R. Torguet

Acoustical image reconstruction in parallel‐processing analog electronic systems

Ultrafast cardiac‐valve ultrasonic tomography requires parallel multichannel processing of received echoes. In parallel processing the level of secondary ’’ghost’’ images due to spatial undersampling is much higher than in slower series processors which use a selective field insonification. The paper describes a 20‐channel moving‐focus parallel‐processing analog electronic system, which is realized in our laboratory. It is shown analytically that the secondary‐image level is reduced and remains unchanged when the receiver angular aperture (aperture relative to distance) is limited and kept constant during the whole observation time.

The infinite planar baffles problem in acoustic radiation and its experimental verification

In acoustics, the analysis of pressure distribution in a field of surface sources is most frequently performed using the assumption that the source constitutes a part of an infinite planar rigid baffle (Rayleigh’s approximation). However, in many practical cases of ultrasonic echography, assumption of a soft pressure‐release baffle (the Rayleigh‐Sommerfeld approximation) or of free‐field conditions (the Kirchhoff approximation) seemed to be better matched to real conditions. A theoretical survey of the planar baffles problem is given in this paper, and its practical aspects in acoustic source radiation are discussed. Some experiments, showing the influence of different boundary conditions onto radiation patterns, verify the theoretical predictions.

Anomalous behavior in the radiation pattern of piezoelectric transducers induced by parasitic Lamb wave generation

In order to improve ultrasonic image quality, transducers, similar to those currently used in nondestructive testing and medical imaging applications, have been studied extensively. Attention has been given to single isolated transducers, for which anomalous radiation patterns have been obtained, and to transducers integrated in an array structure, where additional problems occur due to coupling effects between nearest neighbors. An experimental study of these phenomena has made it possible in both cases to find the origin of the perturbations with respect to the theoretical situations which occur in the radiation pattern. Solutions for increasing these perturbations are also suggested.

Independent acoustooptic modulation of the two wavelengths of a bichromatic light beam.

When a single acoustooptic modulator is used to independently modulate the two nearly equal optical wavelengths of a bichromatic laser beam, each with a given acoustic frequency bandwidth, both the acoustic frequencies as well as the transducer width must be large. A general expression for this bandwidth, valid for both isotropic and anisotropic acoustooptic interactions and whatever the acoustic power and the bandwidth definition are, is derived. In uniaxial crystals, a special configuration of anisotropic interaction is shown to lead to a small acoustic frequency bandwidth with a reasonable transducer width at moderate central frequency. A practical 6-MHz bandpass modulator using the slow shear wave in paratellurite is designed, and experimental results for independent modulation of 0.5145- and 0.4880-μm wavelengths of an Ar laser are reported.

Optimization of a transmission acoustic microscope

The acoustic field focused by a spherical solid‐water interface has been computed and the results have been used to optimize the geometry of a transmission acoustic microscope. An experimental study has been performed, which enables the sampling of the focused acoustic field, as may be realized by using the Fourier‐transform‐operation concept for an ideal lens.

Optoacoustic testing of microsound devices

Acoustic surface wave (ASW) components have been tested by visualizing the acoustic field amplitude and phase distribution in real time in the 15–25‐MHz frequency range. The properties of some useful devices are reported here.

SUNTRACKER FOR ATMOSPHERIC REMOTE SENSING

A heliostat is designed and built to track the sun for optical remote sensing of the stratosphere from a balloon-borne pointed gondola. The tracking mechanism is controlled by two direct torque motors used to drive a single flat acquisition mirror. A horizontal turntable, rigidly attached to the azimuth drive, supports the elevation assembly. A position sensor receiving a small part of the solar beam reflected off the main acquisition mirror is used for fine servo control. Using a CCD camera prepointing of the acquisition mirror is achieved when the sun is in the field of view of the heliostat. This system is coupled with a high- resolution (0.02-cm-1) Fourier transform IR (FTIR) spectrometer to retrieve stratospheric trace species concentration profiles. The suntracker directs the solar radiation in a stable direction along the spectrometer optical axis. The pointing precision is 1 arcmin from a stratospheric gondola, which has static and dynamic angular excursions up to 6 deg. The heliostat coupled to the Limb Profile Monitor of the Atmosphere (LPMA) instrument performs successfully on several balloon flights. The description, ground tests, and balloon flight results of the suntracker are presented.

On a general treatment of acousto‐optic interactions in linear anisotropic crystals

In arbitrary anisotropic crystals exhibiting optical activity, the normal modes of light propagation are elliptically polarized. These modes are studied using a new gyration pseudotensor affecting the relative dielectric impermeability tensor, defined here for convenience. The propagation of light inside a first‐order acoustically perturbated medium is then considered. The equations are solved by using a normal‐mode expansion of the electric‐displacement vector, assuming a stationary regime of diffraction and slowly spatially varying amplitudes. A coupled first‐order partial derivative set is so obtained, which describes the acousto‐optic interactions. The case of acoustic plane bulk wave modulation is finally emphasized and all the classical approximate results, deduced from theories with more restricted areas of applications, are retrieved using suitable assumptions in our general equations.

Visualization of thick specimens using a reflection acoustic microscope

A reflection acoustic microscope may be used to image structural details under the apparent surface of a thick sample. To design such an apparatus, the geometrical parameters of the acoustic lenses must be carefully defined. The acoustic field distribution has been computed in the previous structure for this purpose. Some preliminary experimental results are also reported here.

Space-integrating acousto-optic processor for rapid Global Positioning System signal acquisition

A space-integrating acousto-optic processor conceived for Global Positioning System (GPS) signal acquisition is presented. The main advantage of the proposed system, as compared with traditional ones, is that only one parameter estimation, Doppler shift, is needed, instead of two. The GPS clear and acquisition (C/A) codes are matched to a paratellurite Bragg cell in time-compression hardware with a compression factor of 20. This technique is further exploited for accelerating Doppler estimation. Experimental results show an acquisition time of at most 1 ms.