Noncontact Ultrasonic Detection in Low-Pressure ... - OSA Publishing

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 35, NO. 23, DECEMBER 1, 2017

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Noncontact Ultrasonic Detection in Low-Pressure Carbon Dioxide Medium Using High Sensitivity Fiber-Optic Fabry–Perot Sensor System Junfeng Jiang, Tianhao Zhang, Shuang Wang, Kun Liu, Chao Li, Zixu Zhao, and Tiegen Liu

Abstract—For the application of ultrasonic detection in lowpressure carbon dioxide medium, we propose a fiber-optic FabryPerot (F-P) ultrasonic sensor system based on an edge-stretched circular polymer diaphragm. The diaphragm is fabricated with pre-tension of 200 N/m. The demonstrated demodulation system is based on quadrature phase-shifted demodulation method. By setting the F-P cavity length and the wavelengths of distribute feedback (DFB) semiconductor lasers, two quadrature-phase interference signals can be obtained. Experimental results showed that the proposed sensor system can have a sensitivity of 10.4 nm/kPa and a signal-to-noise ratio (SNR) of 14.45 dB under gas pressure of 270 Pa in low-pressure carbon dioxide. The unique advantages of the proposed sensor system make it attractive for non-contact ultrasonic detection in low-pressure gas medium. Index Terms—Carbon dioxide, fabry–perot interferometer, low pressure, ultrasonic sensor.

I. INTRODUCTION LTRASONIC is a kind of high-frequency sound wave with longer propagation distance for its directionality and penetrating ability [1]. It can be used for ranging, velocity measurements, topographical reconnaissance, local wind field measurement [2]. Propagation of ultrasonic wave is closely related to atmospheric composition, pressure, temperature, humidity and frequency of the ultrasonic waves themselves [3]. The mean atmospheric pressure on Mars is about 600 Pa, and the atmospheric pressure can change violently. The main component of the Mars atmosphere is carbon dioxide. It can be predicted that

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Manuscript received July 10, 2017; revised September 19, 2017 and October 18, 2017; accepted October 18, 2017. Date of publication October 22, 2017; date of current version November 16, 2017. This work was supported in part by the National Natural Science Foundation of China under Grants 61505139, 61227011, 61378043, 61675152, and 61475114, in part by the National Instrumentation Program of China under Grant 2013YQ030915, in part by the Tianjin Natural Science Foundation under Grants 13JCYBJC16200 and 16JCQNJC02000, and in part by the Shenzhen Science and Technology Research Project under GrantJCYJ20120831153904083. (Corresponding authors: Shuang Wang and Kun Liu.) J. Jiang, T. Zhang, S. Wang, K. Liu, Z. Zhao, and T. Liu are with the College of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China, and also with the Key Laboratory of Optoelectronics Information Technology, Ministry of Education, Tianjin 300072, China (e-mail: [email protected]; [email protected]; sarahwang02166@ gmail.com; [email protected]; [email protected]; [email protected]). C. Li is with the Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2017.2765693

the attenuation of ultrasonic waves on Mars is much larger than that on Earth [4]. Other propagation features will also be different. Many numerical simulation analyses have been done on propagation characteristics of acoustic waves in low pressure carbon dioxide medium, but there is no relevant experiment to verify it [5]. In order to cope with the needs of studying ultrasonic propagation characteristics in Mars atmosphere, it is necessary to find a suitable sensor system, which can be proper functioning in the ultrasonic detection experiments in this environment. Fiber-optic interferometers have been extensively studied for ultrasonic detection. Fiber-optic sensors have unique advantages over traditional electrical sensors, such as small-size, lightweight, being immune to electromagnetic interference, remote detection and multiplexing capability [6]. Fiber optic F-P sensors, Bragg gratings (FBG) ultrasonic sensors and Sagnac ultrasound sensors are three kinds of extensively studied sensors for ultrasonic detection. FBG sensors and Sagnac sensors can be self-adaptive by using tunable lasers with complex feedback loops or tunable optical band-pass filters. These methods also bring poor stability and bulky problems [7], [8]. Moreover, in the case of non-contact detection, the detection sensitivities of these sensors are significantly limited due to the all-fiber structure [9]. Fiber optic F-P sensors show unique advantages in noncontact ultrasonic detecting for its simple signal processing and high sensitivity at the quadrature point (Q point) with proper diaphragm [10]. In recent years, various materials have been used as one of the mirrors of the F-P cavity, such as silver [11], silica [12] and graphene [13]. Sensitivity of these sensors can be improved by using diaphragms with small thickness and large diameter. In some researches, silver diaphragms with small thickness and small diameter were fabricated by introducing MEMS technology, which can achieve a high detecting frequency [14]. However the small thickness designed to achieve high sensitivity makes it uneasy to fabricate, and the small diameter limits its sensitivity to some extent. Also the enclosed F-P cavity structure makes the sensor unsuitable for applications where gas pressure changes greatly. In other researches, ultra-thin silver diaphragms were fabricated with diameter of 1.6 mm, which achieved a higher sensitivity [15]. However the low resonant frequency of this diaphragm makes it unsuitable for the detection of ultrasonic waves. To improve the detection sensitivity, another method is to stabilize the Q point. Some researchers

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introduced Grating-assisted demodulation using diffraction grating and feedback control system [16]. Some other researchers used broadband light sources and tunable interference filters or fixed dense wavelength division multiplexers (DWDM) to construct two quadrature-phase interference signals [17]–[19]. However, these devices, such as tunable filters, are unstable and susceptible to environmental interference. These methods also can cause huge optical power waste. Therefore, these sensitivity improving methods are not suitable for applications in hash conditions such as Mars atmosphere, where the atmospheric pressure is low and changes violently. In this letter, we present a fiber-optic F-P ultrasonic sensor based on an edge-stretched ultra-thin circular polymer diaphragm. Diaphragm of this sensor is fabricated with pretension. To have a stable output even if the initial cavity length drifts [19], we introduce a quadrature phase-shifted demodulation method. The thickness and radius of the sensing diaphragm are 1.2 μm and 2.45 mm, respectively. The value of the pretension can be set according to the frequency of the ultrasonic wave. The intrinsic low acoustic impedance of this diaphragm optimizes our coupling to extremely low acoustic impedance gas environment. Vent hole on the sensor allows it to operate at different gas pressures, so that it can adapt to different pressures. The vent hole makes the proposed sensor applicable to ultrasonic detection experiments in low pressure environments. To achieve a more accurate demodulation and maintain a high sensitivity, quadrature phase-shifted demodulation is introduced. By matching the center wavelengths of the two narrowband DFB laser sources with the F-P microcavity length, two quadraturephase interference signals can be obtained. The performance of the proposed sensor is verified by comparing it to PZT sensor in a series of experiments. These experiments were not carried out under standard atmospheric pressure and compositions, so the sound pressures at sensor position in these experiments were unknown. Experimental results show that this sensor works well in carbon dioxide medium under gas pressure of 270 Pa. Diaphragm deformation under this test condition is ±5.2 nm, the estimated sensitivity is 10.4 nm/kPa, indicating that the sensor system is suitable for ultrasonic detection in low pressure gas medium and can achieve a high detecting sensitivity. II. PRINCIPLE OF THE F-P SENSOR SYSTEM Ultrasonic wave has characteristics of small intensity and high frequency in low-pressure carbon dioxide medium. Organic macromolecule polymer has advantage of large elastic modulus, and it is easy to process into ultra-thin films. It is very suitable for the usage as an ultra-thin film for weak acoustic signal detection. Unlike edge-clamped circular diaphragm fabricated by MEMS technology such as silica or silver, it is easy to apply pre-tension on polymer diaphragm during the manufacturing process. In fact the upper limiting frequency of an edge-stretched circular diaphragm is proportional to T1/2 /R [20]. The detecting upper frequency limit of the sensor should be less than f0 . 1.2 f0 = πR

T σ

(1)

Fig. 1. (a) Upper limiting frequency under different diaphragm radiuses and pre-tensions; (b) Schematic of the fiber-optic F-P sensor; (c) Interference spectrum of F-P sensor.

Where R is the diaphragm radius and T is the pre-tension applied on the diaphragm. The material areal density σ is related to the material’s properties and thickness. The areal density is proportional to the membrane thickness h. We choose a polyphenylene sulfide (PPS) polymer diaphragm with thickness of 1.2 μm, which is the thinnest diaphragm that the PPS could be made into. Areal density of this material is 1.62 g/m2 . Fig. 1(a) shows the upper limiting frequency changes as radiuses and pre-tensions changes. For the application of 40 kHz ultrasonic detection, pre-tension can be set to 200 N/m with radius of 2.45 mm so that the upper limit frequency is 54.3 kHz. The frequency of 40 kHz is chosen for its good directionality and relatively small acoustic absorption coefficient in low pressure carbon dioxide gas according to the numerical simulation [2], [5]. The structure of the F-P ultrasonic sensor designed for this application is shown in Fig. 1(b). Sensitive element is the proposed ultrathin polymer diaphragm, forming a microcavity with the fiber end face. The polymer diaphragm is pressed and fixed to the ferrule while a tension is uniformly applied on it. The

JIANG et al.: NONCONTACT ULTRASONIC DETECTION IN LOW-PRESSURE CARBON DIOXIDE MEDIUM

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condition: ϕ1 =

4πL0 , λ1

Δϕ = 4π

ϕ2 =

4πL0 λ2

λ2 − λ1 π L0 = nπ + , λ1 λ2 2

n = 0, 1, 2, 3 . . . . . . (2)

The two interference signals which meet this orthogonal condition can be expressed as: f1 = A1 + B1 cos ϕ(t) f2 = A2 + B2 sin ϕ(t)

(3)

Where A1 and A2 is the constant term, B1 and B2 is the amplitude of the interference term, ϕ(t) is the phase information modulated by acoustic signal: ϕ(t) = 2kL(t), k = 2π/λ1

(4)

Where L(t) is the F-P cavity length modulated by ultrasonic signal. By extracting the normalized AC signals of the two interfering signals, expressed as g1 = cos ϕ(t), g2 = sin ϕ(t), the phase information can be calculated: t ϕ(t) = [g1 g 2 − g 1 g2 ] dt (5) Fig. 2. (a) Schematic of quadrature phase-shifted demodulation system; (b)Simulation curve of demodulation result under different cavity length changes with initial cavity length of 73 μm.

ferrule, D-type capillary and the fiber are fixed together by epoxy. The cavity length is adjusted by a nanometer displacement table during the bonding process of fiber and D-type capillary. The gap between the D capillary and ferrule forms the vent hole. This F-P microcavity is connected to the outside environment through this vent hole to balance the internal and external air pressure. The vent hole allows the sensor to accommodate different ambient pressure. When using SLD source with Gaussian Spectrum, the interference spectrum of proposed sensor is shown in Fig. 1(c). We introduce a quadrature phase-shifted demodulation system to achieve high detection stabilization and accuracy. Schematic of the proposed system is shown in Fig. 2(a). Two laser beams with different wavelengths from two narrowband DFB lasers are bundled into a single mode fiber through a dense wavelength division multiplexer (DWDM) and then incident into the optical fiber F-P sensor. The reflected optical signal, which carries the ultrasonic signal, contains two wavelengths. The signal is separated by a DWDM and then converted into electrical signals by photodiode (PD) and enlarged by amplifiers (AMP). The gain of this AMP is 10 dB. Data acquisition (DAQ) card collects the two interference signals and transmits them to a computer for further calculation. By setting the wavelengths of the DFB lasers and F-P cavity length, two interference signals of quadrature-phase can be obtained. In this proposed system, to obtain quadrature phase-shifted signals, relationship between the two wavelengths λ1 and λ2 of DFB sources and the F-P cavity length L0 matches following

0

In following experiments, the cavity length of the F-P microcavity is set to 73 μm. Because we found that under this cavity length, two reflected beams have the same intensity. Center wavelengths of the DFB lasers are set to λ1 = 1546.02 nm, λ2 = 1550.12 nm. According to (2), these two interference signals can match the orthogonal condition. We analyzed the demodulation result changes when initial cavity length drifts. The excitation signal is a 40 kHz sin-signal. Fig. 2(b) shows the demodulation results under different cavity length changes. The initial cavity length is 73 μm. The ΔL represents the amount of drift in cavity length. The attenuation ratio of the demodulation results is only 0.2‰ when ΔL is 1 μm and 5.4‰ when ΔL is 5 μm. III. EXPERIMENTS AND ANALYSIS In order to prove that the proposed sensor and demodulation system can work well in low-pressure carbon dioxide medium, a series of experiments were carried out. All the following results from F-P sensor are calculated from the quadrature phase-shifted demodulation method by (5). We first compare the performance of proposed sensor and system with PZT sensor in an open environment. Experiment schematic is shown in Fig. 3. Ultrasonic driving and emission section is used for generating specific ultrasonic wave signals. The ultrasonic emission part is a transmitting PZT to achieve the conversion from electrical signal to mechanical vibration. Signal generator (SG) and power amplifier (PA) inject sin-signal into the transmitting PZT. Mechanical vibration of transmitting PZT energizes surrounding gases to emit ultrasonic wave. As comparison, a receiving PZT of the same operating frequency is placed in front and aligned with the transmitting PZT. To ensure the coupling efficiency between transmitter and receiver, emission surface and

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Fig. 3.


Schematic of open-environment experimental setup. Fig. 5.

Schematic of low pressure experimental setup.

Fig. 6. Responses from (a) F-P sensor and (b) PZT sensor to ultrasonic signals under different pressures in carbon dioxide gas environment.

Fig. 4. (a) FFT of the output signals of the sensor and the original signals (inset) for the continuous actuation; (b) Long term (5 hours) stability experiment of F-P sensor; (c) FFT of the output signals of the sensor and the original signals (inset) for the burst actuation at atmospheric pressure.

receptive surface are parallel to each other. Fiber-optic F-P sensor is placed close to the receiving PZT. They are separated by a buffer layer (sound-absorbing sponge) to make sure that there is no vibrations pass through the solid structure. To ensure the simultaneous detection of ultrasonic wave, diaphragm of the F-P ultrasonic sensor is placed in the same plane as receptive surface of receiving PZT. We first tested the response of the F-P sensor and demodulation system to continuous sinusoidal ultrasonic signals with frequency of 40 kHz to verify the accuracy and stability of sensor response. In this experiment, the transmitting PZT was powered by continuous signal with a peak-to-peak voltage of 1V. The normalized demodulation results and the FFT of these results from the F-P sensor and the PZT sensor are shown in Fig. 4(a). Signal waveforms from both sensors are similar to each other. They are also consistent with the source signal well, which clearly indicate that the F-P sensor system successfully detected the ultrasonic wave. The strongest peak on spectrum is at 40 kHz and clearly indicates that the F-P sensor response is with the same frequency of the ultrasonic signal. Another peak shows at 80 kHz is the second-order harmonic signal with much lower amplitude than the strongest peak. This harmonic peak is believed to be a result from the nonlinear behavior of ultrasonic source and F-P sensor. We then measured the ultrasonic signal for 5 hours to support that the system is stable. Experimental result in Fig. 4(b) shows that the system has a stabilized output signal for long term stability.


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Fig. 7. (a-b) F-P sensor response and time-frequency representation under 600 Pa pressure in carbon dioxide; (c-d) F-P sensor response and time-frequency representation under 270 Pa pressure in carbon dioxide.

Ultrasonic pulse is helpful for the study of transmission characteristics of ultrasound in time domain. It is widely used in ranging and obstacle avoidance. Therefore, in another experiment, the F-P ultrasonic sensor was used for the detection of ultrasonic pulse. Pulse signal was generated from the transmitting PZT powered by a repeating 1 V peak-to-peak burst signal with 10 cycle counts and a center frequency of 40 kHz. The normalized signals and the FFT of these signals from PZT and F-P sensor are shown in Fig. 4(c). This figure shows that there is only one signal output from F-P sensor. The time delay between source signal and sensor output is corresponding to the sound velocity in air, indicating that there is no ultrasonic signal transmitted through the slide rail. These signals agree well with each other again, indicating that the fiber-optic ultrasonic system successfully detected the burst signal. The signal from F-P sensor exhibits a series of wave packets. The first one with the largest amplitude corresponds to the ultrasonic pulse that directly impinged onto the sensor. The following ones correspond to the ultrasonic pulses due to the reflections from the edges of the receiving PZT. The strongest peak on spectrum is also at 40 kHz. Further experiments were carried out in a pressure chamber. We changed the gas pressure to test the performance of proposed F-P sensor and demodulation system. The experiment setup, as shown in Fig. 5, consists of the ultrasonic driving and transmitting section, pressure chamber and pressure control system (PCS) and fiber-optic ultrasonic sensor system. Both fiber-optic F-P sensors and piezoelectric transducers are fixed on an automatic sliding rail with electric machinery. The distance between transmitter and the receiver can be controlled by machine controller (MC). Experiment was carried out in a pressure chamber. This chamber is 1.2 m in length and 0.9 m in diameter. Pressure control devices were used to provide a closed low-pressure

environment in which pressure can be controlled. During the experiment, the chamber was filled with carbon dioxide gas. Fiber-optic ultrasonic sensors and PZT sensors were placed in this pressure chamber, connected to external devices by optical fibers and electric wires via aviation connectors. Transmitting PZT was powered by a repeating burst signal with 10 cycle counts with peak to peak value of 230 V. During the experiment, gas pressure in the pressure chamber decreased from 10 kPa to 270 Pa. The responses of F-P sensor and PZT sensor to ultrasonic pulse from a distance of 40 cm are shown in Figs. 6 (a)–(b). The distance of 40 cm is the maximum distance the system can achieve. Results from both F-P sensor and PZT sensor decreased when gas pressure dropped from 10 kPa to 3 kPa. Results drop from 0.575 rad to 0.148 rad by F-P sensor system and from 0.372 mV to 0.118 mV by PZT sensor. The SNR of response from F-P sensor system was 32.16 dB when gas pressure is 3 kPa. However, it is difficult to distinguish the response of PZT sensor from noise when gas pressure drops to 3 kPa. Above shows evident advantage that the F-P sensor can have more efficient outputs under this experiment condition compared to PZT sensor. Considering that electrical devices are susceptible to electromagnetic interference, long distance transmission wire, aviation connectors contacted to the chamber and operation of the slide rail motor were all believed to be the possible causes of this obvious noise. There are tailings occurred in the response of F-P sensor under low gas pressure. These tailings are caused by the enlargement of free damped vibration, which is associated with the decrease in gas density as gas pressure drops. Different sensor locations and different modes of ultrasonic waves in the pressure chamber are also responsible for the differences occurred between the responses of F-P sensor and PZT sensor.

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TABLE I SNRS OF F-P SENSOR AND PZT SENSOR RESPONSES UNDER DIFFERENT GAS PRESSURES

study of propagation characteristics of ultrasonic wave in lowpressure carbon dioxide medium. The unique advantages of simple structure and high sensitivity make the proposed sensor and demodulation system attractive in non-contact ultrasonic detecting applications in low pressure gas environment. REFERENCES

We kept extracting until gas pressure was set to 600 Pa. Result from F-P sensor system showed in Figs. 7(a)–(b) demonstrates that there still has an effective signal output. The peak-to-peak value is 0.029 rad and maintains a SNR of 16.52 dB, clearly indicating that we detected the ultrasonic pulse successfully. Considering the special structure of the proposed sensor, it should survive at lower gas pressure. Then we reduced the pressure to the limit of the pressure control device, which is 270 Pa. The result kept decreasing to 0.021 rad as shown in Figs. 7(c)–(d). The diaphragm deformation calculated according to the output signal is ±5.2 nm according to (4). SNR of F-P sensor system under this experiment condition is 14.54 dB. The SNRs of F-P sensor and PZT sensor under different gas pressures are shown in Table I. It clearly shows that the SNRs of F-P sensor are significantly higher than the SNRs of PZT sensor. There is no reference sensor under these test conditions can be found to achieve secondary calibration. Sound pressure at the sensor position is unable to be tested in low pressure carbon dioxide. So the specific value of sensitivity under these test conditions cannot be calculated. According to numerical simulation analyses [5], the sound pressure is approximately equal to 1 kPa when gas pressure is 270 Pa. So the estimated sensitivity is 10.4 nm/kPa. The SNR of output signal and calculated diaphragm deformation indicate that the proposed sensor and demodulation system can work well in carbon dioxide medium under gas pressure of 270 Pa. All these facts indicate that the proposed F-P sensor and demodulation system can achieve a high detecting sensitivity in low-pressure carbon dioxide medium. IV. CONCLUSION In order to answer the requirement of studying propagation characteristics of ultrasonic waves in low-pressure carbon dioxide medium, we demonstrated a non-contact fiber-optic F-P sensor system including an edge-stretched ultra-thin polymer diaphragm based sensor. For the detection of 40 kHz ultrasonic wave, we set the thickness and radius of the sensing diaphragm to 1.2 μm and 2.45 mm with pre-tension of 200 N/m so that the upper frequency limit is 54.3 kHz. The proposed sensor is able to adapt to extreme conditions and shows high stability and sensitivity. The demodulation system based on quadrature phase-shifted demodulation method is introduced to stabilize the output signal from F-P sensor. The experimental results from both PZT sensor and the F-P sensor agree well with each other. In the ultrasonic detection experiment in low-pressure carbon dioxide medium, the sensor system can still maintain a high SNR of 14.45 dB under gas pressure of 270 Pa. Diaphragm deformation under this test condition is ±5.2 nm. The estimated sensitivity is 10.4 nm/kPa when static gas pressure is 270 Pa. The entire experiment provided data support for the

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Junfeng Jiang received the B.S. degree from the Southwest Institute of Technology, Mianyang, China, in 1998, and the M.S. and Ph.D. degrees from Tianjin University, Tianjin, China, in 2001 and 2004, respectively. He is currently an Associate Professor at Tianjin University. His research interests include fiber sensors and optical communication performance measurement.


Tianhao Zhang was born in Shandong, China, in 1992. He received a B.Eng. degree from Tianjin University, Tianjin, China, in 2015. He is currently working toward the M. Eng. degree in optical engineering from Tianjin University. His research interests mainly focus on F-P fiber sensing. He is currently a Professor at Junfeng Jiang’s student.

Chao Li, biography not available at the time of publication.

Shuang Wang was born in Tianjin, China, in 1982. She received the B.S. degree from Shandong University, Shandong, China, in 2005 and the M.S. degree from Tianjin University, Tianjin, China, in 2007. She is currently a Lecturer at Tianjin University. Her research interests include optical fiber sensing and demodulation algorithm.

Zixu Zhao, biography not available at the time of publication.

Kun Liu received the B.Eng., M.Eng., and Ph.D. degrees from Tianjin University, Tianjin, China, in 2004, 2006 and 2009, respectively. He is currently an Associate Professor at Tianjin University. His research interests include fiber physics and chemistry sensing systems.

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Tiegen Liu received the B.Eng., M.Eng., and Ph.D. degrees from Tianjin University, Tianjin, China, in 1982, 1987 and 1999, respectively. He is currently a Professor at Tianjin University. He is also a Chief Scientist of the National Basic Research Program of China under Grant 2010CB327802. His research interests include photoelectric detection and fiber sensing.