TRANSD 默语 COUSTIC WAVE FIGURE 48.4(a) Resonator structure;(b)acoustic probe;(c)acoustic delay line or optical modulator Thus, while the input impedance is infinite as in a parallel resonant circuit at o it is zero as in a series resonant circuit at a slightly lower frequency where the bracketed term in Eq.(48. 14)is zero. When losses are present or there is radiation out of an acoustic port, a resistive term is included in the reactive expression of Eq (48.14) Behavior analogous to that of coupled tuned electrical circuits for multipole filters can be achieved by subdividing the electrodes of Fig. 48.4(a)into different areas, each of which will act separately as a tuned circuit but if they are close enough together there will be acoustic coupling between the different radiators controlling this coupling, narrowband filters of very high Q and of somewhat tailored frequency response can be built in the megahertz and low gigahertz range. The basic geometry of Fig. 48.4(c) gives an electric-to-electric delay line whose delay is given by the lengt of the medium between the transducers divided by the phase velocity of the acoustic wave and would be on the order of 2 us/cm. Since the solid has little dispersion, the bandwidth of the delay line is determined by that of the transducers. Here it is necessary to choose the characteristic impedances and thicknesses of the backing and matching layers in Fig. 48.2 in such a manner that the conversion of the electrical energy incident on the electrical port to the acoustic energy out of acoustic port 2 of Fig. 48.3 is independent of frequency over a large range about the resonant frequency of the piezoelectric transducer itself. Varying the matching and backing .m ers is equivalent to varying the terminating impedances on the acoustic line of Fig. 48.3. The matching is en assisted by lumped elements in the external electrical circuit a The geometry of Fig. 48.4(c)is also the prototype form for acousto-optic interactions. Here the second ansducer is not relevant and can be replaced by an acoustic absorber so that there is no reflected wave present in the active region. An optical wave coming into the crystal as shown in Fig. 48.4(c)sees a propagating periodic perturbation of the medium, and if the photoelastic coefficients of the solid are large, the wave sees appreciable ariations in the refractive index and hence a moving diffraction grating. The angle of deflection of the output optical beam and its frequency as produced by the grating depend on the amplitude of the various frequency components in the acoustic beam when the optical beam traversed it. Thus, for example, the intensity versus angular position of the emerging optical beam is a measure of the frequency spectrum of any information modulated on the acoustic beam As noted previously, ultrasonic waves are often used as probes when the wavelength and attenuation are appropriate. For these radar-like applications, the acoustic beam is generated by a transducer and propagates in the medium containing the scatterer to be investigated as shown in Fig. 48.4(b). The acoustic wave is scattered by any discontinuity in the medium, and energy is returned to the same or to another transducer. If the outgoing ignal is pulsed, then the delay for the received pulse is a measure of the distance to the scatterer. If the transducer is displaced or rotated, the change in delay of the echo gives a measure of the shape of the scatterer. Any movement of the scatterer, for example, flowing blood in an artery, causes a Doppler shift of the echo, and this ft, along with the known direction of the returned beam, gives a map of the flow pattern. Phasing techniques with multiple transducers or multiple areas of one transducer can be used to produce focused beams or beams electrically swept in space by differential variation of the phases of the excitation of the component areas of the transducer c 2000 by CRC Press LLC© 2000 by CRC Press LLC Thus, while the input impedance is infinite as in a parallel resonant circuit at vo, it is zero as in a series resonant circuit at a slightly lower frequency where the bracketed term in Eq. (48.14) is zero. When losses are present or there is radiation out of an acoustic port, a resistive term is included in the reactive expression of Eq. (48.14). Behavior analogous to that of coupled tuned electrical circuits for multipole filters can be achieved by subdividing the electrodes of Fig. 48.4(a) into different areas, each of which will act separately as a tuned circuit, but if they are close enough together there will be acoustic coupling between the different radiators. By controlling this coupling, narrowband filters of very high Q and of somewhat tailored frequency response can be built in the megahertz and low gigahertz range. The basic geometry of Fig. 48.4(c) gives an electric-to-electric delay line whose delay is given by the length of the medium between the transducers divided by the phase velocity of the acoustic wave and would be on the order of 2 ms/cm. Since the solid has little dispersion, the bandwidth of the delay line is determined by that of the transducers. Here it is necessary to choose the characteristic impedances and thicknesses of the backing and matching layers in Fig. 48.2 in such a manner that the conversion of the electrical energy incident on the electrical port to the acoustic energy out of acoustic port 2 of Fig. 48.3 is independent of frequency over a large range about the resonant frequency of the piezoelectric transducer itself. Varying the matching and backing layers is equivalent to varying the terminating impedances on the acoustic line of Fig. 48.3. The matching is often assisted by lumped elements in the external electrical circuit. The geometry of Fig. 48.4(c) is also the prototype form for acousto-optic interactions. Here the second transducer is not relevant and can be replaced by an acoustic absorber so that there is no reflected wave present in the active region. An optical wave coming into the crystal as shown in Fig. 48.4(c) sees a propagating periodic perturbation of the medium, and if the photoelastic coefficients of the solid are large, the wave sees appreciable variations in the refractive index and hence a moving diffraction grating. The angle of deflection of the output optical beam and its frequency as produced by the grating depend on the amplitude of the various frequency components in the acoustic beam when the optical beam traversed it. Thus, for example, the intensity versus angular position of the emerging optical beam is a measure of the frequency spectrum of any information modulated on the acoustic beam. As noted previously, ultrasonic waves are often used as probes when the wavelength and attenuation are appropriate. For these radar-like applications, the acoustic beam is generated by a transducer and propagates in the medium containing the scatterer to be investigated as shown in Fig. 48.4(b). The acoustic wave is scattered by any discontinuity in the medium, and energy is returned to the same or to another transducer. If the outgoing signal is pulsed, then the delay for the received pulse is a measure of the distance to the scatterer. If the transducer is displaced or rotated, the change in delay of the echo gives a measure of the shape of the scatterer. Any movement of the scatterer, for example, flowing blood in an artery, causes a Doppler shift of the echo, and this shift, along with the known direction of the returned beam, gives a map of the flow pattern. Phasing techniques with multiple transducers or multiple areas of one transducer can be used to produce focused beams or beams electrically swept in space by differential variation of the phases of the excitation of the component areas of the transducer. FIGURE 48.4 (a) Resonator structure; (b) acoustic probe; (c) acoustic delay line or optical modulator