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Abstract
In this study, a novel fiber-optic, multipoint, laser–ultrasonic actuator based on fiber core-opened tapers (COTs) is proposed and demonstrated. The COTs were fabricated by splicing single-mode fibers using a standard fiber splicer. A COT can effectively couple part of a core mode into cladding modes, and the coupling ratio can be controlled by adjusting the taper length. Such characteristics are used to obtain a multipoint, laser–ultrasonic actuator with balanced signal strength by reasonably controlling the taper lengths of the COTs. As a prototype, we constructed an actuator that generated ultrasound at four points with a balanced ultrasonic strength by connecting four COTs with coupling ratios of 24.5%, 33.01%, 49.51%, and 87.8% in a fiber link. This simple-to-fabricate, multipoint, laser–ultrasonic actuator with balanced ultrasound signal strength has potential applications in fiber-optic ultrasound testing technology.
© 2017 Optical Society of America
1. Introduction
Active ultrasound testing techniques have been widely used in nondestructive evaluation, quantitative theory, and structural health monitoring [1–3]. Active ultrasonic testing for detecting and identifying invisible damage in components of aircrafts, bridges, and highways [4, 5] uses ultrasonic actuators that actively excite ultrasound waves within the components and sensors that detect the transmitted, reflected, and scattered ultrasound waves. However, the current piezoelectric (PZT) ultrasonic actuators and sensors require separate electrical wiring and perform poorly in harsh environments, e.g., heavy electromagnetic interference (EMI). Such difficulties can be overcome using fiber-optic ultrasonic actuators and sensors that are stable in harsh environments and have high multiplexibility because of their small size, light weight, and anti-EMI characteristics [6–10].
Fiber-optic sensors for ultrasound detection have developed rapidly [11, 12] but studies on practical fiber-optic ultrasonic actuators are few compared to ultrasonic sensors. For improving photoacoustic conversion efficiency, many photoacoustic conversion materials have been studied, including a graphite and epoxy resin mixture [13], gold nanocomposite and polydimethylsiloxane (PDMS) mixture [14, 15], carbon nanotubes and PDMS mixture [16]. However, these transducers can only implement single-point excitation. Some schemes have been reported to achieve multipoint laser–ultrasonic generation, but success has been limited. For example, a method of directly polishing the fiber cladding till the core and replacing it with photoacoustic material at multiple required locations of the fiber sidewall has been reported [17]. However, controlling the laser energy extracted at every ultrasonic excitation location is difficult, leading to an unbalanced ultrasound wave strength between generation points and weaker signal strengths downstream. The method requires specific fiber polishing equipment and complex preparation, which is not cost-effective. Another method based on a series of different ghost-mode wavelengths tilted fiber Bragg gratings (TFBGs) for multipoint ultrasonic excitation has been reported [18]. Although this method can overcome the problem of unbalanced ultrasound strength and generate ultrasound in a location-scanning manner, its photoacoustic conversion efficiency is low. Moreover, an essential scanning laser and customized TFBGs must be used at a high cost per application.
For multipoint ultrasonic excitation along a fiber, finding a way to effectively tap light from the fiber core into the cladding, which is subsequently utilized by the ultrasonic-generating material, is important. To obtain balanced ultrasound strength, the light has to be tapped at every excitation point in a controllable manner. Therefore, we proposed and demonstrated a novel multipoint, fiber-optic, ultrasonic actuator based on fiber core-opened tapers (COTs) with balanced ultrasonic strength and simple fabrication. The COT structure was fabricated by splicing single-mode fibers (SMFs) using a commercial fiber splicer. This can effectively couple part of the light from the fiber core into the cladding. The coupling efficiency of a COT can be controlled by adjusting its taper length. By linking a series of COTs with the proper coupling ratios in order from small to large, an equal amount of optical power can be provided at each point for ultrasonic generation. Using a pulsed laser amplified by a high-power, erbium-doped fiber amplifier (EDFA), we produced a four-point, laser–ultrasound transducer with high, balanced ultrasonic signal strength. Only standard fiber instruments and materials, including a fusion splicer, cleaver, and SMF, were needed, enabling simple, cost-effective fabrication.
2. Principle and operation
The structural diagram of the COT is shown in Fig. 1(a). It comprises a lead-in SMF and receiving SMF connected by a core-opened fiber taper region. Because of the COT, the condition of single-mode transmission is broken. The light is coupled partly from the core mode into the cladding modes whenever light from the lead-in fiber travels into the COT region. Light in the cladding modes propagates to a certain distance in the coreless cladding and then diffuses. Then, part of the cladding mode light couples back to the core of the receiving fiber and continues steady propagation in the core mode [19]. The remaining cladding mode light, which is a linear combination of the LP0m modes, propagates in the cladding of the receiving fiber. The energy of the cladding modes is absorbed by the ultrasonic-generating materials that replace part of the fiber cladding of the receiving fiber and is transformed by the thermoelastic effect into an ultrasonic wave at the COT point.
The COT was fabricated using a fiber splicer (FSM-80s, Fujikura) in taper-splicing mode [20, 21]. The splicing parameters in this mode were set as follows: Duration time of pre-fusion, 1 s; interval of fiber ends, 10 μm; overlap value, 5 μm; discharge time: 2 s; and power, + 2 bit. Two cleaved fibers were first placed in the splicer and positioned 10-μm apart by the motors of the splicer. For adequately softening the optical fiber ends, the electrode bars perfused the fibers for 1 s, extending the fiber ends into a hemispherical shape. Next, the softened hemispherical fiber ends were pushed 5 μm forward instead of the standard 10 μm by the splicer motors, leading to inadequate contact of the two fiber ends. Then, pullback forces were applied by the motors to the inadequately connected fused fibers, which caused the softened region of the fused fibers to form a taper. In this process, the softened cores of the fiber ends were first stuck together and then immediately pulled apart, forming a small sphere at the center of the taper and creating a COT [Fig. 1(b)]. Note that adjusting the taper welding length value in the taper-splicing mode can create tapers of different lengths, e.g., a 15-μm taper welding length value can form a taper with 80-μm length. Due to the high reproducibility and stability of the splicer’s splicing operation, the COTs fabricated under the same splicing condition have a high degree of consistency both in structure and coupling rate. The taper length effectively influences the COT coupling ratio.
Therefore, to investigate the evolution of the light field through the fiber taper region, we implemented a theoretical analysis using the finite element beam propagation method. To simulate beam propagation in the fiber, we applied the RSoft commercial software, specifically the BeamPROP module. The simulation used Corning SMF-28 parameters. The fiber core diameter and refractive index were set at 8.3 μm and 1.45213, respectively, whereas the cladding diameter and refractive index were set at 125 μm and 1.44692, respectively. The background was set to air, and a COT with a 120-μm taper length was modeled. The taper waist diameter was set at 100 μm according to an actual preparation. The COT structure was set behind a 1000-μm-long lead-in fiber. Fundamental excitation with 1550-nm wavelength was assumed, and the mesh size was set at 0.1 × 0.1 × 0.1 μm.
The longitudinal field distribution of the COT is presented two-dimensionally in Fig. 2(a), and the energy evolution in the core mode, as monitored, is shown in Fig. 2(b). The basic mode (LP01, typically supported by SMF), which has normalized power, steadily propagates up to 1000 μm in the lead-in fiber core until it reaches the taper [Fig. 2(a)]. Because of the opened core of the COT, a series of high-order cladding modes are excited and light travels 120 μm in the coreless cladding taper. The small sphere helps the light converge back into the core. Part of the cladding mode light is coupled back to the right core and propagates in the core mode of the receiving fiber. Because of the total inner light reflection on the cladding–air plane, the remaining cladding mode light makes a forward, lossless propagation in the receiving fiber cladding, on which photoacoustic conversion material is coated for ultrasonic generation. The core mode energy evolution in Fig. 2(b) shows that for a 120-μm taper length, 50% of the energy remains in the core mode of the receiving fiber and can therefore be used to feed subsequent fiber core-offset units.
Fig. 2 Simulation results: (a) Transmission light field distribution of COT; (b) Evolution of core mode energy through the entire structure; (c) Correlation curve between taper length of COT and the energy coupling ratio of the fiber core and cladding.
The correlation between the coupling ratio and COT length was further investigated by simulation. The COT length was set from 0 to 1000 μm, and the COT waist diameter was considered to vary with taper length [(−1/8) taper length + 125 μm] according to a large number of actual repeated experiments. The small sphere diameter in the taper core changed proportional to the COT waist diameter. The COT coupling ratio depends on the taper length and improves rapidly from 0% to 85% as the taper length increases from 0 to 400 μm (Fig. 2(c)). However, the improvement within taper length increments of 400 to 1000 μm is relatively slow. Figure 2(c) shows that a COT with specific coupling ratio can be obtained by adjusting the taper length during the fabrication process. If multiple COT units with specific coupling ratios are set in series according to the mode-coupling ratio, from small to large over one fiber link, equal cladding mode energy can be coupled from each COT unit to further guarantee ultrasonic signal generation with balanced amplitude at each transducer. This in turn improves the energy utilization of the system light source and increases the number of ultrasonic transducers supported by one fiber link system. Unlike ordinary fiber tapers with continuous cores [22–25], this COT has a higher coupling ratio with a shorter taper length because of the opened core, which forms a much more robust and durable structure for miniature device applications.
To achieve multipoint ultrasonic generation with balanced strength, the COTs coupling ratios should be reasonably controlled. For example, we prepared a four-point ultrasonic actuator based on pre-made COTs by adjusting the taper length, with coupling rates of 25%, 33%, 50%, and 99% in order, which ensured that 25% of the total light energy was allocated to each COT point. To accurately evaluate the coupling ratio, the ends of the lead-in and receiving fibers were connected to a narrow linewidth laser and power meter to monitor the power change during fabrication with different taper length. As expected, the final experimental coupling ratios of the four COTs were 24.14%, 33.01%, 49.51%, and 87.8% (Fig. 3). This means that the COTs can couple 24.14%, 25.04%, 25.17%, and 22.30% of the total energy into the cladding, respectively. It is noted that the coupling ratios and the taper lengths of the COTs fabricated in Fig. 3 agree well with the theoretical simulation in Fig. 2(c), which vividly proves the validity of the physical model of the COT.
3. Experiment and discussion
The proposed multipoint, all-fiber, laser–ultrasonic generation system based on COTs is shown schematically in Fig. 4(a). A pulsed laser (VLSS-1550-M-PL–MP, Maxphotonics) with 1550-nm central wavelength, 3-kHz repetition rate, and 5-ns pulse width was used as a seed laser and amplified by an EDFA (MFAS-1550-B-HP-PL, Maxphotonics) with a maximum average output power of >1 W. A high-power fiber isolator was inserted behind the EDFA to impede the backward reflection of light. Four COTs with the above-mentioned coupling ratios were chained together and linked after the isolator. To improve the powerutilization ratio of the cladding modes, we pre-reduced the fiber cladding diameter of the COT behind the fused taper joint to 55 μm using a 40% hydrofluoric acid solution. A typical micro picture of the COT after etching is shown in Fig. 4(b). For efficient laser–ultrasound generation, graphite powder and epoxy resin mixture (Duralco 4460) was used as the laser-responsive, ultrasonic-generating material, which was also used to embed the etched region of COT in a 200-μm-deep slot that was pre-machined into a 0.15 × 5 × 5 cm aluminum plate. The material was then solidified at a high temperature (120°C) for 4 h [Fig. 4(c)]. The graphite concentration and the thickness of the graphite powder and epoxy mixture are 3% and 75 μm, respectively. In the experiments, each of COT was embedded and solidified within an individual aluminum plate as shown in Fig. 4(a). So we can implement multipoint ultrasonic measurement by individually detect the generated ultrasound signal from each COT. The fiber length between each COT is about 1 m. To detect the ultrasound of the proposed actuator, a PZT sensor (V120-RB, Olympus) with a 7.5-MHz resonant frequency was placed under the aluminum plate (on the side opposite the buried fiber) and connected to an electrical amplifier (5660C, Olympus). The generated ultrasound was detected and converted into an electrical signal, which was then amplified by 60 dB and recorded using an oscilloscope (MDO3104, Tektronix).
Fig. 4 Experimental demonstration: (a) Schematic of experimental setup; (b) COT etched after 900-μm taper region; (c) Prepared transducer unit based on COT. AP:Aluminum Plate.
The laser source characteristics were studied before ultrasound detection. The spectrum of the pulse laser after it passed through the EDFA was measured using an optical spectrum analyzer (AQ6370C, YOKOGAWA) [Fig. 5(a)]. The laser pulse after the EDFA was received by a photoelectric detector (PDB410C, Thorlabs) and recorded by an oscilloscope [Figs. 5(b) and (c)]. Figure 5(a) shows that the laser source has a 1,550.2-nm central wavelength, 0.12-nm 3-dB line width, and 1.12-nm wide base. The pulse duration and repetition frequency are 5 ns and 3 kHz as shown in Figs. 5(b) and 5(c), respectively. It should be noticed that the pulse duration of the seed laser is 50 nm, which is a relatively symmetrical Gaussian profile with a steep rising edge (7 ns) and a slow falling edge (40 ns), as shown in the inset of Fig. 5(b). However, the laser pulse after EDFA is not a symmetrical Gaussian profile as shown in Fig. (b). The reason is that the amplification of EDFA we used can't maintain the pulse shape well. Only a small fraction near the peak of the pulse is effectively amplified, leading to a narrowed laser pulse of 5 ns with a small narrow peak in the left side (the not amplified rising edge of the original pulse) and a flat peak in right side (the not amplified falling edge of the original pulse). However, this does not affect the effective laser-ultrasound generation. The average EDFA output power in the experiment was tuned to 120 mW, and per-pulse energy corresponding to 0.04 mJ was calculated. Each COT could couple 25% of the total energy into the cladding; the per-laser pulse energy coupled into the cladding modes at each COT point was ~0.01 mJ.
Fig. 5 Demonstration of laser characteristics and generated ultrasound signal: (a) Laser spectrum after passing through EDFA; (b) Single laser pulse after EDFA, inset: single seed laser pulse; (c) Pulse sequence in a large view with a 3-kHz repetition rate after EDFA; (d) Ultrasonic pulse sequence generated by the first COT with a 3-kHz repetition rate.
The coupled energy in the cladding was absorbed by a graphite/epoxy mixture and transformed into heat. It was then partially converted into an ultrasound wave because of the epoxy’s thermoelastic properties. A series of narrow ultrasonic pulses with 3-kHz repetition frequency is generated from the first COT, which is the same repetition pattern as the laser source, proving that the ultrasonic signal was excited by the seed laser [Fig. 5(d)]. To verify the balanced excitation of the ultrasonic signals, those signals generated by each COT unit were detected and their single ultrasonic pulses are shown in Figs. 6(a)–(d). Ultrasound wave signals with 515-, 543-, 453-, and 450-mV peak-to-peak values from first excitation point to the end were generated, showing strong, balanced, multipoint generation of the proposed actuator. Their Fourier transforms are shown in Figs. 6(e)–(h), wherein the generated ultrasound waves of broad and relatively flat spectrum below 15 MHz and a small peak near 30 MHz. The intensity of the high frequency peak near 30 MHz is much lower (about 40 dB) than the main frequency band below 15 MHz. Because the detected ultrasound wave is influenced by both the ultrasonic actuator and the structure under test [26, 27]. Such spectral characteristics of detected ultrasonic signal are determined by both the COT and whole structure of the aluminum plate. It is noted that the response time of COT3 is slower than the one of COT2 and COT4. This is caused by the slight inconsistency in the COT fabrication process, such as the thickness of the converted material, and the planeness of the aluminum plate. In addition, we have tested the damage threshold of the graphite/epoxy coated on COT. In the experiment, one COT with 50% coupling ratio and 55 μm etched fiber diameter was tested with 5-ns width, 3-kHz repetition rate pulsed laser after the EDFA. The tested COT began to burn when the measured output average power of EDFA was increased to 450 mW, which means 225 mW average power can be coupled into cladding mode and be absorbed by graphite because of 50% coupling ratio COT. Thus the damage threshold of absorbed laser is calculated as 0.075 mJ. There is a maximum of the generation points of the proposed system because the COT actually had a minimum coupling ratio of 12% in the experiments with the proposed method. This means that the maximum of the balanced ultrasound generation points is 8.
Fig. 6 (a)–(d) Ultrasonic pulse signals generated from four COTs; and (e)–(h) their Fourier transforms.
4. Conclusion
Herein, a novel multipoint all-fiber optic laser–ultrasound actuator with balanced signals at each excitation point based on COTs was proposed and demonstrated. Through simulations, the light field distribution in the COT was investigated, indicating that light can be effectively and quantitatively tapped from the fiber sidewall, thus proving the feasibility of multipoint ultrasound excitation with balanced signal strength. A four-point ultrasonic actuator with stable and balanced ultrasonic signal amplitude was experimentally demonstrated. Common materials and simple fabrication make it a potential candidate for use in all-fiber ultrasound testing technology.
Acknowledgment
National Natural Science Foundation of China (NSFC) (61675055, 61575051); Shenzhen Municipal Science and Technology Plan Project (JCYJ20150529114045265, JSGG20150529153336124).
References and links
1. R. J. Colchester, E. Z. Zhang, C. A. Mosse, P. C. Beard, I. Papakonstantinou, and A. E. Desjardins, “Broadband miniature optical ultrasound probe for high resolution vascular tissue imaging,” Biomed. Opt. Express 6(4), 1502–1511 (2015). [PubMed]
2. B. Lin and V. Giurgiutiu, “Development of optical equipment for ultrasonic guided wave structural health monitoring,” Proc. SPIE 9062, 90620R (2014).
3. J. B. Spicer, A. D. W. McKie, and J. W. Wagner, “Quantitative theory for laser ultrasonic waves in a thin plate,” Appl. Phys. Lett. 57(18), 1882–1884 (1990).
4. K. Diamanti and C. Soutis, “Structural health monitoring techniques for aircraft composite structures,” Prog. Aerosp. Sci. 46(8), 342–352 (2010).
5. N. Takeda, Y. Okabe, J. Kuwahara, S. Kojima, and T. Ogisu, “Development of smart composite structures with small-diameter fiber Bragg grating sensors for damage detection: Quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing,” Compos. Sci. Technol. 65(15), 2575–2587 (2005).
6. P. A. Fomitchov, A. K. Kromine, and S. Krishnaswamy, “Photoacoustic probes for nondestructive testing and biomedical applications,” Appl. Opt. 41(22), 4451–4459 (2002). [PubMed]
7. E. Biagi, M. Brenci, S. Fontani, L. Masotti, and M. Pieraccini, “Photoacoustic generation: optical fiber ultrasonic sources for non-destructive evaluation and clinical diagnosis,” Opt. Rev. 4(4), 481–483 (1997).
8. Y. Hou, J. S. Kim, S. Ashkenazi, S. W. Huang, L. J. Guo, and M. O’Donnell, “Broadband all-optical ultrasound transducers,” Appl. Phys. Lett. 91(7), 073507 (2007).
9. L. Belsito, E. Vannacci, F. Mancarella, M. Ferri, G. P. Veronese, E. Biagi, and A. Roncaglia, “Fabrication of fiber-optic broadband ultrasound emitters by micro-opto-mechanical technology,” J. Micromech. Microeng. 24(8), 085003 (2014).
10. C. Hu, Z. Yu, and A. Wang, “An all fiber-optic multi-parameter structure health monitoring system,” Opt. Express 24(18), 20287–20296 (2016). [PubMed]
11. T. Liu, L. Hu, and M. Han, “Adaptive ultrasonic sensor using a fiber ring laser with tandem fiber Bragg gratings,” Opt. Lett. 39(15), 4462–4465 (2014). [PubMed]
12. T. Liu, L. Hu, and M. Han, “Multiplexed fiber-ring laser sensors for ultrasonic detection,” Opt. Express 21(25), 30474–30480 (2013). [PubMed]
13. E. Biagi, F. Margheri, and D. Menichelli, “Efficient laser-ultrasound generation by using heavily absorbing films as targets,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 48(6), 1669–1680 (2001). [PubMed]
14. X. Zou, N. Wu, Y. Tian, and X. Wang, “Broadband miniature fiber optic ultrasound generator,” Opt. Express 22(15), 18119–18127 (2014). [PubMed]
15. N. Wu, Y. Tian, X. Zou, and X. Wang, “Fiber optic photoacoustic ultrasound generator based on gold nanocomposite,” Proc. SPIE 8694, 86940Q (2013).
16. H. W. Baac, J. G. Ok, A. Maxwell, K. T. Lee, Y. C. Chen, A. J. Hart, Z. Xu, E. Yoon, and L. J. Guo, “Carbon-nanotube optoacoustic lens for focused ultrasound generation and high-precision targeted therapy,” Sci. Rep. 2, 989 (2012). [PubMed]
17. V. Kochergin, K. Flanagan, Z. Shi, M. Pedrick, B. Baldwin, T. Plaisted, B. Yellampalle, E. Kochergin, and L. Vicari, “All-fiber optic ultrasonic structural health monitoring system,” Proc. SPIE 7292, 72923D (2009).
18. J. Tian, Q. Zhang, and M. Han, “Distributed fiber-optic laser-ultrasound generation based on ghost-mode of tilted fiber Bragg gratings,” Opt. Express 21(5), 6109–6114 (2013). [PubMed]
19. C. Shen, Y. Wang, J. Chu, Y. Lu, Y. Li, and X. Dong, “Optical fiber axial micro-displacement sensor based on Mach-Zehnder interferometer,” Opt. Express 22(26), 31984–31992 (2014). [PubMed]
20. C. Y. Shen, J. L. Chu, Y. F. Lu, D. B. Chen, C. Zhong, Y. Li, X. Y. Dong, and S. Z. Jin, “High Sensitive Micro-Displacement Sensor Basedon M-Z Interferometer by a Bowknot Type Taper,” IEEE Photonics Technol. Lett. 26(1), 62–65 (2014).
21. B. Sun, Y. Huang, S. Liu, C. Wang, J. He, C. Liao, G. Yin, J. Zhao, Y. Liu, J. Tang, J. Zhou, and Y. Wang, “Asymmetrical in-fiber Mach-Zehnder interferometer for curvature measurement,” Opt. Express 23(11), 14596–14602 (2015). [PubMed]
22. D. Wu, T. Zhu, M. Deng, D. W. Duan, L. L. Shi, J. Yao, and Y. J. Rao, “Refractive index sensing based on Mach-Zehnder interferometer formed by three cascaded single-mode fiber tapers,” Appl. Opt. 50(11), 1548–1553 (2011). [PubMed]
23. S. Zhang, W. Zhang, P. Geng, and S. Gao, “Fiber Mach-Zehnder interferometer based on concatenated down-and up-tapers for refractive index sensing applications,” Opt. Commun. 288, 47–51 (2013).
24. Z. Tian, S. S. H. Yam, J. Barnes, W. Bock, P. Greig, J. M. Fraser, and R. D. Oleschuk, “Refractive index sensing with Mach–Zehnder interferometer based on concatenating two single-mode fiber tapers,” IEEE Photonics Technol. Lett. 20(8), 626–628 (2008).
25. G. Yin, Y. Wang, C. Liao, J. Zhou, X. Zhong, G. Wang, B. Sun, and J. He, “Long period fiber gratings inscribed by periodically tapering a fiber,” IEEE Photonics Technol. Lett. 26(7), 689–701 (2014).
26. C. H. Yew, K. G. Chen, and D. L. Wang, “An experimental study of interaction between surface waves and a surface breaking crack,” J. Acoust. Soc. Am. 75(1), 189–196 (1984).
27. C. B. Scruby, R. J. Dewhurst, D. A. Hutchins, and S. B. Palmer, “Quantitative studies of thermally generated elastic waves in laser-irradiated metal,” J. Appl. Phys. 51(12), 6210–6216 (1980).
