Interferometric Acoustic Sensor Research Articles

A Grating Interferometric Acoustic Sensor Based on a Flexible Polymer Diaphragm.

This study presents a grating interferometric acoustic sensor based on a flexible polymer diaphragm. A flexible-diaphragm acoustic sensor based on grating interferometry (GI) is proposed through design, fabrication and experimental demonstration. A gold-coated polyethylene terephthalate diaphragm was used for the sensor prototype. The vibration of the diaphragm induces a change in GI cavity length, which is converted into an electrical signal by the photodetector. The experimental results show that the sensor prototype has a flat frequency response in the voice frequency band and the minimum detectable sound pressure can reach 164.8 µPa/√Hz. The sensor prototype has potential applications in speech acquisition and the measurement of water content in oil. This study provides a reference for the design of optical interferometric acoustic sensor with high performance.

High-Stability Polarization-Based Interrogation Method With Cross-Correlation for Optical Fiber Interferometric Acoustic Sensors

In this work, we propose and demonstrate a high-stability polarization-based interrogation method using cross-correlation for optical fiber interferometric acoustic sensing. Specifically, an extrinsic Fabry-Perot interferometric (EFPI) sensor based on polyphenylene sulfide (PPS) diaphragm is firstly used for signal detection, and then a cross-correlation interrogation system is formed with a birefringent crystal based on polarization low-coherence interference technology. Finally, the direct current (DC) component of the interference signal can be acquired in real-time with the help of the birefringent crystals block of known length. The system does not require an orthogonal relationship between the interference signals. Moreover, it can still work when the initial cavity length of the sensor drifts due to the influence of the environment. Theoretical simulations and a series of experiments have been carried out to verify the feasibility of the proposed scheme. And experimental results show that the sensor's initial cavity length shifts from 90.082 μm to 88.451μm or 91.692 μm under 1kHz acoustic signal, the maximum relative demodulation error is 0.4539 nm, and the signal-to-noise ratio(SNR)can reach 80 dB. Compared with the reported schemes, the proposed interrogation scheme simplifies the system configuration, and has properties of low cost, strong robustness, and high-frequency, so it has great potential in practical engineering applications.

Deep Learning-Based Speech Enhancement of an Extrinsic Fabry-Perot Interferometric Fiber Acoustic Sensor System.

To achieve high-quality voice communication technology without noise interference in flammable, explosive and strong electromagnetic environments, the speech enhancement technology of a fiber-optic external Fabry-Perot interferometric (EFPI) acoustic sensor based on deep learning is studied in this paper. The combination of a complex-valued convolutional neural network and a long short-term memory (CV-CNN-LSTM) model is proposed for speech enhancement in the EFPI acoustic sensing system. Moreover, the 3 × 3 coupler algorithm is used to demodulate voice signals. Then, the short-time Fourier transform (STFT) spectrogram features of voice signals are divided into a training set and a test set. The training set is input into the established CV-CNN-LSTM model for model training, and the test set is input into the trained model for testing. The experimental findings reveal that the proposed CV-CNN-LSTM model demonstrates exceptional speech enhancement performance, boasting an average Perceptual Evaluation of Speech Quality (PESQ) score of 3.148. In comparison to the CV-CNN and CV-LSTM models, this innovative model achieves a remarkable PESQ score improvement of 9.7% and 11.4%, respectively. Furthermore, the average Short-Time Objective Intelligibility (STOI) score witnesses significant enhancements of 4.04 and 2.83 when contrasted with the CV-CNN and CV-LSTM models, respectively.

An SOI-MEMS Acoustic Sensor Based on Optical Grating Interferometer

Capacitive microelectromechanical system (MEMS) microphone in a small package encounters performance limits associated with damping in the air gap between diaphragm and backplate. In this work, we explored an optical-readout technique by fabrication of an optical interferometric acoustic sensor. The designed sensor chip forms a grating interferometer by a diffraction grating integrated backplate and a pressure-sensitive diaphragm. A scalar diffraction theory and a finite-element method have been applied to optimize the sensor parameters. The sensor device was fabricated on a silicon-on-insulator (SOI) wafer using complementary metal–oxide–semiconductor (CMOS) compatible processes. The performance of the sensor was studied using an acoustic testing system. The sensor demonstrates a good linear relationship with applied sound pressure. The frequency response is relatively consistent with a commercial reference microphone for audible range. The signal-to-noise ratio (SNR) of the sensor is about 43 dB at a frequency of 1 kHz. The proposed design shows good potential for high-performance acoustic sensors.

Large-Dynamic-Range and High-Stability Phase Demodulation Technology for Fiber-Optic Michelson Interferometric Sensors.

A large-dynamic-range and high-stability phase demodulation technology for fiber-optic Michelson interferometric sensors is proposed. This technology utilizes two output signals from a 2 × 2 fiber-optic coupler, the interferometric phase difference of which is π. A linear-fitting trigonometric-identity-transformation differential cross-multiplication (LF-TIT-DCM) algorithm is used to interrogate the phase signal from the two output signals from the coupler. The interferometric phase differences from the two output signals from the 2 × 2 fiber-optic couplers with different coupling ratios are all equal to π, which ensures that the LF-TIT-DCM algorithm can be applied perfectly. A fiber-optic Michelson interferometric acoustic sensor is fabricated, and an acoustic signal testing system is built to prove the proposed phase demodulation technology. Experimental results show that excellent linearity is observed from 0.033 rad to 3.2 rad. Moreover, the influence of laser wavelength and optical power is researched, and variation below 0.47 dB is observed at different sound pressure levels (SPLs). Long-term stability over thirty minutes is tested, and fluctuation is less than 0.36 dB. The proposed phase demodulation technology obtains large dynamic range and high stability at rather low cost.

High-Sensitivity Silicon Cantilever-Enhanced Photoacoustic Spectroscopy Analyzer with Low Gas Consumption.

A silicon cantilever-enhanced photoacoustic spectroscopy (PAS)-based trace gas analyzer with low gas consumption is presented. A silicon cantilever-based fiber-optic Fabry-Perot (F-P) interferometric acoustic sensor with a compact structure and high sensitivity is designed for photoacoustic signal detection. The non-resonant photoacoustic cell (PAC) is a cylindrical copper tube with a volume of 0.56 mL. A near-infrared laser with a center wavelength of 1532.83 nm amplified using an erbium-doped fiber application amplifier is used as the excitation light. The wavelength modulation spectroscopy (WMS) technique is employed in the present work for second-harmonic photoacoustic signal detection. The experimental results show that the minimum detection limit of C2H2 is 199.8 parts per trillion (ppt) with an average time of 60 s. The normalized noise equivalent absorption coefficient is calculated as 1.72 × 10-9 cm-1 W/Hz1/2. Furthermore, the proposed silicon cantilever-enhanced non-resonant PAS-based gas analyzer can not only analyze the gas concentration in a closed small-capacity PAC with low gas consumption but also detect target gas leakage in real time at a long distance.

An Optical Acoustic Detection System Based on Fabry Pérot Etalon Stability Structure.

The optical acoustic detection system based on the Fabry Pérot Etalon (FPE) with high quality–factor (High Q) and stability structure is described and tested. The FPE contains two high–reflectivity Plano–Concave lenses, achieving high fineness and stability. The protective structure of the confocal stabilized FPE is composed of an invar tube, copper sheath, Bakelite sheath and aluminum housing to protect the sensor from the effects of ambient temperature and vibration. The audio signal is injected into the cavity through the sound hole located in the center of the cavity. Acoustic waves induce the vibration of the medium in the cavity, which leads to a simultaneous change in the FPE optical path and a shift of the interference spectrum. The acoustic detection system is built, and the frequency of the laser is locked on the resonant frequency points of the FPE by using phase modulation technology, so as to detect acoustic signals of different frequencies and amplitudes. In addition, the sensitivity of the proposed sensor exceeds 34.49 mV/Pa in the range of 20 Hz–20 kHz. A Signal-to-Noise Ratio (SNR) of 37 dB can be achieved at 20 Hz. Acoustic signal detection technology based on the FPE stability model is used to test the theoretical feasibility of the future high sensitivity Fabry Pérot Interferometric (FPI) acoustic sensors.

Sparsity-Based Recovery of Three-Dimensional Photoacoustic Images from Compressed Single-Shot Optical Detection.

Photoacoustic (PA) imaging combines optical excitation with ultrasonic detection to achieve high-resolution imaging of biological samples. A high-energy pulsed laser is often used for imaging at multi-centimeter depths in tissue. These lasers typically have a low pulse repetition rate, so to acquire images in real-time, only one pulse of the laser can be used per image. This single pulse necessitates the use of many individual detectors and receive electronics to adequately record the resulting acoustic waves and form an image. Such requirements make many PA imaging systems both costly and complex. This investigation proposes and models a method of volumetric PA imaging using a state-of-the-art compressed sensing approach to achieve real-time acquisition of the initial pressure distribution (IPD) at a reduced level of cost and complexity. In particular, a single exposure of an optical image sensor is used to capture an entire Fabry–Pérot interferometric acoustic sensor. Time resolved encoding as achieved through spatial sweeping with a galvanometer. This optical system further makes use of a random binary mask to set a predetermined subset of pixels to zero, thus enabling recovery of the time-resolved signals. The Two-Step Iterative Shrinking and Thresholding algorithm is used to reconstruct the IPD, harnessing the sparsity naturally occurring in the IPD as well as the additional structure provided by the binary mask. We conduct experiments on simulated data and analyze the performance of our new approach.

High dynamic range distributed acoustic sensing based on dual-wavelength fiber Bragg grating pairs.

A high dynamic range distributed acoustic sensing (DAS) system is demonstrated based on a dual-wavelength (2λ) interferometric acoustic sensor array, constructed using 2λ fiber Bragg grating pairs (FBGPs). Each FBGP is composed of two weak FBGs presenting -30-dB reflectivity and center wavelengths of 1535 nm and 1565 nm. With the phase demodulation results from the two wavelengths, 2λ linear regression phase unwrapping is implemented, which significantly enhances the dynamic range of the DAS system. By examining the four time-division-multiplexed sensors, an average background noise level of -105.2dBrad/Hz at 1 kHz and a cross talk level lower than -70dB at 1 kHz are achieved.

Ultrasensitive fiber optic Fabry Pérot acoustic sensor using phase detection

In this paper, a fiber optic Fabry–Pérot interferometer (FPI) acoustic sensor is demonstrated based on a molybdenum disulfide (MoS2) diaphragm with a thickness of 500 nm. An air cavity-based fiber-tip FPI is implemented to detect acoustic signal frequency and corresponding pressure. Subsequently, the MoS2 is deposited on the air cavity with a fiber tip-MoS2 membrane distance of ~57 µm and ~2.52 mm dia. In fact, MoS2 nano sheets (NS) acts as a membrane sensitive to the acoustic signal. The results give out the acoustic sensitivity of the proposed sensor to be 18.8 rad/Pa at acoustic frequency of 1 kHz and 100 mPa acoustic pressure respectively. This sensor benefits a nearly flat frequency response ranging 8 Hz–10 kHz sustaining a high temperature stability over 20°−80 °C.

Fabry–Perot acoustic sensor based on a thin gold diaphragm

An extrinsic Fabry–Perot interferometer (EFPI) acoustic sensor based on a gold diaphragm with an about 100 nm thickness has been proposed and demonstrated. The Fabry–Perot (F-P) cavity consists of a gold diaphragm and a fiber endface. Experimental results illustrate a static pressure sensitivity of about 19.5 nm / kPa in the range from 0 to 100 kPa with linearity of about 0.99. Meanwhile, the flat acoustic frequency response of about 35.0 mV / Pa between 0.2 and 2.0 kHz is achieved, and the largest acoustic pressure sensitivity is about 80.6 mV / Pa at 2.6 kHz. The noise-limited minimum detectable pressure is 1.3 mPa / Hz1/2 @ 2.6 kHz. The proposed EFPI acoustic sensor may have potential application in security and surveillance systems.

Fiber optic Fabry–Pérot acoustic sensor using PVC and GO diaphragms

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.

Optical Fiber Fabry–Perot Acoustic Sensors Based on Corrugated Silver Diaphragms

In this article, we fabricated and tested a series of extrinsic fiber Fabry–Perot interferometric acoustic sensors based on the corrugated silver diaphragms. These diaphragms were fabricated through a simple and low-cost method. The corrugations were first etched on the silicon surface permanently and then transferred to the silver diaphragm with the help of photoresist as the sacrificial layer. The sensors with different corrugation depths were fabricated. The experimental results showed the same trend with the theoretical results on the mechanical sensitivities of the fabricated corrugated diaphragms, which first increased and then decreased with the increase in the corruption’s depth. Other characteristics of the fabricated optical acoustic sensors based on the corrugated silver diaphragms were also tested in detail. The proposed fabrication method of the corrugated diaphragm showed the advantages of simplicity and low cost, and the acoustic sensors showed high sensitivity, which was suitable for weak acoustic’s sensing.

Fiber-Optic Interferometric Sensor Based on the Self-Interference Pulse Interrogation Approach for Acoustic Emission Sensing in the Graphite/Epoxy Composite

Fiber-optic acoustic emission sensors are highly attractive for non-destructive testing applications in composites due to their small size, galvanic isolation, and immunity to electromagnetic interference (EMI). The significant part of such sensors is based on the Fabry-Perot interferometers (FPIs) because of their small size, high sensitivity, and their ease of embedding into a composite structure. However, linear demodulation techniques of their signals suffer from the limited dynamic range and require an additional operating point control. In this paper, we propose the novel self-interference interrogation approach for the fiber-optic FPI-based acoustic emission sensor, which allows the FPI to be interrogated as a two-beamlike interferometer with current-induced frequency modulation of the light source. Then, with using of a PGC-ATAN demodulation algorithm or similar, phase signals can be measured independently of the interferometer operating point with the extended dynamic range. The sensor is based on the fiber-optic FPI that consists of the optical fiber segment bounded by two TiO2 mirrors: the semitransparent one (20%-25% reflecting and -1.6-dB loss) in the fiber and the external one (99.9% reflecting) at the fiber tip. The mirrors are spaced 7 mm apart. Two FPI sensors were fabricated and experimentally investigated. One sensor was mounted on the surface of the graphite/epoxy composite plate, and another one was embedded into its structure. The experimental investigation results of two considered interferometric acoustic emission sensors in comparison with the reference piezoelectric transducer GT301 are presented. Obtained results confirmed the efficiency of the proposed interrogation approach in terms of FPI usage as a two-beamlike interferometer for acoustic emission signals detection and might be used for the implementation in existing and new FPI-based acoustic emission sensing systems.

Spherical microcavity-based membrane-free Fizeau interferometric acoustic sensor.

The monitoring of acoustic waves is essential for a variety of applications, ranging from photoacoustic imaging to industrial non-destructive evaluation and monitoring. Optical acoustic sensors have many advantages over electrical counterparts such as passivity and immunity to electromagnetic interference, and can be quite compact with all-fiber structures. A common approach is to optically detect the acoustically induced mechanical movement of a cantilever or a reflective membrane. However, sensors based on moving mechanical parts have limitations, because they are influenced by the mechanical properties of the structures involved that behave as a coupled spring-mass system. Here a new type of optical acoustic sensing concept based on a spherical microcavity fiber Fizeau interferometer is described. The sensor is fabricated by inserting a section of single-mode fiber into a spherical microcavity to form a Fizeau interferometer. Thanks to the elasto-optic effect, the sound pressure modifies the refractive index of the microcavity; then a corresponding change in the interferometric spectrum can be observed. The sensor was successfully used to detect acoustic waves under the whole audible region, from 20Hz to 20kHz. The experimental results show that the sensitivity can be improved via altering the length of microcavity. The structure is membrane-free, and the sensitivity will not be limited by these size restrictions which makes the technology an interesting alternative to conventional transducers for acoustic wave measurement.

Study of gain factor effect of erbium doped fiber amplifier on noise floor level of fiber-optic interferometric sensor

Subject of Research. The paper presents experimental study results of the gain factor effect of an erbium doped fiber amplifier on the noise floor level of a fiber-optic interferometric acoustic sensor. Fiber-optic sensor is based on a Michelson interferometer. The optical amplifier is located behind the compensating interferometer which induces the auxiliary phase modulation to the interference signal. The homodyne demodulation algorithm is used to recover the sensor phase signal. Method. During the experiment there were no external acoustical impacts on the sensor. The power control of optical signals from the fiber amplifier was performed. Noise signals from the sensor were written into data files under the different values of the fiber amplifier gain factor. Spectral estimations of the noise floor level of the fiber-optic interferometric sensor were performed by the averaged modified periodogram method under the different values of the fiber amplifier gain factor. Obtained results were used to define the dependence of the noise floor level of the fiber-optic interferometric sensor on the fiber amplifier gain factor. Main Results. Mean noise floor levels were equal to 64 urad/Hz0.5 at 355 Hz, 68 urad/Hz0.5 at 450 Hz and 66 urad/Hz0.5 at 500 Hz. Experimental results showed insignificant increase of the noise floor level of the fiber-optic interferometric sensor with the growth of the optical amplifier gain factor from 14.6 dB to 25.8 dB. The gain factor increase was about several percent and did not exceed inaccuracies of performed measurements. Practical Relevance. The absence of significant changes in the noise floor level of the considered fiber-optic sensor with the changing of the fiber amplifier gain factor is caused by the intensity noise suppression of the used demodulation scheme. The considered fiber amplifier might be used for the amplification of optical signals from the fiber-optic multiplexed array of fiber-optic sensors without the significant deterioration of their noise performance on condition that the homodyne demodulation algorithm with the intensity noise suppression is used.

Phase Demodulation of Short-Cavity Fabry–Perot Interferometric Acoustic Sensors With Two Wavelengths

A phase interrogation method for a fiber-optic acoustic sensor based on a short-cavity extrinsic Fabry–Perot interferometer (EFPI) is proposed with two wavelengths. A multichannel tunable optical filter is employed, which selects out two monochromatic beams from the broadband interference spectrum of EFPI with fixed wavelength interval. The influence of the wavelength interval on the recovery of the phase signal has been theoretically and experimentally analyzed. To get the best sensitivity, the wavelength interval is fixed at one quarter of the period of sensor interference fringes. An optimized differential cross multiplication algorithm is utilized to demodulate the acoustic signal, which can eliminate the impact of optical power imbalance between the two light paths. This system may be a universal phase demodulation unit for short-cavity EFPI acoustic sensors. The EFPI acoustic sensing head is formed by an aluminum-attached polyethylene terephthalate membrane and a cleaved fiber tip. The sensor interrogated by the proposed demodulation system demonstrated large dynamic range in low-frequency domain under high sound pressure. A signal-to-noise ratio (SNR) of ∼53 dB is obtained at 80 Hz. These features make the sensor especially suitable for noise detection.

Extrinsic Fabry-Perot fiber acoustic pressure sensor based on large-area silver diaphragm

An extrinsic fiber Fabry-Perot interferometric acoustic sensor based on a large-area silver diaphragm is proposed. The thickness and diameter of the silver diaphragm are 160nm and 2.4mm, respectively. The flatness of the diaphragm is <14μm. Experimental results showed that the sensor has the acoustic sensitivity of −124.8dB re 1V/μPa with a high linear pressure response in the range of 2.5–268mPa. The mechanical sensitivity of the diaphragm is 28.57nm/Pa. The sensor exhibits a flat frequency response in the range of 0.2–2.8kHz. The noise equivalent acoustic signal level is 83μPa/Hz1/2 at 1kHz. The sensor is expected to be used for weak acoustic sensing.

MEMS-Based High-Sensitivity Fabry–Perot Acoustic Sensor With a 45° Angled Fiber

A high-sensitivity cross-axial extrinsic fiber Fabry–Perot interferometric acoustic sensor with a 45° angled fiber is proposed. The thickness of pressure-sensing silver diaphragm is 95 nm. The diameter is 1.6 mm yet the flatness of the diaphragm is $ . The measured dynamic sensitivity of the diaphragm is 67.01 nm/Pa. The sensor exhibits a flat frequency response in the range of 1–6 kHz and a dynamic range of 66.43 dB. The minimal detectable pressure is $\sim 107~\mu $ Pa/Hz $^{1/2^{^{^{}}}}$ at 3 kHz. The fabricated sensor is expected to be useful for highly sensitive acoustic measurement.

Analyzing the applicability of miniature ultra-high sensitivity Fabry–Perot acoustic sensor using a nanothick graphene diaphragm

A miniature Fabry–Perot interferometric acoustic sensor with an ultra-high pressure sensitivity was constructed by using approximately 13 layers of graphene film as the diaphragm. The extremely thin diaphragm was transferred onto the endface of a ferrule, which had an inner diameter of 125 μm, and van der Waals interactions between the graphene diaphragm and its substrate created a low finesse Fabry–Perot interferometer with a cavity length of 98 μm. Acoustic testing demonstrated a pressure-induced deflection of 2380 nm kPa−1 and a noise equivalent acoustic signal level of ~2.7 mPa/Hz1/2 for a 3 dB bandwidth with a center frequency of 15 kHz. The sensor also exhibited a dynamic frequency response between 1 and 20 kHz, which conformed well to the result obtained by a reference microphone. The use of a suspended graphene diaphragm has potential applications in highly sensitive pressure/acoustic sensors.