|
[Microacoustic
instruments inc.]
Focusing
of Ultrasonic Waves in Air
Using a Micromachined Capacitance Transducer and an
Integrated Micromachined Fresnel Zone-Plate:
Figure 1 (a)
a photograph of a micromachined Fresnel zone-plate aperture
which was manufactured using photolithography and wet-chemical
etching. This aperture is placed in direct contact with
the front generating surface of a micromachined air-coupled
capacitance transducer in order to focus ultrasonic
plane waves of wavelength lambda at a predetermined
focal length z0 (see Figure 1(b), below).
>>
Back to MicroAcoustic home page
Figure
1 (b):
>>
Back to MicroAcoustic home page
Figure
2 :
Figure 2:
Measured ultrasonic intensity in front of the micromachined
Fresnel zone-plate aperture when excited by ultrasonic
plane-waves at a frequency of 580kHz; r is the radial
distance off axis of the zone-plate, whereas z is the
distance along axis. The intensity is seen to rise dramatically
on the zone-plate's axis at a focal length of 7.5mm.
The spot size of the ultrasonic beam (i.e., the width
of the beam in the r direction) is 600micron. The depth
of field (i.e., the axial distance over which the ultrasonic
field is reasonably intense) is approximately 3mm.
Conclusion
A well-focused ultrasonic beam can be created in a gas
by using a micromachined Fresnel zone-plate mounted
upon a micromachined air-coupled capacitance transducer.
Such a well-focussed field is ideal for a wide variety
of non-contact imaging applications, such as high-resolution
ultrasonic imaging of solid surfaces in air. (For further
information on the air-coupled Fresnel zone-plate, see
the following technical publications: (1) D.W. Schindel,
A.G. Bashford and D.A. Hutchins, "Focusing of ultrasonic
waves in air using a micromachined Fresnel zone-plate,"
Ultrasonics, vol.35, no.4, pp.275-285, September 1997;
and (2) D.W. Schindel, "Ultrasonic imaging of solid
surfaces using a focused air-coupled capacitance transducer,"
Ultrasonics, vol. 35, no. 8, pp.587-594, March 1998.
>>
Back to MicroAcoustic home page
|
