Ultrasound, an acoustic wave with a frequency exceeding 20 kHz, is widely utilized in nondestructive testing, medical imaging, medical treatment, and other sectors owing to its considerable penetration depth, excellent resolution, good directivity, and minimal radiation. As the key component of ultrasound equipment, the ultrasound transducer is used for bidirectional conversion between ultrasound waves and electrical or visual signals. When the ultrasound transducer is used as a transmitter, the transducer converts the input excitation signal into the mechanical structure vibrations of the transducer element, which then releases ultrasound waves into the medium. Conversely, when the transducer is used as a sensor, the incident ultrasonic wave causes the structure deformation of the transducer element, causing the corresponding change in the electric, magnetic, or optical field inside the transducer.
Currently, piezoelectric ultrasound transducers represent the state of the art but have many drawbacks, such as enormous size, narrow bandwidth, and electromagnetic sensitivity. An optical fiber transducer offers an alternative to challenge the present piezoelectric hegemony. A fiber-optic ultrasound transducer, a newly created transducer, is used to convert optical or ultrasound signals to one another. Compared with standard piezoelectric ultrasound transducers, fiber-optic ultrasound transducers have various advantages. For example, most fiber-optic ultrasound transducers are the same size as the normal single-mode fiber (125 μm), making them suitable for minimally invasive detection. Furthermore, ultrasonic detection devices based on fiber-optic ultrasound transducers usually have remarkable resolutions owing to broad bandwidth properties. Easy multiplexing is another benefit of fiber-optic ultrasound transducers, which fits the need for rapid ultrasound imaging without mechanical scanning. To date, ultrasound transducers based on optical fiber have been widely used in photoacoustic/ultrasound imaging, nondestructive testing, structure safety monitoring, and other applications. Although these new technologies or applications exhibit numerous benefits and great development potential, several obstacles and challenges still exist. Therefore, the current development status of fiber-optic ultrasound transducers must be examined and described, providing a development path for further optimizing their performance and expanding their applications.
A fiber-optic ultrasound transducer comprises two parts: a transmitter and a sensor. The research development of two types of fiber-optic ultrasound transducers is summarized below, and the application of fiber-optic ultrasound transducer is reviewed.
As a type of ultrasound transducer, an ultrasound transmitter is used to convert a visual signal into an ultrasound signal. Several optical-fiber-based ultrasonic transducers have been proposed, exhibiting apparent performance differences, such as emission pressure, emission bandwidth, and conversion efficiency, owing to their diverse constructions and materials (Table 1). The materials with a high thermal expansion coefficient, low specific heat capacity, and high light absorption coefficient bring higher excitation efficiency. Currently, ultrasound transducer materials have progressed from single-component materials and are heading for the era of composite materials with improved performance. Several composite materials have been demonstrated, including polydimethylsiloxane (PDMS) + gold nanoparticles, PDMS + carbon black, and PDMS + carbon nanotubes. Among them, the fiber-optic ultrasonic transmitter based on PDMS + gold nanoparticles exhibits the highest excitation efficiency up to 0.073 MPa·mJ-1·cm2 and emission pressure of 0.64 MPa with a measured distance of 1 mm (Fig. 3).
A fiber-optic ultrasound sensor, as another type of ultrasound transducer, is also systematically explained (Table 2). To meet the requirements of high-precision ultrasound imaging, fiber-optic ultrasound sensors with high sensitivity, broad bandwidth, and small size are required. To achieve these goals, various fiber-optic ultrasound sensors based on diverse sensing concepts have been proposed, including the types of intensity, phase, and wavelength modulations. Among them, the phase-modulated Fabry-Perot interferometer (FPI) ultrasound sensor exhibits high sensitivities and wide detection bandwidth. The noise equivalent pressure as low as 1.79 mPa/Hz1/2 and -6 dB detection bandwidth of 34 MHz have been established [Fig. 6(c)].
Another interesting study is the integrated fiber-optic ultrasound transducer. The first to be examined is the discrete device-based ultrasound transducer, which comprises an ultrasound transmitter and an ultrasound sensor, and has been used to image biological tissue. In the pursuit of smaller size, fiber-optic ultrasound transducers with high integration have been developed by combining the ultrasound transmitter and ultrasound sensor onto a single optical fiber. Currently, a fiber-optic ultrasound transducer with a diameter of 125 μm has been developed [Fig. 9(b)], which exhibits an emission pressure up to 846 kPa and a noise equivalent pressure as low as 1.7 kPa.
Fiber-optic ultrasound transducers have been used in industrial and medical fields, such as nondestructive testing, medical diagnosis, and therapy. In medical diagnosis, several imaging techniques based on fiber-optic ultrasound transducers have been developed, including photoacoustic microscopy, photoacoustic tomography, endoscopic imaging, and ultrasound imaging. Particularly, fiber-optic ultrasound transducers have considerable potential in endoscopic imaging owing to their tiny size of only 3.2 mm in diameter and excellent spatial resolution of 46 μm [Fig. 12(a)]. In the realm of nondestructive testing, fiber-optic ultrasonic transducers are progressing toward large-scale array multiplexing. Currently, a nondestructive testing system based on fiber Bragg grating sensors has been able to detect and locate faults. Moreover, partial discharge location with a positioning accuracy of 0.3 m has been realized using fiber-optic ultrasonic transducers [Fig. 15(e)].
To summarize, although the fiber-optic ultrasound transducer technology has achieved remarkable advances, such as high-pressure ultrasonic emission and broadband ultrasound detection, certain flaws still exist, such as low excitation efficiency, low detection sensitivity, and inadequate multiple abilities. Therefore, new materials must be developed and the structure of transducer elements must be optimized to obtain the high efficiency of ultrasound emission. Further, the ultrasound detection sensitivity needs to be further enhanced using resonant structures and new types of sensing materials. Parallel multichannel ultrasound detection should be researched to improve detection speed, which necessitates developing the demodulation technology. In the future, fiber-optic ultrasonic transducers will exhibit great potential in industrial, medical, and other industries.
