A high-resolution fiber-optic ultrasonic sensor based on a suspended-core fiber was designed and experimentally demonstrated. The intrinsic Fabry–Perot interferometer consisting of a micro suspended-core from acid corrosion of a grapefruit fiber proved highly sensitive to a wide range of ultrasonic wave (UW) frequencies. A compact interrogation system using spectral sideband filtering was constructed for UW detection. The sensor exhibited significantly improved spatial resolution and detection sensitivity by etching the suspended-core diameter to few microns. Sensor fabrication involves only fiber splicing and corrosion, which provide a self-shielding cladding surrounding and protecting the core from collisions. This sensor is an excellent candidate for high-quality UW detection.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Ultrasonic wave (UW) detection can provide the structural information and mechanical characteristics of specific objects and fields by employing an array of ultrasonic transducers or a single-element receiver to detect the UW signals from thermoelastic expansion (photoacoustic imaging, PAI) or surface pulse echoes (ultrasonic imaging of seismic physical models, SPMs) [1–4]. Piezoelectric transducers (PZTs) are employed in most traditional approaches [5,6]. However, these current-driven transducers have some inherent drawbacks, such as, narrow detection frequency bandwidths and low resistances to electromagnetic disturbances. Especially for PAI, the large diameter or length in the millimeter range limits the spatial resolution, and the detection sensitivity decreases with decreasing size . High-resolution PAI requires a point-like detector to provide the near omnidirectional response. Correspondingly, the detector size needs to be comparable to the wavelength of the photoacoustic signals (subsequent ultrasonic pulses with frequencies in the range of tens of megahertz) [8,9]. It is a massive challenge to fabricate PZTs with element dimensions of tens to hundreds of micrometers and adequate detection sensitivity simultaneously. As an alternative, the optical mean can be exploited to provide significantly smaller dimensions with considerable detection sensitivity for high-resolution PAI [10–13]. For instance, a type of all-optical Fabry–Perot interferometer (FPI) sensor consisting of a thin polymer film spacer sandwiched between two dichroic mirrors provides element dimensions of tens of micrometers and a detection sensitivity two orders of magnitude higher than that of a PZT of sub-100-µm diameter . FPI etalon sensors are utilized as photoacoustic scanners for 3D PAI of soft tissues and can readily realize spatial resolutions ranging from tens to hundreds of micrometers. Nevertheless, a higher spatial resolution on the order of 10–20 µm requires decreasing the system size by reducing the sensor film thickness, interrogation beam spot size, and scan step size, which may become problematic. Fiber-optic sensors in UW detection have attracted significant attention owing to their distinct advantages, such as, their increased responses with controllable cross-sensitivity, micron-level waveguide structures for single-point sensing in restricted areas, geometric versatility that can be configured into arbitrary shapes, dielectric construction, and stabilized UW transmission [15–17]. Thus, fiber-optic UW sensors may even provide the dual advantages of higher spatial resolution and detection sensitivity through approaches more direct and simple than existing techniques.
Recently, fiber-optic FPI sensors have attracted increasing research interest due to their compactness, easy fabrication, high precision, and good stability (immunity to low-frequency disturbances), and most of them are well applied in SPM detection [18–20]. A variety of optical thin films with low Young’s moduli have been developed as Fabry–Perot (FP) reflectors, such as, graphene , silica , parylene-C polymer , and gold or silver diaphragms , forming fiber-optic diaphragm-based FPIs (classified as extrinsic FPIs, EFPIs) ranging from ferrule-based (millimeter) to fiber-tip-based (micrometer) UW sensors. In a probe based on gold film, a ceramic tube is typically utilized to hold the fiber and film, to obtain an air FP cavity and detect weak UWs in air . However, the sensor size is limited to the millimeter scale by the supporting tube. A hollow core fiber section coated with an epoxy resin diaphragm can be used for the scan imaging of SPMs in water . On the downside, a capillary glass tube and polyethylene film are required for its waterproof packaging, which decrease the SPM spatial resolution. Moreover, some polymer materials and metallic films exhibit poor structural integrity, chemical stability, and heat resistance (undergoing, for example, physical collisions, moisture absorption, and atmospheric oxidation). Due to the restricted cavity size and reflection offset from the fabricated assembly and multilayer structure, most EFPI sensors present limited sensitivities with signal-to-noise ratios (SNRs) less than 70 dB. According to the diaphragm deflection model [27–29], these film reflectors must be sufficiently thin (nanometer scale or less) to be deformed significantly by ultrasonic strain waves, which may introduce inconvenience into the film fabrication process and bonding operation. Meanwhile, it is not easy to adjust the length of an FP cavity to obtain a linear spectral slope for filtering interrogation (the input laser wavelength is tuned to the linear range of one interference notch to perform intensity-referenced demodulation) [30,31]. Most such sensors also require additional waterproof packaging so that they can work underwater to realize effective ultrasonic transmission and coupling. The multi-step sensor fabrication process may introduce difficulties in sensing reproducibility. Consequently, it is urgent to explore a new feasible approach to enhance the acoustic performances of fiber-optic FPI sensors in terms of spatial resolution, detection sensitivity, and sensor fabrication for high-quality UW detection.
In this paper, a micro all-fiber suspended-core sensor produced by acid corrosion of a grapefruit photonic crystal fiber (PCF) is proposed and its application for UW detection is discussed. The micron-scale suspended-core forms an intrinsic FPI (IFPI) and is highly sensitive to a wide range of UW frequencies. A compact interrogation system using spectral sideband filtering is employed to characterize its UW responses. Sensor fabrication requires only fiber fusion splicing and corrosion, and the suspended-core fiber provides the dual benefits of improved spatial resolution and increased response sensitivity by simply reducing the suspended-core diameter through acid corrosion.
2. Sensor fabrication and sensing mechanism
Figure 1(a) presents a schematic diagram of the proposed sensor using a suspended-core PCF. The effective sensing element consists only of a superfine suspended core, which is formed by etching the grapefruit PCF of the fusion-spliced single-mode fiber (SMF)-PCF in Figs. 1(b) and 1(c). In particular, the residual outer cladding after corrosion serves as a self-shielding layer surrounding and protecting the suspended core from collisions. Thus, large bend or deflection of the suspended core will not happen in the experiment. As illustrated in Fig. 1(a), the suspended core functions as an FP cavity with two reflections, one from R1 (the splicing point) and the other from R2 (the fiber end face). Eventually, a well-defined IFPI probe is achieved for UW detection, as shown in Fig. 1(k). Compared with the sensors described in our previous works [3,20,25,26,32], this sensor possesses the dual benefits of increased spatial resolution and higher detection sensitivity, which can be readily obtained by decreasing the diameter of the suspended core via acid corrosion.
A cross-sectional image of the grapefruit PCF (Shanghai Fsphotonics Technology Ltd.) is provided in Fig. 1(b). The fiber core is germanium-doped silica with a diameter of 3 µm and a refractive index (RI) of 1.479. Six air holes with a major axis length of about 35 μm are arranged periodically around the core within the fiber cladding. The outermost layer is 20-µm-thick cladding. This thin-walled grapefruit PCF was chosen for its heavy-gauge air holes and micro core so that the suspended core after corrosion (less than 10 µm) would be able to sense weak ultrasonic vibrations. It should be noted that there is an inner cladding wrapping the core in Fig. 1(b) with a diameter and RI of 15 µm and 1.457, respectively, which are comparable to the core parameters of the lead-in/lead-out SMF (9 µm and 1.450, respectively). Before fusion splicing, the end faces of both the SMF and PCF are modified slightly by arc discharge using a Fujikura arc fusion splicer (FSM-100 P + ) at a discharge power of −30 bit to modulate the RI difference and improve the optical reflectivity of the fusion interface, according to the Fresnel reflection mechanism . Then, the two modified fiber tips are spliced via a core self-alignment process under the selected splicing conditions (a discharge power of −30 bit and discharge time of 400 ms), as shown in Fig. 1(c).
Figures 1(d)–1(i) depict the acid corrosion evolution of the SMF-PCF structure to obtain a suspended-core fiber (the PCF is immersed in 49% hydrofluoric acid for various time periods). The acid corrosion is processed at room temperature (25 °C), and the etching rate of the 49% hydrofluoric acid is about 130 µm/h at 25 °C. Since the six segmented claddings separating the air holes are ultra-thin (with a thickness of 1.8 µm) and have a smaller RI than the germanium-doped core, they present a faster corrosion rate as the time increases from 10 s to 70 s. After 40 s, the segmented claddings are almost etched completely, as shown in Fig. 1(g). As the corrosion time progresses, the inner cladding is partly etched at 50 s to a diameter of 7.2 µm. Eventually, the whole periodic structure of the air holes, inner cladding, and fiber core is etched thoroughly, and it disappears at 70 s. The ultrasonic strain sensitivity is highly dependent on the diameter of the suspended core (the relationship of the strain sensitivity to the fiber diameter will be discussed later in Section 2). A thinner diameter may enable a higher UW sensitivity to be achieved. Thus, a corrosion time of around 50 s is an optimal choice for suspended-core fabrication.
The sensor fabrication process involves only fiber splicing and acid corrosion. There is no need for film preparation, manual FP cavity adjustment, or protective packaging due to the self-shielding outer cladding. No mechanical or bonding structure (such as an aluminum cone  or a thin film ) is used, and the assembly of a structure from different materials (such as metal , polymethyl methacrylate , or polypropylene ) is also abandoned. Only a tube is employed experimentally to fix the fiber pigtail to avoid wobbling vibrations from water (water is utilized as the propagation medium for UW detection).
To demonstrate the interference mechanism further, the fiber structures before and after corrosion were each illuminated by a He-Ne red laser to observe the beam distribution visually, as shown in Figs. 1(j)–1(l). Two bright red spots are focused on R1 and R2 both in Figs. 1(j) (SMF-PCF) and 1(k) (the suspended-core sensor). Thus, the input light is clearly reflected by R1 and R2, and the interference pattern is indeed that of an IFPI. Furthermore, the light intensity level on R2 in Fig. 1(l) (SMF-PCF) shows that the light intensity is mainly focused in the core, inner cladding, and six segmented claddings (the bright white area), which demonstrates that besides the core-guided core mode contributing to the IFPI, cladding modes are excited by core diameter mismatch and participate in the IFPI. Besides, the side view of the sensor in Fig. 1(k) and the end-face image in Fig. 1(h) further demonstrate that the suspended core after corrosion keeps straight.
Interference spectra of SMF-PCFs with different PCF lengths are plotted in Fig. 2(a), and the free spectral range (FSR) versus PCF length is plotted in Fig. 2(b). The experimental data agree well with the theoretical curve (solid line), which was obtained by employing the typical FSR equation for an FPI:
Figure 3(a) shows the spectral evolution from the SMF-PCF (the blue curve) to the suspended-core sensor in air (the red curve) and water (the green curve), and the corresponding Fourier transforms into the spatial frequency domain are presented in Fig. 3(b). Since a larger spatial frequency represents a higher-order cladding mode for a fixed PCF length of 240.8 µm, the amplitude peaks in Fig. 3(b) indicate that different-order cladding modes are excited and involved in IFPIs (the excited cladding modes interfere with the core mode and each other) . These cladding modes are weak and participate in modifying the main interference pattern, leading to the inhomogeneous interference spectra in Fig. 3(a). Specifically, the cladding mode group of the suspended-core sensor in air is largely weakened because of the acid corrosion. When immersed in water, the air cavity of the suspended-core PCF is filled with water (the suspended-core structure consisting of the core and partial inner cladding plays the role of a new core, and the surrounding water serves as the new cladding). Then, the mode recoupling to the lead-out SMF is relatively increased, causing the mode excitation and extinction ratio to be improved in water. Compared with the situation in air, the fringe contrast of the interference pattern is largely promoted by using water as the UW couplant, which favors highly sensitive sideband filtering interrogation with a large dynamic range . Thus, the interference patterns in Fig. 3 are the superimposed IFPI patterns of the core mode and excited cladding modes, and the spectral modification of the cladding modes varies with the corrosion and surroundings, in agreement with the visual beam distribution in Fig. 1.
The sensing mechanism of the well-designed sensor can be characterized by analyzing the interaction between the suspended core and UW. In the experiment the PZT source mainly emits longitudinal waves for UW detection. Since the UW wavelength (typically 4.67 mm in water for 300 kHz UW) is much larger than the sensing region of the suspended core, the UW can be regarded as a plane wave and series of homogeneous ultrasonic expansions and compressions. When the UW is applied on the sensor in water, it periodically deforms the suspended core (axial tension or compression). Therefore, the interaction between the suspended core and UW can be modeled as a single degree of freedom mass-spring vibration system, as shown in Fig. 4.
According to the Hooke’s law, the spring stiffness of the suspended core can be expressed asFig. 5.
For the theoretical vibration model in Fig. 4, the axial length of the suspended core is modulated as follows:Eq. (5) and (7), the pressure sensitivity of the sensor can be derived to be
3. Experimental results and discussion
The fiber-optic UW detection system is shown in Fig. 6. A tunable laser (Santec, TSL-710) with a 100 kHz linewidth as the light source is launched into the sensor through an optical circulator (OC), and the reflected light power is converted into an electrical signal by a photodetector (PD, New Focus) with a bandwidth of 10 MHz and finally monitored by an oscilloscope (RIGOL, DS2302A). A water tank is employed because of the improved interference fringe contrast and small acoustic impedance of 1.48 × 106 kg/(m2·s) in water. A PZT held on a motorized positioning stage is driven by a function generator, and the PZT end face is immersed in water for UW emission. The acoustic energy is firstly transduced into optical power by the fiber sensor, then the voltage signals by the PD. The sensor is fixed on an adjustable stage at the tank bottom. To ensure stability and robustness during real-time detection, the SMF pigtail is compactly packaged into a thin steel tube and the free-sensing gauge is about 10 mm (the suspended-core structure connected to one SMF section). The centers of the sensor and PZT are kept in a line perpendicular to the experiment platform, and their vertical distance can be precisely controlled as needed. According to the mounting locations of the motorized stage and PZT, the sensor is vertically fixed and directly faces the emitting end of PZT to obtain the UW response as large as possible. The UW detection was processed in water at room temperature. The water with a relatively large specific heat capacity can provide an almost constant temperature environment around the sensor.
The response of the suspended-core sensor to a 300 kHz sinusoidal UW is presented in Fig. 7, where the output power of the tunable laser was 20 mW, the band-pass filtering of the PD was set from 100 kHz to 1 MHz for denoising, and the vertical distance between the sensor and PZT was fixed as 5 cm. The real-time response in Fig. 7(a) is a quasi-sinusoidal profile with a peak-to-peak voltage of 19.97 V. The frequency-domain spectrum shown in Fig. 7(b), which was obtained by taking the Fourier transform, clearly reveals the vibration frequency of 300 kHz, which is coincident with the emission frequency of the PZT source.
The PZT source was further driven by a square-wave pulse with a frequency of 300 kHz. To demonstrate the response characteristics of the suspended-core sensor, another PZT receiver was used as a reference and compared with the sensor under the same detection conditions. Figure 8(a) shows the difference between the responses of the sensor (the red curve) and PZT (the green curve). The two receivers exhibit comparable UW responses (peak-to-peak voltages of 14.43 V for the sensor and 16.82 V for the PZT). Nevertheless, the suspended-core sensor presents significantly lower resonant noise. Moreover, compared with the fiber-optic devices investigated in previous research [3,20,25,26,32], the sensor exhibits a smoother real-time response and has fewer resonant components in the time domain. Therefore, the proposed sensor is capable of providing strong UW reconstruction for an emission source with small signal distortion. By further increasing the driving voltage of the PZT source with the frequency of the pulsed UW fixed to 300 kHz, the response sequence presented in Fig. 8(b) was obtained, with a peak-to-peak voltage of 24.78 V and an SNR of 81.86 dB. The uniform pulse array confirms the sensing stability, thanks to the support of the tube packaging. In addition, the pulse repetition rate was calculated to be 199.2 Hz, which is coincident with the signal repeat rate of 200 Hz from the UW generator.
The sensor also presents a response to the 5 MHz UW in Fig. 9. As can be seen, the suspended-core sensor can accurately detect the original UW signals. The SNRs of the UW response at 5 MHz (72.15 dB for the continuous UW and 68.81 dB for the pulsed UW) are lower than the SNRs at 300 kHz (79.98 dB for the continuous UW and 81.86 dB for the pulsed UW). Higher ultrasonic emission frequency means larger transmission and coupling loss in water, leading to the decreased SNRs as the emission frequency increases at a fixed driving voltage. Based on the −6 dB widths of the frequency-domain response to continuous UW [35,36], the fractional bandwidths can be calculated as about 25.46% for the 300 kHz UW (−6 dB bandwidth of 76.4 kHz), and 16.2% for the 5 MHz UW (−6 dB bandwidth of 810 kHz), respectively. Moreover, the time-domain spectra for pulsed UW in Figs. 8(a) and 9(b) demonstrate that the resonant components significantly increase as the emission frequency increases, mainly resulting from the PZT source that might launch multiple resonances close to the main frequency. Thus, the suspended-core sensor has a wideband frequency response up to 5 MHz. Unfortunately, the noise equivalent pressure and frequency bandwidth of the sensor could not be characterized now because of the lack of the PZT sources and matching calibration microphones. The further improvement will be ongoing.
A time-domain output of the sensor in response to a 300 kHz sinusoidal UW is acquired at room temperature in Fig. 10. Figure 10(a) shows the time-domain response of 700 µs. The time-domain response from 300 µs to 400 µs marked in Fig. 10(a) is extracted and magnified in Fig. 10(b), and the corresponding fluctuations of signal peak voltages are shown in Fig. 10(c). The maximum fluctuation of the output voltage is only 0.143 V. Compared with the signal voltage (about 9.87 V), the fluctuation can be neglected. The good stability is mainly attributed to the tunable laser source (Santec, TSL-710), which possesses a built-in wavelength monitoring with high wavelength stability of ± 1 pm and power stability of ± 0.01 dB. The output wavelength of the tunable laser can be well located at the optimum bias point of the interference spectrum for high sensitive UW detection.
As the driving voltage of the UW applied to the sensor increases continuously from 25 V to 250 V (with the emission frequency fixed at 300 kHz), the peak-to-peak voltage of the time-domain response increases linearly, as shown in Fig. 11(a). Thus, the output power of the sensor (converted into the electrical voltage) is proportional to the UW strain field (directly determined by the driving voltage at a fixed frequency). The slight fluctuation of the data is the result of the nonlinear increase in acoustic pressure from the PZT source. Moreover, the detection aperture (the angular range of UW reception determined by the sensor size) is another key factor in UW detection . Using the configuration depicted in Fig. 6, the PZT was moved horizontally along the motorized stage (in the Z-direction) while the sensor was fixed underwater, as illustrated in the inset of Fig. 11(b). With the PZT displacement varying from −30 mm to 30 mm in 5 mm steps (and the corresponding direction angle θ varying from −63.5° to 63.5° around the sensor), the signal voltage presents an orientation-dependent response, indicating that the sensor possesses wide directivity for UW detection. Generally, the maximum response at 0° is adopted in UW detection, i.e., the PZT and sensor are face to face in the same line. The slight asymmetry of the response in Fig. 11(b) may result from the fact that the whole displacement process was not strictly symmetrical relative to the sensor.
4.1 Spatial resolution
The suspended-core sensor with a diameter of 7.2 µm (or smaller due to the controllable corrosion process) can enable high-resolution UW detection. If the sensor is used for ultrasonic scan imaging (such as PAI and the SPM), the spatial resolution is a key parameter for characterizing the minimum separation necessary to distinguish two point targets clearly . Theoretically, the spatial resolution of the suspended-core sensor can be estimated by using the following two equations:3,20,25,26,32]. By comparison, the suspended-core sensor with a reduced diameter of 7.2 µm can enable higher-resolution UW detection. Given the UW frequency (from 300 kHz to 5 MHz in ultrasonic imaging of seismic physical models) and the sensor direction angle in water (−63.5°–63.5°), the spatial resolution of the suspended-core sensor can be calculated to be from 2.5 mm (300 kHz) to 151.05 µm (5 MHz), which is improved compared with one previous value of 5 mm . When used in photoacoustic imaging, it may provide an expected value of about several to tens of microns (from 75.52 µm to 7.56 µm).
The suspended-core sensor is highly sensitive to UW, mainly due to its micro diameter. The suspended-core sensor possesses a largely reduced size of several micrometers by acid corrosion, leading to a significant response to UW. Thus, a high SNR of more than 80 dB was achieved in UW detection. Here the SNR is proposed to characterize the UW response of the sensor. The SNR highly depends on the initial noise of experimental setups, especially introduced by the power fluctuations of laser source, transmission line and sensor stability. Thus, the SNR of the proposed sensor is compared to SNRs of other UW sensors in our recent works [3,20,25,26,32]. These sensors are all characterized in the same experimental setup with almost the same noise level of about 2 mV. The SNR of the suspended-core sensor is much larger than SNRs of our previous sensors (27.96 dB for a fiber Bragg grating-FP sensor , 24.08 dB for a micro-bubble FPI sensor , 62.21 dB for a diaphragm FPI sensor ). These results clearly reveal that the proposed sensor is more sensitive to UW. Moreover, the optical reflectivity of the IFPI sensor can be increased by end-surface coating of the suspended core. We believe that the SNR of the sensor will be much larger than the present value.
In conclusion, a fiber-optic suspended-core sensor was designed for high-quality UW detection. The sub-10-µm suspended core proved highly sensitive to a wide range of UW frequencies. The sensor interrogation only requires one PD together with a tunable laser or sideband filter. Both an improved spatial resolution and enhanced detection sensitivity can be simultaneously achieved simply by reducing the suspended-core diameter through acid corrosion. Furthermore, the compact all-fiber sensing structure is convenient for sensor repetition and mass production. In terms of its spatial resolution, response sensitivity, and sensor fabrication, this sensor significantly promotes SPM detection; in particular, it has the potential to provide superior imaging resolution for PAI. Further work will focus on increasing the reflectivity of the IFPI sensor and enhancing the sensitivity by surface-coating of R2.
National Natural Science Foundation of China (61735014, 61327012, 61275088); the Scientific Research Program funded by Shaanxi Provincial Education Department (08JZ58); the Northwest University Graduate Innovation and Creativity Funds (YZZ17088); China Scholarship Council.
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