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Abstract
Multipoint acoustic sensing system plays an important role in industrial applications. Here, a diaphragm based optical fiber sensor array is proposed, in which each sensor tip is made of 10-layer graphene diaphragm and optical fiber pigtail, with the compact size of about 2.5 mm in diameter. In particular, coherent phase detection is adopted to improve detection signal-to-noise ratio (SNR) and eliminate the demodulation dependence on structural parameters of sensor tips, and thus to achieve the multiplexing ability. Through time division multiplex (TDM), a multiplexing capacity up to 248 in theory can be realized, which is the first time to theoretically demonstrate large-scale acoustic sensor array for diaphragm based fiber sensor by phase detection, to the best of our knowledge. A prototype of 2 *2 sensor array is built to demonstrate the acoustic sensing performance. The field test results show excellent acoustic sensitivity of higher than −136 dB re 1 rad/μPa within the frequency range of 300 Hz~15 kHz, as well as the MDP of only 75 μPa/Hz1/2. Besides, good temperature stability and wide directivity are demonstrated. The proposed sensor array is promising in sound source localization, where the positioning accuracy of 3.55 cm is successfully realized.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Acoustic sensors have been widely used in scientific researches and industrial applications, such as the study of vector sound field, acoustic imaging [1], underwater detecting [2], nondestructive testing (NDT) for large structures [3], partial discharge (PD) detection in power transformers [4,5] and photoacoustic gas detection [6]. In addition to high sensitivity, the real-time multipoint measurement is also a common demand in the above applications. Acoustic sensor array can map the distribution of the whole sound field, thus to better locate and identify the characteristic events. Piezoelectric or capacitive acoustic sensors represent the current state of the art. However, the commercial microphones usually work separately and are expensive to form a microphones array. The capacitive micro-machined ultrasonic transducers (CMUTs) emerges [7], making it possible to achieve two-dimensional sensor array with small size and low cost. Unfortunately, the electronic elements suffer from electromagnetic interference (EMI), and therefore fail to work under the extreme environments with strong electromagnetic, such as in power transformers.
Optical fiber acoustic sensors have unique advantages of multiplexing, high sensitivity and excellent immunity to EMI. Much work has been done in the study of optical fiber acoustic sensors. Among them, the interferometric fiber acoustic sensors are mature developed and widely used, including Sagnac, Mach–Zehnder (MZ), and Michelson interferometers [8–10]. These acoustic sensors own the superiorities of simple-structured, wide frequency response range and long sensing distance, especially the distributed acoustic sensing ability [11,12]. However, the interferometric sensor is not compact, and the spatial resolution is not ideal. Besides, its sensitivity mainly depends on the length of the sensing fiber, which is hard to be improved. Fiber Bragg gratings (FBGs) acoustic sensors [13–15] have multiplexing nature in wavelength domain, but also suffer from low sensitivity [16]. Sensors based on fused-tapered optical fiber coupler [17,18] can be embedded into engineering structures to monitor distributed acoustic emissions (AE). Yet, the tapered fiber sensor is fragile and too sensitive to environmental fluctuations, which limit the development in practical applications.
Diaphragm-based extrinsic Fabry-Perot interferometric (EFPI) fiber acoustic sensors have attracted great attentions recently, due to their compactness and the ultra-high sensitivity. Numerous diaphragms have been reported with different materials, such as chitosan [19], nano-layer silver [20], gold-coated PET [21], multilayer graphene [22] and UV adhesive [23] diaphragms. Unfortunately, the sensing performance of EFPI sensor is cavity-length-dependent [24] by employing the intensity demodulation method, which is the major drawback of this scheme. The working point can easily drift from the optimized Q-point when the background temperature or pressure changes, leading to a degradation in acoustic sensitivity. Moreover, it is quite difficult to fabricate multiple EFPIs with identical cavity lengths. So the Q-points of different sensors are inconsistent, which eliminates the possibility of multiplexing or multipoint detection. There are few reports about the multipoint diaphragm based acoustic sensor. The two-wavelength quadrature method [25] provides the possibility to achieve multipoint detection. However, the system is complex and expensive, where multiple photo detectors are used. The other scheme based on Cepstrum-Division-Multiplexing can improve the multiplex capacity to 20~39 [26]. While, the sensitivity and capacity will be severely restricted by the spectral resolution and range.
Significantly, the improvement of demodulation method is the key to solve the multiplexing issue. Phase-demodulated scheme [24,27,28] can recover acoustic waves from the optical phase change, which is generated by the displacement or vibration of diaphragm. The phase change is independent of structural parameters of sensors, thus to provide more stable measurement and multiplexing ability. However, there are few reports on phase demodulation based multi-point acoustic sensing scheme, which will be a great challenge to be solved in the research.
In this study, we propose a diaphragm based optical fiber acoustic sensor array. With the help of time division multiplex (TDM) and coherent phase detection techniques, signals from multiple sensing units can be demodulated simultaneously. The sensing unit of the array is composed of a 10-laryer graphene diaphragm and an optical fiber pigtail, with only 2.5 mm in diameter. Excellent acoustic sensitivity with wide directivity is experimentally demonstrated, as well as the superior temperature stability. Besides, the capacity of proposed acoustic sensor array has been systematically discussed to be 248. A prototype sensor array is developed and successfully applied to realize sound source location.
2. Acoustic sensing principle
The schematic diagram and photograph of one sensor tip are depicted in Fig. 1(a) and 1(b), respectively. A ceramic sleeve with the side opening is tightly assembled to a ceramic ferrule, in which an optical fiber pigtail is fixed central. The facet of fiber pigtail is polished with an angle of 8° to reduce the Fresnel reflection from the fiber end. A 10-layer graphene diaphragm (JCNANO Tech) is glued on the top of the sleeve with UV-curable adhesive. The adhesive force causes the graphene to laminate onto the substrate and slightly wrap down the sidewall of the cavity. As shown in Fig. 1(c), the force stretches the diaphragm and applies tension on it, thus to keep the diaphragm flat. The extremely thin graphene diaphragm is covered with PMMA, with the total thickness of about 5 nm. The sensor tip is about 2.5 mm in diameter, while the distance between graphene diaphragm and fiber facet is several hundred micrometers. Further, plenty of sensor tips can be manufactured and complexed to form the sensing array, as shown in Fig. 1(d) with a 2 *2 sensor array.
Fig. 1 (a) Schematic diagram of the sensor tip; (b) photograph of the sensor tip; (c) schematic of the applied tension on graphene diaphragm; (d) 2 *2 sensors array
When the acoustic wave is applied on the sensor tip, the diaphragm vibrates accordingly, resulting in the length change between the diaphragm and fiber facet. In this way, the optical phase modulation of the reflected light from the diaphragm is achieved. According to ref [29], the transferred graphene diaphragm has unique properties. Firstly, the diaphragm will wrap down onto the sidewall of cavity because of the Van der Waals force, so there is pre-stress between the diaphragm and sidewall. Secondly, at the thickness level of nanoscale, the acoustic sensitivity of diaphragm has little dependence on the thickness. The relationship between graphene diaphragm deflection h and the applied acoustic pressure p can be established as Eq. (1).
Where represents the adhesion energy per unit area, and a is the radius of the diaphragm. Then, the acoustic sensitivity S can be derived as the following equation, which is proportional to the square of the diaphragm radius a, and inversely proportional to the adhesion energy.Where, is the optical phase change of reflected light, is the refractive index of optical fiber, and is the wavelength of probe light, h is the deflection of graphene diaphragm. Owing that is hard to be precisely controlled during the fabrication process, small difference exists in the sensitivities of different sensor tips.3. Demodulation system setup for multipoint sensor array
The system configuration of the acoustic sensor array is illustrated schematically in Fig. 2. Multiple sensor tips are connected by a 1*N coupler with single mode fiber (SMF) delay line (DL) in different length, and multiplexed through TDM. Due that the reflection signal from graphene diaphragm is weak, coherent phase detection technique for improving the signal-to-noise ratio (SNR) is employed for the sensor array, which up-converts the acoustic signal into the high sideband frequency to reduce the noise. Hence, the laser with narrow linewidth outputs continuous light of 1550 nm, which is split into the probe light and the local-oscillator light by 99:1. The probe light is modulated into pulses with the width of 10 ns through an acoustical optical modulator (AOM), as well as being frequency shifted by 200 MHz. The repetition rate of light pulse is defined here as the optical sampling rate, which can be adjusted according to the actual demands. After amplified by an erbium-doped optical fiber amplifier (EDFA), the probe light pulses launch into the sensing fiber through a circulator. The polarization state of the injected light is adjusted by a polarization controller (PC) to eliminate the influence of polarization dependent fading. Then, the pulses are partly reflected by a fiber Bragg grating (FBG) and enter into the sensor array. Note that the FBG here serves as the referenced reflection point with fixed optical phase. The reflected pulses from sensor array are carried with acoustic signals and interfere with the local-oscillator light. The generated beat frequency signals are received by a balanced photo detector (BPD) and collected by a data acquisition card (DAQ module from National Instrument with sample rate of 2 GS/s). Due to the different time delay of reflected pulses, beat frequency signals from different sensor tips can be distinguished in time domain, as shown in Fig. 3(a). The signal intensities of graphene diaphragm is slightly weaker than that of the FBG, due to the optical couping loss induced by the divergence angle from the diaphragm to fiber core. To prevent signals from overlapping, the length difference of the DL between adjacent sensor tips must satisfy the following expression.
Where is the width of light pulse and n is the refractive-index of SMF.The electric field intensities of the reflected light pulses from FBG and the nth diaphragms can be expressed as:
while, the intensity of local-oscillator light is:In which is the frequency-shift of the probe light, represents the optical phase of the local-oscillator light, is the optical phase of the nth reflected pulses and is of FBG in particular. N is the total number of sensor tips. Therefore, the output signal of BPD can be derived as:The phase-extraction algorithm is proposed to recover the acoustic signals, and the flow chart is shown in Fig. 3(b). A band pass filter (BPF) is used to extract the intermediate frequency (IF) component, so as to eliminate the phase noise. From the filtered data, we can get the reflected Signal(n) of each diaphragm in chronological order. Besides, a pair of orthogonal reference functions can be generated with the usage of Hilbert transform. Multiply the reference function and the Signal(n), orthogonal signals are generated. Then, the differential cross-multiplying (DCM) process is carried out to obtain the optical phase change and recover acoustic signal from it. The optical phase change of the sensing array can be derived as:
It can be seen that the FBG here can eliminate the phase noises from light source or other disturbance on optical fibers in the central office. Besides, the fiber DL for each sensor tip should be coiled and protected in an acoustic shield from the irrelevant environmentally vibration. Since that the phase sensitivity is irrelevant to the absolute value of the distance between the diaphragm and the fiber facet, slight fabricate deviation in different sensor tips is tolerable, and the measurement stability can be improved as well.We have studied the theoretical capacity of the acoustic sensor array. Assuming that the reflected signal can be effectively distinguished when its intensity is ten-fold stronger than that of the Rayleigh backscattering [30]. The signal intensity PS of the nth sensor tip and the Rayleigh backscattering PN can be respectively derived as:
Where P0 is the power of input light, 1/N is the insertion loss from 1*N coupler, the reflectivity R and r of the FBG and graphene diaphragm are 1% and 2%, respectively. α is the attenuation coefficient of the fiber, and are the length of fiber and light pulses. αR and Sn represent the Rayleigh scattering coefficient and the ratio of backscattered Rayleigh scattering, which is about 0.03210−3/m and 10−3 respectively in SMF. Thus, the number N of multiplexed sensors can be deduced as follow.Remarkably, the multipath-reflections exist in this system. The first order multi- reflection can be expressed as:
Where the number of paths M = N-1. However, the multipath-reflection will be seriously reduced by the second weak reflection of FBG and diaphragm, let alone the serious optical loss from the 1*N coupler. Therefore, the multipath-reflection is about weaker than the signal, which could be neglected. The theoretical capacity is calculated to be 248 according to Eq. (11). And the multiplexing capacity can be further increased by using a sensing diaphragm with higher reflectivity. This is the first time to achieve large-scale acoustic sensor array for diaphragm based fiber sensor, to the best of our knowledge.4. Experimental results and discussion
4.1 Frequency response and the minimum detectable pressure (MDP)
One of the sensor tips in the array is selected to demonstrate acoustic sensing performance. The sensor tip is placed in an anechoic test box, where sine acoustic waves of different frequency are applied, produced by a signal generator and a speaker. The demodulated signals are shown in Fig. 4(a)-4(d), with the optical sampling rate of 60 kHz. It can be seen that the measured waveforms of 400 Hz and 2 kHz are regular and smooth without being filtered, showing competitive performance in high-fidelity signal recovery. While the curves of 60 Hz and 20 kHz have a little distortion, which are mainly caused by the unpurified frequency of the sound speaker and the insufficient optical sampling rate of demodulation system, respectively.
The frequency response of the sensor is measured under the calibration of commercial microphone (B&K 4191). As shown in Fig. 5(a), the result exhibits relative high sensitivity of larger than −136dB re 1 rad/μPa within the full measured frequency range from 300 Hz to 15 kHz. And a resonance peak appears at 1.2 kHz with the sensitivity up to −119 dB re 1 rad/μPa. Although the fundamental resonance frequency of 1.2 kHz is not high enough, it can be optimized by adjusting diaphragm parameters for different applications. For example, the resonance frequency can be set higher to detect ultrasound by using a thicker and smaller diaphragm, as well as choosing a material with higher Young’s modulus [22].
Fig. 5 (a) Frequency response from 300 Hz to 15 kHz; (b) Power spectrum of the measured signals when an acoustic pressure level of 25.7 mPa at 3 kHz is applied.
Besides, we measured the power spectrum of output signals when acoustic wave of 25.7 mPa (62.2 dB) at 3kHz is applied. The noise floor is about −82 dBm for a frequency resolution of 20 Hz. It can be seen that the SNR is about 37.68 dB, so that the MDP is calculated to be only 75 μPa/Hz1/2. This excellent MDP makes it favorable for acoustic detections with specific frequency.
4.2 Temperature stability
Temperature stability is of great significant to acoustic sensors. To achieve a reliable acoustic detection, the phase sensitivity of the sensor or the output signals from acoustic sensing system should remain constant under large temperature variation. An experiment is carried out to demonstrate the temperature stability of the proposed sensor tip.
The sensor tip is fixed on a thermo-electric cooler (TEC), which is employed to change the background temperature. The temperature of the TEC is set to be 0°C initially and then gradually turned to 60°C with the increasing step of 1°C. Acoustic waves of 3 kHz with constant pressure are applied to the sensor tip. And the output signals are real-time monitored during the temperature change, as depicted in Fig. 6. The fluctuation of the output signal is about 0.66 dB, proving high measurement stability under the large variation of temperature. The fluctuation is mainly ascribed to the thermal-expansion coefficient inconsistency between different materials of diaphragm and sleeve, where the change of pre-stress brings uncertainty to the acoustic sensitivity. Besides, the phase-noise from fiber DL also induces demodulation error.
4.3 Directivity
Directivity is another important index of acoustic sensor. Most acoustic applications require the point-like omnidirectional receivers, which have uniform acoustic response within a wide angular range. For example, the acoustic sensor with ideal directivity provides excellent imaging SNR in diagnostic ultrasound imaging [31]. Besides, it is suitable for the mapping of sound field distribution, for the reason that the measured value of acoustic sensor is only related to the distance, but independent of the angle between the sound source and sensor tip.
Figure 7(a) depicts the experiment setup for the directivity test. The included angle between sensor tip and the sound source is adjusted from −90° to 90°, while keeping the distance between them unchanged. As shown in Fig. 7(b), the directivity of the sensor at 1 kHz and 2kHz are measured and depicted. Though the directional response of two frequencies differ slightly, they both exhibit excellent and flat acoustic response with wide directivity. The normalized acoustic sensitivities are optimal at 0° and only decrease to 0.72 (−2.85 dB) within the angle range of ± 90°.
Fig. 7 (a) Schematic of directivity experiment; (b) Normalized sensitivity with different acoustic incident angle at 1 kHz and 2 kHz.
4.4 Multipoint acoustic detection
To demonstrate the multipoint sound detection ability of the sensor array, a 2 *2 array is designed and manufactured. The acoustic waves of 3.4 kHz are applied to four sensor tips simultaneously, with the acoustic pressure of 0.6 Pa, 0.9 Pa, 1.2 Pa, 1.52 Pa, 1.89 Pa, 2.14 Pa and 2.4 Pa. The relationship between the measured phase and the applied acoustic pressure has been fitted and plotted in Fig. 8(a). The result shows good linearity and similar sensitivity, while the slight inconsistency of sensitivity is caused by the adhesion energy differences of diaphragms.
Fig. 8 (a) Fittings curves of 2 *2 sensor array at 3.4 kHz; (b) Output signals from four sensor tips with different sound waves applied (Sensor1: 1 kHz; Sensor2: 2 kHz; Sensor3&4: no signal).
Besides, every sensor tips in the sensor array is capable of working independently. We put sensor 1 and sensor 2 into two anechoic test boxes, where sine acoustic waves of 1 k Hz and 2 kHz are applied respectively. Meanwhile, no signal is applied to sensor 3 and sensor 4. The measured acoustic wave of four sensors in 5ms are plotted in Fig. 8(b), demonstrating excellent multipoint acoustic response without crosstalk. This characteristic enables application in distributed detection of acoustic emission (AE), as well as the multi-dimensional sound source localization.
4.5 Sound source localization
Sound source localization is a typical application for multipoint acoustic sensing, and three sensor tips can form the smallest sensor array system for two-dimensional (2D) sound source localization. As shown in the Figs. 9(a) and 9(b), three sensors are placed at the three vertices of a square, with the coordinates of S1(0, −60), S2(−60, 0) and S3(0, 0), respectively. A point acoustic source is set at the position of A (−60, −45) and emits the sound wave at one specific moment, then the sound waves propagate through air and detected by the sensor array simultaneously. As shown in Fig. 9(c), the waveforms measured by three sensor tips have similar morphological characteristics but different in timing. While, the time difference of arrival (TDOA) can help to calculate the distance difference of every two sensors to the sound source.
where ~ represent the time at which sound waves reach three sensors, and is the propagation speed of sound wave in the air. In the experiment, hyperbolic positioning algorithm is adopted to locate the sound [17]. Take (S1, S3) as the focal points, a pair of hyperbola in blue can be drawn according to the calculated . In the same way, another pair of hyperbola in red with the focal points of (S2, S3) can be drawn. As shown in Fig. 9(b), the intersection of the two hyperbola is the location of sound source. The calculated coordinate A’ (−59.74, −44.67) is in good agreement with the actual position A (−60, −45).Fig. 9 (a) Schematic diagram of sound source localization; (b) Result of sound source localization with hyperbolic positioning technique; (c) Waveforms detected by sensor array in time-domain.
The theoretical spatial resolution dx = νS /f is calculated to be 0.68 cm, with the optical sampling rate f of 50 kHz. In order to test the localization accuracy, we conducted repeated experiments. The sound source is set at certain position, while its coordinate is estimated by the acoustic sensor array. The test is repeated for 10 times at 16 different locations, respectively. The results are plotted in Fig. 10, in which the measured positions are marked with red crosses. The diameters of blue circles represent the positioning errors, which are less than 3.55 cm. It can be noticed that the minimum positioning error of 0.83 cm is close to the theoretical spatial resolution of 0.68 cm. The error distribution indicates that the positioning error increases with the distance of the sound source to the sensors. When the sound source is far from the sensors, the distance differenceof each sensor will be too small to be recognized, due to the limited temporary resolution. Actually, the localization precision can be improved by increasing the sampling rate and the number of sensor tips. Meanwhile, according to the frequency response of sensor, this acoustic detection system can accurately locate the sound source with the broad bandwidth of 300 Hz~15 kHz.
Another noteworthy potential is that the size of single unit is tiny, hundreds of sensor tips can be bunched together to form the two dimensional (2D) acoustic sensor array, providing the real-time mapping of the 2D imaging of sound field. Moreover, the networking of the sensor array can significantly reduce the cost and technical complexity of sensing system compared with discrete piezoelectric detectors [32]. Therefore, this sensor array is promising in sound filed mapping, such as the biomedical imaging.
5. Conclusion
We have reported a graphene diaphragm based optical fiber sensor array, as well as the coherent phase demodulation system to achieve real-time multipoint acoustic detection. The sensor tip is tiny with 2.5 mm in diameter. High-sensitivity with wide directivity of higher than −136 dB re 1 rad/μPa within the frequency range of 300 Hz~15 kHz is demonstrated, as well as the MDP of 75 μPa/Hz1/2 at 3 kHz. The sensors also exhibit high temperature stability with only 0.66 dB fluctuation when temperature changes from 0 to 60°C. The theoretical multiplexing capacity of sensor array is up to 248, which breaks through the technical bottleneck in multipoint measurement of diaphragm based fiber sensor. Acoustic sensor array can work synchronously or independently, so that it has certain potential in AE detection, acoustic imaging and sound source localization. Meanwhile, the field test of 2D sound source localization is carried out, demonstrating the positioning accuracy of 3.55 cm.
Funding
National Natural Science Foundation of China (NSFC) (61775072); Science Fund for Creative Research Groups of the Natural Science Foundation of Hubei (2018CFA004); Major Projects of Technical Innovation of Hubei (2018AAA040); Fundamental Research Funds for the Central Universities (2017KFXKJC002).
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