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Abstract
Here, a low cost high sensitive fiber optic Fabry–Pérot interferometer (FPI) acoustic sensor is developed. A novel polyvinyl chloride (PVC) diaphragm-based fiber-tip FPI is implemented for the sake of acoustic sensing with high sensitivity. The PVC diaphragm has been coated to the pigtail of a standard single-mode fiber (SMF). Subsequently, the multilayer graphene oxide (GO) is deposited on the PVC cavity. The configuration of (SMF + PVC + GO) attests a sensitivity higher than (SMF + PVC). In fact, GO acts as the acoustic membrane and PVC features as the cavity to demonstrate a sensitive FPI acoustic sensor. The results also give out the acoustic sensitivity of the (SMF + PVC + GO) sensor equivalent to 685 mV/Pa (19.9 rad/Pa) at 1 kHz and a 100 mPa pressure with a frequency response ranging 0.1–10 kHz.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical fiber sensors are defined as means through which physical, chemical, and biological measurands interact with light guided through an optical fiber to produce a modulated optical signal coupled with information related to the measurement parameter [1–3]. Presently, acoustic sensing is a very in-demand field, which plays an important role in modern society, with applications spanning from structural health monitoring and underwater surveillance to medical imaging and even automobile industries [1–3]. Currently, numerous works in the optical fiber acoustic sensor are reported based on FPI [4–16]. In fact, FPI sensor usually employs a facet of an optical fiber and a deflectable reflecting diaphragm separated by an air cavity utilizing a diaphragm sensitive to the acoustic pressure. Gong et al. reported a FPI acoustic sensor based on parylene-C diaphragm with a sensitivity of ∼100 mV/Pa at 1kHz [5]. Wu et al. demonstrated a FPI acoustic sensor made up of GO diaphragm that benefits a minimum detectable pressure of $10.2 \mathrm{\mu} \textrm{Pa}\textrm{.H}{\textrm{z}^{ - 0.5}}$ [8]. Despite the sensitivity of the recent sensors is relatively high, however those are difficult to manufacture. On the other hand, a comprehensive study on FPI acoustic sensor has not been performed yet to attain both the low manufacturing cost and ultrahigh sensitivity. Here, a novel and simple method for fabrication of high sensitive FPI acoustic sensor is primarily demonstrated. The proposed sensor is based on a PVC cap created on the end of pigtail of a SMF. This can detect acoustic wave in the frequency range of 0.1-10 kHz with sensitivity of 500 mV/Pa (15.5 rad/Pa) at 1 kHz and 100 mPa pressure. In fact, a material with a very low young’s modulus like PVC, acts both as a cavity and acoustic sensitive diaphragm. In the following, the role of GO on the sensitivity of FPI sensor is investigated using the PVC cavity. The results attest when GO is deposited on PVC cavity, higher sensitivity will be obtained rather than (SMF + PVC) configuration. The typical sensitivity of sensor with GO deposition on PVC is measured to be 685 mV/Pa (19.9 rad/Pa) at 1 kHz frequency and 100 mPa pressure.
2. Sensor fabrication
Figure 1 (a) depicts an image of the proposed FPI sensor including PVC cavity. A few layers of commercial PVC diaphragm are folded together to constitute a multilayer PVC by stacking at the end of a pigtail of SMF. It is handmade to avoid air gap between PVC layers. In this process, PVC diaphragm is solidified up to ∼ 70µm thick. The diaphragm forms a PVC cap on the end of pigtail of the SMF. Each layer of PVC is 14 µm thick. The thickness of PVC cap can be changed by adjusting the number of the PVC layers. Next, a ∼500 nm GO diaphragm (GO is purchased from GrapheneX Co), is prepared and deposited on the PVC cavity as described in [8]. In fact, the GO solution with 0.25 mg/mL concentration is dropped onto a small copper (Cu) foil and then the GO solution is dried by placing the Cu foil on a heater. Cu foil is etched by ferric chloride solution and next, GO membrane is rinsed in deionized water. A pigtail of SMF is moved towards the floating GO membrane for covering the end face of SMF pigtail (fishing method). Finally, the GO membrane is adhered onto the end face of the SMF pigtail according to van der Waals forces. Figure 1(b) illustrates the image of the proposed FPI sensor including a 56 µm thick PVC cavity with a ∼500 nm GO deposited on it.
Fig. 1. Schematics of proposed sensors and corresponding cross sections of the tip endface under optical microscope for (a) (SMF + PVC) and (b) (SMF + PVC + GO) configurations. The bright spots on the image are effects of the microscope LED.
3. Experimental results and discussion
Figure 2 illustrates the experimental setup of the FPI acoustic sensor. A 1550nm distributed feedback laser diode (DFB-LD) with narrow linewidth of ∼5kHz (NKT photonics) generating 8mW power is launched into the FPI via an optical circulator. Then, the laser beam propagates into FPI sensor. The different acoustic pressure levels deform the diaphragm, leading to change of the FP cavity length and reflected intensity. Consequently, the demodulated signal is detected due to the reflected intensity change according to the interferometric demodulation mechanism. An InGaAs PIN photo detector (PD, 1811-FC New Focus) receives the reflected light from the FPI acoustic sensor through the optical circulator. The PD is used to convert light into electrical signal. The amplified electrical signals are collected by a data acquisition (DAQ) unit and then sent to the computer. After the calibration of the FPI sensor by commercial microphone (B&K 4189 with a sensitivity of 50 mV/Pa), the frequency response of the FPI sensor is measured. Another experimental setup was employed in Fig. 2 to examine the wavelength shift of FPI resonant peak. The setup features a superluminescent diode light source (SLD) accompanying an optical spectrum analyzer (OSA) (Model 203B, Thorlabs).
Fig. 2. Experimental Setup of FPI acoustic sensor including distributed feedback laser diode (DFB-LD), superluminescent diode (SLD), circulator, sensing probe, photo detector (PD), data acquisition (DAQ) unit, optical spectrum analyzer (OSA) and function generator (FG).
The optimal thickness of FP cavity arises from the nature of the maximum extinction ratio (ER). According to the two-beam interference, maximum ER can be obtained when the intensities of two reflected light beams get equal [8]. Figure 3(a) and (b) display, the calculated ER in terms of cavity length according to the beam divergence inside the PVC cavity and Fresnel reflections at the bounds. Hence, the optimal PVC thickness for SMF + PVC and SMF + PVC + GO configurations are obtained to be 68.2µm and 58.1µm respectively. Figure 3(c) and (d) show the reflection spectra of proposed sensors when no acoustic signal is applied and in the case of acoustic signal application at 1kHz and 100 mPa respectively. The length of the PVC cavity L can be obtained by L=λ2/2nδλ, where λ, n and δλ denote to be the dip (or peak) wavelength, refractive index of cavity and the wavelength spacing between the two successive fringes respectively [15].
Fig. 3. The calculated ER in terms of cavity length for (a) SMF + PVC and (b) SMF + PVC + GO configurations. Reflection spectra of the proposed sensors in the static mode, when (i) no acoustic signal is applied and (ii) acoustic signal is applied at 1 kHz frequency and 100 mPa pressure in configurations; (c) SMF + PVC and (d) SMF + PVC + GO.
Regarding Fig. 3(c) in static mode, the cavity length consists of 5 PVC layers which is determined to be 70µm. The cavity length from Fig. 3(d) in static mode, is determined to be 56µm (the closest value to calculated optimal thickness equal to 4 PVC layers).
The performance of PVC cavity length on sensor sensitivity is primarily investigated at 1 kHz frequency for 100 mPa acoustic pressure. Figure 4 depicts the sensitivity of FPI sensor versus PVC thickness. The highest sensitivity is achieved in favor of 70 µm PVC.
Fig. 4. Sensor sensitivity taken from FPI sensor with PVC diaphragm versus PVC thickness at 1kHz frequency and 100 mPa acoustic pressure. The error bar indicates the standard deviation of the experimental results.
When the acoustic pressure is applied to PVC (or GO) membrane, the change of cavity length dL and corresponding optical phase difference dφ, can be expressed as [15,16]:
where, dλ, denotes to be the wavelength shift of the interference fringe. The change of interference intensity dI in FPI is obtained as follows [16]: where, ${I_1}$ and ${I_2}$ ascertain the intensities of two successive reflections of FPI. Here, the measured sensitivity (mV/Pa) is normalized to the phase sensitivity (rad/Pa) by eliminating the effect of the incident light power.On the other hand, the wavelength shifts of resonant peak in FPI for two mentioned configurations are monitored. In fact, FP cavity acts as a spectral filter. This, would decrease the optical intensity of interferometric valleys or (peaks). In this regime, with applying acoustic frequency, a time mapping causes to change the optical intensity in time domain. This optical intensity alteration is in agreement with excitation acoustic frequency.
Figure 5(a) shows the original acoustic signal at 2 kHz and 100 mPa. Moreover, the performance of GO deposition on PVC cavity is individually studied. Figures 5(b) and 5(c) illustrate, the sensor output in terms of acoustic signal frequency typically at 2 kHz frequency and 100 mPa pressure in two different configurations. The thickness of PVC in PVC and PVC + GO configurations are given 70 µm and 56µm respectively. The thickness of GO in Fig. 5(b) is set ∼500 nm. As a consequence, the sensitivity of FPI sensor achieves the higher value in favor of the (SMF + PVC + GO) configuration rather than (SMF + PVC). The sensor outputs regarding SMF + PVC and SMF + PVC + GO give out to be 1.06 rad and 1.81 rad respectively. This delineates that by utilizing the PVC as the cavity and GO as the diaphragm (sensitive material to pressure) a more sensitive sensor is obtained. The low elasticity of PVC cavity (several tens of Mpa) causes a significant deformation of cavity with respect to applied acoustic pressure. The GO diaphragm enlarges the acoustic impedance matching during the acoustic pressure transfer to PVC cavity. This means acoustic impedance of GO and PVC are closer together rather PVC and air. Thus, when an acoustic signal impacts the FPI diaphragm, then the elastic PVC cavity undergoes a pressure exerted by the acoustic wave. Subsequently, the latter forms an effective deformation on the cavity.
Fig. 5. (a) Original acoustic signal at 2 kHz and 100 mPa and sensor output for (b) PVC cavity and (c) PVC cavity deposited by GO, typically at 2 kHz for a certain acoustic pressure of 100 mPa.
Fresnel reflection coefficients for SMF-PVC, PVC-air and PVC-GO interfaces are given to be 0.04%, 4.3% and 1.6% respectively. Reflected intensities at SMF-PVC, PVC-air and PVC- GO interfaces are calculated to be ${I_{SP}} = 0.0004 \times {I_0},\,\,{I_{PA}} = 0.0096 \times {I_0} \times 0.043 \times \cos (\frac{{4\pi L{n_{PVC}}}}{\lambda })\,$ and ${I_{PG}} = 0.0096 \times {I_0} \times 0.016 \times \cos (\frac{{4\pi L{n_{PVC}}}}{\lambda })$ where ${I_0}$ is the optical intensity reached to SMF-PVC interface. ${I_{SP}}\; ,\; {I_{PA}}$ and ${I_{PG}}$ denote to be the reflected intensities at SMF-PVC, PVC-air and PVC-GO interfaces respectively. The ${I_1}$ and ${I_2}$ are proportional to Fresnel reflection coefficients. For instance, the following calculations are carried out to obtain 1.99 rad (19.9 rad/Pa at 100 mPa). Sensor output (mV) and measured reflected power for reference (standard fiber reflector) are determined to be 24 mV and 0.18 mW. Sensor output (mV) for SMF+ PVC + GO configuration is measured to be 69 mV. Using above proportionality, the reflected power $dI$ of SMF + PVC + GO configuration will be 0.52 mW. ${I_1}$ and ${I_2}$ in Eq. (4) are proportional to ${I_{SP}}$ and ${I_{PG}}$ respectively. $\sqrt {{I_1}{I_2}}$ is measured to be 0.2 mW. Using dI, Eq. (4), ${I_{SP}}$ and ${I_{PG}}$, $dL$ will be obtained 160.5 nm. Using Eq. (2), $d\varphi$ is measured to be 1.99 rad.
Figure 6(a) plots the frequency response of the proposed sensors ranging 100 Hz to 10 kHz at 100 mPa characterizing optical phase change of light during propagation through the FPI. Figure 6(b) displays the wavelength shift of the resonant peaks at 1550 nm. This attests an obvious peak at 1 kHz.
Fig. 6. (a) Sensitivity of proposed sensors in terms of rad/Pa at 100 mPa pressure. Note that the results are extracted from Fig. 5. (b) Wavelength shift of proposed sensors at 100 mPa pressure based on OSA data.
We propose acousto-optical coupling ratio (AOCR) as a criterion of trade off between acoustic impedance matching and optical Fresnel reflections. Both of these parameters are effective in sensor sensitivity. According to AOCR, dL can compensate the effect of low reflections. AOCR is defined as $AOCR = \frac{{dL.\sqrt {{R_1}{R_2}} }}{{{{[N(\Delta AI)]}^2}}}.$ dL and R1 denote, cavity length change and reflection coefficient between SMF and PVC respectively. R2 is reflection coefficient between PVC and air (or PVC and GO) for PVC + air and PVC + GO configurations respectively. $N\Delta (AI)$ term is normalized acoustic impedance difference for PVC and air or PVC and GO interfaces. This ratio is proportional to acoustic sensitivity. According to the Table 1, the higher coupling ratio causes to obtain the larger sensitivity. Note that the experiment for SMF + air + GO is performed for an optimal air cavity length equal to 100µm.
Table 1. AOCR and sensitivity for different configurations.
Figure 7 shows the output of our home made sensors under various acoustic pressure levels for two different configurations of interest at 1 kHz.
Fig. 7. Output of the proposed sensors in terms of applied acoustic pressure at 1kHz for both configurations of interest.
Directivity is an important index of acoustic sensor. Most acoustic applications require the point-like omnidirectional receivers, which have uniform acoustic response within a wide angular range. Figure 8 illustrates the sensor output at various alignment angles. This is performed by changing the angle of speaker relative to the sensor head from −90° to 90°. In fact, the omnidirectional acoustic sensitivity is tested. Here, the (SMF + PVC + GO) configuration deals with angular response at 1 kHz frequency and 100 mPa pressure. The directional response of acoustic sensor exhibits nearly flat acoustic response with wide directivity. The acoustic sensitivity is optimal at 0° and only decreases to 1.3 rad (1.82 dB) within the angles ranging ±90°.
Figure 9 indicates the linewidth of detected acoustic signal for two sensors under investigation. The linewidth is considered as the width at which the amplitude reaches 0.37 of the maximum amplitude.
Fig. 9. Linewidth of detected acoustic signal in terms of frequency at 100 mPa for both configurations.
The linewidth of detected acoustic signal is a criterion of FP structure (cavity + diaphragm) effect on the linewidth of excitation acoustic signal. This can be attributed to the multiple reflections of light from the cavity and the diaphragm. When PVC is used both as cavity and diaphragm, these reflections exhibit a nearly invariant linewidth through wide frequency range, attributed to the uniform elastic behavior of PVC, wheras in (PVC + GO) configuration, the linewidth linearly increases with acoustic frequency. In fact, Utilizing multilyer graphene as FP diaphragm attests the random changes in the reflected light intensity in time domain which in turn generates a varying frequency-dependent linewidth of the acoustic signal in the course of sensing.
In summary, we have shown that the acoustic effect of GO diaphragm is remarkable, because the GO diaphragm enlarges the acoustic impedance matching during the acoustic pressure transfer to PVC cavity. However, the optical resonance in FP cavity demonstrates to be more effective to enhance the acoustic sensitivity of the sensor with (SMF + PVC + GO) configuration against (SMF + PVC) configuration, mainly due to larger refractive index difference takes place in PVC/GO interface than that of SMF + PVC boundary.
4. Conclusion
High-sensitive low-cost FPI acoustic sensor is developed here based on PVC + GO diaphragms. This performs properly as an acoustic cavity relying on GO vibration under acoustic wave signal and simultaneously acts as a FPI optical resonator. The experimental results indicate that the proposed sensors demonstrate good performance characterizing linear acoustic pressure response with high sensitivity over acoustic frequencies. Furthermore, making use of PVC as the acoustic cavity + GO as membrane, the performance of the FPI sensor is notably improved. The sensor exhibits a high frequency response ranging 100 Hz to 10 kHz and the sensitivity in the order of 685 mV/Pa (19.9 rad/Pa) at 1 kHz and 100mPa. Finally, this sensor demonstrates nearly 14 folds more sensitive than B&K 4189 commercial microphone.
Disclosures
The authors declare no conflicts of interest.
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