Fiber Optic Accelerometer Research Articles

Sensitivity Enhancement of a Semicircular Curved Hetero-Core Optical Fiber Accelerometer With Low Cross-Axis Sensitivity

In this article, the frequency, amplitude, and cross-axis sensitivity characteristics of a hetero-core fiber optic accelerometer subjected to mechanical vibrations are presented. The proposed accelerometer is based on a fixed-support structure and can be used for structural health monitoring. It consists of a semicircular curved hetero-core optical fiber with both ends fixed to a base, which operates as a vibration detector. To increase its sensitivity and reduce its cross-axis sensitivity, a hetero-core portion is placed near one of the fixed ends. By fabricating an ideal semicircular beam-shaped acceleration sensor with both ends fixed, good agreement between the experimentally obtained spectrum results and finite element analysis results when the accelerometer is subjected to mechanical vibrations is observed. The accelerometer’s sensitivity is clearly stable at frequencies less than approximately a quarter of the first natural frequencies of 1810, 850, and 350 Hz with sensitivities of 1.45 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{-3}$ </tex-math></inline-formula> , 2.80 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{-3}$ </tex-math></inline-formula> , and 6.16 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{-3}$ </tex-math></inline-formula> dB/g for fiber curvature radii of 7, 10, and 15 mm, respectively. Owing to its highly linear acceleration characteristics, it is possible to efficiently convert the acceleration to optical loss. The cross-axis sensitivity is in the range 4.27%–15.9%, which is better than that of previously reported accelerometer structures, which use a hetero-core fiber sensor located at the center between the two fixed ends without a free supporting point. Therefore, the proposed accelerometer, which consists only of a semicircular-shaped optical fiber, is capable of improving the sensitivity and reducing the cross-axis sensitivity by adjusting the placement of the hetero-core fiber sensor.

Strain distribution characteristics of sensing fiber and influence on sensitivity of a fiber-optic disk accelerometer

In a fiber-optic disk accelerometer, the strain distribution of sensing fiber is crucial for the improvement of sensitivity. The distribution characteristics of axial and radial strain in the sensing fiber are analyzed by the finite-element method, and the influence of strain distribution on the sensitivity of accelerometer is studied. Sensors with different outer radii of sensing fiber coils are designed and manufactured, and their performance is tested. The resonant frequencies are greater than 200 Hz, and the sensitivity increases as the outer radius of the sensing fiber coil increases. The dynamic range of the sensor with maximum sensitivity is 145.8 dB@100 Hz, and the transverse cross talk is 32.5 dB. Among the sensing fiber strain calculation methods we tested to predict the value of sensitivity, using the strain of sensing fiber directly obtained by the finite-element analysis method shows the smallest error with experimental results (within 7%). It is concluded that in the optical fiber strain disk, the absolute values of axial strain and radial strain of sensing fiber decrease with the increase of disk radius, while the signs of axial strain and radial strain of the ipsilateral sensing fiber are opposite and remain unchanged. The sensitivity can be further improved by optimizing the inner and outer radius of the sensing fiber coils, which is very important for the research and design of high-sensitivity fiber-optic accelerometers.

Shallow seafloor seismic wave monitoring using 3-component fiber optic interferometric accelerometer

A fiber optic interferometric seismic monitoring system developed by us was deployed on the shallow seafloor near the Wuchang oceanic monitoring station off the Hainan Island, in the South China Sea, in 2017. In the past three plus years, it has recorded more than 40 teleseismic activities of magnitude 6.0+. We analyze these shallow seafloor observed seismic signals monitored by the fiber optic interferometric three-component accelerometer from the perspective of analyzing the underwater ambient seismic noise and the attenuation of the noise in the shallow seafloor seismic signals. The ambient seismic noise produced by wind, wave, shipping, and on-shore manmade activities admixed in the seismic signal is analyzed and discussed in detail. And the attenuation of such noise in the shallow seafloor seismic data using a hybrid denoising algorithm is demonstrated by applying it to three representative seismic events having different distances and magnitudes, with results showing that the noise in the shallow seafloor seismic signal is effectively reduced, and the ratio of the maximum acceleration value of seismic wave to the noise level is improved by more than 32 dB, rendering the seismic waves clearly observable.

Orientation-dependent fiber-optic accelerometer based on eccentric fiber Bragg grating.

A highly localized eccentric fiber Bragg grating (EFBG) accelerometer was proposed, and its orientation-dependent measurement results were demonstrated experimentally. An EFBG was inscribed point-by-point (PbP) in a single-mode fiber (SMF) using a femtosecond laser, and the cladding mode was recoupled to excite the ghost mode through an abrupt taper. Owing to the asymmetry caused by the lateral offset of the EFBG, the ghost mode showed a significant directional response to acceleration. Furthermore, monitoring the fundamental core mode resonance can help calibrate accidental power perturbation or cross-sensitivity.

Low-Cost Fiber Optic Cantilever Accelerometer With a Spherical Tip Based on Gaussian Beam Focusing

We propose and demonstrate a low-cost and simple fiber optic cantilever accelerometer with a spherical tip based on Gaussian beam focusing. The accelerometer consists of ceramic ferrule, ceramic sleeve, receiving fiber and emitting fiber, where both fibers are single mode fiber, the ferrule and sleeve have characteristics of high precision, which reduce the difficulty of optical alignment. The end of the emitting fiber is made into a spherical tip for focusing the Gaussian beam to improve sensitivity. When the accelerometer is in operation, the emitting fiber acts as a cantilever beam, the acceleration can be measured by detecting the transmission power. Further, our experimental results show that the spherical fiber tip can improve the acceleration sensitivity by 67% over 10 Hz-1000 Hz without reducing the working bandwidth. In addition, it is found that the fiber accelerometer has a high signal-to-noise ratio (SNR) up to 60 dB, and a low harmonic distortion of better than -30 dB, rendering a quasi-8-shaped directionality at the working frequency ranging from 10 Hz to 1200 Hz. This clever sensor structure may have potentials for developing high-performance and cost-effective accelerometers and hydrophones.

Hydroacoustic Pressure Gradient Recording by a System of Two Fiber-Optic Accelerometers

Hydroacoustic Pressure Gradient Recording by a System of Two Fiber-Optic Accelerometers

Common-mode noise self-suppressed 3-component fiber optic accelerometer based on low-reflectivity Bragg gratings.

Common-mode noises (CMNs) are frequent in the fiber optic accelerometer, and their suppression is extremely important, particularly in the ultra-weak signal detection application, e.g., micro-seismic monitoring. This Letter proposes a 3-component (3C) low-reflectivity fiber Bragg gratings accelerometer for CMN self-suppression. When compared with the traditional CMN suppression method, the proposed 3C accelerometer is able to improve the CMN suppression effect by an average value larger than 4.5 dB in three axes, as well as double the effective signal amplitude due to the push-pull structure, which brings an enhanced signal-to-noise ratio. Besides, the proposed 3C accelerometer does not need an additional reference interferometer to achieve such a CMN suppression effect; hence, it largely reduces the volume and cost of the sensing system, which shows huge advantages, particularly in the large-scale quasi-distributed array. The proposed 3C accelerometer provides a promising candidate for the weak vector vibration detection.

Train-Induced Vibration Monitoring of Track Slab under Long-Term Temperature Load Using Fiber-Optic Accelerometers.

The train-induced vibration response provides a flexible solution for the real-time monitoring deformation of high-speed railway track slab in actual operation. This paper proposes a long-term real-time monitoring method for track slab deformation based on wavelet packet energy (WPE) using fiber optic accelerometers to record train-induced vibration. We found that the vibration response law of track slab deformation could be established by using the WPE of the frequency band covering the first- and second-order frequencies induced by the adjacent carriages. A field test was carried out for more than one year on the Beijing–Shanghai high-speed railway to investigate the train-induced vibration response law of track slab that was continuously deformed under a long-term temperature load. The maximum values of the WPE characteristic index appeared in winter and summer, and they were positively correlated with the temperature difference between the air environment and the track slab under the daily temperature load. These results were demonstrated to be consistent with the track slab deformation law for long-term and daily temperature loads. The novel method based on fiber optic accelerometers and WPE provides a new method for the long-term and real-time monitoring of track slab deformation.

Highly Sensitive FBG Seismometer With a 3D-Printed Hexagonal Configuration

We have designed and developed a compact fiber optic accelerometer for vibration detection. The mechanical vibrating element is a hexagonal structure which is fabricated using 3D-printing technology. Such a structure can efficiently convert vertical vibrations into periodic deformations of a longitudinally positioned fiber structure into which a fiber Bragg grating (FBG) is inscribed. By demodulating the wavelength shift of the reflection spectrum, we established the relationship between vibrational acceleration and wavelength shift. The measured sensitivity is 421.4pm/g. A second FBG inscribed in the fiber enables temperature compensation. The proposed design has achieved a working bandwidth from 10 to 210Hz with a resonant frequency of 124.9 Hz. Both laboratory and in-field measurements using our sensor have been compared with those of a traditional electromagnetic accelerometer to validate the effectiveness of the derived theoretical models and the simulation results. The comparison has effectively supported the parameter determination of the sensor design.

Recording of Hydroacoustic Signals Using a Fiber-Optic Accelerometer

A prototype of a mobile fiber-optic accelerometer in which a multiturn optomechanical transducer placed in a fiber-optic Mach–Zehnder interferometer interferometer arm is used as a sensitive element is developed and studied. Passive phase demodulation using a fiber-optic splitter 3 × 3 maintains stable accelerometer operation in the case of a temperature drift of an operating point. The detectability of weak hydroacoustic signals using a fiber-optic interferometric accelerometer is shown.

Experimental Study of Dynamic Deformation Processes in Gas Pipeline

This paper is concerned with an experimental investigation of vibrations occurred in gas pipelines under different impact loads. The mechanical state of these objects is determined by the interaction between the processes taking place in the metal pipe, the ground in the vicinity of the pipe and the gas flow inside it. In the experiments, different pipeline vibration modes were investigated. Application of fiber-optic accelerometers made it possible to record vibrations from shutoff valves, as well as gas leak-generated vibrations. It was found that the loads applied to the pipe wall or surrounding soil induced pipe vibrations in the local region near the impact site (the signal effective distance is about 60 meters). A much stronger effect on the pipe wall was exerted by the jumps in pressure via the shut-off valves (the distance measurement of 2 km). The data obtained can be used to construct a structure diagram for the online monitoring system needed to control the deformation state of gas pipelines, to assess the sensitivity of sensors to various factors, and to verify the available mathematical models describing vibration processes in gas pipelines.

Performance Optimization of Fiber Optic Interferometric Accelerometer Based on Phase Noise Analysis

The phase noise of a Michelson interferometer-based fiber optic accelerometer is analyzed to determine the optimal modulation depth of the laser light frequency and the length difference between the interference arms. The output intensity fluctuation of the interferometer, originating from the phase noise of the modulated laser source, is examined theoretically, revealing that both the interference intensity noise power and the phase modulation depth of the interferometer are proportional to the frequency modulation depth of the laser and the interference arm length difference. In addition, it is determined experimentally that the laser linewidth, or equivalently, the laser phase noise, varies approximate linearly with the light frequency. As the optimum phase modulation depth is a constant, the interference intensity noise power has a minimum value, when the light frequency modulation depth is at its maximum. The proposed theory is demonstrated experimentally, with results showing that a high-performance broadband fiber optic interferometric accelerometer achieves the lowest noise level when the light frequency modulation depth of the source laser is at its maximum value.

Adaptive bias and attitude observer on the special orthogonal group for true-north gyrocompass systems: Theory and preliminary results

This article reports an adaptive sensor bias observer and attitude observer operating directly on for true-north gyrocompass systems that utilize six-degree-of-freedom inertial measurement units (IMUs) with three-axis accelerometers and three-axis angular rate gyroscopes (without magnetometers). Most present-day low-cost robotic vehicles employ attitude estimation systems that employ microelectromechanical system (MEMS) magnetometers, angular rate gyros, and accelerometers to estimate magnetic attitude (roll, pitch, and magnetic heading) with limited heading accuracy. Present-day MEMS gyros are not sensitive enough to dynamically detect the Earth’s rotation, and thus cannot be used to estimate true-north geodetic heading. Relying on magnetic compasses can be problematic for vehicles that operate in environments with magnetic anomalies and those requiring high-accuracy navigation as the limited accuracy ( error) of magnetic compasses is typically the largest error source in underwater vehicle navigation systems. Moreover, magnetic compasses need to undergo time-consuming recalibration for hard-iron and soft-iron errors every time a vehicle is reconfigured with a new instrument or other payload, as very frequently occurs on oceanographic marine vehicles. In contrast, the gyrocompass system reported herein utilizes fiber optic gyroscope (FOG) IMU angular rate gyro and MEMS accelerometer measurements (without magnetometers) to dynamically estimate the instrument’s time-varying true-north attitude (roll, pitch, and geodetic heading) in real-time while the instrument is subject to a priori unknown rotations. This gyrocompass system is immune to magnetic anomalies and does not require recalibration every time a new payload is added to or removed from the vehicle. Stability proofs for the reported bias and attitude observers, preliminary simulations, and a full-scale vehicle trial are reported that suggest the viability of the true-north gyrocompass system to provide dynamic real-time true-north heading, pitch, and roll utilizing a comparatively low-cost FOG IMU.

Highly sensitive fiber-optic accelerometer using a micro suspended-core fiber.

A compact fiber-optic accelerometer was proposed and demonstrated experimentally based on Fabry-Perot interference (FPI). The device consists of a suspended-core fiber embedded in a hollow-core fiber, forming an enclosed cavity structure. A short section of multi-mode fiber (MMF) was spliced on the leading-in single-mode fiber (SMF), which worked as a micro lens to focus the light to decrease the transmission loss. A well-defined interference spectrum was achieved by a low-fitness FP interferometer formed by both the end-face of lead-in fiber and the end-face of suspended-core fiber. Thanks to the outstretched FP cavity by suspended-core fiber, the sensor is highly sensitive to vibration along the fiber axis. Moreover, a one-dimensional mechanical transducer was used to improve the frequency band of the sensor. By the side-band filtering technology, the vibration was detected and analyzed by a simple intensity interrogation technology.

Real-time acceleration sensing with an arctan algorithm based on a modal interferometer.

This study proposes a fiber-optic accelerometer for low-frequency vibration signal detection. The phase velocities of the polarization eigenmodes are affected differently by signals, leading to a polarization rotation of the transmitted lights. The orthogonal square roots of the photovoltages are utilized for an arctan demodulation scheme. Experimental results show that it provides a flat response of 75.04 mrad/g, an average resolution of 13.44 μg/√Hz, and a dynamic range of 111.62 dB below 180 Hz. The environmental instability and sensor complexity are significantly reduced, so that the sensor can be further used in the warning of coal and gas outburst.

МОДЕЛИРОВАНИЕ ВОЛОКОННО-ОПТИЧЕСКОГО АКСЕЛЕРОМЕТРА МАЯТНИКОВОГО ТИПА

Analysis of scientific literature has shown that there is a lack of attention to the processes used in fiberoptic accelerometers, which operate under concentrated impact of destabilizing factors generated by compactly arranged equipment of power unit. Constructive and technological features of accelerometers are not considered, and there is no assessment of their metrological and operational characteristics in these conditions. Accordingly, there is no systematic description of the processes in fiber accelerometers and, in general, the approach to the solution of this important issue. The author studies the work of pendulum scheme accelerometer to control high frequency vibration in special conditions. In the developed scheme of the accelerometer the performance of optical fiber in the form of laminated layered structure allows to create and use discrete design diagram. According to this diagram: a fiber is replaced by a cylinder, consisting of a series of concentric cylindrical layers, within each of which the voltage is considered to be constant. In each layer the refractive index is constant and the field is described by the ψ function; to estimate the magnitude of the refractive index in each layer the method of equal volume sections can be used; the task of coordination of two waveguide structures is considered as an excitation of the receiving antenna by some given field of the radiating antenna. Based on the modified theory of related modes in tunnel-coupled optical fibers, the coefficient of radiation modes coupling in a coaxial structure, created under the influence of oscillatory processes, has been determined. The model of fiber-optic accelerometer, which has allowed to develop a measuring converter model for laboratory measurements and realize the devices modeling for their effective design, is further developed.

A MEMS Accelerometer Based on Wavelength Modulation Using an Integrated Blazed Grating

This paper reports a microelectromechanical systems (MEMS) optical fiber accelerometer with an integrated blazed grating as a wavelength modulation component. The accelerometer includes a V-shaped cantilever beam, a vertically symmetrical proof mass, and an integrated blazed grating. The accelerometer prototype is fabricated with bulk micromachining processes and is tested. The bandwidth of the accelerometer is 200 Hz. The optical sensitivity and nonlinearity of the measure range ±5.2 g are 1.63 nm/g at 100 Hz and 0.06%, respectively. The x- and y-axes cross-axis sensitivities are 2% and 4.5%, respectively. In the full temperature range of -20 °C-200 °C, the temperature sensitivity is 2.8 pm/°C. Thanks to the above-mentioned characteristics, the proposed MEMS accelerometer could have a wide range of applications, such as rail traffic and attitude heading sensing applications.

High-accuracy transient response fiber optic seismic accelerometer using a shock-absorbing ring as a mechanical antiresonator.

This study proposes a high-accuracy transient response fiber optic seismic accelerometer based on the resonance suppression mechanism. A shock-absorbing ring is embedded in the accelerometer structure, which acts as a mechanical antiresonator. The experimental results show that the sensitivity at the resonance frequency is suppressed by 21.79dB, and the 3dB operating bandwidth is extended without reducing the average sensitivity. Under this condition, the high-accuracy transient response is obtained during the vibration-event test. This study provides a practical seismic acquisition technique solution for vertical seismic profiling monitoring in the smart oilfield.

Ultra-Low-Frequency Tri-Component Fiber Optic Interferometric Accelerometer

We demonstrate a high-performance ultra-low-frequency tri-component fiber optic interferometric accelerometer (FOIA), designed particularly for application in seismic observation of ocean-floor deployment, with high sensitivity, low noise level, and large dynamic range. Experimental results show that the sensitivity of the FOIA is 57 dB re rad/g, and the noise levels of the three orthogonal components are 0.163, 0.181, and $\mathrm {0.153~ \mu g}$ , respectively, within the operating frequency band (0.005–50 Hz). The dynamic range of the system is higher than 117.03 dB. Actual observational results of an M4.3 seismic event show that the prototypical system can clearly record the seismic wave, with unambiguous demarcation of S-wave and P-wave, and locate the hypocenter of the earthquake.

Micro all-glass fiber-optic accelerometers

A micro all-glass fiber-optic accelerometer based on the extrinsic Fabry–Perot interferometer (EFPI) is developed. A microcantilever beam is fabricated using a femtosecond laser and assembled with a silica tube and a silica inertial mass. Two mirrors, the end face of the leading single-mode fiber and the inner glass/air interface of the cantilever beam, form an EFPI. When the accelerometer is subjected to the vibration along the fiber axis, the vibration is detected by interrogating the variation in the cavity length of the EFPI. The proposed accelerometer has a compact structure and high signal-to-noise ratio. Experimental results show that the sensitivity of 2.9 nm/g@500 Hz within the acceleration range of 0 to 3 g is achieved. The accelerometer can work within the frequency range of 100 to 1500 Hz.