This dissertation consists of two topics in the area of surface acoustic waves (Rayleigh waves) and laser range-finding systems. The technology of surface acoustic waves (SAW) is widely used in a number of applications, such as detection of surface cracks or flaws, touch panels, and SAW filters. The propagation of SAWs on the surface of solid materials has been used to detect hidden cracks or other surface discontinuities in a medium. They are also readily attenuated when a soft object touches the surface of medium. For a piezoelectric material, SAWs can be generated using a transducer of interdigitated electrodes fabricated on the surface. However, most materials are nonpiezoelectric. A method to produce SAWs on a non-piezoelectric material is from converting bulk acoustic waves, generated by acoustic transducers, using the equivalence of Snell’s law in optics.
This dissertation first reports the input impedance matching technique of acoustic transducers. It uses an inherent impedance property of transducers and thus does not need an external electric matching circuit or extra acoustic matching section. The transducer size can be properly chosen so that the impedance at off-resonant frequencies is close to 50 Ω, the output impedance of signal generators. At this off-resonant operating frequency, the reflection coefficient of the transducer is minimized without using any matching circuit. Other than the size, the impedance can also be fine tuned by adjusting the thickness of material that bonds the transducer plate to the substrates.
Another concern is the acoustic prisms to generate SAW on glass, from bulk waves which are generated by acoustic transducers. To satisfy the Snell's law, longitudinal bulk wave propagating in the acoustic prism must be slower than the SAWs on the nonpiezoelectric material. This posts a challenge because the SAW velocity is approximately 50% of the longitudinal wave velocity in the same material. It is very hard to find a material in which the longitudinal wave velocity is slow and the propagation loss is acceptable. For nearly all SAW-based touch panels on the market, the acoustic prisms are made of Lucite. An issue with Lucite is its high coefficient of thermal expansion (CTE), which is much higher than that of glass panels and piezoelectric ceramics. In outdoor environments with large temperature variations, this large CTE mismatch can cause the Lucite prism to break off the glass panel and the piezoelectric transducer to break away from the Lucite prism. After an extensive search, we indentified liquid crystal polymer (LCP) and bismuth (Bi) as potential acoustic prism materials having low CTE. Following the Telcordia GR-468-Core recommendation, the wedge modules went through thermal cycling between -45°C and 85°C for 1,000 cycles. The wedge modules with the Lucite prism broke before 50 cycles. The wedge modules with LCP and Bi prisms survived the 1,000 cycles. In terms of acoustic performance, the wedge module with the LCP prisms is as good as the wedge module with the Lucite prism. Accordingly, LCP is the best prism material for SAW-based touch panels in outdoor applications.
The second topic evaluates and implements the use of silicon photomultiplier (SiPM) array module as the receiver for laser range-finding experiments to simultaneously measure the range of multiple targets. The array consists of 4 x 4 SiPMs on a single chip. Each SiPM is followed by a trans-impedance amplifier to have an adequate output signal voltage. The 4 x 4 array is capable of measuring the range of 16 targets. A SiPM is an avalanche photodiode that operates in Geiger mode where a huge multiplication factor, as high as 1,000,000, is obtainable. Thus, it can achieve high sensitivity. Our performance evaluation results show that the sensitivity of the SiPMs measured actually approaches the quantum limit. The SiPM can detect as little as one single photoelectron in the depletion region. Each SiPM has a large active area, 8 mm2, and can collect much more returned light compared to typical Si-APDs. The rise time of the output voltage waveforms from SiPMs vary from 6 ns to 300 ns, depending on the incident light power. The rising edges of the output waveforms are compared for time measurement. Using a digital oscilloscope, the time of flight can be measured with 1 ns accuracy. This corresponds to a range resolution of 15 cm.