Air-coupled fiber Fabry-Perot ultrasonic sensor formed by diaphragm for seismic physical model imaging

Air-coupled fiber Fabry-Perot ultrasonic sensor formed by diaphragm for seismic physical model imaging

Optik - International Journal for Light and Electron Optics 168 (2018) 794–799 Contents lists available at ScienceDirect Optik journal homepage: www...

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Optik - International Journal for Light and Electron Optics 168 (2018) 794–799

Contents lists available at ScienceDirect

Optik journal homepage:

Original research article

Air-coupled fiber Fabry-Perot ultrasonic sensor formed by diaphragm for seismic physical model imaging


Xiaobo Liu, Tingting Gang, Rongxin Tong, Xueguang Qiao, Chi Zuo, Xiaohong Bai, ⁎ Ce Bian, Manli Hu School of Physics, Northwest University, Taibai Beilu 229, Xi’an, Shaanxi 710069, China



Keywords: Fabry-Perot interferometer Fiber-optics sensors Ultrasonic wave imaging Air-coupled

We presented a miniature fiber ultrasonic sensor based on the principle of Fabry-Perot(F-P) interferometer, which comprises a single-mode fiber(SMF) spliced with a hollow core fiber (HCF), whose end face is adhered with UV glue(NOA68). The interface of NOA68 diaphragm-air as one of the two interfaces of F-P cavity is sensitive to ultrasonic wave due to its low Young’s modulus. Thus, it has the ability to detect weak ultrasonic wave reflected from seismic physical model (SPM) in air. Furthermore, the compact sensor has a characteristics to identify orientation of ultrasonic source because of its perfectly symmetrical structure and is capable of detecting wide ultrasonic frequency range from 300 KHz to 5 MHz. The spectral band-side filter technique is used for the interrogation of ultrasonic signal. As expected, using the designed sensing probe, we can reconstruct the SPM in 2D and 3D by the time-of-flight approach.

1. Introduction The ultrasonic imaging of seismic physical model (SPM) is a significant procedure of pre-experiment for oil and gas exploration. This means that the information of oil and gas stored underground can be obtained indirectly according to the stratigraphic texture of SPM imaged by the ultrasound wave simulating seismic waves. The response sensitivity of sensor for acoustic wave is a key to collect the signal of acoustic wave and image different SPM in two-dimensional(2D) or three-dimensional(3D), which contribute to the oil and gas resources exploration and development [1,2]. So far, ultrasonic field detection was mainly performed by using a piezoelectric transducer (PZT) [3,4]. Unsatisfactorily, the sensitivity of the PZT is inversely proportional to the lateral resolution and directly proportional to the size respectively. On the other hand, PZT is susceptible to electromagnetic interference owing to fabrication materials and has a narrow resonant frequency range in virtue of resonance mechanism [5]. In addition, because of its low sensitivity, it is necessary for PZT to use some coupling agent in the water tank to obtain better ultrasonic echo signals. One way to replace PZT as detector of UW is fiber-optics sensors, which have been widely used. Fiber-optics sensor has some advantages such as high resistance to electromagnetic interference and high lateral resolution because the diameter of fiber is generally less than 200 μm ect. At present, the fiber-optics ultrasonic sensors can usually be divided into fiber Bragg grating (FBG) [6,7] and interferometer. Normally, the spectral side-band filter technique is used for the signal demodulation of fiber-optics ultrasonic sensor because its sensitivity can be improved by increasing spectral slope. In FBG ultrasonic probe, the length of the grating should be large as much as possible for increasing the spectral slope. However, when the length of the grating is larger than the ultrasonic wavelength, the sensitivity of the sensor to detect UW will reduce considerably [8]. Moreover, the higher Young’s modulus of optical fiber material limits the sensitivity improving of the FBG ultrasonic sensor. This is the reason that the sensor of FBG can’t ⁎

Corresponding author. E-mail address: [email protected] (M. Hu). Received 10 February 2018; Received in revised form 27 April 2018; Accepted 2 May 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.

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Fig. 1. Fabrication process.

generally realize imaging in air. There are usually four types of interferometric ultrasonic sensors: Sagnac interferometer, Mach-Zehnder interferometer (MZI), Michelson interferometer (MI) [9], and Fabry-Perot interferometer (FPI). As the previous article reported, the fiber-optic FPI has exhibited advanced characteristics of small size, flexible structure, and good stability [10,11]. In addition, the fiber-optic FPI has enhanced sensitivity for change of the optical path length resulted from UW. When ultrasonic radiation acts upon the sensor, the change of the cavity length leads to a drift in the interference spectrum [12]. Some other FPI sensors based on multiple diaphragms have been reported in ultrasonic detection, such as thin silica film [13], par-c polymer [14], and thin silver diaphragm [15] and 353ND diaphragm [16]. But, these sensor have low sensitivity and small signal-to-noise ratio (SNR) because the diaphragms of the sensors have large Young’s modulus. Therefore, ultrasonic detection and imaging usually use coupling agent to reduce transmission loss, the coupling agent, such as water, its acoustic impedance is similar to that of optical fiber, is usually used for the ultrasonic sensors to reduce transmission loss in ultrasonic detection and imaging. However, this process requires waterproof packaging for the sensor and leads to a decrease in sensitivity. Moreover, the waterproof packaging of the sensor will cause a lot of noise and resonance, and so degrade image quality. Therefore, no coupling agent fiber ultrasonic sensor with ultra-high sensitivity is necessary to glean UW signals containing the structure information of SPM. In this paper, we proposed a UW sensor based on FPI and demonstrate that the sensor can be used for SPM imaging in air. The sensor is made of a section of hollow core fiber (HCF) coated with NOA68 diaphragm. The spectral side-band filter technique is adopted in the sensor system. Finally, the 2D and 3D images of the SPM are achieved by reconstructing the detected UW signals using the time-of-flight approach. 2. Sensor design and principle The structure and fabrication process of the sensor is demonstrated as Fig.1.The process is mainly divided into four steps. The first step is to splice the HCF (the internal and external diameters of HCF are 100 and 170 μm, respectively) with 100 microns single-mode fiber to form HCF-SM structure, the excess HCF is cut off with a fiber cutter, leaving 100 microns in length. Then, the structure is fixed on the left of a commercial fusion splicer (FSM-60S, Fujikura, Tokyo, Japan), other single-mode optical fiber, whose end face is cleaned and coated with NOA 68, is fixed on the right end of the fusion splicer. Next, let the single-mode fiber with NOA 68 move towards the right end of the HCF accurately by the fusion splicer, and then separate after the end of HCF and the single-mode fiber have a slight touch, some of the NOA 68 remain at the end of HCF. An ultraviolet lamp is used to illuminate NOA 68 for 30 min [17], a smooth NOA 68 diaphragm can be formed on the end of the HCF. Therefore, the two interfaces of fiber-to-air and NOA 68-to-air form an Fabry-Perot interferometer. Of course, in order to obtain stable characteristics of the sensor, the finished sensor needs to be placed

Fig. 2. (a) micrograph of the UW sensor structure. (b) Schematic diagram of the UW sensor structure. (c) Interference spectrum of the sensor. 795

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for one week in order to form a permanent chemical bond between the fiber and NOA68. The photo of the sensor structure and the interference spectra are shown in Fig. 2. NOA68 is a clear, colorless, and liquid photopolymer which becomes glassy under UV light. The Young’s modulus of NOA68 (∼0.1 GPa) is much lower than that of other materials reported earlier, such as silver (73.2 GPa), silica fiber (70 GPa), and 353ND diaphragm (∼1 GPa). Because of the fact that detection sensitivity of the sensor is inversely proportional to Young's modulus of the material forming the interference cavity, the sensor is able to detect weak ultrasonic when the low Young’s modulus is improved the acoustic response. Therefore, the functioned diaphragm formed by NOA 68 is suitable for fabricating fiber optic ultrasonic sensors which are used for imaging SPM in air. The response mechanism of the sensor is demonstrated theoretically by analyzing the interaction between the ultrasonic and the diaphragm. In this process, in order to simplify the discussion, we consider only the axial strain of the NOA68 diaphragm induced by the UW pressure and ignore the shear stress because the diaphragm is circular symmetric and thin. The deformation of diaphragm affected by ultrasonic sound pressure can cause variations of the cavity length of the sensor. According to the principle of elasticity, the relationship between pressure change and cavity length change can be expressed as follows:

Δl =

3 (1 − μ2 ) r 4 ⋅Δp Eh3 16


Where μ , r and E is the Poisson ratio, the effective radius and the Young’s modulus of the NOA68 diaphragm respectively, and Δp is the change in pressure. The reflected lights in the cavity of the fiber-optic FPI ultrasonic sensor produce interference, and the output light intensity from the cavity is relative to the deformation caused by UW. As the two interfaces of SMF-air and diaphragm-air have low reflectivity, the output light intensity of the sensor can be expressed as follow according to the principle of dual-wave interference.

I = 2R (1 − cos

4πnl ) I0 λ


where R is the reflectivity of the two interface of the cavity, I0 is the incident light intensity, n is the refractive index, l is the cavity length of the sensor, and λ is the laser wavelength. From Eq. (1) and Eq. (2), we can obtain the output light intensity changing with the sound pressure while the ultrasound acts on the sensor.

ΔI =

3πnRI0 (1 − μ2 ) r 4 8πnRI0 Δl = ⋅Δp 2λEh3 λ


Eq. (3) shows that the output intensity is inversely proportional to the Young’s modulus of the functional diaphragm. Therefore, when the ultrasonic energy and the incident light energy remain constant, we can increase the output energy of sensor by using lower Young’s modulus diaphragm. 3. Experiment and discussion The schematic configuration of the UW sensing system based on the FPI is shown in Fig. 3. Light wave with a narrow line width of 0.1 pm and power of 20 mW was emitted by a wavelength tunable laser (TSL710, Santec, Aichi Prefecture, Japan) into the FPI probe. The light wave wavelength was held at the 3 dB position of interference spectrum. The reflected light from the sensor is transmitted to the photodetector (PD, NEW Focus, San Jose, CA, USA) through a circulator. Then, the final signal was transmitted to the oscilloscope (DS2302 A, RIGOL, Beijing, China). The sensor and PZT were connected to the 2D displacement platform above the air tank through the connecting rod in the model. The PZT source and fiber sensor were held at an electric-driven stage, which could move in two dimensions (with a spatial resolution of 2 μm for point-to-point scanning). The distance between the PZT and sensor was 3 cm. The air gap between the model and sensor was 3 cm.

Fig. 3. Schematic diagram of the fiber-optic ultrasonic detection systems. 796

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Fig. 4. Sensor detect ultrasound in time domain signal with the frequencies of 300 kHz (a) and 1 MHz (b) and 5 MHz (c) UW. The distances between PZT and sensor are 1 cm, 4 cm, 7 cm, respectively.

To measure the sensitivity of the sensor in the air, we fixed the sensor on a 1D displacement platform in the same height with the PZT. The distance between the sensor and the PZT was precisely controlled by the 1D displacement platform. Fig. 4 shows the time domain signal at the different distance between the sensor and the PZT, in which ultrasonic frequencies are 300 kHz, 1 MHz, and 5 MHz, respectively. The three experimental results show that all the three frequencies of the ultrasonic signal have high sensitivity (the SNR of 300 kHz sensor is 48.13 dB) in air, which indicates that the sensor can measure 4.7 MHz frequency range of UW. As shown in Fig. 4, with the increase in distance between the sensor and the PZT, the voltage signal detected by the PD becomes lower because the loss of ultrasonic energy increases in the air. Fig. 5 is the frequency domain spectrum obtained by Fourier transform for the time-domain signal. It can be seen that the main response frequency of the sensor is 300 kHz, 1 MHz, and 5 MHz, which is in good agreement with the resonance frequency of the PZT source. In the frequency spectra, there are other resonance signals surrounding the main peaks of 300 kHz, 1 MHz, and 5 MHz because the resonant frequency of the PZT has a certain bandwidth. The SPM imaging is demonstrated as follows. The Figs. 6 and 7 show the UW images of SPMs reconstructed by the time-of-flight approach. As expected, Fig. 7(a) clearly shows the presence of two interfaces and the inclination of the model. Fig. 7(b) shows the reconstructed 3D outline of the model. We can distinguish the upper surface of the clinohedral and the surface of the spherome by reconstructing the image. In conclusion, the interfaces of two physical models are imaged clearly by reconstructing their reflected UW including the amplitude and time-of-flight. The key to success of the proposed sensor in detecting UW in air is its ultra-high sensitivity to a weak UW field. The functioned diaphragm formed by NOA68 with low Young’s modulus produces greater deformation under the same energy of ultrasound,

Fig. 5. Frequency domain spectra of UW at the following frequencies: (a) 300 kHz, (b) 1 MHz, and (c) 5 MHz. 797

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Fig. 6. Diagrammatic sketch of two physical models: (a) a rectangular plexiglass block with a length, width, and thickness of 50, 50, and 5 cm, respectively, is placed by gradient. (b) Organic glass model consists of a spherome with diameter of 12 cm and a clinohedral. (c) The imaging area is 3 cm × 3 cm, starting from the vertex of the spherome.

Fig. 7. Images of physical models: (a) organic glass plate and (b) organic glass model.

resulting in an increase in the output light energy; therefore, the proposed sensor has a higher sensitivity. In our experiment, the 300kHz UW has a large energy loss during the propagation process, and there is a large acoustic impedance difference between the atmosphere and the plexiglass, but the surfaces and boundaries of the two physical models were imaged clearly by reconstructing their reflecting UW signal because of the sensor’s excellent characteristics mentioned above. When the ultrasound emitted from the PZT reaches the upper surface of the physical model, a portion of the ultrasonic energy is reflected, and the remaining energy continues to be transmitted into the model through the upper surface. Moreover, the ultrasonic energy is reflected to the sensor when it reaches the lower surface. First, the sensor's compact structure and good directivity result in good spatial resolution of the sensor. Second, by using NOA68 with low Young’s modulus material, the sensitivity of the sensor becomes higher, which makes the reflection surface of the model more visible. The third is the use of band-pass filters around 300 kHz to eliminate the unwanted noise frequency of ambient electromagnetic interference and piezoelectric resonance. In addition, because of the physical model of the model conversion and the SPM material causing refraction and sound velocity changes, noise will be produced; therefore, digital filtering is required for denoising. 4. Conclusion In this paper, we proposed a fiber-optic FPI probe-based functioned diaphragm for non-contact UW imaging of SPMs. The sensor is based on the NOA68 diaphragm with low Young’s modulus, which can be used for UW imaging in air. The 3D images of the model were reconstructed in the proposed UW scanning system. The SNR of sensor reached 48.13 dB for 300 kHz ultrasound. The spectral side-band filter technique is used to interrogate the sensor. Compared with PZT imaging device, the proposed sensor has the advantages of fabricating easily, high resolution, high signal-to-noise ratio and low cost. And moreover, our some experiments show that this sensor is also applicable to the nondestructive testing in other areas for its high sensitivity, such as mural disease ultrasonic detecting, unearthed ceramic damage ultrasonic diagnosis, and so on. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 61377087) and Shaanxi Natural Science Foundation (No. s2010jc3655). References [1] M. Urosevic, G. Bthat, M. grochau, Targeting nickel sulfide deposits from 3D seismic reflection data at Kambalda, Australia, Geophysics 77 (2012) 123. [2] S. Dou, S. Nakagawa, D. Dreger, J. Ajo-Franklin, A rock-physics investigation of unconsolidated saline permafrost: P-wave properties from laboratory ultrasonic measurements, Geophysics 81 (2016) WA233. [3] S. Park, S. He, Standing wave brass-PZT square tubular ultrasonics motor, Ultrasonics 52 (2012) 880.


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