Bragg Grating-based Sensor Research Articles

Few-pumped flat Raman fiber amplifier for parallel distributed FBG-based sensor system

Multi-pump technique is frequently adopted to improve the flatness of Raman fiber amplifier (RFA), but brings the issues in terms of cost and algorithm complexity. In this article, we limit the number of pumps and the few-pumped RFA is proposed and investigated by optimizing the power and wavelengths. We further define three new factors to evaluate the improvement ratio of gain and flatness and determine the advisable number of pumps. By using the designed RFA, an experiment of distributed fiber Bragg grating-based sensor system is developed, and the flatness of less than ±0.44dB is obtained with the gain of about 3dB in the range of 1510–1570nm.

Design and performance investigation of a highly accurate apodized fiber Bragg grating-based strain sensor in single and quasi-distributed systems.

In this work, fiber Bragg grating (FBG) strain sensors in single and quasi-distributed systems are investigated, seeking high-accuracy measurement. Since FBG-based strain sensors of small lengths are preferred in medical applications, and that causes the full width at half-maximum (FWHM) to be larger, a new apodization profile is introduced for the first time, to the best of our knowledge, with a remarkable FWHM at small sensor lengths compared to the Gaussian and Nuttall profiles, in addition to a higher mainlobe slope at these lengths. A careful selection of apodization profiles with detailed investigation is performed-using sidelobe analysis and the FWHM, which are primary judgment factors especially in a quasi-distributed configuration. A comparison between the elite selection of apodization profiles (extracted from related literature) and the proposed new profile is carried out covering the reflectivity peak, FWHM, and sidelobe analysis. The optimization process concludes that the proposed new profile with a chosen small length (L) of 10mm and Δn_ac of 1.4×10^-4 is the optimum choice for single stage and quasi-distributed strain-sensor networks, even better than the Gaussian profile at small sensor lengths. The proposed profile achieves the smallest FWHM of 15GHz (suitable for UDWDM), and the highest mainlobe slope of 130dB/nm. For the quasi-distributed scenario, a noteworthy high isolation of 6.953dB is achieved while applying a high strain value of 1500μstrain (με) for a five-stage strain-sensing network. Further investigation was undertaken, proving that consistency in choosing the apodization profile in the quasi-distributed network is mandatory. A test was made of the inclusion of a uniform apodized sensor among other apodized sensors with the proposed profile in an FBG strain-sensor network.

A Novel Wireless Mobile Platform to Locate and Gather Data From Optical Fiber SensorsIntegrated Into a WSN

This paper presents a novel design for a wireless mobile platform to locate and gather data from different types of optical fiber sensors, thereby enabling the more effective integration of a number of such optical fiber sensors with an advanced mobile wireless sensor network (WSN). This then more readily permits potential applications, such as monitoring in remote and harsh environments and tracking, exploiting fully the combined advantages offered both by the mobile WSN and the advanced optical fiber sensing technologies. The platform which was designed and implemented consists of an optical fiber sensor module and a smart mobile WSN module, which shows important advantages for mobile sensing and tracking and mesh networking. In this paper, a fiber Bragg grating-based temperature sensor and an intrinsic pH optical fiber sensor were specially designed and integrated successfully into the optical fiber sensor module as an exemplar to investigate the performance of the integrated system based on the mobile WSN platform. The positive experimental results obtained have confirmed the functionality of the platform designed and demonstrated its capacity for real-time optical fiber sensor data monitoring, processing, and wireless transmission. The successful creation of this type of wireless mobile platform with optical fiber sensors can thus be expected to make an important impact on a number of sectors, where either conventional optical sensor designs or WSNs alone cannot meet the systems requirements.

Development of a Bridge Weigh-in-Motion Sensor: Performance Comparison Using Fiber Optic and Electric Resistance Strain Sensor Systems

This paper addresses the problems of effective <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> measurement of the real-time strain for bridge weigh in motion in reinforced concrete bridge structures through the use of optical fiber sensor systems. By undertaking a series of tests, coupled with dynamic loading, the performance of fiber Bragg grating-based sensor systems with various amplification techniques were investigated. In recent years, structural health monitoring (SHM) systems have been developed to monitor bridge deterioration, to assess load levels and hence extend bridge life and safety. Conventional SHM systems, based on measuring strain, can be used to improve knowledge of the bridge’s capacity to resist loads but generally give no information on the causes of any increase in stresses. Therefore, it is necessary to find accurate sensors capable of capturing peak strains under dynamic load and suitable methods for attaching these strain sensors to existing and new bridge structures. Additionally, it is important to ensure accurate strain transfer between concrete and steel, adhesives layer, and strain sensor. The results show the benefits in the use of optical fiber networks under these circumstances and their ability to deliver data when conventional sensors cannot capture accurate strains and/or peak strains.

Characterization of Encapsulating Materials for Fiber Bragg Grating-Based Temperature Sensors

An experimental study has been carried out for the characterization of encapsulating materials for fiber Bragg grating-based temperature sensors to prevent the formation of micro-cracks and devitrification on the fiber surface at elevated temperatures by making use of a rigid probe. The developed sensor probe was configured by encapsulating a type 1 fiber Bragg grating with an aluminum nitride tube and is used to measure temperatures from 20°C to 500°C. The adapted encapsulation technique validated the sensor to achieve linearity of 99.979%, sensitivity of 14.03 ± 0.02 pm/°C, and good repeatability; its practical use in harsh environments is predicted.

Optical fiber-based sensors with flexible encapsulation for pavement behavior monitoring

Summary Owing to the increasing needs for robust highway pavement structural monitoring tools with high accuracy and functionality, this paper introduces some results from the research on developing optical fiber-based pavement behavior monitoring sensors. A new flexible encapsulation technique for optical fiber sensing element has been proposed to help the sensor withstand the harsh working environment and guarantee measuring accuracy. Both fiber Bragg grating and Brillouin optical time domain analysis/reflectometry-based sensors have been produced in a lab with the proposed encapsulation for high accuracy and distributed measurement, respectively. The precision of the fiber Bragg grating-based sensor has been specially calibrated in a lab. Lab and field tests have been carried out to study the effectiveness of the proposed sensors. It has been found that the proposed sensors have high survival rate (>75%) and are efficient for pavement structural behavior monitoring and damage detection. Copyright © 2014 John Wiley & Sons, Ltd.

Inference of bond slip in prestressed tendons in concrete bridge girders

Prestressed concrete (PC) bridge girders have extra flexural strength due to embedded prestressing tendons. The level of strength depends on the integrity of the concrete-tendon bond in pretensioned PC structures, and this bond can be degraded in the presence of an adequately large mechanical force or over time because of accumulated damage. Bond degradation is characterized by the bond slip of the tendon from the host concrete, and in regards to PC structures, bond slip has only been measured from a global perspective. In this paper, a novel method was developed to measure for the first time the local strain of a prestressing tendon during bond slip. Fiber Bragg grating-based strain sensors were installed directly onto prestressing tendons within a PC girder to provide a local perspective of bond slip as the girder was loaded to failure. Measurement of the local strains in selected tendons, from the beginning of bond slip to the complete loss of the concrete-tendon bond, and the failure of the girder was enabled by the method. Copyright © 2014 John Wiley & Sons, Ltd.

Sensing RF and microwave energy with fiber Bragg grating heating via soft ferromagnetic glass-coated microwires

We present results from a fiber Bragg grating-based microwave energy sensor. The sensor relies on a soft ferromagnetic glass-coated microwire that is bonded to the cladding of the grating. The microwire absorbs microwave energy and heats up thus raising the temperature of the fiber Bragg grating. Compared to a similar sensor that uses gold to absorb electromagnetic radiation, the microwire yields a sensor with greater sensitivity (∼10 times at f=3.25GHz) relative to the perturbation of the microwave field. With the sensor reported here, the best sensitivity to electromagnetic radiation corresponds to AC electric fields that have an average electric energy density of approximately 1.3μJ/m3.

Natural frequency detection of smart composite structure by small diameter fiber Bragg grating

Structural health monitoring (SHM) in-service is very important and definitely demanded for the safe working of high performance composite structures in situ. Fiber Bragg grating-based sensors were usually embedded inside a composite to compose a smart structure for a real time monitor. However, the mechanical performance of the materials would be unfavorably influenced. To lower the influence, a smaller diameter optical fiber is preferred. In this paper, a new non-destructive evaluation sensor system based on small diameter fiber Bragg grating (SDFBG) was proposed. The impact results show that the SDFBG sensor could be used in dynamic strain measurement and detect the natural frequency of the structure. Furthermore, the sensor would be applied in SHM.

Fiber Bragg grating-based high temperature sensor and its low cost interrogation system with enhanced resolution

A fiber Brag grating-based high temperature sensor together with a low cost, high speed and compact size interrogation system using a long period grating was designed, developed and tested. The designed sensor measures the temperature from room temperature to 550 °C. The sensor head was configured by encapsulating an fiber Brag grating (type-I) of Bragg resonance wavelength at 1552.88 nm in a capillary tube made of copper. Long period grating with peak transmission loss at 1550 nm was employed to convert the wavelength information from fiber Brag grating into an intensity modulated signal. Temperature related optical intensity information was again converted into its equivalent electrical signal by using a photodiode. The achieved resolution of the sensor was found to be 0.5 °C.

A new approach to ballistocardiographic measurements using fibre Bragg grating-based sensors

A method for acquiring a ballistocardiographic (BCG) signal from the feet of a standing person and the back of a sitting or lying patient is described. The measurements are carried out using in-house constructed fibre-optic sensors interrogated with a commercially available system. The sensor head consists of a fibre Bragg grating (FBG) attached to an elastic board that is placed between the monitored person's body and a soft surface, enabling the board to deform in an unobstructed way. The body's movements, including the BCG component, exert pressure on the board and make it deform along with the attached FBG. The changes to the Bragg wavelength are proportional to the body's movement and a BCG signal can be extracted from the obtained recording. The measuring capabilities of the sensors were evaluated by comparing the heart rate (obtained on the basis of the BCG signal) with the reference signal registered by an ECG recorder. An RMS value of the relative error is below 1.8% and statistical analyses show a satisfactory reconstruction of measurements. Tests carried out in the MRI environment proved the method to be immune to strong electromagnetic fields. The presence of the sensor in an MRI scanner does not affect the quality of imaging.

Study of the nonuniform behavior of temperature sensitivity in bare and embedded fiber Bragg gratings: experimental results and analysis

This paper presents an experiment and analysis on the factors affecting nonlinear evolution of Bragg wavelength with change in temperature in typical bare and embedded fiber Bragg grating-based (FBG) temperature sensors. The purpose of the study was to find the constants in the function required to evaluate temperature from Bragg wavelength shift. The temperature sensitivity of bare FBGs was found to increase with temperature elevation, and is different for FBGs written in different fiber types. The average temperature sensitivity increased by about 20% when the bare FBG temperature was elevated from 25°C to 525°C. The average temperature sensitivity of the embedded FBG sensor, investigated in the temperature range of 30°C-90°C, was a factor of 2-3 times larger than for bare FBG, depending on its fastened length with the substrate. Analytically, it is shown that the nonuniform behavior of temperature sensitivity in bare FBGs is the result of both the thermal expansion effect of the fiber and the temperature derivatives of the effective refractive index. The strain transfer and temperature coefficients of thermal expansion of the substrate affect the nonuniform behavior of temperature sensitivity in embedded FBG sensors.

Shear stress sensing with Bragg grating-based sensors in microstructured optical fibers

We demonstrate shear stress sensing with a Bragg grating-based microstructured optical fiber sensor embedded in a single lap adhesive joint. We achieved an unprecedented shear stress sensitivity of 59.8 pm/MPa when the joint is loaded in tension. This corresponds to a shear strain sensitivity of 0.01 pm/µε. We verified these results with 2D and 3D finite element modeling. A comparative FEM study with conventional highly birefringent side-hole and bow-tie fibers shows that our dedicated fiber design yields a fourfold sensitivity improvement.

A Novel Optical Sensor Platform Designed for Wireless Sensor Networks

This paper presents a novel design of an optical sensor platform, enabling effective integration of a number of optical fibre ('wired') sensors with wireless sensor networks (WSNs). In this work, a fibre Bragg grating-based temperature sensor with low power consumption is specially designed as a sensing module and integrated successfully into a WSN, making full use of the advantages arising from both the advanced optical sensor designs and the powerful network functionalities resident in WSNs. The platform is expected to make an important impact on many applications, where either the conventional optical sensor designs or WSNs alone cannot meet the requirements.

Fiber Bragg grating-based sensor for monitoring respiration and heart activity during magnetic resonance imaging examinations

We present a fiber-optic sensor for monitoring respiration and heart activity designed to operate in the magnetic resonance imaging (MRI) environment. The sensor employs a Plexiglas springboard, which converts movements of the patient's body lying on the board (i.e., lung- and heart-induced vibrations) to strain, where a fiber Bragg grating attached to the board is used to measure this strain. Experimental studies are carried out during thoracic spine MRI examinations. The presence of the metal-free sensor construction in the MRI environment does not pose a threat to the patient and has no influence over the quality of imaging, and the signal is identical to that obtained without any electromagnetic interference. The results show that the sensor is able to accurately reflect the ballistocardiographic signal, enabling determinations of the respiration rate (RR) and heart rate (HR). The data delivered by the sensor are normally distributed on the Bland-Altman plot for the characteristic point determination and exhibit clear dependence on the RR and HR values for the RR and HR determinations, respectively. Measurement accuracies are better than 7% of the average values, and thus, with further development, the sensor will be implemented in routine MRI examinations.

Performance analysis of peak tracking techniques for fiber Bragg grating interrogation systems

In this paper, we propose a spectral correlation-based technique for tracking the wavelength shift of a fiber Bragg grating. We compared this approach, by means of a Monte Carlo numerical simulation, to the typical peak tracking techniques applied in classic interrogation systems. As result, we obtained a considerable gain in terms of noise tolerance (about 20 dB), which can be further incremented by selecting large-bandwidth gratings. This permits to increase the power budget of a fiber Bragg grating interrogator without changing the optical layout, overcoming classical limitations of commercial and custom systems. Penalties due to the non-idealities have been evaluated through the same Monte Carlo approach. Finally, we discuss a practical application of the peak tracking techniques to a fiber Bragg grating-based weight sensor, in which we applied the spectral correlation to track both the Bragg wavelength position, spectral deformations due to high strain, and spectral non-linearity.

Fiber Bragg Grating-Based Three-Dimensional Multipoint Ring-Mesh Sensing System With Robust Self-Healing Function

By utilizing the cyclic filtering and free spectral range characteristics of an arrayed waveguide grating (AWG), we propose and experimentally demonstrate a novel 3-D ring-mesh sensing system with comprehensive self-healing function. The Bragg wavelengths of the employed fiber Bragg grating-based optical sensors are periodically assigned, with respect to the AWG. The proposed ring-mesh topology is constructed by 2 × 2 optical switches to link the ring-based subnets in a meshed architecture. Moreover, a simple and cost-effective signal sensing processor, provided by the AWG, is employed for classifying the sensing signals and monitoring the sensing elements. With the same constructional building block, the proposed sensing system can be easily scaled up which provides more self-healing paths in a self-reproduced manner. In addition, we also bring up a routing algorithm to seek for an optimal sensing path that has the least accumulated power loss to deal with any possible sensing point outage when the sensing system is scaling up.

Fiber Bragg Grating-Based Quasi-Distributed Temperature Sensor for Down-Hole Monitoring

Fiber Bragg Grating-Based Quasi-Distributed Temperature Sensor for Down-Hole Monitoring

Monitoring Respiration and Cardiac Activity Using Fiber Bragg Grating-Based Sensor

This paper shows the design of a fiber-based sensor for living activities in human body and the results of a laboratory evaluation carried out on it. The authors have developed a device that allows for monitoring the vibrations of human body evoked by living activities--breathing and cardiac rhythm. The device consists of a Bragg grating inscribed into a single mode optical fiber and operating on a wavelength of around 1550 nm. The fiber Bragg grating (FBG) is mounted inside a pneumatic cushion to be placed between the backrest of the seat and the back of the monitored person. Deformations of the cushion, involving deformations of the FBG, are proportional to the vibrations of the body leaning on the cushion. Laboratory studies have shown that the sensor allows for obtaining dynamic strains on the sensing FBG in the range of 50-124 μ strain caused by breathing and approximately 8.3 μstrain induced by heartbeat, which are fully measurable by today's FBG interrogation systems. The maximum relative measurement error of the presented sensor is 12%. The sensor's simple design enables it to be easily implemented in pilot's and driver's seats for monitoring the physiological condition of pilots and drivers.

Control Over the Pressure Sensitivity of Bragg Grating-Based Sensors in Highly Birefringent Microstructured Optical Fibers

We present fiber Bragg grating (FBG)-based hydrostatic pressure sensing with highly birefringent microstructured optical fibers. Since small deformations of the microstructure can have a large influence on the material birefringence and pressure sensitivity of the fiber, we have evaluated two microstructured fibers that were made from comparable fiber preforms, but fabricated using different temperature and pressure conditions. The magnitude and sign of the pressure sensitivity are found to be different for both fibers. We have simulated the corresponding change of the Bragg peak separation with finite-element models and experimentally verified our results. We achieve very high experimental sensitivities of -15 and 33 pm/MPa for both sensors. To our knowledge, these are the highest sensitivities ever reported for birefringent FBG-based hydrostatic pressure sensing.