Producing Bessel Beams With an RF Transformer

Bessel beams are exact solutions to the isotropic/homogeneous wave equation. In theory, they can propagate to infinite distances without diffraction. In practice, when produced with a finite aperture, they have a very large depth of field, i.e., they can maintain a small beamwidth over a large distance. In addition, they have a self-healing ability after encountering an obstacle. Because of these properties, Bessel beams have applications in optics, electromagnetics, ultrasound, quantum communications, electron beam guidance, and so on. Previously, in ultrasound, Bessel beams were produced with an annular array transducer driven by multiple independent high-voltage radio frequency (RF) power amplifiers that were bulky, heavy, and consumed a lot of power, which limited the Bessel beams in applications such as wearable medical ultrasound imaging and wearable super-resolution imaging. In this article, pulse (broadband) Bessel beams were produced by a single high-voltage RF power amplifier in combination with an RF transformer, reducing the size, weight, and power consumption. Experiments were performed to produce the pulse Bessel beams in water with a custom RF transformer and a custom ten-ring, 50-mm diameter, 2.5-MHz center frequency, and broadband (about 72% -6 dB relative one-way bandwidth) 1–3 lead zirconate titanate (PZT) ceramic/polymer composite annular array transducer driven by a commercial RF power amplifier at about ±90 V. The results show that the pulse Bessel beams produced were very close to those generated with ten independent high-voltage RF power amplifiers, computer simulations, and theory, and the pulse Bessel beams had a -6-dB beamwidth of about 2.53 mm ( $4.22\lambda $ ) and a depth of field of about 216 mm ( $360\lambda $ ). The reduced number of high-voltage RF power amplifiers makes it easier to apply Bessel beams in applications such as wearable medical ultrasound imaging and wearable super-resolution imaging, as is illustrated in examples where three-dimensional (3-D) or multi-plane images can be produced using a Bessel beam and a mechanically scanned multi-directional vibrating reflector.