Interferometric Optical Sensor Research Articles

Nonlinear error elimination using the fusion of PGC-DCM, geometric fitting, and Atan algorithms.

A phase generated carrier (PGC) demodulation scheme is always accompanied by nonlinear errors. We propose a fusion of PGC differential and cross multiplying (PGC-DCM), geometric fitting, and arctangent (Atan) algorithms for fiber optic interferometric sensors to eliminate nonlinear errors. The output amplitude of the PGC-DCM algorithm is used to judge whether the Lissajous figure of quadrature signals is larger than 1/2 ellipse arc. When the Lissajous figure exceeds 1/2 ellipse arc, the contaminated quadrature signals are corrected by the ellipse correction parameters calculated from the geometric fitting; otherwise, the previous fitting parameters are employed for correction. Geometric fitting is realized by minimizing the Sampson error, and its failure problem under small signals is solved by using the temporary stability of fitting results. Finally, desired signals are extracted from the corrected quadrature signals by the Atan algorithm. Experimental results show that the fusion combines the merits of the three algorithms and expands the application of the geometric fitting in PGC demodulation schemes.

Improved PGC demodulation algorithm for fiber optic interferometric sensors.

An improved phase generated carrier arctangent demodulation algorithm based on harmonic mixing and phase quadrature technology (PGC-Arctan-HP) is proposed in this paper, which can eliminate the effects of modulation depth shift, carrier phase delay, and light intensity disturbance (LID) on the demodulation results. The simulation results are consistent with theoretical analysis, and indicate that the PGC-Arctan-HP algorithm can achieve optimal demodulation compared with other demodulation algorithms. The results of interferometric experiments show that the demodulated waveforms of the improved algorithm are relatively stable with an amplitude error of 0.0294%. The total-harmonic-distortion (THD) and the signal-to-noise-and-distortion (SINAD) can reach -60.0286 dB and 59.5388 dB.

Ameliorated 3 × 3 coupler-based demodulation algorithm using iteratively reweighted ellipse specific fitting.

Passive demodulation scheme using 3 × 3 coupler has been widely used in phase-sensitive optical time-domain reflectometry (φ-OTDR), interrogation of fiber Bragg gratings or fiber optic interferometric sensors, and sensor multiplexing. However, the asymmetry of the 3 × 3 coupler in real applications affects the demodulation performance seriously. We proposed an ameliorated 3 × 3 coupler-based demodulation algorithm using iteratively reweighted ellipse specific fitting (IRESF) to overcome the drawback. IRESF combines iterative reweight technology with ellipse specific fitting, which decreases the weights of high noise points and always outputs ellipse solutions. Any two output signals from the 3 × 3 coupler-based interferometer are fitted by the IRESF and then corrected as a pair of quadrature signals. The stability of the fitting parameters is utilized to resolve the failures of IRESF under small signals. A real-time 1/4 ellipse arc judging module is designed, if the Lissajous figure is larger than 1/4 ellipse arc, IRESF is executed to offer ellipse correction parameters. Otherwise, the fixed parameters preset in the algorithm are used. The fixed parameters are mean values of the fitting parameters of IRESF under a large stimulus. The desired phase signal is finally extracted from the corrected quadrature signals. Experimental results show that the ameliorated algorithm does not require strict symmetry of the 3 × 3 coupler and can work under small signals. The noise floor of the proposed algorithm is -112 dB re rad/√Hz and the demodulated amplitude is 23.15 dB (14.37 rad) at 1 kHz when THD is 0.0488%. Moreover, the response linearity is as high as 99.999%. Compared to the algorithm using direct least squares, the proposed demodulation algorithm is more robust and precise, which has broad application prospects.

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.

Optical Interferometric MEMS Accelerometers

AbstractLaser interferometry is one of the most accurate measurement technologies whose displacement resolution can reach the femtometer (fm) level. With the development of Micro‐Electro‐Mechanical Systems(MEMS) technology, many excellent optical interferometric MEMS sensors have emerged, which play an essential role in intelligent manufacturing, aerospace, and many other fields. Among them, optical interferometric MEMS accelerometers develop rapidly due to their high sensitivity, low noise, and anti‐electromagnetic interference advantages. Lots of research in optical interferometric chip design, micro‐nano fabrication process, signal demodulation algorithm, and range extension technology are performed, and different types of MEMS accelerometers based on the optical interferometric principles are developed, which have been demonstrated and applied in the fields of disaster monitoring, equipment operation and maintenance, and resource exploration. This paper reviews the development status of optical interferometric MEMS accelerometers, mainly focusing on the critical problems and solutions that need to be solved in the development process of optical interferometric accelerometers, and finally introduces the typical application scenarios.

All-sapphire-based optical fiber pressure sensor with an ultra-wide pressure range based on femtosecond laser micromachining and direct bonding

An all-sapphire extrinsic Fabry-Perot interferometer (EFPI) optical fiber pressure sensor with ultra-wide pressure range and high temperature resistance is proposed and experimentally demonstrated. The sensor is fabricated by direct bonding three sapphire wafers, including the sapphire substrate, the sapphire wafer with a through hole, and the sapphire pressure-sensitive diaphragm. A femtosecond (fs) laser is used to inscribe a through hole in the center of the sapphire wafer and roughen the outer surface of the sapphire pressure-sensitive diaphragm. By using original polished surfaces of sapphire wafers with low surface roughness as reflective surfaces of the Fabry-Perot (FP) cavity, the high-quality interference signal can be obtained, thereby improving the measurement accuracy of the sensor. The optical cavity length (OCL) of the proposed sensor changes linearly with the applied pressure in the wide range of 0 - 50 MPa at room temperature, and the pressure sensitivity is 0.0921 µm/MPa. The pressure measurement accuracy reaches 0.31%FS (full scale). High temperature experiments show that the sensor can work stably at 1000 ℃.

Ameliorted algorithm for PGC to eliminate the influence of carrier phase delay with FFT

In the real environment, the carrier phase delay (θ) is difficult to accurately maintain at nπ (n is an integer), which results in the demodulation error of the phase generation carrier (PGC) demodulation algorithm. In this paper, the fast Fourier transform (FFT) technique is used to extract 1θ0, 2θ0, 3θ0, 4θ0 from the first to fourth harmonic signals of the interference signal, then introduce 1θ0, 2θ0, 3θ0, 4θ0 into the mixed signal as the initial phase, and finally perform a mixing operation to real time eliminate the carrier phase delay. The experiments show that when the carrier delay is in the range of 0 to π, the SINAD of the ameliorated algorithm reaches 43.66 dB, which is 19.31 dB higher than the PGC-Arctan algorithm on average. The THD of the ameliorated algorithm can be stabilized at 0.183 %, which always maintains the best level of the PGC-Arctan algorithm. The ameliorated algorithm eliminates the carrier phase delay and does not have the phase singularity, thus expanding the application range of the fiber optic interferometer sensor.

High Precision and Stabilization PGC Demodulation Scheme for Fiber Optic Interferometric Sensors

Ellipse fitting algorithm (EFA) has attracted tremendous interest in phase demodulation schemes to eliminate nonlinear errors, however, there is an inherent drawback that the EFA fails to work when the desired phase signal is small. A high precision and stabilization phase generated carrier (PGC) demodulation scheme using hyper least squares (LS) fitting of ellipse is proposed for the first time, to our best knowledge. The contaminated quadrature signals obtained after mixing and low pass filtering are processed by a hyper LS based EFA to eliminate nonlinear errors. Hyper LS relies on algebraic distance minimization with a carefully chosen scale normalization and statistical bias is eliminated up to second order noise terms, which is far superior to the traditional LS. The temporary stability of the fitting result is used to resolve the drawback of the EFA. A module for judging the ellipse arc of quadrature signals is developed. The EFA is executed when the Lissajous figure is larger than 1/4 ellipse arc, otherwise the fitting result of the previous executed EFA is used to correct the ellipse. Experimental results show that the proposed scheme is highly precise and stable regardless of the desired signal amplitude. It has a high response linearity of 99.997%, a minimum detectable phase of 3.16 μrad and a large dynamic range of 123.52 dB @1 kHz & 1% THD.

High resolution and accuracy model fitting interrogation method for optical fiber Polarization interferometric sensor

Interrogation methods largely determine the performance of the entire sensing system. However, interrogation methods alone are unlikely to provide very good results. An accurate model for the optical fiber polarization interferometric sensor (PIS) has been meticulously formulated within the frequency domain, incorporating the mode birefringence dispersion term of polarization maintaining fibers (PMFs). A novel model-fitting interrogation approach for the PIS is initially proposed and validated through experimental demonstration, which is bifurcated into parameter initialisation and model fitting interrogation process during the operation of the PIS. Subsequently, the parameters are determined at varying temperatures using this method. The interrogation capabilities of the proposed method are then experimentally contrasted with the valley wavelength tracking method. Results corroborate that the measurement range of model-fitting approach is no longer restricted by the spectrum bandwidth. The resolution and accuracy of our approach are 89% and 8 times higher than the valley wavelength tracking method, respectively.

Diamond coated fiber optic interferometric sensors: fabrication and application

Diamond films were deposited by chemical vapor deposition (CVD) on the tip of Fabry-Perot (FPI) and multi-mode (MMI) optical fiber interferometers. Diamond provides a robust interface capable of forming covalent bonds between atoms on its surface and receptor molecules, required for biosensing applications. The films were characterized by optical and scanning electron microscopy (SEM), optical profilometry and Raman spectroscopy. The diamond-coated interferometers were tested with different refractive index solutions. The sensors response was 40 ± 1 dB/RIU and −987 ± 70 pm/ RIU for the FPI and −11 ± 1 dB/RIU for the MMI.

Multi-Point Optical Fiber Fabry-Perot Curvature Sensor Based on Microwave Photonics

This article reports a multi-point curvature sensor system based on multiplexed optical fiber Fabry-Perot interferometric (FPI) sensor devices and a microwave photonics interrogation technique. The FPI sensor is fabricated with the assistance of a capillary tube, where a short section of the capillary is sandwiched between two single-mode fibers, forming the air-gap Fabry-Perot cavity. Bending of the FPI device leads to changes in the fringe contrast of its reflection spectrum. Based on the microwave photonics filtering technique, variations of the fringe contrast are encoded into the changes in the peak magnitude of the passband in the frequency response of the FPI device. By multiplexing such FPI devices with different cavity lengths, multi-point measurements of curvature can be realized by tracking changes in corresponding passbands in the frequency response of the system. The FPI curvature sensor is easy-to-manufacture and cost-effective, and the microwave photonics-based system provides an alternative and robust solution to interrogating the multiplexed FPI sensors for multi-point curvature sensing that could be desired in structural health monitoring, human-machine interface sensing, and other related fields.

High-sensitivity fiber optic Fabry-Perot ultrasonic sensor based on a grooved silicon diaphragm for partial discharge detection.

An extrinsic fiber optic Fabry-Perot interferometric (EFPI) ultrasonic sensor based on a grooved silicon diaphragm for partial discharge (PD) detection has been proposed. The size of the groove is determined by finite element simulation, which allows the resonant frequency of the sensor to meet the requirements of PD ultrasonic detection and improves the sensitivity of the sensor by 5.07 times compared with that based on a traditional circular diaphragm. The microelectro-mechanical system process is used to fabricate the diaphragm on a silicon-on-insulator wafer, and the prepared diaphragm has a grooved section with a diameter of 829.34µm and a thickness of only 2.09µm. At its resonant frequency of 61.5kHz, the acoustic pressure sensitivity of the sensor is 172.42mV/Pa. The ultrasonic signal detection capability of the sensor is verified in the PD experiment. Furthermore, the characteristics of the corona discharge are successfully manifested based on the ultrasonic waves detected by the EFPI sensor. It is demonstrated that the proposed sensor is suitable for PD detection due to its high sensitivity, simple production process, and good resistance to environmental interference.

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.

ZIF-90-Derived Porous ZnO Coated Optical Microfiber Interferometer Sensor for Enhanced Humidity Sensing and Breath Monitoring.

Humidity plays an important role in many fields, and the realization of high sensitivity and fast response simultaneously for humidity detection is a great challenge in practical application. In this work, we demonstrated a high-performance relative humidity (RH) sensor made by supporting zeolitic imidazolate framework-90 (ZIF-90)-derived porous zinc oxide (ZnO) onto an optical microfiber Sagnac interferometer (OMSI). The ZIF-90-modified OMSI (ZIF-90-OMSI) sensor was in situ heated at different temperatures to obtain porous ZnO, and their humidity-sensing properties were investigated ranging from 25 to 80% RH. The experimental results showed that the porous ZnO fiber sensor prepared at 500 °C (Z500-OMSI) exhibited best humidity-sensing performance with a high sensitivity of 96.2 pm/% RH (25-45% RH) and 521 pm/% RH (50-80% RH) and ultrafast response/recovery time (62.37/206.67 ms) at 22.3% RH. These performances were attributed to the complete transformation of ZIF-90 to ZnO at 500 °C. The obtained Z500 not only retained the high porosity and specific surface area of ZIF-90 but also exhibited the exceptional hydrophilicity of ZnO. In addition, the signals of the proposed Z500-OMSI sensor changed with different breathing patterns, indicating the possibility for human respiration monitoring. This work provided a reliable candidate for an effective RH monitoring system with potential application in medical diagnoses, industrial production, environmental detection, and human health monitoring.

Ameliorated PGC demodulation scheme using Taubin least squares fitting of ellipse in fiber optic interferometric sensors

In this paper, we propose an ameliorated phase generated carrier (PGC) demodulation scheme using Taubin least squares (LS) fitting of ellipse to eliminate the nonlinear errors. Taubin LS has carefully chosen a normalization matrix and the ellipse fitting parameters are directly obtained by solving the generalized eigenvector problem, leading to a high accuracy result. Quadrature signals are extracted from the signal fundamental and the sidebands of the fundamental of the carrier, and then corrected by the ellipse fitting algorithm (EFA) based on Taubin LS. A combined internal modulation is used to ensure the proper operation of the EFA under small signals. And the desired phase signal is extracted from the corrected quadrature signals by the differential-cross-multiplying method and evaluated by the total harmonic distortion. Experimental results show that the proposed scheme is superior and highly stable, it has a phase resolution of 3.16 μrad/√Hz, a large dynamic range of 120 dB @ 1 KHz and a good linearity of better than 99.99%.

Simultaneous and Multiplexed Measurement of Curvature and Strain Based on Optical Fiber Fabry-Perot Interferometric Sensors

Optical fiber sensors that have a compact size and the capability for multi-parameter sensing are desired in various applications. This article reports a miniaturized optical fiber Fabry-Perot interferometric sensor with a length of hundreds of µm that is able to simultaneously measure variations of curvature, temperature, and strain. The sensor is easy to fabricate, requiring only the fusion splicing of a short section of the silica capillary tube between two single-mode fibers (SMFs). The combined mechanism of the Fabry-Perot interference occurred in the two interfaces between the capillary and the SMFs, and the antiresonant guidance induced by the capillary tube makes the device capable of realizing multi-parameter sensing. A simplified coefficient matrix approach is developed to decouple the contributions from different parameters. In addition, the capability of the device for multiplexing is investigated, where four such prototypes with different air cavity lengths are multiplexed in a system in parallel. The spectral behavior of an individual device for measuring curvature and strain is reconstructed and investigated, showing reliable responses and little crosstalk between different devices. The proposed device is easy to fabricate, cost-effective, robust, and could find potential applications in the field of structural health monitoring and medical and human–machine interactive sensing.

High-sensitivity weighing sensor based on broadband optical coherence displacement measurement of a dual-beam elastic element

A novel weight sensor based on the Michelson interferometer is proposed in this study to meet the demands of complex environments. The sensor uses a dual-beam type elastic element, optimized to enhance its sensitivity. Its deformation under load is sensed by a broadband optical coherence displacement measurement system, which, in combination with the spectrum correction method, enables displacement sensing down to sub-nanometer resolution. An optical interferometric weight sensor with a rated load of 6 kg is fabricated under laboratory conditions, and its performance is verified experimentally. The proposed weight sensor demonstrates good stability and achieves a sensitivity of 146.814 μm/kg, theoretical resolution of 0.004 g, nonlinearity error of 0.034%, maximum repeatability error of 0.036%, and maximum hysteresis error of 0.055%, with the relative error of the identification threshold not exceeding 0.1%. The proposed weighing method and sensor represent a significant improvement over other reported interferometer-type weight sensors.

Self-Vernier Effect-Assisted Optical Fiber Sensor Based on Microwave Photonics and Its Machine Learning Analysis

Optical Vernier effect has been recently demonstrated as a tool to enhance the sensitivity of optical fiber interferometric sensors and has become a hot topic in the last few years. The generation of the Vernier effect relies on the superposition of interferograms of two interferometers (a sensing one and a reference one) with marginally different optical path differences (OPDs), where an amplitude modulation-like signal is sustained in the output spectrum. The Vernier modulation envelope exhibits significantly magnified sensitivity in response to external perturbations, compared to the individual sensing interferometer, providing a new route to new generations of ultra-sensitive optical fiber sensors. However, the construction of a Vernier effect-based sensor needs careful integration of two interferometers with a precise OPD deviation, which is challenging if not impossible sometimes. Additionally, the interrogation of a Vernier effect-based sensor requires the acquisition of its spectrum in a broad wavelength (frequency) window with a large number of sampling points, followed by cumbersome processing for the extraction of the Vernier envelope signal. In this article, we propose a new concept of the self-Vernier effect for the sensitivity improvement of optical fiber sensors. A proof-of-concept demonstration is performed using a single low-coherent microwave-photonic Fabry-Perot interferometer (FPI) with an addition of an auxiliary delay path. The self-Vernier effect is obtained by the overlap between the microwave interferogram of the FPI and its slightly time-delayed one. Detailed analyses regarding the principle, mathematical modeling, and experimental investigation are given. Moreover, we propose and demonstrate a new demodulation approach to the Vernier effect-based sensor through machine learning analysis by directly and statistically learning the embedded relation between the measurand of interest and the output spectrum. It is appreciated that the necessity of the frequency observation window and the number of sampling points can be remarkably reduced through machine learning analysis. The present interrogation strategy is highly generalizable and can be readily applied to optical-domain Vernier effect-based optical fiber sensors.

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.

Optical Fiber Magnetic Field Sensors Based on 3 × 3 Coupler and Iron-Based Amorphous Nanocrystalline Ribbons

Optical fiber interferometric magnetic field sensors based on magnetostrictive effects have several advantages, e.g., high sensitivity, strong adaptability to harsh environments, long distance transmission, etc. They also have great application prospects in deep wells, oceans, and other extreme environments. In this paper, two optical fiber magnetic field sensors based on iron-based amorphous nanocrystalline ribbons and a passive 3 × 3 coupler demodulation system were proposed and experimentally tested. The sensor structure and the equal-arm Mach-Zehnder fiber interferometer were designed, and the experimental results showed that the magnetic field resolutions of the optical fiber magnetic field sensors with sensing length of 0.25 m and 1 m were 15.4 nT/√Hz @ 10 Hz and 4.2 nT/√Hz @ 10 Hz, respectively. This confirmed the sensitivity multiplication relationship between the two sensors and the feasibility of improving the magnetic field resolution to the pT level by increasing the sensing length.