Depolarizer improvement for optical fiber vibration sensor

This paper proposes the use of an fiber optic sensor (FOS) and electrorheological (ER) fluid actuator in the vibration active monitoring of smart composite structures. The intensity modulated fiber optic vibration sensor is developed following the face coupling theory. The characteristics of this optical fiber vibration sensor have been tested. The experimental results show that the new type intensity modulated optical fiber vibration sensor has higher sensitivity to measure the vibration frequency. At same time, our experimental investigations are focused on evaluating the elastodynamic response characteristics of a beam fabricated in an electrorheological fluid. It is noted that the most significant change in the material properties of ER fluid is the change of material stiffness and damping, which varies with the electric field intensity imposed upon ER fluid. Finally, the vibration of smart composite beam with embedded intensity modulated optical fiber vibration sensor and electrorheological fluids can be detected and controlled actively.

In recent years, phase demodulation methods for optical fiber vibration sensors (OFVS) have attracted more and more attention, aiming to accurately detect vibration signals. The principles of phase modulation and demodulation in OFVS are firstly introduced. Then, three classical phase demodulation methods are described, including phase generated carrier (PGC) demodulation method, <inline-formula> <tex-math notation="LaTeX">${3}\,\,\times \,\,{3}$ </tex-math></inline-formula> coupler demodulation method, and in-phase and quadrature (IQ) demodulation method. Furthermore, the state of art of phase demodulation methods in various OFVS are reviewed. As the results of analysis, different structures of OFVS need appropriate demodulation methods for vibration detection. The Fourier transform method, fringe counting method and PGC demodulation method have strong applicability for Fabry-Perot interference OFVS. The scalability of interferometers provides convenience for PGC demodulation method and <inline-formula> <tex-math notation="LaTeX">${3}\,\,\times \,\,{3}$ </tex-math></inline-formula> coupler demodulation method in the OFVS with composite structure. PGC demodulation method, <inline-formula> <tex-math notation="LaTeX">${3}\,\,\times \,\,{3}$ </tex-math></inline-formula> coupler demodulation method and quadrature demodulation method are good choices for dynamic sensing in FBG-OFVS. The IQ demodulation method is a popular choice for phase sensitive optical time domain reflectometry at present. This paper provides a reference for the OFVS to realize vibration detection with appropriate phase demodulation method.

Optical fiber vibration detection has the advantage of large-scale monitoring, immunity to electromagnetic, excellent flexibility and thus show considerable potential for a variety of practical application. The research of optical fiber sensor in vibration detection is mainly centralized in fiber optic sensing technologies, sensor related signal processing algorithms and some typical applications. It is lack about the comparative study with other vibration sensor, such as piezoelectric sensor. In this paper, comparative experiments of optical fiber sensor and piezoelectric sensor based on vibration detection is designed. The qualitative and quantitative analysis between optical fiber vibration signal and piezoelectric vibration signal is studied, though Fourier analysis and crosscorrelation analysis. The result shown that both methods can extract the main frequency components of the vibration source in frequency domain. However, compared to piezoelectric vibration methods, the optical fiber vibration contains extra low frequency vibration and high frequency vibration. The piezoelectric vibration is slightly better than optical fiber vibration in cross-correlation analysis.

The detection of acoustic signals in the Martian environment is significant to understand the issues such as the evolution of the universe, structure of matter, origin of life, and future migration of humans. We proposed a method for testing ultrasonic wave by using fiber-optic Fabry-Perot vibration sensors in a low-pressure CO₂ environment. We conducted high-precision sound wave amplitude and speed measurement tests with different center frequencies at 21, 25, 34, and 40 kHz under different gas compositions, pressures, and distances. Results showed that under the conditions of 15 &#x00B0;C and gas pressure in the range of 600 Pa to 1 MPa, the measured average sound velocity of ultrasonic signals at each frequency was 268.79 m/s in the CO₂ environment, which was lower than the velocity of 336.18 m/s measured in the air environment. The results in this study can be applied to theoretical and experimental studies of future Mars probes pertaining to ultrasonic positioning and detection.

The paper proposes a version of realization for a fiber optic vibration sensor. The sensor possesses a wide range of operation frequencies with such interrogation devices as optical power meters. In comparison to spectral measuring complexes, the sensor imposes lower requirements on operating conditions. The authors investigated a sensing head based on a fiber SMF-MMF-SMF structure and a fiber Bragg grating inscribed in this structure. The external vibration frequency applied to the tested sensor structure is obtained using the Fourier transform of the signal received from the photodetector. The structure designed in this study with a fiber Bragg grating inscribed in it can be used as a sensing head of a fiber optic vibration sensor. It is demonstrated that the sensor based on the developed sensitive structure is able to obtain the external vibration frequency in the range of 20-9 000 Hz with accuracy up to 1 %. The research results are essential for monitoring systems for the state of structural elements of buildings and structures. The implementation of the vibration sensor in the format of a fiber optic device allows overcoming the limitations of piezoelectric sensors, providing high noise immunity and resistance to harsh environmental effects.

Accurate in-situ acquisition of high-frequency vibration signals remains a critical technical challenge. Conventional vibration sensors suffer from limited bandwidth and insufficient dynamic response, making them inadequate for effectively capturing high-frequency signals. To address these limitations, this study proposes a novel fiber-optic vibration sensor based on the Fabry-Pérot (F-P) interference principle. The sensor adopts an integrated design in which a resonant cavity, sensing diaphragm, and hollow cavity are directly fabricated onto the tip of a single-mode optical fiber using two-photon 3D printing technology. Finite element analysis (FEA) was employed to optimize the structural parameters. Experimental results demonstrate that the sensor effectively captures both continuous high-frequency vibrations and transient impact signals. Specifically, the sensor exhibits a broad frequency response ranging from 115 kHz to 305 kHz, and maintains an average sensitivity of 25 mV/g within the range of 0.5 to 10 kHz. The signal-to-noise ratios (SNR) of the sensor are as high as 45.5 dB@115kHz and 58.8 dB@305kHz, respectively. This performance demonstrates that proposed sensor is among the most advanced in the family of fiber-optic vibration sensor, with significant potential for applications such as structure health monitoring, ultrasonic detection, and mechanical fault diagnosis.

The mine hoist operation status is closely related to the vibration signal of the hoist various components. using optical fiber sensing technology, this paper designed a hoist fault diagnosis system based on vibration spectrum analysis. Through rapid demodulation of real-time vibration signal, the system realized vibration spectrum analysis to various parts of the hoist. The test results show that the system can achieve effective monitoring of the various parts of the hoist operating status, provide an important basis for fault diagnosis.

Bearings are crucial components in mechanical equipment, and bearing failure is one of the reasons for mechanical equipment safety accidents. In this work, an optical fiber vibration sensor based on Sagnac interferometer (SI) is proposed and applied to bearing fault detection. The polarization-maintaining fiber (PMF) is spliced between two single-mode fibers (SMFs) to form a SMF-PMF-SMF (SPS) fiber structure, which is connected with a 3 dB coupler to form a SI based on SPS fiber structure. The mass block is fixed in the middle of the PMF. When the monitored surface or structure vibrates, the stress of PMF will change and the Sagnac interference spectrum will be shifted, so that the vibration can be measured. The fabrication technology of sensor based on 3D printing is studied, and the structural parameters of the sensor are optimized through experiments and theoretical analysis. A vibration detection system and a rolling bearing vibration test platform based on SI and fiber ring laser are built. The experimental results show that the relative error between the experimental and theoretical results of healthy and faulty bearings is less than 1.60%, and the fault detection of bearings is realized. Temperature cross-sensitivity and stability of the sensing system are analyzed. The sensor has the advantages of simple structure, easy fabrication, anti-interference, etc, and has wide application prospects in engineering, machinery, security and other fields.

Vibration detection plays a vital role in the safety monitoring of engineering areas such as structural deformation of large buildings and railway tracks, and oil and gas spills. Herein, a temperature-insensitive optical fiber vibration sensor based on Mach-Zehnder interference is proposed and validated, which is achieved by utilizing a dispersion-compensated fiber (DCF). Specifically, single-mode fiber (SMF) is fused to both ends of a DCF to form an optical fiber vibration sensor, which is SMF-DCF-SMF (SDS) sensor. Under this sensing structure, this sensor’s vibration sensing characteristics, amplitude-frequency response, stability, repeatability and temperature-insensitive characteristics are investigated. The sensor can measure vibration frequency signals in the large frequency range of 0.1 Hz to 47 kHz by intensity demodulation. When the vibration frequency is 600 Hz, the maximum signal-to-noise ratio (SNR) of the sensor reaches 69 dB. In addition to that, the sensor can capture low amplitude vibration frequency signals down to 0.01 V. The proposed sensor has a wide measurement frequency range that effectively reduces temperature cross-sensitivity, and it has the advantage of being compact, low cost and easy to manufacture, with potential applications in health inspections, machinery and building saturation.

Intensity-based optical fiber vibrations sensors (OFVSs)are used in damage monitoring of fiber-reinforced plastics, invibration sensing, and location of impacts. OFVSs were constructed byplacing two cleaved fiber ends in a capillary tube. This sensor isable to monitor structural vibrations. For vibration sensing, theoptical fiber sensor was mounted on the carbon fiber reinforcedcomposite beam, and its response was investigated for free and forcedvibration. For locating impact points, four OFVSs were placed atchosen positions and the different arrival times of impact-generatedvibration signals were recorded. The impact location can bedetermined from these time delays. Indentation and tensile tests wereperformed with the measurement of the optical signal and acousticemission (AE). The OFVSs accurately detected both free and forcedvibration signals. Accurate locations of impact were determined on anacrylate plate. It was found that damage information, comparable inquality to AE data, could be obtained from the OFVS signals.

Analysis of the vibrational characteristics of materials or mechanical systems had the promise of providing useful information of their integrity. Acoustic monitoring of heavy machine bearings is a well-established technique for detecting the onset of wear [1]. Damage to materials [2] or structures [3] results in local stiffness reductions which may be detected by measuring their dynamic response. Vibration sensors could also be used to monitor the passage of traffic over bridges and roadways or for security applications such as detecting forced entry into buildings. Vibration sensing is most commonly performed by electromechanical devices such as piezoelectric, piezoresistive or capacitative accelerometers. Whilst this is reliable and tested technology it has a potential drawback. Susceptibility to electromagnetic interference limits application around strong electric fields and the need for either strongly shielded leads or a power supply for a built-in amplifier results in relatively heavy, bulky cables. If an extensive network of sensors is to be incorporated into a structure, this may prove to be a considerable problem, even more so if there is a premium on weight, as in aerospace structures. Optical fibre sensors have been shown to be suited for embedment in composites [4] and cementitious materials [5]. They have largely unaffected electromagnetic interference and in some cases they may be multiplexed to give a distributed sensing system. Optical fibre sensors tend to be more expensive than their conventional counterparts, mainly because the equipment needed to interrogate the sensor is often delicate and complex. To measure vibration, either a strain gauge-type device is attached intimately to the structure to measure surface acoustic waves, or a fibre-optic accelerometer is employed in place of a conventional one. The device presented in this latter modulates the intensity of light transmitted through it and thus requires only simple and cheap instrumentation. It is illustrated schematically in Fig. 1. The details of its construction and operation are as follows. Two 50=125 multimode optical fibres are cleaved and the protective polymer jacket of one of the fibres is stripped back a short length. The prepared fibres are then inserted into a precisiondrawn glass capillary, their end-faces separated by a few tens of microns, and fixed with epoxy resin adhesive. When the sensor is subjected to an acceleration prependicular to the axes of the optical fibres, the motion of the unsupported fibre lags behind. As it moves out of alignment, relative to the fixed fibre, the transmitted optical power decreases. The power variations are detected with a fastresponse photodiode and amplifier and sent to a personal computer for further processing. The response of the sensor to sinusoidal and transient excitation was investigated. First, the sensor was mounted on an electromagnetic coil which was driven by a signal generator and amplifier and excited over a range of frequencies from 100 to 10000 Hz. The sensors which provided the results presented here had resonant frequencies of 3 Hz and 6 Hz, but this value can be varied by changing the length of the cantilevered fibre or by choosing fibres of different diameters. To test the response to transients, sensors of 3 Hz resonant frequency were bonded to the surface of 70 3 70 mm 16-ply crossply carbon-fibre reinforced plastic (CFRP) with a cyanoacrylate adhesive. An impulse was provided by either dropping a 6.3 mm diameter steel ball onto the panel from a height of 25 cm, giving an impact energy of 2.5 mJ, or a standard pencil break test [6]. Examples of sensor signals from these events are given in Fig. 2. The impact of the steel ball, considerably the more energetic of the two events, gave a greater signal amplitude, as well as a characteristic shape different from that of the pencil break. Results were consistent within both series, with the pencil break showing a little more variability.

This paper proposes the use of a fiber optic sensor (FOS) and electrorheological (ER) fluid actuator for vibration monitoring of smart composite structures. A new intensity modulated fiber optic vibration sensor is developed following the face coupling theory. It has high sensitivity similar to the traditional piezoelectric sensor. Also it is lower in cost. The experiment of vibration control of smart composite beam with embedded intensity modulated optical fiber vibration sensor and ER fluids are described in this paper. It is noted that the most significant change in the structural properties of smart composite beam is the change of structural damping and natural frequency, which varies with the electric field intensity imposed upon ER fluid. So the structural vibration can be monitored and controlled effectively utilizing FOS and ER fluids.

The loose failure of transmission tower bolts may lead to structural instability and safety risks, so effective monitoring methods are crucial to the stable operation of transmission lines. The purpose of this study is to explore the effectiveness of vibration signal technology in monitoring the loose bolts of transmission tower. Based on vibration theory, the principle of bolt loosening of transmission tower is analyzed, vibration signal data is collected by means of vibration exciter and optical fiber vibration sensor, and then the monitoring test of transmission tower loosening fault is carried out. The obtained test results show that the time domain waveform is significantly different before and after excitation, and the wavelength after excitation has a significant mutation, increasing from 1550 nm to 1553 nm, and slowly decreasing to the original wavelength, which also means that the transmission tower bolt loosening fault monitoring system has a good monitoring ability and can be used for vibration measurement. According to these monitoring results, the conclusions can be obtained as follows: first, the frequency domain data amplitude changes before and after loosening can be used to judge whether the bolt is loose, so as to achieve the monitoring purpose; Second, the strength of the vibration signal is large, the vibration signal change caused by the loosening of the bolt is submerged, and the installation of excitation at the sensor should be avoided to ensure that the monitoring is not disturbed by external factors. The research provides a new technical way for real-time monitoring of loose bolt fault of transmission tower, which has practical value and popularization prospect.

In recent years, traditional perimeter security system is gradually replaced by optical fiber distributed vibration sensing system, as it has superior advantages such as high sensitivity, fast response, and simple structure. However, it is still challenging to accurately realize multievent pattern recognition in practical applications. Accurate pattern recognition can reduce the false alarm rate and significantly increase the stability of the optical fiber system. In this article, we proposed a pattern recognition approach based on short-time Fourier transform (STFT) and Resnet 152-based neural network. First, the vibration signal containing high-frequency information was extracted through a median filter. Second, STFT was used to convert a 1-D time-domain signal to a 2-D time–frequency signal. The feature dimension of optical signals was expanded. Third, the redundant information would be removed by dividing the high-, medium-, and low-energy segments. Finally, the preprocessed optical signals were sent to Resnet 152 convolutional neural network (CNN) model for pattern recognition. To verify the effectiveness of the proposed scheme, field tests with nine sensing events (climbing, crashing, cutting, kicking, knocking hard, knocking lightly, no intrusion, pulling, and waggling) have been experimentally carried out. It is demonstrated that the average recognition accuracy of the nine common sensing events is 96.67%, and the detection time is 0.2391 s. The feasibility of deep CNN in solving pattern recognition has been proved.

In the petrochemical industry, delayed cooking is an important production process. This paper analyzes the principle of FBG vibration sensor. The automatic monitoring system of hydraulic defocusing is studied based on fiber optic vibration sensor. The fiber optic sensing system can detect the removal of coke in the coke tower automatically, and it can reduce the labor intensity of workers. It can also improve the automation level of real-time monitoring of hydraulic decoking system.