Ultrasonic low-loss, phase continuous Fresnel lens for focal plane array imaging

Although pulsed echo, line by line imaging is yet the mainstream fruitful technology in this field, a FPA (focal plane array) imagery is an emerging (or revisiting) technology to innovate them to reject cumbersome beam-forming step as well as to have orthographic image rather than tomographic. The challenge here is to realize a design rules for low-loss image formation lenses, using laboratory available basic materials and technologies. First step decision is to adopt Fresnel lens structure rather than entirely concave or spherical structure, to minimize the thickness or the usage of lossy material and hence to minimize the insertion loss. A 5 cm dia. f=2 Fresnel lens by RTV-60 silicone rubber, designed for 3 MHz narrow band operation, has been fabricated using mould die made by precision NC lathe. The point of design is to adjust the height of each step of the Fresnel contour to be one wavelength equivalent. In other word, the sound wave via inside and outside the groove meets together in an "in-phase" manner at every step of the groove, in order not to be destructive meeting. This phase-continuous design solves major known problems of Fresnel structure such as unexpected dark ring, however, at cost of narrow band operation. Its insertion loss and near-axis resolution are almost as theoretical estimation, however, off-axis degradation of resolution is more than expected which limits the maximum usable viewing angle merely within big postage stamp size at focal plane. It has been suggested that wide view-angle and Fresnel concept are basically incompatible. However, it could be excellent to visualize small viewing angle in precision, such as Doppler imaging of moving fetal heart transabdominally.