Performance comparison of Raman/erbium-doped-fiber hybrid-amplification-based long-distance fiber Bragg grating sensor systems

A study regarding the performance comparison of three different types of long-distance fiber Bragg grating (FBG) sensors based on bidirectional Raman and erbium-doped fiber (EDF) amplification has been carried out to determine the most power-efficient and accurate configuration. The three sensor system configurations studied were a bidirectional hybrid Raman amplifier with (Type I) an EDF located after the distributed Raman amplification section, (Type II) an EDF located before the distributed Raman amplification section, and (Type III) without an EDF. The FBG sensor systems are based on a demodulation scheme employing radio frequency power measurements of a beat signal, in which a sine-modulated amplified spontaneous emission from a directly modulated reflective semiconductor optical amplifier is employed as a broadband light source. The results of our experimentation showed that the sensor system with the EDF located prior to the Raman amplification section provided the best performance in regards to its pump power efficiency, electrical signal-to-noise ratio, and measurement accuracy.     

Fabrication and Sensing Performance of Smart Composite Structures Using Optical Fibre Sensors

This paper determines the performance of Fibre Bragg Grating (FBG) sensors for strain sensing applications in carbon fibre composite materials. Carbon fibre laminates in either cross-plied or quasiisotropic stacking sequences were fabricated using T300/Hexcel 914 prepregs. The FBG optical sensors were either surface attached, or embedded within laminates. The sensor orientation was aligned either parallel or transverse to the adjacent carbon fibre layers. The composite structures with integrated FBG sensors were subjected to static tensile loading. A scanning fibre Fabry-Perot filter was used to monitor the reflected Bragg wavelengths. The optical sensor embedded between two 90° carbon fibre plies shows a high sensitivity to multi-site cracking formed in the transverse plies. The embedding in 90° plies seems to change the local stress distributions and to become a source of crack initiation. Efficient stress transfer from the host materials to the sensors is dependent upon incorporation methods, the thickness of the adhesive layers, and the location of the sensors.     

Application of Bragg grating sensors in the characterization of scaled marine vehicle models

Experiments using two different fiber-optic Bragg grating strain sensor systems to measure dynamic loads, slamming forces, and bending moments on a scaled catamaran model are reported. The system designed to measure slamming forces is based on dynamic locking of the laser diode wavelength to the peak wavelength of a Bragg grating mounted in a force cell. The sensor system has a dynamic range of 27 dB (1-550 N) and a bandwidth in excess of 1 kHz. For the measurement of bending moments, a time-domain multiplexing system, which used a unique pulsed rf modulation technique, of two Bragg grating sensors mounted on a steel beam was designed. The dynamic range of this system was also 27 dB [17 Nm (Newton meter) with a 0.03-Nm resolution]. The bandwidth was limited to approximately 50 Hz.     

Simultaneous determination of curvature,plane of curvature,and temperature by use of a miniaturized sensing head based on fiber Bragg gratings

We present what we believe to be a novel miniaturized sensor configuration based on fiber Bragg gratings for simultaneous measurement of curvature and temperature in smart structures and composite materials. Because of the particular geometry of the sensing head, it is possible not only to measure the radius of curvature but also to determine the plane of curvature. We arrange three Bragg gratings in the vertices of the smallest equilateral triangle that can be defined by the cross sections of the fibers. The set is then inserted into a glue-filled capillary stainless-steel tube to provide both suitable protection for the Bragg sensors and rotational symmetry to the sensing head. This tube also ensures isolation from axial strain, allowing for the additional determination of temperature. The proposed sensing head is particularly well suited for applications in smart structures because it can be embedded along any layer of a composite material (including the neutral line) without special concern for the relative orientation of the Bragg gratings and the composite layers. We can also use this sensing configuration to implement more-sophisticated sensors dedicated, for example, to the measurement of multiaxial acceleration or flow and temperature.     

Multiplexed Long-Base Flexible Optical Fiber Extensometers and Temperature Bragg Sensors Interrogated by Low-Coherence Interferometry

We present a novel concept of quasi-distributed flexible optical fiber extensometers fully compensated from thermal variations. Developed for structural health monitoring applications, the sensors are composed of a combination of intrinsic Fabry-Perot cavities as long-base extensometers, and point-like Bragg gratings inserted along the same fiber used as temperature sensors. This configuration enables a high degree of multiplexing, thus quasi-distributed sensing, and very efficient temperature compensation. Both types of sensors are read by a fiber-optic low-coherence interferometer, used in an original way to measure simultaneously the length variations of the cavities and the wavelength shifts of the Bragg gratings. Finally, we present the experimental validation of the whole measurement system, suitable for concrete structures instrumentation, as it includes an original optical fiber sheath packaging the optical fiber as a flexible sensor.     

Bragg光栅式应变传感器的传感特性研究

光纤Bragg光栅是一种新型的传感元件,其原理是利用波长解调的方法将被测的信号转化为光栅反射波长的偏移量,同时反射波长不受入射光的功率波动或光路系统损耗的影响。光纤Bragg光栅应变式传感器因此具备良好的可靠性、不受电磁干扰、不易腐蚀,同时最大的优点是在一根光纤上将多个光纤光栅应变传感器串接组成构成阵列开展分布式测量等优点,在逐渐应用于各类力学检测领域。本文着重对光纤Bragg光栅温度和应变的传感特性简要介绍并进行试验分析.     

Applications of remote sensing techniques to geology

Remote sensing is a new emerging field of technological development and has made a very significant impact on the geological surveys and studies. The work done so far in geological remote sensing has indicated the scope, utility and limitations of these modern techniques in different geological problems. The utility of airborne surveys and aerial photography has now been well established whereas satellite remote sensing at present has two main constraints—resolution and lack of stereoscopy. With the developments in sensor technology to provide sensors with improved resolution, more spectral bands and stereoscopy, substantial new results are anticipated in the geological remote sensing from space. Brief overview of applications of remote sensing techniques to geology is discussed in this paper.     

Digital demodulation system for low-coherence interferometric sensors based on highly birefringent fibers

A new type of demodulation system for low-coherence interferometric sensors based on highly birefringent fibers is presented. The optical path delay introduced by the sensor is compensated in four detection channels by quartz crystalline plates of appropriate thickness. The system can be used to decode a single-point sensor with a resolution of 2.5 x 10(-3) or two serially multiplexed sensors with decreased resolution. In a multiplexed configuration each sensor is served by two detection channels. By tilting the quartz plates, we can tune the initial phase shift between interference signals in successive channels to differ by pi/8 or pi/4, respectively, for a single-point or a multiplexed configuration. We transferred the sinusoidal intensity changes into digital pulses by appropriate electronic processing, which eventually allows for an unambiguous phase-shift measurement with a resolution of 1/8 or 1/4 of an interference fringe. The system performance for the measurement of hydrostatic pressure changes and simultaneous changes of hydrostatic pressure and temperature is demonstrated. The pressure sensors are based on side-hole fiber to ensure high sensitivity and an operation range of 2.4 MPa. A new configuration for temperature compensation of hydrostatic pressure sensors is proposed, which is better suited for dynamic pressure measurements. In this configuration the sensing and compensating fibers are located in the same compartment of the sensor housing.     

Fibre bragg gratings optical sensing technology

Fibre optic technology is a well established field in the world of smart structures. While optical communications systems date back as far as the late 1700s when the French engineer Claude Chappe invented the ‘optical telegraph’, advances in optical fibres did not really take off until Corning Glass Works (now Corning, Inc.) developed single-mode fibres with attenuation below 20 dB/km in 1970¹. Now fibre optics are widely used in the telecommunications industry. This breakthrough also led the way for the development of fibre optic sensors. While there are many types of optical sensors, the focus of this article is on the fibre Bragg grating sensor. In-line fibre Bragg gratings (FBGs) were developed as narrow band optical filters for the telecommunications industry². Television commercials by the major telephone companies describe the capability of transmitting as many as 16 telephone calls on the same optical fibre link, a capability made possible with wavelength division multiplexing using Bragg grating or equivalent spectral filtering technology. This intrinsic multiplexing capability and their sensitivity to thermo-mechanical loads also render in-fibre Bragg gratings one of the most promising distributed sensor technologies for structural systems. There are many other applications of FBGs including shape, moisture and corrosion sensing.     

Optical sensor interrogation with a blazed fiber Bragg grating and a charge-coupled device linear array

We present what is to our knowledge the first comprehensive investigation of the use of blazed fiber Bragg gratings (BFBGs) to interrogate wavelength division multiplexed (WDM) in-fiber optical sensor arrays. We show that the light outcoupled from the core of these BFBGs is radiated with sufficient optical power that it may be detected with a low-cost charge-coupled device (CCD) array. We present thorough system performance analysis that shows sufficient spectral-spatial resolution to decode sensors with a WDM separation of 75 rhom, signal-to-noise ratio greater than 45-dB bandwidth of 70 nm, and drift of only 0.1 rhom. We show the system to be polarization-state insensitive, making the BFBG-CCD spectral analysis technique a practical, extremely low-cost, alternative to traditional tunable filter approaches.     

Acousto-Ultrasonic Optical Fiber Sensors: Overview and State-of-the-Art

This paper gives a review of acoustic and ultrasonic optical fiber sensors (OFSs). The review covers optical fiber sensing methods for detecting dynamic strain signals, including general sound and acoustic signals, high-frequency signals, i.e., ultrasonic/ultrasound, and other signals such as acoustic emissions, and impact induced dynamic strain. Several optical fiber sensing methods are included, in an attempted to summarize the majority of optical fiber sensing methods used to date. The OFS include single fiber sensors and optical fiber devices, fiber-optic interferometers, and fiber Bragg gratings (FBGs). The single fiber and fiber device sensors include optical fiber couplers, microbend sensors, refraction-based sensors, and other extrinsic intensity sensors. The optical fiber interferometers include Michelson, Mach-Zehnder, Fabry-Perot, Sagnac interferometers, as well as polarization and model interference. The specific applications addressed in this review include optical fiber hydrophones, biomedical sensors, and sensors for nondestructive evaluation and structural health monitoring. Future directions are outlined and proposed for acousto-ultrasonic OFS.     

Impact localisation with FBG for a self-healing carbon fibre composite structure

We demonstrated the localisation of impacts in orthotropic carbon fibre reinforced polymer (CFRP) composite materials to within a few centimetres, using a sparse array of fibre Bragg grating (FBG) sensors. This type of sensor is easily embedded in composites permitting the development of structures with integrated sensing capability. Impact location was determined by measuring the differences in time-of-flight of ultrasonic Lamb waves at three surface-mounted sensors. An algorithm was developed taking into account the angle dependence of the optical fibre sensor sensitivity and the variation of Lamb waves propagation velocity with direction and wave mode. The performance of FBG sensors for impact localisation was compared to that of standard piezoelectric transducers (PZTs), which are already widely used for that purpose. The FBG-based system showed promising potential for a non-intrusive impact detection system applied to self-healing composite structures.     

Impact localisation with FBG for a self-healing carbon fibre composite structure

We demonstrated the localisation of impacts in orthotropic carbon fibre reinforced polymer (CFRP) composite materials to within a few centimetres, using a sparse array of fibre Bragg grating (FBG) sensors. This type of sensor is easily embedded in composites permitting the development of structures with integrated sensing capability. Impact location was determined by measuring the differences in time-of-flight of ultrasonic Lamb waves at three surface-mounted sensors. An algorithm was developed taking into account the angle dependence of the optical fibre sensor sensitivity and the variation of Lamb waves propagation velocity with direction and wave mode. The performance of FBG sensors for impact localisation was compared to that of standard piezoelectric transducers (PZTs), which are already widely used for that purpose. The FBG-based system showed promising potential for a non-intrusive impact detection system applied to self-healing composite structures.     

基于F-P标准具和Music算法的FBG动态应变传感系统

提出了基于可调光纤环形激光器结构,并采用F-P标准具解调的光纤布拉格光栅(FBG)动态应变传感系统,具有高达60 dB的光学信噪比.F-P标准具用来作为一个边缘滤波器探测FBG波长的漂移,这样的解调方式具有稳定性高的优点.为了提高动态应变测量系统的分辨率,采用了Music算法进行频谱分析.实验结果显示,在700Hz时的动态应变分辨率达到了0.1με/Hz~(1/2),是传统FFT算法的10倍.     

Design of X-band complementary metal-oxide semiconductor-based frequency-modulation continuous-wave sensor

This study presents an X-band complementary metal-oxide semiconductor-based frequency-modulation continuous-wave (FMCW) sensor system for transportation management. The proposed sensor system has two antennas, one to transmit signals and the other to receive them. The complete radio frequency (RF) transceiver is based on standard 0.18 μm one-poly six-metal (1P6M) CMOS technology with a chip area of 1.68 mm X 1.6 mm. Two planar leaky-mode antenna arrays with a gain of 18 dB are also designed. Experimental results indicate that the isolation between two antenna arrays that are 5.0 mm apart exceeds 42.0 dB at 10.5 GHz. The prototype of the FMCW sensor system is used in the range measurement of multiple lanes for the transportation management system (TMS). The major contribution of this study is that it integrates a 0.18 μm CMOS transceiver and antenna arrays into an FMCW RF front end, and employs an IF amplifier and a digital signal processor to demonstrate that the beat frequencies are linear. Measurements made in field tests agree closely with the simulation results.     

Best Practice Guidelines for Pre-Launch Characterization and Calibration of Instruments for Passive Optical Remote Sensing

The pre-launch characterization and calibration of remote sensing instruments should be planned and carried out in conjunction with their design and development to meet the mission requirements. The onboard calibrators such as blackbodies and the sensors such as spectral radiometers should be characterized and calibrated using SI traceable standards. In the case of earth remote sensing, this allows inter-comparison and intercalibration of different sensors in space to create global time series of climate records of high accuracy where some inevitable data gaps can be easily bridged. The recommended best practice guidelines for this pre-launch effort is presented based on experience gained at National Institute of Standards and Technology (NIST), National Aeronautics and Space Administration (NASA) and National Oceanic and Atmospheric Administration (NOAA) programs over the past two decades. The currently available radiometric standards and calibration facilities at NIST serving the remote sensing community are described. Examples of best practice calibrations and intercomparisons to build SI (international System of Units) traceable uncertainty budget in the instrumentation used for preflight satellite sensor calibration and validation are presented.     

Brillouin-Based Distributed Temperature Sensor Employing Pulse Coding

A distributed temperature sensor based on spontaneous Brillouin scattering and employing optical pulse coding has been implemented and characterized using a direct-detection receiver. The signal-to-noise ratio (SNR) enhancement provided by coding is analyzed, along with the influence of coding in stimulated Brillouin threshold. Simplex-coding using 127 bit codeword provides up to 7 dB SNR improvement, allowing for temperature sensing over 21 km of dispersion shifted fiber with 3.1 K resolution and 40 m spatial resolution, permitting to avoid the use of optical pulse amplification.     

一种基于光纤F-P腔的石墨烯谐振式压力传感器

谐振式传感器具有良好的重复性、分辨率和稳定性,全光纤谐振式微型传感器则集合了微纳结构与光纤传感特性两者的优点。石墨烯作为近年来发现的一种新型材料,具有良好的热学和力学特性,可以作为谐振式传感器中的敏感元件。本文提出了一种基于光纤F-P结构的石墨烯谐振式压力传感器,其兼具微机械传感器和光纤谐振器的优点,具有较高的谐振频率,在测量压力的实验中展现出良好的性能,压力灵敏度最高可达2. 93Hz/Pa,具有重要的应用价值。     

Miniaturized Optical Fiber Sensor Interrogation System Employing Echelle Diffractive Gratings Demultiplexer for Potential Aerospace Applications

To enable the application of optical fiber sensors to aerospace vehicles, the sensor interrogation or data acquisition system has to meet small size and low weight requirements. This paper presents the developmental work of an echelle diffractive gratings (EDGs) demultiplexer-based optical fiber sensor interrogation system. The operation principle of this system and its application to fiber Bragg grating (FBG) sensor interrogation are presented. The experimental results have shown that the developed interrogator (not including the electronic controller) weighs less than 60 g and provides better than 1 pm measurement resolution and better than plusmn10 pm measurement accuracy.