A fiber optic vibration sensor based on SMF-MMF-SMF structure and a tilted fiber Bragg grating

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.

Optical cable technology — testing

Fiber optic sensors, prized for their light weight, compact size, high temperature resilience and resistance to electromagnetic interference, find extensive utility in various measurement applications. The performance of these sensors is primarily contingent on their sensitive units, with distinct structures of these units yielding varied performance outcomes. Traditional design methods primarily rely on finite element simulation and optimization, which are subjective and inefficient. Thus, the efficient on-demand design of sensitive structures is essential for different application scenarios. Here, we present a novel approach for both forward performance prediction and inverse structure design employing deep learning techniques based on symmetric bidirectional neural networks, with fiber optic vibration sensors serving as a design example. The proposed method can address the non-unique solution in traditional deep learning techniques for inverse design of three-dimensional (3D) complex structures. By learning the underlying relationships between complex non-intuitive sensitive structures and their performances, the approach can eliminate the need for numerous costly calculations that heavily depend on human experience or intuition. Furthermore, compared to the response surface optimization method, this approach saves 21.1 times the computation time and has an accuracy improvement of 34.6% when dealing with six samples. The results show that the efficiency of fiber optic sensor design can be significantly improved by employing this novel deep learning technique, which offers new insights for the rapid advancement of the fiber optic sensing field.

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 °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.

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.

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.

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.

A new fiber-optic extrinsic Fabry-Perot microinterferometer cantilever beam type vibration sensor (EFPI-V) is described. For eliminating signal fading effects a two-wavelength demodulation scheme is implemented. The theoretical model of this sensor with pronounced eigenfrequency spectrum in the acoustic range is confirmed by experimental results. Initial field tests in an experimental airport ground traffic surveillance system for vehicle defection via monitoring of structural vibrations demonstrate the application as traffic sensor.

A simple to fabricate, cost-effective, robust, and highly accurate fiber optic temperature sensor is proposed and experimentally demonstrated. The sensor head consists of an in-fiber Fabry-Perot interferometer (FFPI) embedded in a mold made of a polyester resin. FFPI, which in general present good strain sensitivity and low-temperature sensitivity, was assembled by splicing a microsegment of capillary fiber of 14.66 μm between two single-mode fibers. The reflected spectrum of the FFPI exhibited a single interference dip within a span of 80 nm. The dimensions of the resin mold can be modified by changing the ambient temperature, which in turn changes the length of the FFPI air-cavity. The dip of the reflected spectrum is shifted toward shorter (larger) wavelengths when external temperature is decreased (increased). The large free spectral range (FSR) allowed to monitor the temperature changes by either a wavelength or an intensity interrogation scheme. Using a laser and an optical power meter it was possible to demonstrate a real-time temperature sensor with a sensitivity of -60.79 nW/°C and a resolution of 0.5 °C, in the range of 10-30 °C. We believe that the practicality of this real-time sensor can be very attractive for environmental sensing application.

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.

An intensity modulation-based fiber optic vibration sensor (FOVS) using serpentine spring with proof mass structure is proposed and demonstrated via experiments. The structure, operating principle, and characteristics of the proposed sensor are given.

Cerebral edema is a significant cause of morbidity and mortality in diverse disease states. Currently, the means to detect progressive cerebral edema in vivo includes the use of intracranial pressure (ICP) monitors and/or serial radiological studies. However, ICP measurements exhibit a high degree of variability, and ICP monitors detect edema only after it becomes sufficient to significantly raise ICP. The authors report the development of 2 distinct minimally invasive fiberoptic near-infrared (NIR) techniques able to directly detect early cerebral edema. Cytotoxic brain edema was induced in adult CD1 mice via water intoxication by intraperitoneal water administration (30% body weight intraperitoneally). An implantable dual-fiberoptic probe was stereotactically placed into the cerebral cortex and connected to optical source and detector hardware. Optical sources consisted of either broadband halogen illumination or a single-wavelength NIR laser diode, and the detector was a sensitive NIR spectrometer or optical power meter. In one subset of animals, a left-sided craniectomy was performed to obtain cortical biopsies for water-content determination to verify cerebral edema. In another subset of animals, an ICP transducer was placed on the contralateral cortex, which was synchronized to a computer and time stamped. Using either broadband illumination with NIR spectroscopy or single-wavelength laser diode illumination with optical power meter detection, the authors detected a reduction in NIR optical reflectance during early cerebral edema. The time intervals between water injection (Time Point 0), optical trigger (defined as a 2-SD change in optical reflectance from baseline), and defined threshold ICP values of 10, 15 and 20 mm Hg were calculated. Reduction in NIR reflectance occurred significantly earlier than any of the ICP thresholds (p < 0.001). Saline-injected control mice exhibited a steady baseline optical signal. There was a significant correlation between reflectance change and tissue specific gravity of the cortical biopsies, further validating the dual-fiberoptic probe as a direct measure of cerebral edema. Compared with traditional ICP monitoring, the aforementioned minimally invasive NIR techniques allow for the significantly earlier detection of cerebral edema, which may be of clinical utility in the identification and thus early treatment of cerebral edema.

Laser Lines Ltd has recently increased its activities in fibre optics and communications. New products offered include the GTE FastomericTM splice which is a development of the popular ElastomericTM. It does not need adhesives or special tools for assembly, and as a result represents a major advance in fibre-optic splicing technology. Simply by inserting the prepared fibres into the splice and snapping the end caps into the body, average losses of 0.22 dB (blind) and 0.15 dB (optimised) are achieved. Other new products for installers of LANs, telecoms and data corns systems are low-cost, high-performance optical power meters and light sources from Fibre Optic Engineering Ltd. These units include the new Ml300 and M8513 power meters for use at 1300 nm and dual 850/1300 nm respectively. Operating from -55 to +10 dBm, they feature large germanium sensor elements. This allows accurate measurements to be made, even when using large core fibre. Adaptors are available for most standard connectors. Complete installer kits composed of optical power meter and light source, and everything necessary for mechanical splicing and connectorising are available from Laser Lines Ltd.

A fiber-optic strain sensor is demonstrated by using a short length of highly birefringent photonic crystal fiber (PCF) as the sensing element inserted in a fiber loop mirror (FLM). Due to the ultralow thermal sensitivity of the PCF, the proposed strain sensor is inherently insensitive to temperature. When a DFB laser passes through the FLM, the output power will only be affected by the transmission spectral change of the FLM caused by the strain applied on the PCF. Based on intensity measurement, an optical power meter will be adequate to deduce the strain information and an expensive OSA would not be needed.

A novel fiber optic vibration sensor is presented, based on the motion detection of an elastic cantilever by means of the optical triangulation technique. The sensor was designed to work at a fixed frequency of 100 Hz for applications in high power electrical plants where insulation and EM immunity requirements make fiber optic sensing the most suitable choice. The working linear range reaches a peak amplitude of 0.25 mm; resolution is 0.1 microm; accuracy is better than +/-3%; and sensitivity is 10 mV/microm. Design details and experimental results are reported.