High-frequency acousto-optic modulator with acoustic mode conversion

An acousto-optic modulator based on a TeO₂ crystal for the frequency range 200–570 MHz with acoustic mode conversion upon reflection from the oblique face of the acousto-optic cell is fabricated and experimentally studied. The modulator operates with a collimated light beam ∼0.8 mm in diameter, and no beam focusing is required. For a sequence of control signal pulses at f > 250 MHz, a sharp (several times) reversible drop in the diffraction efficiency is observed within T₁ = 3 – 20 μs after the onset of diffraction. The fall time front is shorter than 0.5 μs. The value of T₁ depends on the control signal parameters. The upper limits of the frequency range of the modulator in the continuous regime and in the regime of a pulse train of the control signal are determined at 360 and 570 MHz, respectively. The attenuation coefficients of the transverse slow acoustic mode in TeO₂ along the direction of its wave vector, which has an angle of inclination with the [110] axis of 16.80°, are measured: σ(360 MHz) = 1.8 ± 0.4 Np·cm^-1, and σ(570 MHz) = 5.5 ± 0.8 Np·cm^-1. The exponent in the frequency dependence σ(f) ∼ fⁿ is estimated to be 2.4 ± 0.4. Possibilities and features of the TeO₂ backscattering experiment are discussed based on the results obtained.