Photoacoustic ultrasound excitation has great potential in structural nondestructive testing and applications for medical treatments as a promising alternative to electrical ultrasound. This study proposes and demonstrates a multipoint optical fiber laser-ultrasonic transducer system, wherein the fiber-optic ultrasonic transducer is fabricated by a coreless fiber segment’s fusion with single-mode fibers at each end. Simulation and experiment results show that the transducer coupling ratio is dependent on the coreless fiber’s length. The structure of such an ultrasonic transducer is easily manufactured. Thus, the structures of these optical fiber ultrasonic transducers with different coupling ratios are connected in the order of small to large coupling ratios. In this manner, multipoint ultrasonic excitation with equal intensities at each excitation point can be obtained using this simple and low-cost method. Using laser guidance through the optical fiber to generate ultrasound can efficiently solve some shortcomings of traditional ultrasonic transducers, such as large volume, small bandwidth, and electromagnetic interference. Moreover, this type of fiber-optic ultrasound transducer has higher mechanical strength than other fiber-optic ultrasound transducers and is expected to be useful in structural health-monitoring of buildings.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The structural health of facilities in influential sectors, such as civil construction, industrial engineering, and aerospace structure, has attracted an increasing amount of attention in the recent years [1,2]. A series of environmental factors may contribute to damaging the health of these structures, including high temperatures, strong corrosion, and severe collisions . Therefore, the detection of structural health and performance is particularly important, especially with respect to timely discovery and repair of the initial damaged structure to avoid casualties and economic losses. Nondestructive testing (NDT) technology is a noninvasive structural health-monitoring method, which enables the detection of a wide range of structural damage types without causing damage to the inspected structure . In many nondestructive testing technologies, inspection methods based on the principle of ultrasound are widely applied owing to their characteristics of active material penetration, high sensitivity, and fast detection speed [5,6].
Currently, the electrical ultrasonic transducer is widely used, with ultrasonic signals generated via piezoelectric, electromagnetic, and capacitive effects [7–10]. Moreover, ultrasonic transducers with wide bandwidth, high frequency, and compact size are most desirable for advanced applications such as ultrasonic nondestructive testing and medical ultrasound imaging . However, the piezoelectric ultrasonic transducer (PZT), the typical representative of an electrical ultrasonic transducer, has the disadvantages of large volume and limited bandwidth, making it difficult to meet the above-mentioned requirements . The laser-ultrasonic transducer using the photoacoustic conversion effect provides an alternative scheme. All fiber transducers have the advantages of high temperature and corrosion resistance together with high resolution and immunity to electromagnetic interference . Additionally, owing to their small volume and the light weight of optical fiber, they are more suitable for embedded and integrated use .
The two main features determining the functional operation of the optical fiber photoacoustic transducer are the efficiency of the photoacoustic conversion material and the structure of the generator . In the former, the thermal absorption and expansion coefficients of the photoacoustic material determine the efficiency of the laser-ultrasound conversion . Much research has focused on optimization of the conversion efficiency in photoacoustic materials. A series of photoacoustic materials with high thermal absorption or high thermal expansion coefficients have been proposed, such as gold nanoparticles , the graphite–epoxy resin mixture , and carbon nanotube composites . Moreover, the amplitude and spectral width of ultrasonic signals increase following pulsed laser excitation. Regarding the latter aspect, the types of the photoacoustic transducers can be roughly divided into two categories, namely the end face ultrasound generation structure [20,21] and the side wall ultrasound generation structure [22,23]. An ultrasonic generator based on the fiber–end face-type acoustic optical modulator can control the ultrasonic propagation direction and ultrasonic wave shape [24,25], obtaining high laser-ultrasound conversion efficiency. However, for health-monitoring of building structures and precise biological imaging, this single point measurement technology can hardly meet the increasing and urgent demand for multipoint monitoring. The photoacoustic ultrasonic transducer based on the fiber side wall ultrasound generation structure has the ability to realize multipoint ultrasonic excitation. The side wall excitation of the fiber was achieved by Zhou et al. by lapping the side wall of the fiber and applying a gold nanocomposite to the polished surface . This method of polishing the sidewall of the fiber can indeed serve to employ light from the optical fiber to achieve ultrasonic excitation, but it is nevertheless difficult to control the coupling ratio of the laser energy at a single point. Another method permitting precise control involves combining the scanning pulse laser light source and ghost-mode of the tilted fiber Bragg gratings (TFBG) in order to realize multipoint ultrasonic excitation . However, the scanning laser is relatively expensive and the cladding mode in the TFBG affects the coupling ratio of other ultrasound excitation points. Achieving equal ultrasonic intensity at each point can increase the versatility of the system and reduce costs. In multipoint structural health-monitoring, the requirements for each signal excitation point are the same, such that an equal intensity at each point is imperative. Other ultrasound generator structures, such as the core-opened taper (COT)  and core-offset splicing (COS)  structures, achieve multipoint ultrasonic excitation with an equal intensity at each point, but their weak mechanical properties pose other challenges. Therefore, the key to realize uniform ultrasound intensity of all points along the fiber is to find an efficient method to achieve side wall structure excitation with a controllable coupling ratio of the laser light.
In this paper, a novel optical fiber multipoint laser-ultrasound transducer is proposed by directly splicing a segment of coreless fiber between two pieces of single-mode fiber (SMF). This fused structure (SMF–coreless fiber–SMF, SCS) is obtained by fiber cleaving under a microscope followed by splicing using a fiber splicer. The coreless fiber can effectively couple the light transmitted from the lead-in SMF into a series of high-order modes. The coupled light energy is applied for ultrasonic excitation, the mode coupling ratio being dependent on the length of the coreless fiber. A series of coreless fibers with proper length are formed into SCS structures, which are then connected in the proper order for ultrasonic excitation in an equal-strength manner. A five-point ultrasonic transducer system coated with a mixture of graphite and epoxy is proposed and demonstrated, and each transducer is constructed according to the SCS structure with a different coreless fiber length. The proposed multipoint ultrasonic excitation not only efficiently utilizes laser resources and enhances mechanical strength, thereby reducing the cost, but also enhances the potential for engineering application and facilitates standardization. This excitation method with equal signal intensity is expected to be combined with ultrasonic detection to realize fully distributed fiber structural health-monitoring.
2. Fabrication and simulation
A schematic diagram of the components of the multipoint ultrasonic excitation system is shown in Fig. 1(a), and comprises a pulsed laser, high-power erbium-doped fiber amplifier (EDFA), an isolator, and a series of ultrasonic transducers based on the SCS structure. The system workflow progresses from left to right in Fig. 1(a). Primarily, the pulse laser emits a laser pulse with a certain width and pulse period. Then, the laser pulse is amplified with the high-power EDFA, increasing the average power of the laser to the ideal value. The purpose of the isolator is to prevent damage to the instrument by reflected laser light entering the high-power EDFA and laser source. Finally, the laser pulse reaches the ultrasonic excitation points in each SCS structure, thus generating the ultrasonic signal. Therefore, by precisely setting the coupling ratio of each ultrasonic excitation point, the multipoint ultrasonic excitation will generate an equal intensity of ultrasound at each point.
The SCS structure comprises a section of coreless fiber between a lead-in SMF and a receiving SMF, as shown in Fig. 1(b). The diagram illustrates the specific ultrasonic excitation process in detail. A laser pulse is transmitted through the lead-in fiber, breaking the single-mode transmission condition as it reaches the coreless fiber. The coreless fiber disperses the laser energy throughout the fiber. When the dispersed laser light reaches the receiving fiber interface, part of the light energy enters the core of the SMF to form the basic mode transmission, and the other part enters the cladding of the SMF to form the high-order cladding mode and to transmit along a distance. When the cladding-mode resonance reaches the absorbing layer of photoacoustic material, the photoacoustic material absorbs the energy of the laser pulse. Due to the periodicity of the laser pulse, the photoacoustic material will produce a periodic vibration as a consequence of the thermoelastic effect, that is to say, it will generate ultrasonic signal.
Preparation of SCS structure only requires SMF, coreless fiber (Thorlabs, FG125LA, Ø125 µm), hollow core fiber, a commercial splicer (Fujikura FSM-80s), a microscope, and a displacement platform. The procedure for preparing the SCS structure is shown in Figs. 2(a)–2(e). The splicing point between the SMF and the coreless fiber cannot be clearly detected under the microscope, which presents a challenge to the precise cleaving of the coreless fiber to a desirable length. Thus, in order to accurately measure the length of the coreless optical fiber, it was pre-spliced with a hollow core fiber, making the splicing point between the coreless fiber and hollow core fiber clearly observable. In this manner, this splicing point was used to estimate the length of the coreless fiber with the help of a microscope. Subsequently, a SMF was spliced with the cleaved coreless fiber, selecting the automatic fusion mode of the splicer. Next, the fused structure was placed on a translation stage. By adjusting the translation stage under the microscope, the appropriate cutting point could be easily set to obtain the desired length of the coreless fiber. Thereupon, the coreless fiber was spliced with another SMF, and the automatic fusion mode was maintained, thereby obtaining the SCS structure. The microphotograph of a typical SCS structure obtained by the fiber splicer is shown in Fig. 2(f). Thus, by this micromanipulation, a series of SCS structures with different coreless fiber lengths are easily obtained.
In order to better describe the transmission mechanism within the SCS structure, the finite element beam propagation method was applied to theoretically analyze the optical field transmission in the fiber core and cladding. The BeamPROP module in the commercial software RSoft was used to simulate the transmission of light through the fiber. The fundamental mode of the SMF at 1550 nm wavelength is set as the initial emission field. The refractive index parameters and diameter of the SMF are provided by the Corning company. The lengths of the lead-in and receiving SMFs are set to 1 mm and 10 mm, respectively. The properties of the coreless fiber (FG125LA optical fiber model, Thorlabs Inc.) are specified by as a 125 μm diameter, and a refractive index of 1.444 at 1550 nm. The refractive index in the entire simulation background is set to 1, which is the refractive index of air.
As an example, the length of the coreless fiber is set to 90 μm, and an energy monitoring device is placed on the transmission path of the SMF core. Figure 3(a) shows the propagation of light by means of the distribution of the measured longitudinal field in the SCS structure. When light is transmitted through the lead-in fiber, the distribution of the field remains stable and experiences no loss. When the light reaches the interface between the lead-in SMF and the coreless fiber, it is no more limited by the single-mode condition, and disperses in the direction of propagation because of the mismatch of the fiber core. As the propagating field reaches the interface between the coreless fiber and the receiving SMF, because the mode area in the coreless fiber is larger than that in the SMF, part of the light energy couples back into the single-mode fiber core, while the other part enters into the cladding of the SMF and transmits for a longer distance, generating high-order modes in the cladding. The cladding modes transmitted through SMF cladding are transported smoothly for a certain distance under the constraint of air and optical fiber cladding until they are absorbed by photoacoustic material.
The changes in energy along the propagation direction of the SMF core are simulated, as shown in Fig. 3(b). When the laser pulse passes through the coreless fiber, the light energy in the core decreases sharply, with the coupling ratio of the length of the coreless fiber at about 46%. As the light enters into the receiving fiber, the light energy reaching the core part will become the fundamental mode of the SMF (typical LP01), and the optical energy entering the cladding part of the SMF will become a high-order mode (LP0m mode). The high-order modes in the cladding will interfere with each other and propagate through a long distance. Simultaneously, the energy of the fundamental mode in the core remains unchanged until reaching the next ultrasonic excitation point. The coupling ratio of the SCS structure with different coreless fiber lengths is likewise studied. The length range of the coreless fiber is varied from 0 μm to 300 μm, with a scan interval of 20 μm, and 15 total scans. The simulation results are shown in Fig. 3(c). With the increase in length of the coreless fiber, the light power in the fiber core of the receiving SMF is continuously reduced. Thus, the SCS structure with different coupling ratios can be easily obtained by preparing corresponding lengths of the coreless fiber.
The theoretical analysis demonstrates that the mode coupling ratio of the SCS structure is dependent on the length of the prepared coreless fiber. In the manufacturing process of the SCS structure, it is only necessary to master the cutting of a certain length of the coreless fiber. During cleaving, the length of the coreless fiber is controlled by a micro-translation stage. The coupling ratio of the SCS structure can be estimated by measuring with a narrow linewidth laser and an optical power meter. First, the wavelength of the narrow linewidth laser was set to 1550.2 nm. Then, the power of the narrow linewidth laser was directly measured with a power meter by splicing a piece of lead-in SMF and a piece of receiving SMF. Finally, a coreless optical fiber is fused between the two SMFs to record the optical power, such that the corresponding mode coupling ratio of the SCS structure can be calculated by comparing the measured power with and without the coreless fiber insert.
In order to achieve homogeneous multipoint ultrasonic excitation, each ultrasonic excitation point needs to couple with the almost same optical energy. As a typical example, for the five-point ultrasonic excitation system, the energy consumed by each ultrasonic excitation point should be 20% of the total consumed energy. In this system, five SCS structures with different coreless lengths are prepared, and the corresponding mode coupling ratios are 20.17%, 24.46%, 34.86%, 52.21%, and 88.10%. That is to say, 20.17%, 19.53%, 21.02%, 20.51%, 16.53% of the total laser energy can be extracted from the core at each consecutive SCS point. The microphotographs of the SCS structures are shown in the Fig. 4. The fabricated SCS structure is coated with a layer of photoacoustic material, enabling the generation of ultrasound. When these SCS structures are connected to the optical path in the order of coupling ratio from small to large, the multipoint ultrasonic excitation system is completed.
3. Experiment and discussion
The multipoint ultrasonic excitation system is shown in Fig. 5, with each ultrasonic excitation point made up of five SCS segments with different coupling ratios. As shown in Fig. 5(a), the transducer contains a pulsed laser (VLSS-1550-M-PL-MP, Maxphotonics) as the seed light source, with an output wavelength of 1550 nm, and the repetition rate and pulse width set to 3 KHz and 5 ns, respectively. A high-power EDFA (MFAS-1550-B-HP-PL, Maxphotonics) is employed to amplify the pulsed laser. The maximum average power of the amplifier reaches up to 1 W. The high-power optical isolator, connected to the EDFA, effectively prevents the reflected light energy from causing damage to the device. Subsequently, the pulse light reaches each ultrasonic excitation point for ultrasonic excitation. Hence, each point properly absorbs part of the energy from the fiber core, generating the ultrasonic signal.
Considering the energy distribution at the point where the laser couples from the fiber core into the cladding, it is possible to greatly enhance the utilization efficiency of light energy by properly corroding the cladding of the SMF. As shown in Fig. 5(b), hydrofluoric acid (HF, 40% concentration) was used to corrode part of the cladding of the SMF fiber. The cladding diameter of the SMF is 125 μm before corrosion, and decreases to 55 μm after etching. Post-etching, a transition region with a length of about 423 μm was formed at the etched area of the fiber. The photoacoustic material employed in this system is a mixture of graphite (14734 Graphite powder, Alfa, −200 mesh) and epoxy resin (4460 Low Viscosity Epoxy, Duralco). An effective ultrasonic excitation material is formed by mixing graphite and epoxy resin, because of the high absorption properties of graphite and the large thermal expansion coefficient of epoxy. The materials with high absorptivity, such as gold nanoparticles and carbon nanotubes, and materials with higher thermal expansion coefficient, such as PDMS, can also be used to further improve the energy conversion efficiency of the proposed system.
Each SCS ultrasonic excitation segment is placed onto a 5 cm × 5 cm × 1 mm aluminum plate, where a 200 μm deep groove is pre-etched into the mid-area. Then the prepared SCS structure is placed into the slot and coated with the graphite–epoxy compound. Next, the aluminum plate with the SCS structure is placed on a heating platform and heated at 120 °C for 4 hours to solidify the mixture. The completed SCS structural ultrasonic exciter is shown in Fig. 5(c), with the aluminum plate fixed on a bracket. When the system is in operation, a piezoelectric ceramic transducer (PZT, V120-RB, Olympus) with frequency property of 7.5 MHz is used to detect the ultrasonic signals. The frequency spectrum of PZT is similar to that of Gauss distribution. The ultrasonic signal detected by PZT is first transformed into electrical signal, then amplified by about 60 dB through an electric amplifier (5660C, Olympus). The final waveform is displayed and stored by an oscilloscope (MDO3104, Tektronix). As the coupling ratio of each ultrasonic excitation point is carefully designed, a multipoint ultrasonic excitation with equal intensity can be achieved.
The pulse laser light source is of high importance for the effective operation of this system, and its output is worth researching. The repetition rate of the pulse laser is set to 3 kHz, and the pulse width is set to 5 ns. The laser pulse is amplified by EDFA, and the output spectrum after amplification is measured by a spectrometer (AQ6370C, Yokogawa) with a resolution of 0.02 nm, and depicted in Fig. 6(a). The spectrogram of the light source shows that the central wavelength of the pulsed laser is around 1550.2 nm, the 3 dB linewidth is 0.12 nm, and the corresponding bandwidth is 1.12 nm. Then, in order to investigate the time domain characteristics of the pulsed laser light source, the amplification power of the EDFA is reduced in a controlled manner and a certain loss is added to the output. Subsequently, a photodetector (PD, PDB410C, Thorlabs) is used to measure the output signal exiting the amplifier, and an oscilloscope is used to display and record its waveform. Figure 6(b) shows the recorded signal pulse width of 5 ns, and its maximum amplitude near 280 mV. Figure 6(c) shows that the signal amplified by EDFA has the same repetition frequency as the light source, with the pulse interval at about 3.3 ms. Moreover, each pulse has the same peak power and the same pulse width.
Then, the magnifying average power of EDFA is adjusted to 120 mW, and the corresponding single pulse energy is 0.04 mJ. Because the different SCS constructs have different coupling ratios, the segments are connected to the system in the order of from the small to the large coupling ratio. Thus, according to the design of the five-point ultrasonic excitation system, each of the ultrasonic excitation points should be able to extract approximately 20% of the energy from the fiber core to the fiber cladding. That is to say, every ultrasonic excitation point has the energy of 0.008 mJ to excite the ultrasonic signal. The mixture of graphite and epoxy at each ultrasonic excitation point absorbs the coupled light energy in the form of cladding modes which results in periodic expansion and contraction of the material. The ultrasonic signals generated by the first SCS structure are shown in Fig. 6(d). As can be deduced from the diagram, the ultrasonic signal has the same repetition rate of 3 kHz as the pulsed laser, and the peak to peak amplitude value of each pulse is near the 510 mV. Thus, the ultrasonic signal generated has the same period as the light source, with a similarly stable amplitude of the ultrasonic signal.
Finally, the ultrasonic signals of each SCS structure are measured by PZT, and the detection results are shown in Fig. 7. Among these, Figs. 7(a)–7(e) depicts the time domain signal of a single peak corresponding to each ultrasonic excitation point. All ultrasonic excitation signals have the same repetition frequency of 3 kHz. The peak to peak value of each ultrasonic signal depicted in Figs. 7(a)–7(e) can be calculated as 517 mV, 525 mV, 510 mV, 519mV, and 492mV, respectively, which is near the 510mV value. Therefore, the energy conversion efficiency of the transducer is 64.075 mV/J. The small difference between the peak to peak values of the ultrasonic signal is related to the slight deviation of coupling ratios in the SCS structure, as well as to the design and slight inconsistencies of the coating thickness of the photoacoustic material at the detection point. Each signal is provided with approximately equal excitation energy, and the relaxation time is about 5 μs. A fast Fourier transform is performed for each time domain signal, and the corresponding frequency domain spectrum is obtained, as shown in Figs. 7(f)–7(j). The ultrasonic signal has a very wide spectrum of around 10 MHz and a center frequency is near 4 MHz, as depicted by the graph. The center frequency is mainly related to the process of ultrasonic excitation. Generally, the ultrasonic signal exhibits similar properties and pulse shape, which is a strong indication that multipoint ultrasonic excitation with equal signal intensity is achieved. The pulse width of the generated ultrasound signal is narrow in time domain, which produces a wide frequency. The properties of photoacoustic materials and the corresponding detection system will affect the frequency spectrum of detected ultrasonic signals.
More importantly, we analyzed the key information about pulse width, peak value, peak power, and bandwidth of the generated ultrasound signal from time domain and frequency domain. The relationship between the SCS transducer with different coupling ratios and the corresponding pulse width and the peak value of the ultrasound signal in the time domain is shown in Fig. 8 (a). It can be seen that the deviations for amplitude and pulse width of the ultrasound pulse are relatively small. The standard deviation of pulse width of SCS ultrasonic transducer is 0.2125 and the mean value is 5.4544 μs. The standard deviation of the peak to peak value of SCS ultrasonic transducer is 12.70039 and the mean value is 512.6 mV. It can be seen that the performance of the SCS ultrasonic transducer is homogenous in the time domain. Because the coupling ratio of the last ultrasonic transducer is far from the ideal coupling ratio parameters, and the coupling ratio of the transducer is positively related to the excitation amplitude, the standard deviation of the amplitude is large, but it is still within the acceptable range. Then the corresponding analysis was conducted on the frequency spectrum, as shown in Fig. 8 (b), which shows that the peak power and bandwidth of the spectrum are relatively stable. The standard deviation of the peak power of the SCS structure spectrum is 1.12766, and the mean value is −43.7275 dB. The standard deviation of the bandwidth of the spectrum of the SCS structure is 0.2242, and the mean value is 9.8525 MHz. To sum up, the multi-point ultrasonic transducer constructed by us shows the characteristics of equalizing excitation.
After working for long hours, the amplitude of ultrasound signal of five ultrasonic transducers is stable as shown in Fig. 9. The slight drift of the peak-peak amplitude of the ultrasound signal is mainly caused by the floating of light source power. According to the data analysis, the standard errors of the amplitude fluctuation of each ultrasound excitation point are 0.03225, 0.0242, 0.01031, 0.01252 and 0.01977, respectively.
In order to study the mechanical properties of different structures, we fabricated three kinds of micro-fabricated optical fibers: COS, COT and SCS with the same mode coupling rate of 90%. Then we fix these structures between two micro-displacement platforms for strain testing. The distance between the two micro-displacement platforms was 25 cm. The testing results is shown in Fig. 10. We can find that the average strain tolerances of COS, COT and SCS are 3840, 3640 and 4280, respectively, and the corresponding standard deviations are 12.07, 6.75 and 7.93, respectively. In summary, the mechanical properties of the proposed SCS structure are significantly higher than those of other two micro-machined structures.
In the multipoint ultrasonic transducer system, mechanical strength of the ultrasonic transducing structure is an important design requirement. Each downstream excitation point in the single-mode optical fiber is affected by the light energy transmitted from the upstream excitation point. If the mechanical strength of the ultrasonic transducer is not robust, small stress changes can cause damage to the excitation points of the ultrasonic transducer. The mechanical strength of the COT and COS structures is relatively low, and their bearing capacity for radial stress is very small. The SCS structure proposed in this paper does not require deformation of the optical fiber, and therefore the mechanical strength of the optical fiber remains relatively high. Moreover, the difficulty in preparation of ultrasonic transducers is likewise an important factor to consider when evaluating the application ability of transducers. For engineering applications, the difficulty of preparation determines the transducer capability in application. The splicing of the core-open taper and COS structures require manual operation, which increases the labor cost. However, the SCS structure only requires simple single-mode splicing, and thus low technical operability. Finally, the preparation conditions of ultrasonic transducers are also an important consideration. The manufacture of the COT structure requires conical splicing. While the coupling ratio is sensitive to the shape of the cone region, the fabrication accuracy is low. The fabrication of the COS structure requires accurate motor precision, and larger errors lead to an ultrasonic transducer with a low coupling ratio. On the other hand, the fabrication of the SCS structure only requires simple optical fiber cutting and splicing, hence, a simple operation and low system error.
A multipoint optical fiber ultrasonic generation system based on SCS structure, with attributes of low cost, simple structure, and good mechanical performance, has been proposed and demonstrated. The fabrication of SCS structures only requires simple cutting and splicing techniques for the coreless fiber and SMFs. Both experiment and simulations show that the coupling ratio dependence on the length of the coreless fiber in SCS structures. With the calculated selection of the coupling ratio for each the SCS structure, a five-point ultrasonic signal excitation system is constructed and tested. The results show that the ultrasonic transducer system has an equal peak to peak intensity (around 510 mV) and a relatively flat spectrum. The performance and stability of the proposed system are expected to greatly contribute to the field of structural health detection.
National Natural Science Foundation of China (61675055, 61575051); Shenzhen Municipal Science and Technology Plan Project (JCYJ20170815140136635, JCYJ20150529114045265).
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