Browsing by Author "Bozkurt, Ayhan"

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Open Access
Characterization and imaging with lamb wave lens at gigahertz frequencies
(IEEE, 1994-10-11) Bozkurt, Ayhan; Yaralıoğlu, Göksenin; Atalar, Abdullah; Köymen, Hayrettin; Kramer, K.
Lamb wave lenses with conical refracting surfaces are fabricated for use at 400 MHz and 1 GHz. The conical surfaces are ground and polished with mechanical means and they are sufficiently smooth for the frequencies of interest. The wide bandwidth of transducers allow frequency tuning necessary for Lamb wave lenses. The fabricated lenses show the expected V(Z) performance. At high frequencies the attenuation in the coupling medium can be very high, but due to the smaller wavelength the resolution is better and defocus distance can be reduced. Inherently higher leaky wave sensitivity of Lamb wave lens enables a good V(Z) characterization ability at higher frequencies as compared to the conventional spherical lens. Subsurface imaging with these Lamb wave lenses gives satisfactory results for layered structures. Chosen object has leaky wave modes within the angular coverage of the lens. The images exhibit a resolution close to the diffraction limit. Experimental V(Z) curves obtained with these lenses along with images are presented.
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En acil sorun: korsan kitap
(1998) Bozkurt, Ayhan
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Kaçışsız bir yaşam oyunu şiirleri
(1998) Bozkurt, Ayhan
Open Access
Lens geometries for quantitative acoustic microscopy
(Springer, 1995) Atalar, Abdullah; Köymen, Hayrettin; Bozkurt, Ayhan; Yaralıoğlu, Göksenin; Briggs, A.; Arnold, W.
The purpose of the first Lemons-Quate acoustic microscope(1) was to image the surfaces of materials or biological cells with a high resolution. Unfortunately, competition with the optical microscope was only partially successful due to the high degree of absorption in the liquid-coupling medium at high frequencies. Increasing the resolution beyond optical limits was possible with the use of hot water(2) or cryogenic liquids,(3) at the cost of operational difficulty and system complexity. Meanwhile it was shown that the acoustic microscope can generate information that has no counterpart in the optical world.(4) The presence of leaky waves resulted in an interference mechanism known as V(z) curves. The V(z) method involves recording the reflected signal amplitude from an acoustic lens as a function of distance between the lens and the object. This recorded signal is shown to depend on elastic parameters of the object material. After underlying processes are well understood, new lens geometries or signal-processing electronics are designed to emphasize the advantage of the acoustic lens. In any case, the aim has been to increase the quantitative characterization ability of the microscope.
Open Access
Modeling and characterization of capacitive micromachined ultrasonic transducers
(2000) Bozkurt, Ayhan
The Capacitive Micromachined Ultrasonic Transducer (cMUT) is a device used for the generation and detection of ultrasonic sound waves. The device is constructed on a silicon substrate using a microfabrication process. Individual cells constituting the device are membranes which have dimensions in the order of tens of microns, and are made up of a mechanicalh^ strong compound of silicon. The transducer itself has dimensions measured in centimeters, thus the total number of cells that make up a transducer is in the order of thousands. The excitation/detection of acoustic waves relies on the capacitance between the substrate and membrane: The presence of acoustic waves induces a small -AlC variation on the DC bias on the device, which can be used for detection, while a small -A.C component added to the DC bias by the drive circuit changes the electro-static attraction force on the membrane causing it to vibrate, producing acoustic waves. Basic advantages of cMUT devices include easy patterning of array structures, integration of drive/detection electronics with mechanical structures, and low cost. In this study, basic theory describing the characteristics of cMUT devices were developed. The analytic formulation was used to test the validity of a Finite Element Method (FEM) model. The FEM model, then, was emplo3'ed in the analysis of structures for which no analytical models are present. Specific problems solved using the FEM model included the characterization of cMUT devices with judiciously patterned electrodes. A more specific study showed that the bandwidth of an immersion device with an active area of radius 25 /¿m can be increased by 100% by simply setting the electrode radius to 10 /rm. The FEM analysis was, then, extended to handle the effects of substrate loss, which required the incorporation of an Absorbing Boundary Condition (ABC) into the model. A Normal Mode Theory analysis was conducted to give better insight to the physical nature of the effect of substrate loss to device characteristics. The dominant wavemode for a transducer of central frequency 2.5 MHz was found to be the lowest order anti-symmetric lamb wave mode (AO), for a silicon substrate of thickness 500 //m. A microfabrication process was developed for the production of cMUT devices. Hexagonally shaped transducers of radius 40 p.m were fabrictated on a conducting silicon substrated with silicon nitride as the sacrificial la.j'er and amorphous silicon as the membrane material. Both the gap and membrane thicknesses are set to 0.5 //m. 8, 16, and 24 /im gold plates were deposited as top eletrodes. The total number of active cells were 24 thousand for a substrate size of 0.7x0.7 cm^. Some experimental results were obtained from the fabricated transducers to support the analytical cMUT model. The device is found to have a central frequency of 2 MHz.
Open Access
A new directional acoustic lens: V-groove lens
(IEEE, 1993) Bozkurt, Ayhan; Yaralıoğlu, G. Göksenin; Atalar, Abdullah; Köymen, Hayrettin
A new directional acoustic lens is introduced. The geometry is very similar to the line-focus lens except the lens cavity, which is shaped as a groove with flat-bottom V cross section. The slanted planar edges of the groove are inclined in order to generate waves incident on the object surface at a critical angle. Hence, the edges of the groove act like two wedge transducers facing each other. The cross section of the lens is the same as that of the Lamb Wave Lens. Therefore, it enjoys the same sensitivity to surface wave excitations. On the other hand, since the cross section remains the same along one of the lateral directions, it has directional properties very similar to that of the Line Focus Beam Lens. The waves normally incident on the object surface generated from the flat-bottom, interfere with those at the critical angle, giving rise to a V(Z) effect. Calculated responses of the lens are presented for silicon (001) surface as a function of crystal orientation. The calculated curves are compared with measurement results. The leaky wave velocities are extracted from the measurement results using the conventional FFT algorithm. A new model based algorithm is proposed for extracting the velocity information from V(Z) data.
Open Access
A synthetic aperture imaging system using surface wave modes
(IEEE, 1995) Bozkurt, Ayhan; Arıkan, Orhan; Atalar, Abdullah
A synthetic aperture acoustic imaging system with a novel inversion algorithm is described. Data is obtained by using a transducer insonifying the sample surface at a critical angle which is excited by a short electrical pulse. The critical angle is chosen for a suitable surface wave or Lamb wave mode that exists on the object. The transducer is mechanically scanned in only one direction during which many pulse excitations and subsequent recordings are realized. The received signal is sampled in time and digitized to be processed by using the new inversion approach providing an optimal 2-D image of the surface reflectivity.
Open Access
The V-Groove lens
(1994) Bozkurt, Ayhan
Primarily designed for imaging purposes, the acoustic microscope finds application in the qualitative evaluation of materials, too. The lens response as a function of defocus, which is known as the V{z) curve, is formed by the interference of various wave components reflected from the material surface. Leaky wave velocities of the material can be extracted from this interference pattern. The accuracy of the measurement is heavily influenced by the leaky wave contribution to the V(z) curve. Hence, lens geometries capable of efficiently exciting leaky wave modes need to be designed. If a particular geometry is to be used for measurements on materials exhibiting crystalographic anisotropies, it must be able to couple to modes only in a single direction, as well. The proposed V-Groove lens, combines the directional sensitivity of the Line Focus Beam lens and the efficiency of the Lamb Wave lens. The geometry is able to accurately measure the direction dependent leaky wave velocities of anisotropic materials. A new model based approach improves the accuarcy of the extracted velocities. In this study, the V-Groove lens has been analyzed theoretically. A mathematical model describing the lens response has been developed. The performance of the V-Groove lens has been tested by simulations. A new leaky wave velocity extraction algorithm based on fitting the model curve to actual curves using Nelder-Meade search has been proposed. A prototype lens has been manufactured and performance figures have been verified experimentally. The accuracy of the lens has been compared with those of other various geometries.
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Yaşamın kendisi polisiye
(1996) Bozkurt, Ayhan