Wideband Acousto-Optic Bragg Cells Using Phased Array Techniques: Design And Modeling

We present calculations of the third-order intercepts for planar transmission lines that contain a distributed nonlinear inductance or capacitance. We model such a transmission line as a series of small nonlinear elements that interact only weakly. This approach allows us to use simple lumped-element models to describe nonlinear effects in transmission lines longer than the effective wavelength. We use this approach to calculate the third-order intercepts for a lossless transmission line, and also for a transmission line with finite attenuation. Calculations for the lossless case agree well with third-harmonic measurements of high temperature superconducting transmission lines as a function of line length. For transmission lines with loss, the calculations agree well with third-harmonic measurements of transmission lines of variable length incorporating ferroelectric thin films.

Phased array approach to generate guided waves and extract phase velocities using the V(z) method.

Damage localization in aluminum plate with compact rectangular phased piezoelectric transducer array

We review literature on high voltage transmission lines and their effect on wild reindeer (Rangifer tarandus) migration patterns and area use. We conclude that reliable knowledge is lacking on the effects of transmission lines on reindeer ecology. This condition relates to the lack of long-term monitoring of reindeer migration patterns in relation to existing lines. It is also related to the fact that nothing is known about hearing in reindeer (or any other deer species in Norway) in relation to transmission line noise, which is considered an obstacle for migrating reindeer.We then outline a research programme that includes a laboratory study to determine the audiogram in reindeer and two field studies to examine the proximate effects of transmission lines - independently as well as combined with ambient environmental variables - on reindeer behaviour. The audiogram in two yearling male reindeer has been determined; this part of the study will be completed this year with the determination of a corresponding audiogram in two female yearlings. The behaviour study of domestic reindeer under high voltage transmission lines was completed in September 1999. The behaviour study of wild reindeer in the area crossed by a 420 kV power line will continue this year and its completion is anticipated in 2001.

In order to investigate the switching dynamics of ferroelectric materials at nanosecond time scales, we have measured the nonlinear response of ferroelectric-loaded transmission lines at microwave frequencies. We analyzed the third harmonic and intermodulation products produced by planar transmission lines incorporating ferroelectric thin films, and obtained the rf nonlinear capacitance C(Vrf). We also directly measured the change in capacitance of the same structures due to a dc applied voltage, C(Vdc). By comparing these two experimentally-determined nonlinear capacitances, we are able to directly determine if the mechanisms responsible for the capacitance tuning in these materials are able to responsd on nanosecond time scales.

The classical framework for phased array beamforming, analysis, and design is based on the array factor approximation. As research in antenna arrays has progressed, attention has turned to sophisticated, high sensitivity applications that require more accurate models. Mutual coupling, edge effects, surface waves, and other non-ideal effects can be accounted for full wave numerical methods, but the run time for the model becomes large, and design optimization can be unduly time consuming. A middle ground between the array factor and full wave numerical models is provided by the lossless, resonant, minimum scattering (LRMSA)approximation. Embedded element radiation patterns with open circuit loads on the element ports are approximated using an analytically known or modeled isolated element pattern, and mutual coupling is included to first order using the element pattern overlap matrix and relationships from the network theory of antenna arrays. We give an overview of the LRMSA method for fast phased array modeling and design in this paper, and the presentation will include numerical comparisons and design examples for a variety of phased array application areas.

The radar warning receiver (RWR) is the heart of the self protection system of modern aircraft. The most important receiver structures that can be used as RWR are the crystal video receiver (CVR) and the instantaneous frequency measurement (IFM) receiver. This paper presents a complete design and implementation of the microwave part of the CVR. This CVR is designed to cover a frequency ranging from 1.5 up to 4 GHz. The CVR is implemented using the recently developed microstrip technology. The designed receiver was successfully tested and the results in terms of the output voltage and the voltage standing wave ratio (VSWR) were very good. This paper presents also a complete design and implementation of the critical components of the IFM receiver. These components are the phase correlator, the power dividers, the delay line, the square law detectors preceded by broadband matching circuits, and the low pass filter. They are implemented using the microstrip technology. The input to the correlator is the threatening radar signal and the output comprises four video signals that can be processed to obtain the signal frequency instantaneously. A new design procedure is proposed to overcome the problems, usually encountered when trying to design and implement phase correlators. The designed correlator was successfully tested and found to operate adequately in the whole S-band.

The acoustic hologram has made it possible to shape ultrasound waves and project complex pressure patterns in 2D and in 3D, as we have also recently shown. The hologram contains tens of thousands of elements which can be used to form sophisticated pressure patterns by setting the phase and/or amplitude across the wavefront of a sound wave. It thereby offers more degrees of freedom than conventional phased array transducers (PATs). However, the hologram is generally static, whereas PATs are dynamic. PATs do not only have fewer elements, but they are also more difficult to fabricate and require complex electronics. In this talk, I will describe our efforts to combine the best of both worlds. For this, we are exploring the use of light waves as a means to control ultrasound waves. I will describe direct and indirect approaches that can be used to control and modulate holographic ultrasound fields.

We present an optimized design of an acousto-optic tunable filter (AOTF) using a phased-array transducer for a spectrally-multiplexed ultrafast pulse-shaping RF beamformer application. The momentum-space interaction geometry is used to optimize an AOTF using acoustic beam-steering techniques in combination with acoustic anisotropy in order to linearly map the applied RF frequency to the filtered output optical frequency. The appropriate crystal orientation and phased-array transducer design are determined to linearize the RF to optical frequency mapping even in the presence of optical dispersion of the birefringence. After optimizing the phased-array transducer, acoustic anisotropy, and optical anisotropic diffraction geometry, the designed AOTF will compensate for the birefringent dispersion of TeO₂ to give a linear modulation of RF frequencies onto the corresponding optical frequencies. This linearized frequency mapped AOTF is required for a squint-compensated, wavelength-multiplexed, optically processed RF imager.

A series of ultrasonic elements arranged in a phased array transducer can provide the capability to activate each element separately but in a programmed sequence. This will help the acoustic signal to be generated at desired focusing distances and anticipated angles for specific materials and structures. In case of composite material inspection, this characteristic of the phased array method can improve the undesirable effects of the high attenuation and anisotropic structure of composite materials on response signals. In this study different phased array probes and wedges which are commercially available were evaluated for their response signals’ characteristics. First, the capability and resolution of bulk wave generation were studied for each set of probe and wedge, and the response signals were compared to that of the conventional single element ultrasonic transducers for different thicknesses composite plates. Then the resolution of the response signals and their sensitivity to defect size were evaluated and compared to the single element transducers as well. Next, each phased array probe and wedge set was used to generate plate waves in aluminum plates based on plate wave propagation theory, probe and wedge physical properties and the definition of delay law. Results show a general improvement in response signals’ strength and resolution for phased array method in comparison to the single element transducers. Also some plate wave modes could be generated with optimized signal generation parameters in phased array system.

Present state of the art sizing measurements of defects by ultrasonic techniques has been inadequate, especially in support of a quantitative fracture mechanical analysis of the strength of a material. Present capabilities of defect sizing in some cases do not allow an accept/reject decision to be made. Currently, ultrasonic imaging systems including phased array, synthetic aperture, and wavefront reconstruction systems are being developed to quantitatively measure defect sizes. Also, in the medical field phased array systems have been developed for imaging purposes. Ibis work describes an annular phased array system which is being developed for general NDE applications on metal and ceramic materials. The main usefulness of phased array techniques is the ability of electronically changing the focus of the acoustic beam radiating from a phased array transducer or reflecting from a defect in a solid material.

Previous work has led to the design, simulation, and development of a linear phased array transducer. The intention of the array is to be used as a non-destructive ultrasonic device to monitor and evaluate the health of a given specimen. The phased array has been manufactured and tested for the detection and characterization of defects on a target. The array was fabricated with a four-row stepped design with four wires to transfer data and one wire for grounding. The stepped design allows for the interrogation of a larger region using time delays and beam sweeping without the use of additional electrical channels. The array was designed to be utilized in a water immersion environment with about one inch between the array and the target specimen. An OmniScan MX system was used to operate the phased array and perform real-time linear and sectorial scans on a set of rectangular plates. S-scans allow for beam sweeping over an angle range as well as adjustments for time delays and a true-depth display. The array was operated with sixteen active elements and an angle range of 0 to 30 degrees. The phased array was tested with a variety of targets and was used to investigate and characterize different types of defects such as cracking, warping, and corrosion. The ability of the phased array to distinguish between defect types as well as resolve defect size was evaluated.

This paper presents a pixel structure-based phased array with wide-angle scan and filtering response. A periodically-arranged parasitic pixel surface, which is placed upon a magnetic dipole-based phased array, is used as the wide-angle impedance matching layer to improve the active impedance bandwidth and enhance the wide-angle scanning performance. The phased array is fed by slot coupling. Within the feeding lines, another pixel structure is embedded for the realization of the filtering response. The dual pixel structures are optimized based on the internal multiple port method (IMPM). The final optimized phased array achieves a beam scanning from -60° to +60° within the frequency range of 4.8 to 5.1 GHz. Besides, the phased array exhibits good frequency-selective feature and good out-of-band suppression.

Computation of corona effects in transmission lines using state-space techniques

As phased array ultrasonic transducers become more wide spread in underwater acoustic measurement applications, a better understanding of their basic performance characteristic is of benefit. The beam patterns of a phased array can be approximated by a steady state analysis. This approach indicates the directional response of an array in response to a sinusoidal excitation. However, a steady state examination does not properly describe transient behavior of the transmit waveform and also the received signal that is subsequently beamformed. We further investigate the transient performance of the beamformer using a closed-form solution to the acoustic wave equation for a multielement ultrasonic phased array transducer. The model produces transducer beam patterns and transient beamformer output for a series of reflective targets in the far-field of the transducer. The results show a close agreement with laboratory measurements and provide a tool for further investigation of phased array transducer design and performance.