Tilt sensor with FBG technology and matched FBG demodulating method

A novel kind of fiber optic ultrasonic sensor based on matching fiber Bragg gratings (FBGs) is proposed and demonstrated. The sensors consist of a pair of matching FBGs fixed to a special bracket. The bracket plays a role in stretching and squeezing the FBGs, with the push–pull effect efficiently coupling the ultrasonic signal to the sensor, thus, improving the sensor’s sensitivity. Side-band filtering technology-based intensity interrogation was used to detect ultrasounds in water. With the synergic effect of the matching FBGs, the sensor performed with a high signal-to-noise ratio (56.9 dB at 300 KHz, 53 dB at 1 MHz and 31.8 dB at 5 MHz) and the observed ultrasonic sinusoidal signals were undistorted and distinguishable in the time domain.

Fabrication and verification of novel 3D FBG-based transducer for directly measuring vertical and lateral effective stresses

Fiber Bragg Grating (FBG) technology is very attractive to develop sensors for the measurement of thermal and mechanical parameters in biological applications, particularly in presence of electromagnetic interferences. This work presents the design, working principle and experimental characterization of a force sensor based on two FBGs, with the feature of being compatible with Magnetic Resonance. Two prototypes based on different designs are considered and characterized: 1) the fiber with the FBGs is encapsulated in a polydimethylsiloxane (PDMS) sheet; 2) the fiber with the FBGs is free without the employment of any polymeric layer. Results show that the prototype which adopts the polymeric sheet has a wider range of measurement (4200 mN vs 250 mN) and good linearity; although it has lower sensitivity (≈0.1 nm-N(1) vs 7 nm-N(1)). The sensor without polymeric layer is also characterized by employing a differential configuration which allows neglecting the influence of temperature. This solution improves the linearity of the sensor, on the other hand the sensitivity decreases. The resulting good metrological properties of the prototypes here tested make them attractive for the intended application and in general for force measurement during biomedical applications in presence of electromagnetic interferences.

A fibre Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fibre that reflects particular wavelengths of light and transmits all the others. As such, FBGs can be used as inline optical filters to block certain wavelengths, or as wavelength-specific reflectors. Applications include optical fibre communications, sensors and fibre lasers. This book addresses the critical challenge of developing Fibre Bragg Gratings (FBGs) for applications as sensors in harsh and space environment. Coverage ranges from the basic principles through design, fabrication, and testing to their industrial implementation. A thorough review includes the in-depth examination of the FBGs properties and the most important developments in devices and applications. A particular emphasis is given to the applications of fibre optic sensors in the space environment, which is characterized mainly by vacuum, high thermal gradients, mechanical vibrations and various types of cosmic radiation. The book concludes with a summary and overview of challenges faced by FBG technology. The book is supplemented by an extensive survey of published papers, books and conference reports. As an added benefit, the book is structured in such a way as to provide useful and in-depth training and skills development to graduate/undergraduate students, specialised engineers, and academic/industrial experts.

This work presents a fiber Bragg grating (FBG) wide-range tilt sensor based on a cam structure. Two FBGs are fixed on a specially-designed sensor shell. When one FBG is stretched by the rotating cam driven by tilt, the other FBG moves along a standard semicircle with a constant radius, and compensates for temperature variations of the stretched FBG. By detecting the difference of the central wavelength shifts of two FBGs, variations of the tilt angle can be measured. The detailed mechanical structure design and measurement theory of the sensor is presented, and the sensor prototype is discussed. Testing results of comprehensive performance show that there is a good linear relationship between the tilt angle and the central wavelength of the FBG over three intervals from -90° to 90°. The repeatability and creep resistance of the FBG tilt sensor are good, the measuring accuracy is 1.516°, and the measuring resolution is 0.003°. The measurement range is much larger than comparable existing sensors, and it avoids cross-sensitivity between temperature and strain for FBG. The sensitivity of the sensor can be easily adjusted by changing the outline of the cam, or the length of the optical fiber between two fixed points.

Distinguishable circumferential inclined direction tilt sensor based on fiber Bragg grating with wide measuring range and high accuracy

A new temperature-insensitive inclinometer was fabricated using a 3-D printing method based on fiber Bragg grating (FBG) sensors. This new tilt sensor is characterized by its light weight, immunity to electromagnetic interference (EMI) small size, quick prototyping, and cost effectiveness. FBG sensors were directly inscribed into polylactic acid, which was also used for fabricating all different parts of tilt sensors. Calibration tests of this new tilt sensor were performed in laboratory to examine its measurement performance. Measurement sensitivity of 0.012 nm/° and a maximum tilt angle of 25° of the FBG inclinometer was approached in the calibration experiment. The measured tilt angles were validated by theoretical angles calculated in terms of all physical parameters of internal functional parts of the inclinometer. A monitoring test of a sandy slope model using this new tilt sensor was conducted in laboratory. Finite element analysis was also used to examine the mechanical behavior of the slope model under different loading levels. Monitoring test results indicate that the FBG-based tilt sensor presented no obvious change before a surcharge load approaching 0.245 kN but exhibited step-by-step rise in tilt angle afterward, achieving a maximum tilt angle of around 6°. Horizontal displacement, which was calculated in terms of the tilt sensor at different loading levels, was found to agree fairly well with the displacement data from both finite element analysis and displacement transducers.

Limited by the multiplexing number of fiber Bragg grating (FBG), further improvement in the length of 3D shape sensing based on FBG technology is challenging. In this Letter, a wavelength-division and space-division multiplexing multicore fiber grating method is proposed, which extends the sensing length. Employing the femtosecond-laser point-by-point technology, we inscribed WDM grating arrays in six outer cores of a seven-core fiber, respectively. Three cores were utilized as a segment for shape sensing, and two such segments were offset by a specific length and combined to form a shape sensor. Utilizing an FBG interrogator, the proposed shape sensor achieved 2D and 3D shape sensing at a length of 967 mm and effectively mitigated the effects of temperature variations. In experiments, maximum shape reconstruction errors per unit lengths are 1.89%, 2.72%, and 1.47% for 2D shape, 3D shape, and an arbitrary shape under variable temperature conditions, respectively. The proposed method holds promise for further extending the shape sensing length by utilizing multicore fibers or fiber clusters containing more cores.

The infrastructures for oil and gas production and distribution need reliable monitoring systems. The risks for pipelines, in particular, are not only limited to natural disasters (landslides, earthquakes, extreme environmental conditions) and accidents, but involve also the damages related to criminal activities, such as oil theft. The existing monitoring systems are not adequate for detecting damages from oil theft, and in several occasion the illegal activities resulted in leakage of oil and catastrophic environmental pollution. Systems based on fiber optic FBG (Fiber Bragg Grating) sensors present a number of advantages for pipeline monitoring. FBG sensors can withstand harsh environment, are immune to interferences, and can be used to develop a smart system for monitoring at the same time several physical characteristics, such as strain, temperature, acceleration, pressure, and vibrations. The monitoring station can be positioned tens of kilometers away from the measuring points, lowering the costs and the complexity of the system. This paper describes tests on a sensor, based on FBG technology, developed specifically for detecting damages of pipeline due to illegal activities (drilling of the pipes), that can be integrated into a smart monitoring chain.

CRM (Composite Reinforced Mortar) with embedded FBG (Fibre Bragg Grating) sensors were tested in order to guarantee long-term effectiveness end of reinforcement as well as continuous monitoring, indeed, to measure mechanical parameters for Structural Health Monitoring (SHM) of civil engineering structures, fiber optic sensors based on FBG technology are not infrequently used. FBG sensors are particularly suitable for permanent monitoring applications in civil structures, being able to specifically exploit some unique characteristics: resistance to atmospheric agents, low invasiveness of both the sensors and the wiring.The tests presented in this study highlight the reliability of the proposed CRM-FBG integrated system for both structural reinforcement and long-term health monitoring.

The long-span and high-speed optical transmission networks require high-quality optical cables and components to reduce the detrimental effects of polarization-mode dispersion (PMD) and polarization-dependent loss (PDL) on system performance. In addition, many high-accuracy fiber-optic sensing networks also require low polarization-dependent components. As a result, polarization-insensitive transmission fibers and components have been pursued recently. A well-established technique to produce low-PMD and PDL fibers is to spin the fiber during the fiber drawing process, known as spun fibers. As key optical components, fiber Bragg gratings (FBGs) play important roles in optical fiber communications and fiber sensors. However, FBGs written into traditional single-mode fibers exhibit relatively high polarization dependence due to the birefringence introduced by side UV exposure in FBG fabrication process. In this paper, we report for the first time the characteristics of FBGs written into spun fibers that were formed by spinning fibers with certain intrinsic birefringence. Distinct properties of FBGs in spun fibers are found in the experiments, compared to those in traditional single-mode fibers. Based on Jones matrix method, the FBG spectra are simulated and compared with the measured ones. Preliminary experimental results indicate that the spun fiber gratings suffer lower PMD under the same fabrication conditions. Moreover, our theoretical analysis shows that the PMD and PDL characteristics of spun fiber gratings can be further improved by optimizing the structures of spun fibers.

3 - Grating-based sensors

Fiber Bragg grating (FBG) is an important element in many applications including filters and dispersion compensators in fiber communication systems. With recently developed inverse scattering algorithm, FBGs with desired reflection spectrum and/or dispersion properties can now be designed. However, most of these designs require arbitrary grating amplitude and phase control. Previously, fabrication of such FBGs relies on the accurate control of the temporal variation of the intensity pattern using a piezo electric translation stage. The precision of this fabrication method is limited by the noise in the control voltage, which is usually larger than 1%. The distortion in piezo response also affects the performance. In this paper, we develop and demonstrate a novel writing technique for arbitrary FBG fabrication. Our technique is based on a translate-and-write configuration. The incorporation of a precisely controlled shutter allows the apodization and phase of the FBG to be continuously changed at each grating line. The shutter error mainly results from the control signal's timing jitter, which is normally lower than 0.1%. Using this writing technique, we demonstrate a Hamming apodized grating with 20mm length, -22 dB minimum transmission, and < -25 dB reflection side lobe suppression. Furthermore, phase-shift in a grating can be fabricated by a simple delay in the control signal. We also demonstrate FBGs with π, π/2, 3π/2 phase-shifts, respectively. Our experimental results are in excellent agreement with theoretical predictions. To show the capability to fabricate a FBG with arbitrary structure, we demonstrate a 35 mm long zero dispersion grating.

A high SNR system for intensity demodulation of fiber Bragg grating based on phase-locked loop

In this paper we report the development and field test of a high sensitivity fiber Bragg grating (FBG) geophone for geophysical imaging and monitoring application. A high sensitivity FBG geophone is designed, and its sensitivity is about 1000pm/g. The wavelength change of the FBG geophone is interrogated by using interferometric demodulation method, and the demodulation system noise is below 10-3 pm/ √Hz . And the minimum detectable seismic signal is below 1μg/ √Hz . We are presenting field test results for the FBG geophone and comparing its performance with regular exploration geophones. In comparison, FBG geophone has the advantages of higher signal-to-noise ratio and better low-frequency response. This work shows that using FBG technology to develop geophone for oil and gas exploration is both advantageous and feasible.