Showing papers on "Acoustic interferometer published in 1986"

Laser Generation of Convergent Acoustic Waves and Applications to Materials Evaluation

Abstract: A converging-wave technique for laser generation and detection of focused acoustic waves is described together with a number of applications to the nondestructive testing and characterization of industrial materials. An annular-shaped wave is produced by irradiating the inspected material with a laser beam focused through a suitable combination of spherical and conical lenses. The ultrasonic pulse reaches a relatively high amplitude in the center of convergence, where it is probed by a noncontact interferometric optical sensor. Applications to the ultrasonic characterization of ceramic materials, coating thickness evaluation, detection of delaminations in layered materials as well as crack inspection are described.

Neutron interferometry in a gravity field

Abstract: Quantitative understanding of the effects of external forces in three-crystal neutron interferometry is achieved by realizing that the three-crystal system is in fact an eight-path interferometer and not the two-path interferometer of previous descriptions. The approach explains damping of the interferometer oscillations with increasing applied force, even for an ideal unbent crystal and a monochromatic beam.

Acousto-optic tunable filter with two acoustic channels

Abstract: An acousto-optic tunable filter configuration and a method for the operation thereof, whereby the tuning range is extended. A non-collinear acousto-optic tunable filter crystal has a first transducer bonded to one transducer face of the crystal and a single transducer bonded on the opposite crystal face which crystal face is cut so as to be parallel to the first transducer. The first transducer operates at a center frequency f1 and the second transducer operates at a center frequency f2. By launching acoustic waves having a center frequency f1 into the crystal from the crystal's first side acoustic waves propagate in a first direction such that the interaction of the acoustic wave with the light results in the absorption of a phonon. Acoustic waves having a center frequency f2 are launched into the crystal from the crystal's second side and propagate in a second direction which is anti-parallel to the first direction such that the interaction with the light by the second series of acoustic waves results in the stimulated emission of a phonon.

Optimal crystal cuts and propagation directions for excitation of acoustic waves in LiNbO3

Abstract: Acoustic power flow in a LiNbO3 single crystal was calculated to find the optimal crystal cutting orientation and the wave propagation direction for the efficient excitation of acoustic waves in the crystal. By solving the stiffened Christoffel equations and calculating the acoustic Poynting vectors with piezoelectric effect included, maximum longitudinal acoustic power flow in a 30° Z‐rotated, Y‐cut LiNbO3 crystal was found.

Atmospheric variable compensation method and system for use with automated inspection systems

Abstract: An acoustic inspection system includes acoustic transducers to transmit and receive continuous acoustic waves at a constant frequency. Changes in phase and/or amplitude of acoustic waves reflected at different times from a fixed reference target through an isothermal transmission medium are detected. The temperature of the acoustic transmission path is controlled in response to the detected phase changes to maintain the wavelength of acoustic waves constant independent of changes in humidity and barometric pressure.

Optical Measurements of Acoustic and Photoacoustic Effects

Abstract: Optical sensors for acoustic waves and photoThey are easy to scan and they can have acoustic effects are used because no mechanical contact needs to be made to the solid material being examined. very good definition. The major disadvantage is that they are extremely insensitive compared to piezoelectric detectors. In photacoustic applications, they are employed to detect modulation of t he surface temperature, surface displacement, and surface slope of a solid, as well as the temperature variation of transparent solids, liquids, and gases. A number of well-established optical detectors, their applications, and their limitations are described. Particular a ttention is given to a new type of Wollaston prism detector, used in combination with a semiconductor laser, which is very simple and exhibits very high sensitivity.

Acoustic imaging and the juxtaposition of waves

Abstract: Boundary conditions involving acoustic pressure waves and their normal derivatives are used to develop an acoustic imaging criterion. This criterion is actually a testing procedure which operates upon the sum of the incident and reflected acoustic waves associated with reflection from an unknown acoustic boundary. One of the outputs from this procedure is used to reconstruct the unknown acoustic reflector while a second output from this procedure estimates the acoustic wavespeed on the opposite (transmission) side of the boundary. The calculation of the reflected acoustic wave as a function of spatial position and temporal frequency (which is a prerequisite for applying the testing procedure) is analogous to the downward continuation calculation used in conventional migration of seismic data. After using some simple examples to demonstrate the concepts involved in this procedure, the method is applied to computer generated synthetic data.

A new ultrasonic interferometer for velocity measurement in liquids using optical diffraction

Abstract: A new ultrasonic interferometer for velocity measurement using optical diffraction and an optical heterodyne system is described. The velocity of pure water at 20 degrees C has been measured in the frequency range 0.5-5 MHz, and the velocity obtained increases with decreasing ultrasonic frequency. In order to clarify the behaviour, a new theory for the ultrasonic interferometer is given and the usefulness of the instrument is shown by using it to measure the velocity dispersion in benzene.

Effects of second sound on acoustic transmission at the solid-liquid 4He interface.

Abstract: We have calculated the reflection and transmission coefficients of acoustic waves propagating across a solid-liquid interface of 4He, using an extension of the treatment of Castaing and Nozieres in which we include pressure changes associated with second sound. These calculations account well for the experimental results obtained over the temperature range 0.83 K.:$ T .:$1.46 K and provide an alternative explanation to the one we offered previously for the lack of agreement between experiment and the theory of Castaing and Nozieres.

SAW RF Spectrum Analyzer: Detection Issues

Abstract: RF spectrum analysis using the diffraction of surface acoustic waves has been demonstrated [l]. The input RF signal drives a phased array of SAW interdigital transducers that act like a curved diffraction grating to focus and angularly disperse the acoustic waves with frequency. An array of output transducers sorts the signal spectrum into contiguous output frequency bands. An important application of this device is in EW receivers for fine frequency sorting. To be effective it must operate over a wide dynamic range, with high channel isolation, and high receiver sensitivity. Since the total number of channels in a typical EW application can be large, a simple design is important. This paper discusses the design of the output transducers and the detection circuitry, critical to providing high performance. We present performance results of an experimental device.

Interaction of second sound with acoustic waves in solids

Abstract: An expression has been derived for the collision operator for phonons in a solid, which is valid at very low temperatures. The set of coupled equations for the elastic deformation and the phonon density or second sound has been reduced to a simple tractable form and the dispersion equation for the coupled waves consisting of the acoustic modes and second sound has been derived. It is shown that only the longitudinal mode interacts with the second sound. It is also shown that as a result of the interaction with the second sound, the longitudinal velocity along the principal axis acquires a correction term that is proportional to bothγ 2 andT 4.

Sound Generation in a Thermoacoustic Microscope

Abstract: Using an exact, three-dimensional, Green's function solution to the thermoelastic equations in semi-infinite solid, we have calculated the relative generation efficiencies of thermal waves, longitudinal acoustic waves and transverse acoustic waves induced by a periodically modulated beam of light or particles. The angular distributions and strengths of energy transfer and momentum transfer contributions are compared. Scattering and mode-conversion effects are discussed. Theory "Thermoacoustic microscopes"' are devices which generate sound in solids by means of focused beams of light or particles (either electrons or ions). These beams may be either pulsed or periodically modulated and, loosely speaking, generate sound through the response of the solid to the energy and momentum they deposit in it. The detection of that sound is accomplished by means of an acoustic transducer somewhere on the surface of the solid, and the microscopic image is generated by recording the signal from the transducer as the beam is scanned over the region of interest. In this paper we present a detailed, three-dimensional theory for the generation of sound in such a microscope. The starting point for the calculation is the (linearized) set of equations2 which describe the conservation of energy and momentum in the solid,