Optical Measurements of Ultrasonic Attenuation and Reflection Losses in Fused Silica

Abstract

The ultrasonic attenuation in fused silica was measured by the Bragg diffraction of a 6328-Å He-Ne laser beam; two complementary techniques were used to obtain room-temperature data over the frequency range 200–980 MHz. The frequency dependence of the attenuation is given by f1.97 and f1.98, for longitudinal and transverse waves, respectively. If an exact square-law dependence is assumed, the material loss at 25.0°C can be characterized by the constants B (longitudinal) = (6.1±0.2) × 10−18 dB ⋅ sec, and B (transverse) = (5.7±0.2) × 10−18 dB ⋅ sec, where A (dB/μsec) = B f2. The acoustic-wave reflection losses at the free surface and transducer-bond surface of the sample were evaluated. Although in the former case the loss was negligible, in the latter large reflection losses were observed, in spite of the low conversion efficiency of the transducer. A study of the angular distribution of the acoustic intensity for the generated and reflected beams showed that this loss was mainly dissipative, although a deformation of the plane wavefront was also a contributing factor.