In recent years there has been a growing interest in using the Lamb wave devices to fabricate sensors applied to physical, chemical and biological fields. Because the sensitivity of the sensors is dependent on the shift of the operation frequency induced by the measured physical quantity, the sensitivity of the sensors will increase if the oscillation frequencies increase. Therefore, many researches focused on the increase of the operation frequencies of the devices, however, the fundamental frequencies of the devices cannot increase limitlessly due to the technical limitation for fabricating interdigital transducers (IDT). The propagation characteristics of the high order modes of Lamb waves are studied here, and a new kind of oscillator using the high order modes Lamb wave excited by IDT is presented with which one can obtain a higher phase velocity and a higher oscillating frequency, and then sensitivity will be greatly improved. Due to the dispersive and multi mode property of the Lamb wave, the fabrication of high frequency oscillators by using high order modes of the Lamb wave will be easier than the zero order mode Lamb wave. To select the operating mode of the high mode Lamb wave oscillator, a high order mode selector is inserted between the Lamb wave delay line and the amplifier in the closed oscillating loop. The phase velocity of the operating order modes Lamb wave is much higher than that of the zero order mode Lamb waves. Experimental measurements are performed on a 127. 86° rotated Y-cut, X propagating lithium niobate plate, with the ratio of the plate thickness to the fundamental wavelength h/λ=0. 94. By the measurement results of the dispersion curve of the Lamb wave, the relation between the wavelength and frequency can be calculated, which is used to determine the operating frequency. A high order modes Lamb wave with the phase velocity 19 652 m/s is obtained in experiments,the oscillating frequency is 46. 24 MHz, whereas the oscillation frequency of the oscillator using the a0 mode of the Lamb wave excited by the same IDT is about 8. 75 MHz. Thus, the operation frequency, i. e. , the phase velocity,of the high order Lamb mode oscillator is 5.3 times higher than that of the a0 mode. Therefore the high frequency oscillator with operating frequency above UHF band can be fabricated much easier by this method, then the sensitivity of the sensor using the new oscillator can be increased greatly.
high order mode Lamb waveoscillatorinterdigital transducer
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