Laser Probe Investigation of SAW Filters

A fi beroptic homodyne probe has been constructed to study the acoustic fields of SAW filters. The probe measures the amplitude and phase of the normal surface displacement, corrected for surface reflectivity variations. The optics used to focus the laser beam on the SAW substrate are also used by a video imaging system. The laser beam is visible in the image, simplifying alignment of the probe. A number of practical filter configurations were investigated, including bidirectional apodized IDT’ s, mu1 t i -strip couplers, and unidi rect ional transducers. Wavefront distortion (sometimes referred to as apodization loss) was observed in bidirectional transducers. The beam spreading properties of both tracks of a multi-strip coupler were studied. The directionality of a unidirectional transducer was directly measured by making scans through the axis of the transducer. The probe has also been used to identify misoriented substrates (which cause beam walkoff) and to study excess attenuation.