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Abstract
Fiber optic hydrophones (FOHs) offer the notable advantage of electromagnetic interference resistance. Nevertheless, overcoming the challenge of sustaining stable, high-performance operation in intricate underwater settings at a low cost remains a considerable obstacle for them. To circumvent the restrictions noted above, we employed a miniaturized FOH, utilizing an easily fabricated extrinsic Fabry-Perot interferometer (EFPI) which is made up of a composite chromium-aluminum (Cr-Al) membrane and fiber. The linear demodulation also suppresses the drift issue in the output spectrum. The average sound pressure sensitivity of the sensor, according to experimental findings, is around -139.15 dB re 1 V/µPa, while the equivalent noise sound pressure at 1 kHz is 51.52 dB re 1 µPa/Hz1/2. This sensor has a lot of potential because of features like sensitive low-frequency response and noise performance.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Due to the remarkable advantages of underwater acoustic wave transmission, hydrophones have gained widespread utilization for long-distance detection of underwater targets. Nevertheless, traditional piezoelectric hydrophones face limitations regarding their physical characteristics, including susceptibility to electromagnetic interference, restricted transmission distance and size constraint [1]. In contrast, fiber optic hydrophones (FOH) provide numerous benefits, such as resistance to electromagnetic interference, wide dynamic range and flexible structural design. Consequently, FOH has emerged as a crucial direction in the advancement of hydroacoustic detection technology [2–5]. The FOH contains sensing structures such as fiber Bragg gratings (FBGs) [6–8], fiber lasers [9–11] and fiber interferometers [12,13]. Firstly, FBG-based sensing structures need a large fiber area to gain great sensitivity, and are sensitive to temperature changes. Secondly, fiber laser sensing structures are susceptible to significant low-frequency noise, which restricts their effectiveness in detecting low-frequency acoustic waves. Conversely, the membrane-based fiber interferometer has become a hot spot area of research due to its notable attributes, including high sensitivity, miniaturization and straightforward fabrication.
The sensing characteristics of FOH employing a membrane rely primarily on the mechanical deformation of the membrane. Therefore, a wide range of materials, including silicon [14], polymers such as PET [15], UV adhesives [16], PDMS [17], metals like gold [18] and silver [19], as well as graphene [20], are available for membrane fabrication. However, the use of non-metallic thin membrane materials mentioned above may introduce additional optical loss and noise due to their low reflectivity. Furthermore, the effective properties of certain membranes might be constrained by their limited chemical stability in harsh environments, such as silver and aluminum (Al) membranes. Besides, gold exhibits excellent stability albeit at a higher cost [21]. Hence, it is crucial to explore novel materials or modification methods to address these challenges and enhance the performance of membrane FOH in underwater acoustic detection.
Al possesses favorable properties such as good ductility, film-forming capabilities and excellent optical reflectivity, making it a versatile material. However, it is also characterized by limitations such as chemical instability and poor mechanical strength. We propose a composite membrane based on chromium (Cr) and Al, which effectively alleviate those problems. Existing research indicates that Cr is a metal with good adhesion [22–24]. Depositing a layer of Al on Cr to form a composite Cr-Al membrane can enhance membrane quality, our experimental results also demonstrated it can achieve a more sensitive response to incident acoustic and lower membrane noise levels, outperforming the data reported in some previous studies. More importantly, the Cr layer can prevent direct contact between the Al layer and seawater.
In this paper, we proposed and demonstrated an interferometric FOH based on the Cr-Al membrane structure. Compared with the pure Al membrane, the composite Cr-Al membrane offers superior chemical stability, ensuring reliable operation in diverse underwater environments. In order to balance sensitivity and mechanical strength of the membrane, the layer thickness of Cr and Al was set at 30 nm and 300 nm, leveraging the principle of forced vibration of the membrane.
2. Design principle
The designed sensor structure is shown in Fig. 1. In order to prevent elastic vibration of the cavity wall under acoustic pressure, we utilize a zirconium dioxide (ZrO2) ceramic sleeve to create the rigid cavity wall of an EFPI, which is also employed to secure the edge of the Cl-Al membrane and connect the optical fiber to form the EFPI. The thickness and diameter of the composite membrane were set at 330 nm and 2.5 mm, respectively, where the thickness of the chromium layer was 30 nm and the thickness of the aluminum layer was 300 nm.
Regarding the sensor as a standard Fabry-Perot cavity, its output spectrum can be analyzed by the interference theory. Since the cavity length cannot be ignored for the Fabry-Perot cavity, the optical transmission coefficient (η) needs to be taken into account in the output spectrum calculation. And the interference spectrum (${R_{\textrm{FP}}}$) of the Fabry-Perot cavity can be expressed as Eq. (1) [25]:
In this equation, R1 denotes the reflectivity of the fiber end face with a magnitude of 0.04; R2 denotes the reflectivity of the Cr-Al membrane with a measured magnitude of 0.96@1550 nm, more detailed data of the reflectivity is presented in Fig. 2. By using the Gaussian mode coupling models, η is calculated as 0.9985 [26,27]; n, L denotes the refractive index of the air (≈1) and the cavity length (≈275 µm) of the Fabry-Perot cavity, respectively. λ denotes the incident light wavelength (1550 nm). Figure 2 reveals that the reflectance of the Cr-Al membrane remains remarkably stable in the measurement bands, maintaining a value of 96%, on the other hand, the optical reflectance characteristics of the pure Al membrane exhibit a V-shaped pattern near 1550 nm, displaying greater fluctuations and relatively lower optical stability. All of the optical reflectance date was obtained through ultraviolet-visible and near-infrared spectroscopy (UV-vis-NIR) spectrophotometry measurements [28]. And the testing equipment information can be found in Tabel S2 of Supplement 1.
Besides, Sφ is defined as the normalized optical sensitivity of the sensor, which can be expressed as Eq. (2):
In our sensor design part, we employ a fiber optic EFPI configuration based on membrane. The key component of the sensor is the composite Cr-Al membrane material, chosen for its excellent vibration performance and high optical reflectivity, resulting in superior acoustic sensor capability. By calculating the tension coefficient [30] of the Cr-Al composite membrane, we determined it to be approximately 533, thereby classifying the acoustic membrane as a thin-membrane model. The solution of the vibration equation ur,t and first-order resonant frequency of the membrane f0 can be mathematically expressed as Eq. (4-5) [29]:
In the formula, a is the membrane’s radius, hc, ρc and σ represents the thickness, density and residual internal stress of the membrane, respectively, and r is the horizontal distance from any point to the membrane center. P0 and ω0 represent amplitude and angular frequency of the acoustic wave. The mechanical sensitivity of the membrane SM can be defined as ${\raise0.7ex\hbox{${{u_{r,t}}}$} \!\mathord{\left/ {\vphantom {{{u_{r,t}}} {{P_0}}}} \right.}\!\lower0.7ex\hbox{${{P_0}}$}}$. Figure 3(a-b) illustrate the simulated three-dimensional deformation of both the composite Cr-Al membrane and the pure Al membrane, with identical parameters, as calculated using Eq. (1). Since the membrane is coaxial with the fiber, only the theoretical mechanical sensitivity of the center of the membrane is usually considered at different frequencies, and the comparison results of the two membranes can be seen in Fig. 3(c). Based on those simulation results, it is evident that the vibration performance of the composite Cr-Al membrane is approximately twice as better as that of the Al membrane, and theoretically proven the effectiveness of the composite membranes.
Fig. 3. Comparison of the composite Cr-Al membrane and Al membrane. (a) Simulated three-dimensional deformation of the Cr-Al composite membrane; (b) simulated three-dimensional deformation of the Al membrane; (c) simulated mechanical sensitivity of the composite Cr-Al membrane and Al membrane; (d) comparison of the frequency response transfer function between the composite Cr-Al membrane and Al membrane.
In order to accurately analyze the response of the sensor to the incident acoustic, it is essential to calculate the frequency response transfer function (Hs) of the sensor and its actual mechanical sensitivity (Ss). Since the size of the sensor is small compared with the wavelength of the incident acoustic (range of approximately 1.5 m to 150 m), we choose the equivalent circuit model for the solution [27], and the Hs and Ss can be expressed as Eq. (6)–(7) [30].
In which, Mmem, Mrad, Cmem, Ccav and Rrad are the acoustic components in the equivalent circuit model, the specific definitions and expressions are shown in Table 1. The calculation of Hs are shown in Fig. 3(d) by using Eq. (6). As seen, when the back cavity filling with air, the sensor made up of Cr-Al membrane is still better, and the advantage disappears when water fills the back cavity. For another, the | Hs | is greater when the back cavity is entirely air, regardless of the material. Indeed, the shallow water level causes a relatively low static water pressure during the measuring process, which is insufficient to cause the membrane to break. As a result, the sensor can maintain its closed state, and the cavity remains filled with air throughout the process.
Table 1. Definition and expression of acoustic components in the equivalent circuit model [30]
3. Fabrication
The clean silicon wafer is spin-coated with AZ5214 photoresist as a sacrificial layer. Spin coating parameters are as follows: spin speed of 800 rpm, acceleration of 1000 rpm/s, and spin coating time of 90 s. And the cured temperature and time are set at 100 °C and 120 min, respectively, resulting in a sacrificial layer thickness of approximately 1.6 µm. A composite metal membrane based on Cr-Al is prepared using the electron beam evaporation (EBE) process, with a designated thickness of 30 nm for the Cr layer and 300 nm for the Al layer.
To peel the composite metal membrane from the substrate, a thin layer of epoxy resin 353ND is applied to the prepared sleeve. The structure is then bonded to the Al layer and cured using a hot plate at a temperature of 80°C for 10 minutes. To reinforce the connection, a ring of epoxy resin is applied to the outer edge of the contact area between the substrate and the ceramic sleeve. The curing temperature is maintained, and the curing time is set to 70 minutes, resulting in a bonded connection between the sleeve and the substrate. Subsequently, the entire bonded structure is immersed in an acetone solution to dissolve the sacrificial layer, with a dissolution time of approximately 15 hours. An optical fiber is then inserted into the ceramic sleeve and encapsulated using UV glue. Due to maintaining a certain distance between the fiber end-face and the composite metal membrane, the fiber end-face and the composite metal membrane form a Fabry-Perot interferometer sensor.
Besides, Fig. 4(a) illustrates the sensor fabrication processes, while Fig. 4(b-c) showcase the picture of the FOH and the cross-sectional scanning electron microscope (SEM) characterization of the composite membrane. The deposition of the composite membrane is mainly accomplished using EBE.
Fig. 4. Sensor fabrication and the Cl-Al membrane characterization. (a) Sensor fabrication processes. (b) The picture of the FOH, (c) SEM characterization of cut surfaces of the composite membrane.
4. Testing and demodulation system
As illustrated in Fig. 5, an acoustic demodulation system was constructed to measure the acoustic properties of the Cr-Al membrane. The system employed a linear demodulation method. A tunable laser (TSL) outputted light with a power of 1.1 mW, which was transmitted to the Fabry-Perot cavity after passing through a fiber optic circulator, and was received by a photodetector (PD) after reflecting from the Fabry-Perot cavity, and the output signal was collected by a data acquisition card (DAC). The 5 Hz∼10 kHz measurements were performed by the vibration liquid column method. An arbitrary waveform generator (AWG) generates the output signal, which is then amplified by a power amplifier (AMP) to drive the vibration table (VT) and generate acoustic signals at the same frequency. The calibration is performed using a standard hydrophone on the same horizontal plane. Detailed equipment information used during the testing process can be found in Table S2 of Supplement 1.
For the frequency range of 5 Hz to 8 Hz, the sampling rate is set to 25 KSPS, with a sampling time of 10 seconds and a frequency resolution of 0.1 Hz; for the frequency range of 10 Hz to 10 kHz, the sampling rate and the sampling time were differently set to 125 KSPS and 2 seconds, and the frequency resolution is 0.5 Hz.
5. Experimental results and analysis
The spectral parameters of the Cr-Al membrane-based Fabry-Perot cavity were measured using the experimental setup depicted in Fig. 6(a). The measurement setup involved an amplified spontaneous emission (ASE) light source and an optical spectrum analyzer (OSA) for scanning the Fabry-Perot cavity's spectrum. The wavelength scanning range was set to 1528 nm ∼ 1563 nm with a wavelength resolution of 2.5 pm. The scanned results are presented in Fig. 6(b), showing the free spectral range (FSR) of the completed encapsulated Fabry-Perot cavity to be 4.37 nm. The extinction ratio (ER) is 16.35 dB, indicating the contrast between the peak and background signals. The instability observed in the ER is mainly attributed to wavelength-dependent variations in η and R2 as well as inevitable systematic measurement errors. Furthermore, the reverse derivation result of the cavity length is approximately 275 µm.
Fig. 6. The spectral of the EFPI. (a) Schematic diagram of the spectral testing system. (b) The reflection spectrum of the FOH.
The example demodulated signal measurements of the Fabry-Perot cavity are shown in Fig. 7(a-b), The signal is subjected to bandpass filtering to remove acoustic noise from the environment. The acoustic frequency of Fig. 7(a) is 100 Hz, the demodulated signal intensity is 1.73 dB (re 1 V/Hz1/2), the pressure magnitude is 10.76 Pa, and the total harmonic distortion (THD) is -40.20 dB; the acoustic frequency of Fig. 7(b) is 1 kHz, the demodulated signal intensity is -1.08 dB (re 1 V/Hz1/2), the pressure magnitude is 7.84 Pa, and the THD is -40.51 dB.
Fig. 7. Experiment results. (a) The output time domain signal and its power spectral density (PSD) of the FOH at 100 Hz. (b) The output time domain signal and its PSD of the FOH at 1 kHz. (c) Sound pressure sensitivity level testing results. (d) Relationship between sound pressure and phase-amplitude of the FOH at 1 kHz.
Besides, the schematic diagram of the sound pressure sensitivity level (Mr) of the Fabry-Perot cavity from 5 Hz to 10 kHz is shown in Fig. 7(c), and the average Mr in the frequency range from 10 Hz to 1 kHz is -139.15 dB (re 1 V/µPa), with a maximum fluctuation of 0.83 dB. The observed resonance frequency point is 3.15 kHz, and the corresponding Mr is -121.94 dB (re 1 V/µPa).
Furthermore, the linearity of the sensor at 1 kHz was measured using the least squares linear fitting method. The result is shown in Fig. 7(d). The measured pressure range was 0.31 Pa to 24.08 Pa, and the peak output voltage ranged from 0.037 V to 2.52 V. The fitted sound pressure sensitivity at 1 kHz was 105 mV/Pa, with a goodness of fit of 0.9993.
Finally, the noise was collected 30 times without input sound pressure at a sampling rate of 250 KSPS and a single sampling time of 2 s. Each noise data's power spectral density (PSD) was examined, and the average noise PSD of those 30 measurements was computed (Fig. 8(a)). Then the noise PSD was differenced from the Mr, consequently the following results can be obtained: the 10 Hz equivalent noise sound pressure (NEP) is 75.23 dB (re 1 µPa/Hz1/2); the NEP in 100 Hz is 62.03 dB (re 1 µPa/Hz1/2); and the NEP in 1 kHz is 51.52 dB (re 1 µPa/Hz1/2), as indicated in Fig. 8(b). Figure 8(a) also showed the dynamic range of the sensor system. It can be obtained with a maximum undistorted output of 8.02 dB (re 1 V/Hz1/2, THD = -30.16 dB) and a noise of -87.45 dB re 1 V/Hz1/2 at a frequency of 1 kHz, giving a dynamic range of 95.45 dB.
Fig. 8. Noise property and dynamic range of the FOH. (a) Noise PSD and the output signal PSD of the FOH at 1 kHz. (b) The NEP and the noise PSD of FOH in different frequencies.
Above all, we list the comparison with other EFPI sensors for hydroacoustic sensing in Table 2. Compared with other FOHs based on membrane, the Cr-Al membrane structure has better flatness, wider frequency range, higher dynamic range, and lower detection limit, which can be better used for hydroacoustic detection at low frequency.
Table 2. Performance comparison of FOH based on different membrane
6. Conclusions
In summary, this paper proposed an optical fiber acoustic sensor based on a Cr-Al membrane in nanometer range. The Cr-Al thin membrane, fabricated using electron beam evaporation deposition, has a thickness of 330 nm (Cr 30 nm, Al 300 nm) and a diameter of 2.5 mm. The acoustic signal is demodulated using a linear demodulation algorithm to overcome the instability of the sensor during underwater acoustic detection. Experimental results demonstrate that the sensor achieves an average sound pressure sensitivity of -139.15 dB (re 1 V/µPa) from 10 Hz to 1 kHz, with a variation of 0.83 dB. The NEP of the system at 1 kHz is 51.52 dB (re 1 µPa/Hz1/2). The sensor has the advantages of high flatness and easy batch production, etc. Consequently, it exhibits significant potential for applications in underwater fiber optic acoustic sensing.
Funding
National Key Research and Development Program of China (2022YFE0103300); National Natural Science Foundation of China (11874169); Shenzhen Science and Technology Plan Project (GJHZ20210705142539009).
Acknowledgments
We acknowledge the financial support by the National Key Research and Development Program of China (grant no. 2022YFE0103300) and the National Natural Science Foundation of China (grant no. 11874169). We also acknowledge the Shenzhen Science and Technology Plan Project (GJHZ20210705142539009). The authors thank the Analytical and Testing Center of HUST, the facility support of the Center for Nanoscale Characterization and Devices at Wuhan National Laboratory for Optoelectronics, and the technical support by Experiment Center for Advanced Manufacturing and Technology in the School of Mechanical Science & Engineering of HUST.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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