Orthogonal shear acoustic mode suppression in gallium phosphide acousto-optic Bragg cells

Abstract

The fast shear acoustic modc in Gallium Phosphide, with acoustic propagation in the [ I .-1.01 direction and pcrlari7ation in the [0.0,1 J direction, is particularly attractive for acousto-optic applications because of its anisotropic acoustic "self-collimating" beam property. The orthogonally polari7cd slow shear mode, however, has a similar acoustic velocity (3.09 vs. 4.13 m m / p for the self-collimating mode) and figurc of merit, M2, (4.3 vs. 16.5 for the self-collimating modc). Excitation of this orthogonal mode will lead to unwanted acousto-optic diffraction and subsequent reduction in h e spurious free dynamic range of the acousto-optic device. Both transduccr selection and orientation with respect to the acousto-optic crystal are critical in suppressing the unwanted slow shear mode. An optimum Lithium Niobate transducer cut is described and thc slow shear mode suppression is calculatcd as a function of trancducer misorientation. An alternative optical propagation direction is also examined where the coupling to the slow shear mode vanishes at thc expense of increased accruetic hcam divcrgcnce. The negative phase front curvature of this modc is discussed.

<title>Wideband multichannel acousto-optic spectrometer for millimeter and submillimeter wavelength radio astronomy</title>

“…Bragg cells with center frequencies as high as 4 GHz have been produced, albeit with a very short (<100 us) time aperture for military signal processing applications. Additional factors to be considered in comparing the performance of these two cells is the known ghost mode generation problem in GaP [9] and the low thermal conductivity in LiNbO3 (an order of magnitude lower than GaP). The optimum acousto-optic material and mode must now be found to achieve the device performance we seek.…”

<title>Multichannel acousto-optic spectrometer</title>

“…ilL MULTICHANNEL BRAGG CELL The 4-channel Bragg cell uses the "sell-collimating" shear mode in Gallium Phosphide [4,5]. Each channel of the cell has a 3 dB bandwidth of 1 GHz, a nominal time-aperture of 1 ps, and a diffraction efficiency greater than 3%/W.…”