Directional acoustic signal measurement based on the asymmetrical temperature distribution of the parallel microfiber array

Ran Gao; Jiansen Ye; Xiangjun Xin

doi:10.1364/OE.27.034113

1. Introduction

Sound source localization (SSL) has been extensively applied to applications in partial discharge detection, smart structure monitoring, public security, and so on [1]. Compared with some conventional electronic methods, fiber-optic acoustic sensors offer many advantages, such as high sensitivity, compact size, and immunity to electromagnetic interference [2].

Commonly, sound pressure can be detected by using a microphone, and the SSL can be calculated through an array of two (or more) microphones. To locate the position of an acoustic source, several acoustic sensors placed in different locations are required [3,4]. The time-delay differences between signals received by different sensors may be used to calculate the position of the acoustic source [5,6]. Hence research has been carried out on fiber-optic acoustic sensor arrays, such as the Sagnac sensor array configuration and the diaphragm-based Fabry–Perot interferometer array [7,8]. However, the fiber-optic sensor array must be larger than a critical size in order to improve the accuracy of the SSL, because the direction resolution is dependence of the interaural separation [9]. Therefore, the separation between each sensor in the microphone array is very large, especially for acoustic signals with low frequency. Another type of fiber-optic vector sensor is based on the polarization in the fiber [10]. The SSL can be detected through the different response of two orthogonal polarization modes. In addition, a novel bio-inspired miniature dual-fiber-optic acoustic sensor was fabricated in order to overcome the size constraint of the SSL system [11,12]. This sensor imitates the mechanical coupling of the fly Ormia ochracea to determine the location of the sound source. However, one disadvantage of the sensor is the narrow range of the detection angle (± 30°).

Compared with sound pressure, the acoustic particle velocity (APV) describes the vibrational movements of particles in a medium under an acoustic signal. The APV is a vector physical quantity which contains both the magnitude and the direction of the sound [13]. Hence, detection of the APV allows the SSL sensor to be minimized. The Microflown method has been employed in the past for the measurement of APV [14]. One or two parallel heated wires can generate an amount of heat, leading to a symmetrical temperature distribution in space [15]. Under the applied acoustic signal, the temperature distribution in space is asymmetric, making a temperature variation for two thermal sensors. The APV could be measured through the difference between the two thermal resistances. More importantly, the temperature difference changes when the location of the sound source changes due to the vector physical quantity of the APV. Hence the SSL can be realized by using a Microflown based on parallel heated wires. Various Microflown-based APV sensors have been researched in recent years, including crossed parallel wires [16], low-voltage Wheatstone Bridge wires [17], and 2D orthogonal wires [18]. Moreover, multiple one-dimensional APV sensors and a pressure microphone are combined to realize a three-dimensional SSL [19]. The integrated APV sensor could overcome the size constraint of the microphone-array-based SSL system effectively.

Most thermal-resistance-based APV sensors inevitably suffer from electromagnetic interference. In this paper, a parallel microfiber array for the measurement of directional acoustic signals is proposed and experimentally demonstrated based on a self-heated Co²⁺-doped microfiber. The thermal spatial distribution of the self-heated Co²⁺-doped microfiber changes when the direction of the sound source differs, making the variation of the wavelength interval of two microfiber Bragg gratings (micro-FBGs) on two sides of the Co²⁺-doped microfiber. The all-fiber APV vector sensor with a miniature size offers potential for directional acoustic signal detection in remote detection or in environments with strong electromagnetic interference.

2. Fabrication method of the parallel microfiber array

Figure 1(a) shows the sensing configuration of the parallel microfiber array. A ring-shaped metal plane was fabricated, which contains a square through-hole with a side length of 1.5mm. In the middle of each side, a rectangular groove with a width of 380 um was fabricated by using laser micromachining. The depths of the groove along the x-axis and y-axis are 200 and 400 um, respectively. A well-cleaved Co²⁺-doped fiber (CorActive Inc., attenuation: 9.1dB/cm from 1250 to 1620 nm) with a length of 2 mm was spliced to a single-mode fiber (SMF). Then the Co²⁺-doped fiber was placed into the rectangular groove along the x-axis, and two FBGs in SMFs with the same Bragg wavelength (1529.42 nm) were placed on two sides of the Co²⁺-doped fiber, and were fixed in the groove by using the polyamide. Because the groove and the three fibers were the same size (125 μm for one fiber), the Co²⁺-doped fiber and SMFs were automatically parallel to each other in one rectangular groove. In this way, another two parallel FBGs and one Co²⁺-doped fiber were placed in the other groove along the y-axis, which were perpendicular to the first Co²⁺-doped fiber. In this way, a crossed configuration for two arrays with three parallel microfibers along the x-axis and y-axis, which contains two parallel FBGs and one Co²⁺-doped fiber, was formed, as shown in the insets in Fig. 1(a). It should be noted that the region of the FBG must be placed in the square through-hole. The entire size of the crossed parallel microfiber arrays is less than 1.5mm, which reduce the size of the sensor array significantly compared with conventional microphone array.

Fig. 1. Fabrication process.(a) The plane with the horizontal and vertical fibers with one Co²⁺-doped fiber and two FBGs. Insets show the parallel fiber array with one Co²⁺-doped fiber and two FBGs along the x-axis and y-axis. (b) The SEM of the crossed structure for the three-parallel-microfibers array. Insets show the parallel-microfibers array with one Co²⁺-doped microfiber and two microFBGs along x-axis and y-axis.

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After that, the entire square plane was immersed in hydrofluoric (HF) acid in order to taper the fiber. The Co²⁺-doped microfibers with a diameter of 15μm were formed by immersion in the HF for 105 min. At the same time, two micro-FBGs were also formed on two sides of each Co²⁺-doped microfiber, and the distance between the Co²⁺-doped microfiber and the micro-FBG was 125μm, as shown in Fig. 1(b). Then the microfibers were washed with distilled water three times to remove the remaining HF. From Fig. 1(b) it can be seen that each microfiber has three places that overlap with other microfibers along different axis. However, the depths of two grooves along the x-axis and y-axis are 200 and 400 um, respectively. Figure 2 shows that the heat generated by the Co²⁺-doped microfiber is mainly distributed with the range from -100 um to 100 um. Therefore, the heat generated by the Co²⁺-doped microfiber is hard to affect the micro-FBGs along different axis due to the large height difference (200 um). Besides, because two parallel microfibers are normal to the Co²⁺-doped microfiber along different axis simultaneously, the temperature effect of the Co²⁺-doped microfiber for two parallel microfibers along different axis is same to each other. Hence although two parallel microfibers are affected by the Co²⁺-doped microfiber along different axis, the temperature effect for two parallel microfibers can be compensated due to the same temperature effect.

Fig. 2. Temperature distribution in the spatial region and the normalized temperature at y = 0. (a) and (b) without acoustic wave. (c) and (d) with acoustic wave (1 Pa).

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3. Principle

The Co²⁺-doped microfiber can be heated by a 1480-nm laser launched via the SMF pigtail [20]. The temperatures inside the fiber core ${T_{core}}$ and outside the fiber core ${T_{sp}}$can be expressed as [21]:

(1)$$\begin{array}{l} {T_{sp}}(r) = {T_0} - \frac{{{Q_0}{a^2}}}{{4k}} - \frac{{{Q_0}{a^2}}}{{2k}}\ln (\frac{r}{a}),\begin{array}{{cc}} {}&{} \end{array}(a \le r \le \infty ),\\ {T_{core}}(r) = {T_0} - \frac{{{Q_0}{a^2}}}{{4k}},\begin{array}{{cc}} {}&{} \end{array}(0 \le r \le a). \end{array}$$

where ${T_\textrm{0}}$ is the maximum temperature, ${Q_0}$is the Joule-heat power density, r is the radius in the polar coordination, k is the thermal conductivity, and a is the fiber radius. Figs. 2(a) and 2(b) show the temperature distribution in the spatial region and the normalized temperature at y = 0. Since the length of the microfibers is much larger than the diameter of the microfiber, it can be considered that the temperature along the fiber axial direction is uniform. Therefore, the numerical simulation can be simplified to a two-dimensional structure. From Figs. 2(a) and 2(b), it can be seen that the Co²⁺doped microfiber core generates the maximum heat due to the maximum optical intensity of the pumped light, and the heat decays in the radial direction of the fiber. Inset in the Fig. 2(a) is the closed view of the temperature distribution in the Co²⁺doped microfiber (The white dash line represents the microfiber, and the color bar is enlarged for 5 times in order to show the temperature distribution clearly). The temperature distribution in the Co²⁺doped microfiber is in a good agreement with the calculation of Eq. (1).

Given that the acoustic wave is a periodic signal, the particle velocity of the acoustic wave in the harmonic form is $\mu {\ =\ }{\mu _0}{e^{i2\pi f}}$, where ${\mu _\textrm{0}}$ is the maximum particle velocity and f is the acoustic frequency. Hence ${\mu _0}{e^{i2\pi f}}\cos \theta$ is the APV alone the x direction. According to the heat transfer function, the temperature change alone the x direction is dependence of the forced convection induced by the APV and the thermal diffusion, which can be expressed as [22]:

(2)$$\delta T(x,\theta ){\ =\ }{e^{i2\pi f}}\frac{{{\mu _0}Px}}{{i2\pi klD\overline f }}(1 - \sqrt {i\overline f } {K_1}(\sqrt {i\overline f } ))\cos \theta .$$

where $\overline f { = 2}\pi {x^2}f/D$, D is the thermal diffusion coefficient, P is the dissipation power, and ${K_1}$ is the Bessel function.

Then an acoustic with the pressure of 1Pa and acoustic frequency of 500 Hz is applied in the simulation. Figs. 2(c) and 2(d) show the temperature distribution in spatial region and the normalized temperature at y = 0 under the acoustic wave. Compared with the temperature distribution without the acoustic wave, the temperature distribution is not symmetrical. On the contrary, the temperature at $x{ = 100}\mu m$is higher than that at $x{ = - 100}\mu m$; hence a temperature difference between them is generated. Inset in the Fig. 2(c) is the closed view of the temperature distribution in the Co²⁺doped microfiber (The white dash line represents the microfiber, and the color bar is enlarged for 5 times in order to show the temperature distribution clearly). The temperature distribution in the Co²⁺doped microfiber also becomes asymmetric due to the applied acoustic wave. The temperature difference in different spatial regions under the acoustic signal leads to an opposite wavelength shift for two micro-FBGs. Hence the measurement of APV can be realized through the detection of the wavelength interval of two micro-FBGs, which can be expressed as $\Delta \lambda {\ =\ }{\lambda _{1}}{\ -\ }{\lambda _{2}}{\ =\ (}{\lambda _\textrm{a}}{\ -\ }{\lambda _b}{)\ +\ 2}{n_{eff}}\Lambda (\alpha + \frac{1}{{{n_{eff}}}}\frac{{d{n_{eff}}}}{{dT}})\Delta T$, where ${\lambda _\textrm{1}}$ and ${\lambda _\textrm{2}}$ are the Bragg wavelength under the acoustic signal, ${\lambda _\textrm{a}}$and ${\lambda _b}$are the original Bragg wavelength without the acoustic signal, ${n_{eff}}$and $\Lambda $are the refractive index and period of the micro-FBG, and $\Delta T$is the temperature difference. Due to the vector quantity of the APV, the velocity component along the x direction is changed with the angle ($\cos \theta$ in Eq. 2). Because two micro-FBGs are placed along the x direction symmetrically, the measurement of the temperature difference for two micro-FBGs can be also used for the SSL. The well-known matched FBGs interrogation scheme was employed to detect the mismatching of Bragg wavelengths for two micro-FBGs, which can be expressed as [23]

(3)$${P_D}{\ =\ }{P_0}{R_1}{R_2}\Delta {\lambda _1}\textrm{/}\Delta {\lambda _0} \times \exp [ - 4\ln 2({\lambda _1} - {\lambda _0})\textrm{/}\Delta {\lambda _0}] \times \exp [ - 2\ln 2({\lambda _1} - {\lambda _2})\textrm{/}\Delta {\lambda _1}].$$

where ${P_D}$ is the reflected power, ${P_\textrm{0}}$, ${\lambda _\textrm{0}}$, and $\varDelta {\lambda _\textrm{0}}$ are the total power, centre wavelength, and full width at half-maximum (FWHM) of the optical source, ${R_\textrm{1}}$ and ${R_\textrm{2}}$ are the reflectivity of two micro-FBGs, ${\lambda _\textrm{1}}$ and ${\lambda _\textrm{2}}$are the Bragg wavelengths of the two micro-FBGs, and $\varDelta {\lambda _1}$ is the FWHM of the micro-FBG. Two Bragg wavelengths are overlapped to each other without the acoustic signal, and the reflected power is maximal. Under the applied acoustic signal, the asymmetric temperature difference increases the wavelength interval, making the decrease of the reflected power according to Eq. 3. Due to the vector quantity of the APV, the velocity component along the x direction changes with different angle, and the reflected power also changes along the different directions of the APV. Therefore, the directional acoustic signal measurement can be realized by using the matched FBGs interrogation of two parallel micro-FBGs on two sides of one Co²⁺doped microfiber.

4. Experiment and Discussion

The experimental setup is adapted as the “piston-one-sphere” approach, which exploits the known sound field in front of a loudspeaker placed in a spherical housing, as shown in Fig. 3(a). Such a construction acts as an acoustic monopole on its center axis and has an acoustic impedance accordingly [24]. The rectangular plane with two parallel microfiber arrays were fixed in the center of a rotation stage. A reference pressure microphone (AWA14423, Hangzhou Aihua Instruments Co., Ltd.) was placed close to the plane. There are two purposes for the microphone: (i) The microphone could calibrate the standard acoustic pressure for the proposed APV sensor. (ii) The performance of the proposed APV sensor can be compared with that of the commercial microphone. A loudspeaker connected with a waveform generator (Agilent 33220A) was fixed on an arm of the rotation stage in order to adjust the acoustic position. For low frequency measurements (<1 kHz), the speaker is very close to the proposed sensor (≈2 cm) while the pressure microphone is placed inside the speaker, and the APV can be calculated with the sound pressure measured by using the microphone. In the high frequency range (>1kHz), the proposed sensor is placed at a distance of 25 cm from the speaker while the microphone is very close to the sensor, and the APV is calculated by using both the measured sound pressure and the known acoustic impedance of the acoustic monopole source. The micro-FBGs were interrogated through the matched FBGs interrogation scheme, as shown in Fig. 3(b). An amplified spontaneous emission (ASE) source with a wavelength of 1525–1565 nm and output power of 20 mW was used to propagate a splitter. On each axis, the light was illuminated to one micro-FBG via a fiber circulator (A or C). Then the reflected light propagated back to the other micro-FBG through another circulator (B or D) and was detected by a photodetector. The output of photodetectors are amplified using low noise amplifiers AD8479 with 200× gain setting. On the other hand, a 1480 nm tunable pump laser was illuminated to two Co²⁺-doped microfibers via a splitter and a 1480/1550-nm wavelength division multiplexings (WDMs).

Fig. 3. (a) The experimental setup. (b) The matched FBGs interrogation.

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The reflection spectra of the FBG and micro-FBG inscribed in the Co²⁺-doped fiber before and after immersion in the HF are shown in Fig. 4(a). It can be seen that with the decrease of the fiber diameter, the Bragg wavelength shifts to a shorter wavelength. This is because the fundamental mode energy expands to the silica cladding and then partially into the air. As a result, the modal effective index decreases with the reduction of the fiber diameter. At the same time, the spectral bandwidth of the reflection peak is expanded from 0.16 to 0.32nm for the FBG and micro-FBG, respectively.

Fig. 4. (a) Bragg wavelength of the FBG and micro-FBG before and after immersion in HF. (b) The frequency response with different powers of the pump laser.

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In the experiment, the sensitivity is defined as the reflected power change induced by the wavelength interval with and without the acoustic signal. Besides the temperature change, the micro-FBG can also measure the acoustic pressure directly due to the photoelastic effect, which also induces the wavelength shift of the two micro-FBGs. Hence the acoustic response of the microfiber array with and without the pump laser was investigated first, as shown in Fig. 4(b). The response of the sensor without the pump laser can represent the acoustic pressure response. Obviously, the sensitivity of the sensor without the pump laser (3.4 V/(m/s)) is much lower than that with the pump laser (30.1, 35.7, 40.8, and 44.3 V/(m/s) for 100, 200, 300, and 400 mW). Thus from Fig. 4(b) it can been seen that the influence of the acoustic pressure response of the sensor can almost be neglected. Noted that the low response of the sensor without the pump laser may be also caused by the overlarge wavelength drift of the micro-FBG. Once the center wavelength of one micro-FBG was shifted too much, two Bragg wavelengths are not overlapped any more, resulting a poor acoustic response of the matched FBGs interrogation scheme. The frequency response of the sensor is nearly flat below 1.7 kHz and drops at high frequency due to the thermal mass of the microfiber. Besides, the temperature gradient along the Co²⁺-doped microfiber increases with the pump laser power, causing an average increase in the sensitivity levels of ≈5dB over the entire bandwidth for a 100-mW increase in pump laser power. The APV sensitivity can reaches 44.2 V/(m/s) at a frequency of 1000Hz with the pump laser power of 400mW. Moreover, because the microfibers along the directions of the x- and y-axis are of the same length and the crossed parallel microfiber array has a symmetric configuration, both micro-FBGs along the two axis have exactly the same frequency response. The sensitivity of the parallel microfiber array is higher than that of the commercial microflown sensor (∼12 V/(m/s) @ 1000Hz), but the frequency range (10-1700 Hz) is lower than that of the commercial microflown sensor (20-10000 Hz) [25].

The acoustic pressure response of the sensor was also tested. The acoustic source was driven with the frequency of 1 kHz, while the acoustic pressure was changed from 0Pa to 3Pa in 0.5Pa steps to generate different acoustic pressure (calibrated by using the reference microphone). The acoustic direction is normal to the parallel microfiber array. Figs. 5(a)–5(d) show the sensor response with the acoustic pressure of 0.5 Pa, 1Pa, 2Pa, and 3Pa. A linear relationship between the output voltage and acoustic pressure can be observed at the frequency of 1kHz, as shown in Fig. 5(e). The sensitivity of the fiber APV sensor is 77.3mV/Pa. At the same time, the microphone was also measured with different acoustic pressure. The sensitivity of the microphone is 46.8mV/Pa, indicating the high sensitivity of the proposed fiber APV sensor. The sinusoidal acoustic waves (acoustic pressure is 1Pa) with the frequency of 60Hz, 500Hz, 1.0kHz and 3.0 kHz was generated, respectively. The time-domain signals of the sensor are shown in Figs. 5(f)–5(i), and the corresponding frequency-domain spectra are shown Figs. 5(j)–5(m). The dominant peaks are located at 59.887 Hz, 499.965 Hz, 0.99874 kHz, and 2.99793 kHz, which are in a good agreement with their corresponding driven frequency. Besides, in Fig. 5(k), the applied acoustic wave at 1kHz is 1Pa. The noise floor is about -92 dB for a frequency resolution of 16 Hz, and the SNR is measured as 70.2 dB. Therefore, the minimum detectable pressure (MDP) is calculated to be 77 $\mu Pa/\sqrt {Hz}$ [26]. Figure 5(k) also shows the frequency-domain spectra of the microphone under the applied acoustic wave (acoustic pressure is 1Pa, frequency is 1kHz.). Compared with the MDP of 128 $\mu Pa/\sqrt {Hz}$ for the commercial microphone, the proposed parallel microfiber array can achieves a fiber APV sensor with low MDP.

Fig. 5. The sensor response with the acoustic pressure of (a) 3 Pa, (b) 2 Pa, (c) 1 Pa, and (d) 0.5 Pa. (e) The relationship between the acoustic pressure and the response of proposed APV sensor and microphone. The sensor response with the acoustic frequency of (f) 3 kHz, (j) 1 kHz, (h) 500 Hz, and (i) 60 Hz. The corresponding frequency-domain spectra of the sensor response with the frequency of (j) 3 kHz, (k) 1 kHz, (l) 500 Hz, and (m) 60 Hz.

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The direction response of the crossed parallel microfiber array has been researched. Figs. 6(a)–6(d) show the reflected power change at four frequencies of 200, 1000, 5000, and 8000 Hz. The applied APV was set as 2.4 mm/s with the sound pressure of 1 Pa measured by using the microphone. The reflected power changes at different frequencies have exactly the same direction response: the largest reflected power change is generated when the acoustic direction is normal to two micro-FBGs, while the smallest reflected power change is shown when the acoustic direction is parallel to two micro-FBGs. All direction responses of the micro-FBG array show a typical figure-of-eight pattern, indicating that the SSL can be realized by measuring the reflected power change of the micro-FBG array. The reflected power changes were measured as 75.3, 77.6, 42.7, and 38.8 mV at the frequencies of 200, 1000, 5000, and 8000 Hz. At the frequency of 1000Hz, the maximum reflected power change was 77.6mV (0°) when the acoustic orientation was normal to the Co²⁺-doped microfiber, and the minimum reflected power change was 4.2mV (90°) when the acoustic orientation was parallel to the Co²⁺ doped microfiber. Hence an acoustic direction sensitivity of 0.83mV/deg can be achieved at a frequency of 1000Hz. Moreover, the figure-of-eight pattern direction response of the micro-FBG array along the x-axis is exactly perpendicular to that along the y-axis, proving that two-dimensional directional acoustic signal measurement can be realized by detecting the APV of the crossed parallel microfiber array. Noted that the distance between the Co²⁺-doped microfiber and two micro-FBGs is a little different due to the strain of the microfiber induced by the evaporation of the HF and water, as shown in Fig. 1(b). Hence although the temperature distribution is still symmetrical, the temperature at two micro-FBGs are also different when the acoustic direction is parallel to the parallel microfiber array because the wavelength of two micro-FBGs is not same to each other exactly. Therefore, the reflected power change of the microfiber cannot reach back to zero when the acoustic direction is parallel to the parallel microfiber array (as shown in Fig. 6).

Fig. 6. The reflected power change response at different APV direction with the frequency of (a) 200 Hz, (b) 1000 Hz, (c) 5000 Hz, and (d) 8000 Hz.

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The direction response of the crossed parallel microfiber array with different pumped laser powers was also investigated, as shown in Figs. 7(a)–7(c). All reflected power changes show a clear eight polar patterns. With decreases of the pump light power, the reflected power change of the sensor decreases significantly, which is in good agreement with the experimental result in Fig. 4(a).

Fig. 7. The reflected power change response of the APV direction at different pump laser powers of (a) 400 mW, (b) 300 mW, (c) 200 mW, and (d) 100 mW.

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For optical fiber acoustic sensors, temperature is always a serious cross-sensitivity. Hence the temperature response of the parallel microfiber array was also researched. The proposed sensor was placed in an environment chamber, and the direction response was measured in the temperature range from 20 to 200 ℃ at intervals of 30℃. The variation of the reflected power change with the temperature change is shown in Figs. 8(a)–8(c). Figure 8(d) shows the reflected power change at different temperatures. Although all reflected power changes also show a clear eight polar patterns, it can be observed that the reflected power change of the sensor decreases with the rise in temperature due to the lower heating efficiencies of the Co²⁺-doped microfibers at high temperature, especially when the temperature exceeds 110 ℃. The relationship between the temperature and the reflected power change is $y = - 0.0007{x^2} - 0.02x + 78.64$. Hence, the laser heating power should be adjusted in order to compensate for differences in ambient temperature.

Fig. 8. The response of the APV direction at (a) 20℃, (b) 80℃, and (c) 140℃. (d) The relationship between reflected power change and temperature.

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An electromagnet with a core diameter of 15 mm was fixed below the parallel microfiber array in order to generate the magnetic field. The strength of the applied field was adjusted from 200 Oe to 800 Oe at intervals of 200 Oe, which was monitored with a gaussmeter (HT201, TES). Figs. 9(a) and 9(b) show the reflection power change of the micro-FBGs array at the frequency of 1kHz along the x-and y-axis under different magnetic fields. As expected, a reflection power change with a small standard variation of 1.84% is achieved under different magnetic fields. Besides, the response of the microphone under different magnetic field is also shown in Fig. 9(c) (frequency of 1kHz). The microphone exhibits almost perfect omnidirectional response at the frequency of 1kHz, but shows a high cross-sensitivity of the magnetic field (the standard variation of 25.86% under different magnetic fields). The different response of the magnetic field indicate a good immunity to electromagnetic interference of the parallel microfiber array compared with most conventional electronic sensors.

Fig. 9. The angle response of the crossed parallel microfiber array along (a) x-axis and (b) y-axis under different magnetic fields. (c) The microphone response under different magnetic fields.

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5. Conclusion

In conclusion, an all-fiber APV sensor based on the parallel microfiber array has been proposed. Two FBGs in microfibers were placed on two sides of the Co²⁺-doped microfiber, forming three parallel microfibers. Due to the asymmetrical temperature distribution and thermal change of the Co²⁺-doped microfiber, the parallel microfiber array can measure the temperature difference between the two sides of the Co²⁺-doped microfiber through the matched FBGs interrogation. A two-dimensional SSL can be realized based on the vector characteristic of the APV. The experimental results show that an APV sensitivity of 44.2 V/(m/s) and a direction sensitivity of 0.83mV/deg can be achieved at a frequency of 1000Hz. The frequency, pump laser, and temperature responses of the parallel microfiber array were also researched. The all-fiber APV sensor has the ability to recognize orientation, offering potential for directional acoustic signal detection.

Funding

National Natural Science Foundation of China (61601436); Cultivating funding of Zhengzhou Institute of Technology (GJJKTPY2018K3).

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Directional acoustic signal measurement based on the asymmetrical temperature distribution of the parallel microfiber array

Abstract

1. Introduction

2. Fabrication method of the parallel microfiber array

3. Principle

4. Experiment and Discussion

5. Conclusion

Funding

References

Cited By

Figures (9)

Equations (3)

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