Extending the frequency range of the National Physical Laboratory primary standard laser interferometer for hydrophone calibrations to 80 MHz

Abstract

A method for the primary calibration of hydrophones in the frequency range up to 60 MHz is described. The current National Physical Laboratory (NPL) primary standard method of calibrating ultrasonic hydrophones from 500 kHz to 20 MHz is based on optical interferometry. The acoustic field produced by a transducer is detected by an acoustically transparent but optically reflecting pellicle. Optical interferometric measurements of pellicle displacement at discrete frequencies in tone-burst fields are converted to acoustic pressure, and the hydrophone for calibration is substituted at the same point, allowing sensitivity in volts per pascal to be obtained directly. For calibrations up to 60 MHz, the interferometer is capable of measuring the displacement of the pellicle as a function of frequency in a harmonically rich nonlinear field up to and including the 12th harmonic of the shocked field generated by a 5 MHz focusing transducer, allowing hydrophones to be calibrated by substitution in the same field. Sources of uncertainty in the new method have been investigated. Best combined random and systematic uncertainties at the 95% confidence level for the new method are 7% at 20 MHz, 11% at 40 MHz, and 16% at 60 MHz.

Acousto-optic effect compensation for optical determination of the normal velocity distribution associated with acoustic transducer radiation

“…Since the field determination is absolute, the passive device to be calibrated can be placed in the plane where the normal particle velocity, or displacement, distribution is known, and oriented toward the source transducer, enabling direct measurement of its receiving characteristics. 8,[31][32][33] The main optical methods for measuring the normal velocity distribution on the surface of a radiating transducer or the acoustic particle velocity distribution in the field are based on laser interferometry. Historically, the first of these used an optical Michelson interferometer to detect and measure the displacement of a pellicle, if not that of the transducer surface itself.…”

“…34 It became the basis of a primary calibration method for high-frequency hydrophones. 8,35,36 A second method uses a laser Doppler vibrometer to detect and measure the velocity of a pellicle or transducer surface. 9,33,37 These two interferometric methods involve phase and frequency modulation.…”

“…Applications include acousto-optic modulators (AOMs), 3 e.g., Bragg cells; 2,4 anemometry in air and water; 5,6 calibration of acoustical devices used in medicine 7 apropos of radiation dosage and power delivery, e.g., for diagnostics, treatment by acoustically induced hyperthermia, and lithotripsy; [8][9][10] Schlieren visualization of ultrasonic fields, 11,12 e.g., radiation or scattering of sound; 13,14 and vibrometry to detect and quantify mechanical vibrations, e.g., for materials characterization, 15 non-destructive testing and evaluation, 16,17 and visualization of patterns of surface vibration; 6,18 among other things. There is some question about the influence of the acousto-optic effect when the aim is direct optical measurement of (i) vibrations of a radiating transducer surface, or (ii) acoustically induced vibrations in a very thin, optically reflective, acoustically transparent membrane, called a pellicle, suspended in the acoustic field of the radiating transducer.…”

“…A pellicle was assumed in the pioneering study by Mezrich et al 34 It was used in development of standard calibration methods for hydrophones and transducers, both by homodyne laser interferometry for displacement measurement 8,31 and by heterodyne laser interferometry for velocity measurement. 9,33,39 A number of effects associated with pellicles in the form of a circular membrane have been noted and addressed in the literature.…”

Absolute Hydrophone Calibration to 40 MHz Using Ultrasonic Far-Field

“…3,4) The ultrasonic attenuation increases from 0.8 dB/cm at 20 MHz up to 3.2 dB/cm at 40 MHz. The propagation distance necessary to form an ultrasonic far-field increases in proportion with the ultrasound frequency.…”