Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-02-23
2002-11-05
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S358000
Reexamination Certificate
active
06476541
ABSTRACT:
This invention relates to phased-array ultrasound imaging systems and, more particularly, to methods and apparatus for facilitating communication between an ultrasonic transducer probe and an electronic console that performs beamforming and signal processing.
BACKGROUND OF THE INVENTION
Ultrasonic or acoustic sensing techniques have earned a pre-eminent position in a variety of fields including medicine, nondestructive testing and process monitoring, geophysics, and sonar surveillance. For several decades, applications have exploited the relatively low expense, reliability, and enormous versatility of this modality. A strong theoretical understanding of ultrasonics has been developed in parallel with this practical knowledge, so that improved performance has steadily continued. Much of today's research and development is aimed at increasing the number of elements in an ultrasonic array, decreasing the size of the elements, or achieving both simultaneously. The resulting arrays would provide improved spatial or temporal resolution either by using higher frequencies, or by using true two-dimensional arrays for volumetric imaging. However, such advances present formidable technical challenges. Two major obstacles are element impedance and fabrication issues, and issues concerned with cabling between the sensor head and the electronic console.
Most ultrasonic transducers rely on the piezoelectric effect to detect and generate acoustic waves. The design and fabrication of piezoelectric elements remains as much an art as a science, and proves increasingly difficult as element size is decreased or element number increased. Difficulties are in part mechanical: the actual construction and handling of many extremely small components, the fabrication and “dicing” of multi-element arrays, reproducibility, and yield. Another major concern is electrical: as an element decreases in size, its impedance increases. Impedance matching, critical to signal sensitivity, presents additional complications. In particular, as the element impedance increases relative to the combined impedances of the coaxial line and the receiver circuit, less signal reaches the receiver circuit. Thus, for a given piezoelectric material, a reduction in element size is accompanied by a reduction in signal sensitivity.
The electrical cable bundle linking an ultrasonic array and its electronics also presents problems. Proper shielding is vital, since the cables are a major noise source. Cable length is restricted by the wire impedance relative to the element impedance. Furthermore, fabrication becomes more difficult as the number of array elements, and consequently the number of connecting wires, is increased. To avoid fabricating a hopelessly bulky and unmanageable cable, manufacturers must continually decrease the size of their coaxial wires. Although present technology can enable about 100 coaxial lines to fit into a narrow (a few millimeters in diameter) cable, cable size reductions cannot be continued indefinitely; at such small wire diameters, DC (direct current) resistance becomes significantly high. Additional practical difficulties are presented simply in using extremely small wires, for instance, in wire bonding or soldering.
In response to these challenges, optic, instead of electronic, control of piezoelectric elements has been proposed. Acquisition and control opto-electronics could be coupled to a transducer head via a fiber optic bundle; communication with the compact head would be by optical fibers. With present fiber optic technology, enough fibers for a 100×10 element array could fit inside a cable only a few millimeters in diameter. A thinner, more flexible cable of virtually any length offers added operator convenience, especially for medical use. Medical implementations such as ultrasonic catheters and endoscopes could similarly benefit. Radioactive or other harsh environments could be inspected remotely, without damage to sensitive electronics. Ultrasonic evaluation of large, complex, and limited-access components, such as long tubes, bores, or piping, could be performed more easily. In addition, optical methods of communicating between a piezoelectric transducer array and an electronic console could enable new applications that are not feasible with present technology, for instance in remote sensing or “smart structures”.
To facilitate optical communication between a transducer probe and an electronic console, it has been proposed to detect ultrasound using a micro-cavity laser, which requires only an optical connection to the transducer probe. The proposed prior art method uses a monolithic laser cavity, such as a microchip laser, in place of a piezoelectric crystal. In its simplest form, the microchip consists of small “chip” (of area ≈1 mm
2
) of a lasing medium, which is cut and polished flat on two parallel sides. By depositing dielectric mirror coatings on these flat sides, a laser cavity is defined. Lasing is accomplished by optically pumping with a separate laser tuned to an absorption band of the microchip. The materials that have seen the most development as microchip laser media include neodymium-doped crystals such as Nd
x
Y
3−x
Al
5
O
12
(Nd:YAG) and Nd
x
Y
1−x
VO
4
(Nd:YVO
4
). These crystals have exhibited quite efficient CW lasing (≈30% optical efficiency) when pumped either by a Ti:sapphire laser or a diode laser.
The proposed prior art method relies on the fact that when the cavity length (L) of the laser is changed, the optical frequency (&ngr;
o
) emitted by the laser changes such that the fractional length change is equal to the fractional frequency change as set forth in the following equation:
Δ
v
v
o
=
Δ
L
L
(
1
)
When the laser cavity is placed in a time-varying acoustic field, the cavity length of the laser should be modulated with the same time dependence as the acoustic field and with an amplitude related to the amplitude of the acoustic field. As a result, Eq. (1) shows that the optical energy output of the laser should be frequency modulated with a time dependence and amplitude determined by the respective time dependence and amplitude of the acoustic field. The frequency-modulated optical energy can then be demodulated and converted into an electrical output signal remotely from the microchip for signal analysis. The original time-varying acoustic field can be recovered by frequency demodulating the optical signal using a slope filter.
In order for the aforesaid detection method to be advantageous, the laser detector must be of small size (e.g., active area <1 mm
2
) and free from any electrical connections. Microchip laser technology satisfies these requirements. A microchip laser comprises a “chip” of a lasing medium such as neodymium-doped yttrium aluminum garnet (Nd:YAG), fabricated with dielectric mirror coatings on two ends so that it can be optically pumped. When pumped by a wavelength corresponding to an absorption band, the lasing process can be accomplished with a remotely situated, low-power laser delivered through an optical fiber. The mirror coatings can be arranged so that the microchip laser output energy returns through the same fiber. Since the return light is a different wavelength from the pump light, it can be separated with a wavelength demultiplexer and then frequency demodulated to extract the optical signal component determined by the time dependence and amplitude of the acoustic field.
Equation (1) shows that the frequency shift experienced by the microchip laser output energy depends on the macroscopic strain (&Dgr;L/L) experienced by the microchip. The macroscopic strain is related to the microscopic strain S(x,t) by the following equation:
Δ
L
(
t
)
L
=
1
L
∫
o
L
S
(
x
,
t
)
ⅆ
x
.
(
2
)
where x represents position along the thickness of the micro-cavity laser and t is time. Given any arbitrary acoustic disturbance in the microchip, the change in lasing frequency can be calculated using Eqs.
Duggal Anil Raj
Smith Lowell Scott
Breedlove Jill M.
Budd Mark O.
Cabou Christian G.
General Electric Company
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