Application Cases

Experiment Name: Photoelectric Measurement of Ultrasonic Sound Intensity in Liquids

Research Direction: Photoelectric Measurement

Experimental Objective:
Sound intensity is a fundamental physical parameter for describing acoustic fields, and ultrasonic effects are directly related to sound intensity. For example, in engineering and technological applications, the distribution of the acoustic field in liquids directly influences flow field distribution, while the magnitude of sound intensity affects the efficiency of ultrasonic cleaning, atomization, and emulsification. Sound intensity is also used to characterize the radiation performance of ultrasonic transducers and acoustic artificial structures. Common methods for measuring sound intensity in liquids include calorimetry, radiation pressure, and optical techniques. Optical methods for measuring sound intensity in transparent liquid media rely on the acousto-optic effect. When ultrasonic waves propagate through a medium, the medium alternates between compression and rarefaction, causing periodic changes in its optical refractive index. The acousto-optic effect causes the ultrasonic field to behave like a phase grating with a spacing equal to the acoustic wavelength. When light waves pass through such a medium, diffraction occurs.

Experimental Equipment: Signal generator, ATA-8202 RF power amplifier, ultrasonic transducer, host computer, desktop computer.

Figure: Observation of Far-Field Diffraction Light Field from Ultrasonic Grating

Experimental Procedure:

Figure: Experimental Setup for Photoelectric Measurement of Acoustic Field in Liquids

The experimental setup for photoelectric measurement of the acoustic field in liquids is shown in the figure above. It consists of two channels:

1. Acoustic Channel: Composed of a signal generator, a power amplifier (Aigtek, model ATA-8202), and an ultrasonic transducer (30 mm diameter, collimated beam). The signal generator produces a high-frequency sinusoidal signal, which is amplified by the power amplifier to drive the ultrasonic transducer, emitting ultrasonic waves.

2. Optical Channel: Composed of a laser (He-Ne laser, single-mode output, wavelength 632.8 nm, beam diameter ≤ 1 mm), a lens group, a liquid tank with sound-absorbing material at the bottom, and a CCD camera. The laser emits a Gaussian beam, which is expanded by lens 1 (focal length 50 mm) and lens 2 (focal length 400 mm) before illuminating the liquid tank. The tank is filled with tap water that has been left undisturbed for over 24 hours (to eliminate air bubbles). Under ultrasonic excitation, an ultrasonic grating is formed. The light exiting the tank forms a far-field diffraction pattern on the back focal plane of lens 3 (focal length 800 mm). A cylindrical lens (focal length 50 mm) spreads the diffraction spots into horizontal lines for easier observation. The photosensitive surface of the CCD camera is positioned at the back focal plane of lens 3. The CCD camera is connected to a computer for program-controlled acquisition of diffraction field images.

Experimental Results:

1. Extraction of Characteristic Parameters from Far-Field Images Without Acoustic Field:
   The far-field image formed by passing a collimated laser beam through the liquid tank without an acoustic field, lens 3, and the cylindrical lens is converted into a two-dimensional grayscale data matrix. From the grayscale matrix, the 618th row (middle row) is extracted for Gaussian fitting to determine the background grayscale, peak intensity, center position, and width of the light spot. Additionally, the 309th and 927th rows are extracted for Gaussian fitting to determine their respective spot center positions, and the tilt angle of the fringes relative to the vertical direction of the image is calculated.

2. Extraction of Characteristic Parameters of Diffraction Fringes at Different Orders:
   The finite size of the incident beam diameter leads to overlapping in the distribution of diffraction light fields at different orders. A larger beam diameter reduces this overlap. In the experiment, a lens group is used to expand the incident beam to minimize overlap. The following discussion ignores the overlap between diffraction light fields of different orders. First, the middle row of the diffraction image grayscale matrix is extracted. Discrete Fourier Transform (DFT) is applied to this row to filter out high-frequency components. Inverse Discrete Fourier Transform (IDFT) is then performed to obtain a smoothed curve, and the peak positions are identified. Finally, Gaussian fitting is applied near each peak position to determine the peak intensity and center position of the diffraction spots at different orders.

3. Normalization of Peak Intensities at Different Orders in Diffraction Images:
   Using the center position of the far-field light spot without an acoustic field as a reference and the minimum spacing between diffraction spots as a guide, the diffraction orders of the spots are determined. To eliminate the influence of experimental parameters on sound intensity measurements, the peak intensities at different orders in the ultrasonic grating diffraction images are normalized relative to the peak intensity of the far-field light spot without an acoustic field.

   
   

Figure: ATA-8000 Series RF Power Amplifier Specifications and Parameters