Fiber Bragg Grating Sensor Array Research Articles

Smart structure sensors based on embedded fibre Bragg grating arrays in aluminium alloy matrix by ultrasonic consolidation

Smart structure sensors based on embedded fibre Bragg grating (FBG) arrays in aluminium alloy matrix by ultrasonic consolidation (UC) technique have been proposed and demonstrated successfully. The temperature, loading and bending responses of the embedded FBG arrays have been systematically characterized. The embedded FBGs exhibit an average temperature sensitivity of ∼36 pm °C−1, which is three times higher than that of normal FBGs, a bending sensitivity of 0.73 nm/m−1 and a loading responsivity of ∼0.1 nm kg−1 within the dynamic range from 0 kg to 3 kg. These initial experimental results clearly demonstrate that the UC produced metal matrix structures can be embedded with FBG sensor arrays to become smart structures with capabilities to monitor the structure operation and health conditions in applications.

Integrated monitoring scheme for a maglev guideway using multiplexed FBG sensor arrays

In the structural monitoring of maglev guideways, electromagnetic interference (EMI) can be a significant problem, as the maglev train is powered by high-voltage electric feeding systems. Recently, researchers have successfully applied fiber optic sensors to modern railway structures, mainly because they offer EMI immunity. This study presents an integrated monitoring scheme for a maglev guideway using wavelength division multiplexing (WDM)-based fiber Bragg grating (FBG) sensors. The physical quantities such as strains, curvatures, and vertical deflections are measured in field tests. The strains are directly measured from multiplexed FBG sensors at various locations on a test bridge followed by curvature calculations based on the plane section assumption. Vertical deflections are then estimated using the Bernoulli beam theory and regression analyses. Frequency information obtained from the proposed method is compared with those from a conventional accelerometer. Verification tests were conducted on a newly developed Korean maglev test track at different vehicle speeds.

Thermal characteristics of an end-pumped high-power ytterbium-sensitized erbium-doped fiber laser under natural convection

We investigate the thermal characteristics of a polymer-clad fiber laser under natural convection when it is strongly pumped up to the damage point of the fiber. For this, we utilize a temperature sensing technique based on a fiber Bragg grating sensor array. We have measured the longitudinal temperature distribution of a 2.4-m length ytterbium-sensitized erbium-doped fiber laser that was end-pumped at approximately 975 nm. The measured temperature distribution decreases exponentially, approximately, decaying away from the pump-launch end. We attribute this to the heat dissipation of absorbed pump power. The maximum temperature difference between the fiber ends was approximately 190 K at the maximum pump power of 60.8 W. From this, we estimate that the core temperature reached approximately 236 degrees C.

Optical low-coherence reflectometry for a distributed sensor array of fiber Bragg gratings

An optical low-coherent interferometric technology for fiber Bragg grating (FBG) sensor array is described. A series of identical FBGs with low reflectivity form a sensor array. The first FBG of the array works as the reference FBG and the others work as sensing FBGs. Each sensing FBG and the reference FBG can form an in-fiber Fabry–Perot (F–P) interferometer with a specific cavity length. By scanning a home-made optical low-coherence reflectometry, the interference signals corresponding to different sensing FBGs (F–P interferometers) can be obtained and well distinguished. The mismatch in wavelength between the reference FBG and a sensing FBG induced by the measurand around this sensing FBG will cause the decrease of the intensity of the corresponding interference signal. A measurement of temperature is demonstrated and good performance is achieved. Factors limiting the total number of sensing FBGs are also discussed.

High-Temperature Resistance Fiber Bragg Grating Temperature Sensor Fabrication

Fiber Bragg grating (FBG) temperature sensor and sensor arrays were applied widespread particularly in harsh environments. Although FBGs are often referring to permanent refractive index modulation in the fiber core, exposure to high-temperature environments usually results in the bleach of the refractive index modulation. The maximum temperature reported for the conventional FBG temperature sensor is around 600 degC due to its weak bonds of germanium and oxygen. In this paper, we report design and development of a novel high-temperature resistance FBG temperature sensor, based on the hydrogen-loaded germanium-doped FBG. The refractive index modulation in the FBG is induced by the molecular water. The results of our experiments have shown that the stability of the device is substantially increased at high temperature range. Due to the high bonds energy of hydroxyl and the low diffusivity of the molecular water, the thermal testing results of this temperature sensor show the thermal stability of hydrogen-loaded FBG can be increased by using annealing treatment; moreover, the highest erasing temperature for the device could reach to 1100 degC or more. The reflectivity of this new FBG depends on the concentration of Si-OH and indirectly related to the reflectivity of hydrogen-loaded FBG. Furthermore, the experimental results have provided a better understanding of the formation of the hydrogen-loaded FBGs and the chemical transfers at elevated temperatures in the fiber core

Combined time- and wavelength-division-multiplexing demodulation technique of fiber grating sensor arrays using a tunable pulsed laser

Combined time- and wavelength-division-multiplexing demodulation technique of fiber Bragg grating (FBG) sensor arrays using a tunable pulsed laser is proposed and demonstrated. A tunable fiber laser based on a matched FBG is applied. The wavelengths of the sensing FBGs are obtained by detecting the maximum voltages with a photodiode that avoids the complex demodulation process. The advantages of this scheme are simple structure, high signal-to-noise ratio, and the sensing signals obtained by detecting the maximum voltages.

Passive acoustic fiber Bragg grating sensor measurements of Green function in an instrumented part

The development of fiber Bragg grating (FBG) sensors for acoustic measurements permits the instrumentation of manufactured composite parts for the purposes of nondestructive evaluation (NDE) and lifetime monitoring. The parts can be designed from conception with an array of FBG acoustic receivers integrated into its internal structure for the purpose of multistatic data collection for NDE and monitoring of internal changes. The FBG array is accessed via a port on the outer surface and requires no additional power demands on the operation of the part. Data collection is accomplished by activating sources on the surface of the part and measuring the fields along the FBG array. We are particularly interested in Green function (GF) estimation of a FBG instrumented composite structure. By passively measuring diffuse acoustic fields throughout the volume, we use a technique developed by Lobkis and Weaver [J. Acoust. Soc. Am. 110(6), 3011–3017 (2001)] to estimate the GF within the part and monitor it over time. We describe FBG sensor array operation, review the theory of diffuse field GF estimation, and present examples in heterogeneous multilayered structures. [This work was performed under the auspices of the U.S. DOE by University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.]

Focus on sensors

Focus on sensors

The embedment of fiber Bragg grating sensors into filament wound pressure tanks considering multiplexing

Filament wound pressure tanks are under high internal pressure during the operation and it has the complexity in damage mechanism and failure modes. For this reason, it is necessary to monitor the tank through its operation as well as during the whole fabrication process. A large number of sensors must be embedded into multiple positions of the tank from its fabrication step for monitoring the whole tank. Fiber optic sensors, especially fiber Bragg grating (FBG) sensors are widely used for various applications because of good multiplexing capabilities. However, we need to develop the embedding technique of FBG sensors into harsh inner environment of the tank for the successful embedment. In this paper, we studied the embedding technique of a number of FBG sensors into filament wound pressure tanks considering multiplexing and conducted in situ structural health monitoring of filament wound pressure tanks during water-pressurizing test using embedded FBG sensor arrays. From the experimental results, it was demonstrated that FBG sensors can be successfully adapted to filament wound pressure tanks for their structural health monitoring by embedding.

Fiber Grating Sensor Array Interrogation With Time-Delayed Sampling of a Wavelength-Scanned Fiber Laser

A novel method for fiber Bragg grating (FBG) sensor array interrogation based on a wavelength-scanned fiber laser is presented. The wavelength of the scanning fiber laser is monitored by time-delayed sampling and assigned to return peaks as Bragg wavelengths of the sensor gratings. From experiments with an array of four FBG sensors, the scheme showed the same measurement results using different wavelength scanning waveforms, which verified that this scheme demodulated the sensor signals linearly, regardless of nonlinearities in the wavelength-tuning mechanism. Overall, /spl sim/20 pm of measurement accuracy was obtained which could have been greatly enhanced with faster phase modulation.

Experimental investigation of a 4‐FBG TDM sensor array with a tunable laser source

AbstractThe performance of a time‐division‐multiplexed fiber Bragg grating sensor array that uses a tunable laser source has been investigated. It was found that the wavelength detection accuracy is affected by the extinction ratio of the optical pulse modulator used for input pulse generation. Experiments with a 4‐FBG sensor array demonstrated a strain resolution of 0.7 με with an optical pulse modulator of −24‐dB extinction ratio. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 435–437, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10343

Characterization of crosstalk of a TDM FBG sensor array using a laser source

The crosstalk characteristics of a time-division multiplexing fiber Bragg grating sensor array that uses a laser source are examined. The system performance is found to be limited by the extinction ratio of the input optical pulse modulation and can be improved significantly by properly modulating the laser wavelength.

Multi-point strain measurement of composite-bonded concrete materials with a RF-band FMCW multiplexed FBG sensor array

We report the use of fiber Bragg grating (FBG) sensors for strain measurement in a rectangular composite-bonded concrete beam. Three FBGs are multiplexed by using a frequency modulated continuous wave (FMCW) technique in the RF-band and embedded at three different locations at the interface between the composite reinforcement and surfaces of the concrete beam specimen. Strain measurements were conducted with the concrete beam specimen subjected to a three-point bending test. The strains measured from the FBG sensors are compared with that from the external bonded strain gauges.

Performance analysis of a time-division-multiplexed fiber Bragg grating sensor array by use of a tunable laser source

The results of an investigation of the performance of a time-division-multiplexed (TDM) fiber Bragg grating (FBG) sensor array using a tunable laser source are reported. The system performance is found limited by the extinction ratio of the optical pulse modulator used for pulse amplitude modulation. Formulas that relate the crosstalk to the extinction ratio of the optical pulse modulator, the modulation parameters of the tunable laser, and the optical path differences among sensing channels are derived. Computer simulation shows that an array of 20 FBG sensors with 3 /spl mu//spl epsi/ resolution can be realized with a commercially available single Mach-Zehnder type optical pulse modulator of -35-dB extinction ratio.

Enhanced FBG strain sensing multiplexing capacity using combination of intensity and wavelength dual-coding technique

A novel concept for interrogating a fiber Bragg grating sensor array using intensity and wavelength dual-coded multiplexing (IWDM) is proposed and evaluated. In comparison with conventional wavelength-division-multiplexed sensor systems, IWDM allows the number of sensing gratings in an array to be doubled without sacrificing dynamic range and without additional cost or complexity of interrogation. A practical strain sensing system based on IWDM and using specifically produced gratings with a dual-peaked spectral response has been demonstrated with 50-Hz interrogation rate and 1 /spl mu//spl epsiv/ resolution.

Short-scan interferometric interrogation and multiplexing of fibre Bragg grating sensors

We describe an interferometric technique for the demodulation of serial fibre Bragg grating sensor arrays, yielding absolute measurement of the individual grating mean wavelengths. The composite beam reflected from the array illuminates a scanning Michelson interferometer but, in contrast to spectral measurement by Fourier Transform Spectroscopy, our technique requires an OPD scan far shorter than the coherence length of the grating reflections. The technique is based on the high-resolution measurement of the phases of the complex analytic signals and of the relationship of these phases to interferometric delay. The analytic signals are derived via the Hilbert transform, allowing frequency domain filtering of individual signals within the signal processing. The technique has yielded a resolution of 0.007 nm for an OPD scan of 1.2 mm.

Grating sensor array demodulation by use of a passively mode-locked fiber laser

A fiber Bragg grating sensor array is interrogated by use of a passively mode-locked fiber laser source. A novel demodulation scheme that uses highly dispersive fiber to convert the grating wavelength shift to a temporal shift in the arrival time of the reflected pulses is demonstrated. The source bandwidth of >85 nm permits interrogation of many-grating arrays, and the demodulation technique permits fast sensing of large strains.

Modeling and performance analysis of a fiber Bragg grating interrogation system using an acousto-optic tunable filter

The performance of a fiber grating interrogation system using an acousto-optic tunable filter (AOTF) is analyzed. An equivalent linear circuit is presented to describe the system, which tracks the wavelength of a fiber Bragg grating (FBG) sensor in a FBG sensor array. From the model, the tracking resolution of a shot-noise-limited system has been determined for both transmissive and reflective interrogation of FBGs. An optimum ratio between the bandwidths of the FBG and the AOTF has been derived. Experimental results of a tracking system are presented to validate the theoretical predictions. The theory should also be suitable for interrogation systems using other forms of tunable bandpass filters for tracking the FBG wavelength.

Matched-filter interrogation technique for fibre Bragg grating arrays

A technique for the demodulation of fibre Bragg grating (FBG) sensor arrays that uses a matched grating array to optically filter and track wavelength shifts from the FBG sensors is demonstrated. The use of matched grating arrays in the serial configuration represents an efficient and potentially low-cost interrogation technique for FBG sensors.

Distributed strain sensing with fibre Bragg grating arrays embedded in CRTM™ composites

Fibre Bragg grating (FBG) sensor arrays fabricated during optical fibre drawing have been embedded in fibre-resin composite panels made by the continuous resin transfer molding™ (CRTM™) process to accurately measure distributed strain.