding and diffraction of PSAW

Guiding and diffraction of PSAW are investigated using three different models on the following three cuts: ST-cut quartz ⊥X, 32.8 deg. and 67 deg. YX-cuts in LiTaO₃ and LiNbO₃ respectively. The models are a full wave Green´s function model, a simplified model based on expansion of the excited wave field in a set of waveguide modes, with angular spectrum of waves treatment of the unresolved fields, and a diffraction calculation based only on the angular spectrum of waves. Wave fields from a narrow beam excitation in the guide are obtained from each of the three models on each of the cuts. For the quartz cut the close in near field is corrupted by shallow bulk waves. Otherwise the guided waves model matches the Green´s function results fairly well, whereas the angular spectrum of waves model deviates considerably at larger distances. Compared to the diffraction of normal SAW, the diffraction patterns for PSAW are more uniform due to the high attenuation of off axis plane waves. Especially on the LiNbO3-cut, this effect is strong. Low propagation losses for the on axis waves are ensured through the choice of cut angle. The parabolic diffraction length scaling factor b is found to be about -3.4 for STW on quartz, about -0.84 for the LiTaO3-cut, and about 0.125 for the LiNbO3-cut. A similar factor for wave losses is introduced. The parabolic approximation is shown to have only limited validity on the cuts