Fiber-optic Microphone Research Articles

Highly sensitive ZnO-doped PET diaphragm-based fiber optic microphone

This study presents a highly sensitive Fiber Optic Microphone (FOM) based on a ZnO-doped PET (PET:ZnO) diaphragm for the first time. The FOM was first simulated and then tested experimentally. Under 14 mPa acoustic pressure and in the frequency range of 1 kHz–20 kHz, the highest SNR of 73.57 dB was measured at 5 kHz. The average signal-to-noise ratio (SNR) in the same frequency range was 51.74 dB. Based on the experimental results, the minimum detectable pressure (MDP) measured at 100 Hz resolution was calculated as low as 2 µPa/Hz1/2. The proposed FOM’s sensitivity was determined to be 150.15 mV/Pa. In addition to the static pressure analysis, a dynamic analysis using the PESQ algorithm was performed to measure the sound quality. With a sound quality score of 3.21, the proposed FOM can compete with commercial microphones. It also has advantages in terms of production and cost. Therefore, the proposed FOM has potential use in various applications, such as voice recognition, real-time communication in extreme environments, and long-distance listening in public/military/private security.

Fiber-optic bionic microphone based compact sound source localization system with extended directional range.

Traditional sound source localization (SSL) systems based on electret condenser microphone arrays are bulky because their localization accuracy depends on the size of the array. Inspired by the hearing mechanism of the parasitic fly Ormia ochracea, the localization accuracy of miniature bionic SSL devices breaks through the limitations of device size, but their ability to localize low-frequency sound sources over a wide angular range remains a challenge. In this work, a compact low-frequency SSL system with an extended directional range was prepared using two bionic micro-electro-mechanical system diaphragm based fiber-optic microphones, which form a non-coplanar array with a size of Φ44 mm × 13 mm. An algorithm for quantifying the azimuthal angle of a sound source is established for the prepared SSL system. Simulation and experimental results show that the prepared SSL system is capable of determining the propagation direction of acoustic signals with a frequency of less than 1 kHz in the azimuthal range from -90° to 90°, with a linear response in the range from -70° to 70°, and an angular measurement accuracy of the system within the range of ±7°.

Ultra-high sensitivity fiber optic microphone with corrugated graphene-oxide diaphragm for voice recognition

Ultra-high sensitivity fiber optic microphone with corrugated graphene-oxide diaphragm for voice recognition

A dual‐resonance enhanced photoacoustic spectroscopy gas sensor based on a fiber optic cantilever beam microphone and a spherical photoacoustic cell

AbstractWe propose a novel high‐performance dual‐resonance enhanced photoacoustic spectroscopy (DRE‐PAS) gas sensor based on a highly sensitive fiber optic cantilever beam microphone and a high‐Q spherical photoacoustic cell (PAC). The first‐order resonant frequency (FORF) of the spherical PAC is analyzed by finite element analysis to match the FORF of the cantilever microphone for the double resonance enhancement of the photoacoustic signal. The photoacoustic spectroscopy (PAS) system, including the DRE‐PAS sensor, a 1532.8 nm distributed feedback laser, and a high‐speed spectrometer, has been successfully exploited for trace acetylene (C2H2) detection. The experimental results show that the limit of detection (LOD) is 106.8 parts‐per‐billion (ppb) with an integral time of 1 s, and the LOD can be further reduced to 11.03 ppb by Allan‐Werle deviation for 100 s integral time. The normalized noise equivalent absorption coefficient can be obtained as 2.44 × 10−8 cm−1 WHz−1/2. The reported DRE‐PAS gas sensor has the superior characteristics of photoacoustic signal enhancement, high sensitivity, and strong antielectromagnetic interference capability, which can provide a new solution for PAS development.

Geometry optimization of cantilever-based optical microphones.

The introduction of cantilever-based fiber-optic microphones (FOMs) has proven to be effective in acoustic sensing. Further improvements in cantilevers face two key constraints: the challenge of achieving minimal sizes with sufficient reflective area and the trade-off between sensitivity and response bandwidth. Herein, we present a geometry optimization framework for a cantilever-based FOM that addresses this issue. Employing drumstick-shaped cantilevers housed within a Fabry-Perot (F-P) interferometric structure, we showcase a heightened sensitivity of 302.8 mV/Pa at 1 kHz and a minimum detectable acoustic pressure (MDP) of 2.35 µPa/H z. Notably, these metrics outperform those of the original rectangular cantilever with identical dimensions. Furthermore, our proposed cantilever effectively mitigates the reduction in resonance frequencies, thereby improving the response bandwidth. This geometry optimization framework offers considerable design flexibility and scalability, making it especially suitable for high-performance acoustic sensing applications.

Detection and Recognition of Voice Commands by a Distributed Acoustic Sensor Based on Phase-Sensitive OTDR in the Smart Home Concept.

In recent years, attention to the realization of a distributed fiber-optic microphone for the detection and recognition of the human voice has increased, whereby the most popular schemes are based on φ-OTDR. Many issues related to the selection of optimal system parameters and the recognition of registered signals, however, are still unresolved. In this research, we conducted theoretical studies of these issues based on the φ-OTDR mathematical model and verified them with experiments. We designed an algorithm for fiber sensor signal processing, applied a testing kit, and designed a method for the quantitative evaluation of our obtained results. We also proposed a new setup model for lab tests of φ-OTDR single coordinate sensors, which allows for the quick variation of their parameters. As a result, it was possible to define requirements for the best quality of speech recognition; estimation using the percentage of recognized words yielded a value of 96.3%, and estimation with Levenshtein distance provided a value of 15.

A Frequency-flat Sagnac Optical Fiber Microphone Based on A Composite Sensing Unit

A Frequency-flat Sagnac Optical Fiber Microphone Based on A Composite Sensing Unit

Simultaneous Audio Recording and Magnetic Resonance Imaging of the Temporomandibular Joint

Background: Temporomandibular joint disorder is a complex disease requiring multimodal treatment and often a delayed diagnosis. Purpose: The primary objective of this study is to evaluate the feasibility of simultaneously recording joint sounds while obtaining 3-tesla cine-magnetic resonance imaging (3T cine-MRI) of the temporomandibular joint (TMJ). A secondary objective pertains to optimization of a dynamic TMJ imaging protocol to maximize image quality and speed while maintaining synchronicity. Methods: Investigators enrolled four subjects (August 2018 to August 2019) eliciting audible joint sounds during continuous open-close jaw movements during clinical evaluation at the New York University College of Dentistry. A contact fiber-optic microphone (OptiRhythm 4130, Optoacoustics, Yehuda Israel) was utilized to record joint sounds elicited with functional movements during MR dynamic imaging at NYU Langone Health and all data was recorded appropriately. Results: Results of this feasibility study demonstrate the successful integration of audio and 3-Tesla MR imaging for the TMJ in all subjects. This protocol was shown to be well-tolerated by the subjects. A precise correlation of the movement of TMJ structures within cine-MRI images and with audible observations was achieved and assessed by an experienced oral and maxillofacial surgeon and an experienced radiologist. Conclusion: This protocol for simultaneous audio/cine-MRI is a viable tool for studying TMJ health and disease. In future studies, we anticipate this protocol will ultimately refine our understanding of TMJ sounds and their relation to anatomy, physiology, and pathology. In addition, we hope to establish a diagnostic aid to guide surgical and non-surgical decision-making more precisely.

Fiber-Optic Hydraulic Sensor Based on an End-Face Fabry–Perot Interferometer with an Open Cavity

The paper describes the design and manufacturing process of a fiber optic microphone based on a macro cavity at the end face of an optical fiber. The study explores the step-by-step fabrication of a droplet-shaped macro cavity on the optical fiber’s end surface, derived from the formation of a quasi-periodic array of micro-cavities due to the fuse effect. Immersing the end face of an optical fiber with a macro cavity in liquid leads to the formation of a closed area of gas where interfacial surfaces act as Fabry–Perot mirrors. The study demonstrates that the macro cavity can act as a standard foundational element for diverse fiber optic sensors, using the droplet-shaped end-face cavity as a primary sensor element. An evaluation of the macro cavity interferometer’s sensitivity to length alterations is presented, highlighting its substantial promise for use in precise fiber optic measurements. However, potential limitations and further research directions include investigating the influence of external factors on microphone sensitivity and long-term stability. This approach not only significantly contributes to optical measurement techniques but also underscores the necessity for the continued exploration of the parameters influencing device performance.

High-fidelity optical fiber microphone based on graphene oxide and Au nanocoating

Abstract A high-fidelity optical fiber microphone (HF-OFM) with hybrid frequency and fast response is theoretically and experimentally demonstrated by the nanofabrication techniques for real-time communication, which consists of a graphene oxide (GO) film, an Au nanocoating, and an air cavity. The internal stress of the film is increased by the method of mechanical tensile preparation, and the microphone response flatness is improved. Meanwhile, the structural design of the 3 nm Au nanocoating improves the acoustic pressure detection sensitivity by 2.5 times by increasing the reflectivity. The experimental result shows that single, dual, and triple frequency acoustic signal detection in the frequency range of 0.1 kHz–20 kHz are achieved with acoustic pressure sensitivities of 9.64, 9.66, and 8.9 V/Pa, as well as flat frequency response (<2 dB variation). The minimum detectable pressure (MDP) at 1 kHz is 63.25 μPa/Hz1/2. In addition, the high-fidelity real-time transmission of audio signals over an angle range of −90° to 90° is verified by a self-made acoustic pressure detection device. Such a compact, high sensitivity, and large measurement range HF-OFM is very promising for applications of oil leakage exploration, acoustic source location, and real-time communication.

Dual Fiber-Optic Microphone Based on Coherent Synthesis Technology

This paper proposes a dual fiber-optic microphone (DFOM) based on coherent synthesis technology, in which the sound pickup module mainly consists of two optical fibers and a reflection film, constituting two identical Fabry-Perot (FP) interferometric cavities as the acoustic sensing structure. The use of dual fiber-optic can increase the light intensity of the reflected light received by the photodetector (PD), while offsetting signal distortion caused by factors such as fiber length and temperature variations, and reducing signal interference, thereby improving the purity and accuracy of the signal. The DFOM exhibits strong fault tolerance, in case one of the optical fibers fails, the other one can still perform normal sound collection, thus enhancing the reliability of the microphone. Acoustic tests show that the sound pressure sensitivity of this DFOM at a frequency of 250 Hz is 64.8 mV/Pa. The signal-to-noise ratio (SNR) is as high as 62.45 dB under the action of a 6 kHz sound pressure signal. It maintains a linear sound pressure response and a flat frequency response in the range of 10 Hz to 20 kHz. These results indicate that the DFOM has high sensitivity and a wide frequency response range, making it suitable for acoustic detection within the audible range.

Highly sensitive Fabry-Perot acoustic sensor based on optic fiber spherical end surface

This paper presents and experimentally validates a highly sensitive fiber-optic Fabry-Perot interferometer sound sensor based on a fiber-optic spherical structure. The Fabry-Perot interferometer comprises an aluminum foil diaphragm and a fused fiber-optic microsphere end face, both sealed in a structure made of a glass tube. An optical fiber microsphere structure is used for the first time in acoustic sensing. Based on the strong focusing capability of the microsphere, the high coupling efficiency to the reflected beam, and the extensive range of the focused convergent optical field formed, the focused convergent optical fields facilitate the sensing of an extensive range of motion or deformation of the cavity compared to the divergent optical field of a flat fiber end face, i.e., the acoustic signals at more minor sound pressures can also be well sensed, thus compared to a flat fiber end face fiber The acoustic sensor can monitor acoustic signals over a more extensive frequency range and a larger range of acoustic pressure variations, and the output voltage signal is of a larger amplitude at the same acoustic signal, which allows the monitoring of acoustic signals at acoustic frequencies within the hearing range of the human ear. Experimental results show that the sensor shows a flat frequency response in the range of 20 Hz–20 kHz, covering all frequency components of the human ear hearing range, and the signal-to-noise ratio of the proposed acoustic sensor in the range of 20 Hz–20 kHz is greater than that of the flat fiber end-surface based acoustic sensor and the reference electroacoustic sensor, especially The sensor has the advantages of good high sensitivity, excellent linearity, wide planar response range, and simple manufacturing process. It can potentially be used as a highly sensitive, high acoustic quality fiber optic microphone in practical applications.

Sensitivity-enhanced Fabry-Perot interferometric fiber-optic microphone using hollow cantilever.

Transducer components are crucial in optimizing the sensitivity of microphones. Cantilever structure is commonly used as a structural optimization technique. Here, we present a novel Fabry-Perot (F-P) interferometric fiber-optic microphone (FOM) using a hollow cantilever structure. The proposed hollow cantilever aims to reduce the effective mass and spring constant of the cantilever, thereby enhancing the sensitivity of the FOM. Experimental results demonstrate that the proposed structure outperforms the original cantilever design in terms of sensitivity. The sensitivity and minimum detectable acoustic pressure level (MDP) can reach 91.40 mV/Pa and 6.20 µPa/Hz at 1.7 kHz, respectively. Notably, the hollow cantilever provides an optimization framework for highly sensitive FOMs.

Techniques to enhance the photoacoustic signal for trace gas sensing: A review

Photoacoustic spectroscopy (PAS), which relies on the detection of absorption-induced acoustic waves, is widely used for gas sensing. This paper summarizes and discusses most of the recent techniques for photoacoustic enhancement throughout the overall process from laser-molecule interaction to acoustic wave detection. Considering the theoretical principle and system composition of PAS gas sensor, we classify photoacoustic enhancing techniques into three aspects. The first one is to enhance the photoacoustic excitation by building up the laser power in terms of the positive proportional relation between the optical power and photoacoustic signal. The second one is to amplify the acoustic strength with various kinds of resonators after the photoacoustic generation. The third approach to further improve the photoacoustic detection is to develop more efficient acoustic transducers, for example, custom tuning fork, cantilever and fiber-optic microphone. Finally, the advantages and limitations of these different techniques are analyzed.

Ultra-High-Sensitivity, Miniaturized Fabry-Perot Interferometric Fiber-Optic Microphone for Weak Acoustic Signals Detection.

An ultra-high-sensitivity, miniaturized Fabry-Perot interferometric (FPI) fiber-optic microphone (FOM) has been developed, utilizing a silicon cantilever as an acoustic transducer. The volumes of the cavity and the FOM are determined to be 60 microliters and 102 cubic millimeters, respectively. The FOM has acoustic pressure sensitivities of 1506 nm/Pa at 2500 Hz and 26,773 nm/Pa at 3233 Hz. The minimum detectable pressure (MDP) and signal-to-noise ratio (SNR) of the designed FOM are 0.93 μPa/Hz1/2 and 70.14 dB, respectively, at an acoustic pressure of 0.003 Pa. The designed FOM has the characteristics of ultra-high sensitivity, low MDP, and small size, which makes it suitable for the detection of weak acoustic signals, especially in the field of miniaturized all-optical photoacoustic spectroscopy.

Performance enhancement of polyurethane foam applied to optical fiber microphones.

The application of polyurethane foam to optical fiber microphone sensitization is proposed. In this experiment, the Michelson interference system is used, and polyurethane foam is coated on the optical fiber of the signal arm. By changing the optical fiber material of the signal arm and the reference arm, four sets of comparative experiments are designed to test the sensitivity of the optical microphone. Through a scanning electron microscope (SEM) test and a pore size distribution test, the porous structure and non-closed cell structure characteristics, pore size range, etc., of the polyurethane foam were determined. The average sound absorption coefficient of the polyurethane foam is 0.66 through the sound absorption coefficient and sound insulation test. The sound absorption coefficient of each frequency band is above 0.2, the sound insulation is below 30 dB, and the overall sound insulation performance is poor, which can be regarded as an ideal sound absorption material. The sound-absorbing effect of polyurethane foam is better than that of nylon tight-packed materials in the frequency range of 500-2800 Hz, and it has a significant sensitization effect in this frequency band. In the frequency band above 2800 Hz, the sound-absorbing effect of nylon tight-packed optical fiber is better than that of polyurethane foam, and the optimal combination is determined by lateral comparison as signal arm (nylon tight-packed fiber + polyurethane foam) + reference arm (bare fiber). Finally, with different coating thicknesses as variables, the results show that the optical fiber microphone has the best sound collection effect when the coating thickness of polyurethane foam is 1.5 cm.

Ultra-sensitive ppb-level methane detection based on NIR all-optical photoacoustic spectroscopy by using differential fiber-optic microphones with gold-chromium composite nanomembrane

In this paper, we propose and experimentally demonstrate an ultra-sensitive all-optical PAS gas sensor, incorporating with a near-infrared (NIR) diode laser, fiber-optic microphones (FOMs) and a double channel differential T-type photoacoustic cell. The FOM is realized by Fabry-Perot interferometry and novel gold-chromium (Au-Cr) composite nanomembranes. To meet the demand of high sensitivity and flat frequency response for the FOMs, the Au-Cr composite diaphragm is deliberately designed and fabricated by E-beam evaporation deposition with 330 nm in thickness and 6.35 mm in radius. Experimental results show that the FOM has a sensitivity of about 30 V/Pa and a flat frequency response from 300 to 900 Hz with fluctuation below 1 dB. Moreover, a double channel differential T-type photoacoustic cell is designed and employed in the all-optical PAS gas sensor, with the first-order resonant frequency of 610 Hz. The all-optical gas sensor is established and verified for CH4 detection and the normalized noise equivalent absorption (NNEA) is 4.42 × 10−10 W∙cm−1∙Hz−1/2. The minimum detection limit (MDL) of 36.45 ppb is achieved with a 1 s integration time. The MDL could be further enhanced to 4.87 ppb with an integration time of 81 s, allowing ultra-sensitive trace gas detection.

Study on convolutional recurrent neural networks for speech enhancement in fiber-optic microphones

In this paper, several improved convolutional recurrent networks (CRN) are proposed, which can enhance the speech with non-additive distortion captured by fiber-optic microphones. Our preliminary study shows that the original CRN structure based on amplitude spectrum estimation is seriously distorted due to the loss of phase information. Therefore, we transform the network to run in time domain and gain 0.42 improvement on PESQ and 0.03 improvement on STOI. In addition, we integrate dilated convolution into CRN architecture, and adopt three different types of bottleneck modules, namely long short-term memory (LSTM), gated recurrent units (GRU) and dilated convolutions. The experimental results show that the model with dilated convolution in the encoder-decoder and the model with dilated convolution at bottleneck layer have the highest PESQ and STOI scores, respectively.

A miniature fiber-optic microphone based on plano-concave micro-interferometer.

The sensitive detection of sound waves is essential for a variety of applications. In this work, we propose a miniature diaphragm-free fiber-optic microphone based on a plano-concave optical micro-interferometer. A solid plano-concave micro-interferometer is formed at the end of a cleaved fiber by depositing a tiny volume of liquefied glass. Sound wave induced periodic variation of pressure can significantly modify the refractive index of the plano-concave glass due to the elasto-optic effect, and then, the phase difference between two interferometric beams will be remarkably changed accordingly. The interferometer finally converts the fluctuation of the phase difference into the change in the output optical power. Consequently, the sound wave can be demodulated by detecting the output power of the microphone. The experimental results show that the proposed microphone has the ability to detect sound waves in the whole audible range and almost omnidirectional. The noise-limited minimum detectable sound pressure is around 12 µPa/Hz. In addition, the human voice detection test shows that the performance of our microphone is competitive with the most advanced commercial device. The structure is stable without any movable mechanical parts, and the size is as small as 0.25 mm, which makes the proposed microphone an attractive alternative to the conventional one for sound wave detection.

Fabrication of Glass Diaphragm Based Fiber-Optic Microphone for Sensitive Detection of Airborne and Waterborne Sounds.

A glass-diaphragm microphone was developed based on fiber-optic Fabry-Perot (FP) interferometry. The glass diaphragm was shaped into a wheel-like structure on a 150-μm-thick glass sheet by laser cutting, which consists of a glass disc connected to an outer glass ring by four identical glass beams. Such a structural diaphragm offers the microphone an open air chamber that reduces air damping and increases sensitivity and results in a cardioid direction pattern for the microphone response. The prepared microphone operates at 1550 nm wavelength, showing high stability in a range of temperature from 10 to 40 °C. The microphone has a resonance peak at 1152 Hz with a quality factor of 21, and its 3-dB cut-off frequency is 32 Hz. At normal incidence of 500 Hz sound, the pressure sensitivity of the microphone is 755 mV/Pa and the corresponding minimum detectable pressure is 251 μPa/Hz1/2. In addition to the above characteristics of the microphone in air, a preliminary investigation reveals that the microphone can also work stably under water for a long time due to the combination of the open-chamber and fiber-optic structures, and it has a large signal-to-noise ratio in response to waterborne sounds. The microphone prepared in this work is simple, inexpensive, and electromagnetically robust, showing great potential for low-frequency acoustic detection in air and under water.