“Ghost” modes in shear mode gallium phosphide and tellurium dioxide Bragg cells

The fast shear [1,-1,0] acoustic mode in Gallium Phosphide (GaP) and the slow shear [110] acoustic mode in Tellurium Dioxide (TeO₂ ) are commonly used modes for acousto-optic Bragg cells because of their unique properties-the former because of its anisotropic acoustic “self-collimating” property which allows high channel packing density in multichannel devices and the latter because of its anomalously slow velocity which allows the fabrication of long time-aperture devices. When used in spectrum analyzer applications, these acousto-optic Bragg cells must exhibit high spurious-free dynamic range. In Bragg cells using these modes we have experimentally observed unwanted spurious optical diffraction secondary “ghost” modes that appear spatially separated from the main first-order optical diffraction. The spatial separation of these modes from the main first-order diffraction, as observed on a photodetector array in an acousto-optic spectrum analyzer arrangement, varies nonlinearly with input frequency and are typically only 15 to 25 dB lower in power than the first-order diffraction. These modes thus severely degrade the spurious-free dynamic range performance of an acousto-optic Bragg cell spectrum analyzer. In the case of GaP we attribute the ghost mode to optical diffraction from an unwanted orthogonally polarized shear acoustic mode excited in the cell. In the case of TeO₂ we attribute the ghost mode to optical diffraction caused by acousto-optic coupling to an unwanted elliptically polarized optical mode in the cell. Both theoretical and experimental data explaining these phenomena ate presented as well as recommendations for reducing and/or eliminating these deleterious ghost modes