  Ultrasonic transducer is a sensor that realizes the mutual conversion of
ultrasonic and electrical signals, and it is widely used in quality inspection,
biomedical imaging and other fields. Commonly used ultrasonic transducers have
a small detection range and low sensitivity due to the diffraction of sound
waves. Focused transducers are used to improve detection sensitivity.
Unfortunately, focused transducers have narrow depth of field. Here, we
developed a Bessel ultrasonic transducer for large depth of field by using
conical acoustic lens. An acoustic lens is attached to a unfocused ultrasonic.
And the acoustic lens is a cuboid prism with a concave cone on the bottom, made
of fused silica. Similar to an axicon that can generate a Bessel beam, the
Bessel ultrasonic transducer can produce nondiffracting Bessel ultrasonic
beams. Therefore, extended depth of field with uniformly high resolution and
high detection sensitivity can be obtained. We used COMSOL to simulate the
transmission of ultrasonic field of the designed conical acoustic lens, and
compare it with the spherical focused ultrasonic transducer. The results show
that the depth of field of the Bessel ultrasonic transducer is about 8 times
that of the conventional spherical focused ultrasonic transducer. And the depth
of field of the Bessel ultrasonic transducer can be further adjusted by
adjusting the cone angle of the conical acoustic lens. The Bessel ultrasonic
transducer will help improve the capabilities of the ultrasound probe and
expand its application range. For example, an ultrasonic probe with a large
depth of field will expand the imaging depth of photoacoustic microscopy and
enhance its ability in non-destructive testing.
