Wideband Pulse Generation for Underwater Applications Using Parametric Array

We investigated wideband pulse generation for underwater acoustic applications using a parametric array. We fabricated a transducer consisting of a 3 mm thick 75 mm-by-75 mm square-shaped PZT ceramic plate, which is matched to water media at the radiating face and terminated by a very low impedance at the back. All measurements were made in a large test tank. We transmitted square-root amplitude modulated pulses centered around 855 kHz primary frequency. We showed that phase-sensitive generation of in-phase and out-of-phase bursts suitable for coded transmission using a parametric array is possible. We generated very short duration bursts, as short as half-cycle, at a 10-80 kHz difference frequency range. The definition of the bursts is excellent, e.g., with a normalized cross-correlation of 0.92 with an ideal 2-cycle square burst, for both in-phase and out-of-phase pulses.

Coded Divergent Waves for Fast Ultrasonic Imaging: Optimization and Comparative Performance Analysis

In this paper, we present the optimal use of coded signals in diverging wave transmission for fast ultrasonic imaging. The performance of coded imaging with diverging waves, quantified by SNR, CNR, speckle power and target signal strength, is optimized in terms of code length, wave profile, aperture size and field of view. We obtained one frame in 200 microseconds using coded diverging wave transmission, equivalent to very high 5000 frames/s rate, where the imaging depth is 7.5 cm. The performances of coded diverging wave transmission and conventional single focused phased array imaging are compared on a single frame basis. Complementary Golay sequences with code lengths ranging from 2 to 10 bits are used to code the signal. The signal strength and SNR obtained using synthetic transmit aperture and conventional single focused phased array imaging techniques, respectively, are used as reference in the performance analysis. We present a method to determine the optimum diverging wave profile to maximize the SNR across the entire viewing sector, which matches the received signal strength distribution to that of synthetic transmit aperture imaging. Optimum diverging wave profiles are determined for different sector angles and for different array apertures. For 90{\deg} sector, the SNR of 8-bit coded signal with optimized diverging wave profile is higher than that of conventional single focused phased array imaging at all depths and regions except the focal region, where it is 2-8 dB lower.