A new self-collimating shear acoustic mode in TeO2 for large time-bandwidth multichannel Bragg cell applications

Self-collimating modes in acousto-optics utilize the anisotropy of the crystalline AO material to create collimated acoustic beams. These modes are characterized by an acoustic inverse velocity surface with curvature C=|1-2b|=0, where b is the acoustic anisotropy coefficient. Self-collimating modes are particularly useful in multichannel Bragg cell devices where it is desirable to have a high packing density of channels to utilize the full two-dimensional capability of optical signal processing. Slow shear mode propagation in Tellurium Dioxide (TeO ₂) in the [110] direction is particularly attractive for narrow bandwidth, large time-aperture AO Bragg cell devices because of its anomalously slow acoustic velocity. Acoustic diffraction spreading in this mode, however, is rather large (C=53.8). We have examined other directions of shear mode propagation in the [100]/[010] plane of TeO₂. We have found the curvature is a minimum (C=0.36) for propagation 14.6° from the [100] direction. The acoustic velocity, while not as low as that in the [110] direction, is still relatively slow at 2.31 mm/μs. The acoustic attenuation is -4.0 dB/μs/GHz2. The mode is characterized by a rather large energy walk-off angle of 57°. We have fabricated a four-channel AO Bragg cell using this orientation. The transducers have a height of 0.3 mm with a center spacing of 1.1 mm. Schlieren images were obtained at a frequency of 170 MHz. Experimental results are in good agreement with theoretical calculations. The effect of the large energy walk-off for optical signal processing applications is discussed