Single Fiber Bragg Grating Research Articles

Method for independent strain and temperature measurement in polymeric tensile test specimen using embedded FBG sensors

A novel method to obtain independent strain and temperature measurements using embedded Fibre Bragg Grating (FBG) in polymeric tensile test specimens is presented in this paper. The FBG strain and temperature cross-sensitivity was decoupled using two single mode FBG sensors, which were embedded in the specimen material with a certain angle between them. It is demonstrated that, during temperature variation, both FBG sensors show the same signal response. However, for any applied load the signal response is different, which is caused by the different levels of strain acting in each sensor. Equations to calculate independently the strain and temperature are presented in the article, together with a measurement resolution study.This multi-parameter measurement method was applied to an epoxy tensile specimen, tested in a unidirectional tensile test machine with a temperature controlled cabinet. A full calibration procedure (temperature and strain) was performed to this material-sensor pair, where a calibration error < 1% was achieved. This was followed by a strain-temperature test case, where multiple two loading/strain stages of ε = 0.30% and ε = 0.50% were applied during a continuous variation of temperature, from 40 °C to –10 °C.The consistency of the expected theoretical results with the calibration procedure and the experimental validation shows that this proposed method is applicable to measure accurate strain and temperature in polymers during static or fatigue tensile testing. Two different calibration protocols are presented and analysed.

单层结构光纤布拉格光栅光谱特性研究

单层结构光纤布拉格光栅光谱特性研究

Double fiber probe with a single fiber Bragg grating based on the capillary-driven self-assembly fabrication method for dimensional measurement of micro parts

Focusing on the ultra-precision dimensional measurement of parts with micro-scale dimensions and high aspect ratios, a two-dimensional double fiber probe with a single fiber Bragg grating (DS-FBG probe) is investigated in detail in this paper. The theoretical analysis of the sensing principle is verified by spectrum simulations of the DS-FBG probe with a modified transfer matrix method using the strain distribution within the DS-FBG probe. The fabrication process and physical principle of the capillary-driven self-assembly of double fibers in the UV adhesive with a low viscosity are demonstrated. Experimental results indicate that resolutions of 30 nm in radial direction and 15 nm in axial direction can be achieved, and the short-term displacement drifts within 90 seconds are 28.0 nm in radial direction and 7.9 nm in axial direction, and the long-term displacement drifts within 1 hour are 61.3 nm in radial direction and 17.3 nm in axial direction. The repeatability of the probing system can reach 60 nm and the measurement result of a standard nozzle is 300.49 μm with a standard deviation of 20 nm.

Dynamic optical arbitrary waveform shaping based on cascaded optical modulators of single FBG.

A dynamic optical arbitrary waveform generation (O-AWG) with amplitude and phase independently controlled in optical modulators of single fiber Bragg Grating (FBG) has been proposed. This novel scheme consists of several optical modulators. In the optical modulator (O-MOD), a uniform FBG is used to filter spectral component of the input signal. The amplitude is controlled by fiber stretcher (FS) in Mach-Zehnder interference (MZI) structure through interference of two MZI arms. The phase is manipulated via the second FS in the optical modulator. This scheme is investigated by simulation. Consequently, optical pulse trains with different waveforms as well as pulse trains with nonuniform pulse intensity, pulse spacing and pulse width within each period are obtained through FSs adjustment to alter the phase shifts of signal in each O-MOD.

Simultaneous multichannel carrier-suppressed return-to-zero to non-return-to-zero format conversion using a fiber Bragg grating.

A novel multichannel carrier-suppressed return-to-zero (CSRZ) to non-return-to-zero (NRZ) format conversion scheme based on a single custom-designed fiber Bragg grating (FBG) with comb spectra is proposed. The spectral response of each channel is designed according to the algebraic difference between the CSRZ and NRZ spectra outlines. The tailored group delays are introduced to minimize the maximum refractive index modulation. Numerical results show that four-channel 200-GHz-spaced CSRZ signals at 40 Gbits/s can be converted into NRZ signals with high Q-factor and wide-range robustness. It is shown that our proposed FBG is robust to deviations of bandwidth and central wavelength detuning. Another important merit of this scheme is that the pattern effects are efficiently reduced owing to the well-designed spectra response.

Novel FBG rosette for determining impact location in thin plate-like structure

The paper consists of two interdependent parts. The first part presents numerical simulations of output response of single Fibre Bragg Grating (FBG) sensor which is driven by homogeneous deformation along sensor length. The example of such sensor is FBG sensor glued at only two points. The grating length and modulation depth of the refractive index are the critical parameters contributing to performance of the FBG sensors. Numerical analysis allowed to select an appropriate FBG sensors which will be used in the impact detection problem. In the second part of the paper a novel strain rosette with specific sensor array used in impact localisation problem is discussed and presented. The experiment was carried out on thin composite plate with the use of pulse force excitation. The method is based on estimation of principal direction of perturbed travelling wave initiated at impact point and does not use any information about wave propagation velocity.

Fiber Bragg Grating Sensors for Distributed Torsional Strain Measurements in a (RE) BCO Tape

Fiber Bragg grating (FBG) sensors were used to measure the torsional strain distribution in a second-generation (rare-earth) Ba2Cu3Oy high-temperature superconducting sample tape (SST) at 77 K. Two SST samples were prepared. A slit was fabricated in each sample to allow an optical fiber to be embedded. In the first sample, a 10-mm-long single grating element was inserted into the slit. In the second sample, three grating elements of 10-mm length each were put into the slit. These grating elements were fabricated along a single-mode fiber and could be addressed individually using the wavelength division multiplexing (WDM) technique. Finally, the samples were recoated with a primary and secondary coating to form an embedded sensing system. When the high-temperature superconducting (HTS) tape is subjected to torsion, there is a strain induced in the FBG sensor, which varies the grating period accordingly. It was interrogated using a tunable laser in a WDM scheme. Based on the wavelength shift and the photoelastic properties of the FBG sensor embedded to the HTS tape, the torsional deformation of the HTS tape can be obtained. In this paper, the design and technology requirements to embed the single and WDM FBG sensors into the coated conductors are presented. Initial experiments to measure the torsional strain distribution at 77 K are reported.

Low-velocity impact localization in a stiffened composite panel using a normalized cross-correlation method

This paper presents an experimental study on the low-velocity impact localization of complex composite structures. An impact localization algorithm, which localizes an impact source by comparing the normalized cross-correlation between the reference database and the obtained impact signals, was proposed. The proposed method was applied to a stiffened composite panel that consists of a main spar and stringers. Impact tests were conducted on the composite panel in which four multiplexed fiber Bragg grating (FBG) sensors were attached on the bottom surface. The verification results indicated that 20 verification points were successfully localized with the maximum error of 43.98 mm and average error of 14.23 mm using four FBG sensors. The effect of the number of sensors on the localization performance was also investigated. The comparison results revealed that the proposed method could localize an impact source using a reduced number of sensors. A single FBG sensor covered an area of 600 × 900 mm2 of the stiffened composite panel with a maximum error of 64.76 mm and an average error of 17.86 mm using the proposed method.

Single and Multiphase Flow Characterization by Means of an Optical Fiber Bragg Grating Grid

This study introduces a new approach to characterize single and multiphase flow of water and air/water blends, respectively, by means of the utilization of optical fiber Bragg gratings (FBGs) arranged in a grid pattern. Here, the FBGs act as transducers between the force applied on the optical fiber surface by the liquid or air/liquid flow and the strain-induced Bragg wavelength shift. Since the force is proportional to the square of the velocity, associated to the kinetic energy, it is possible to establish a relationship between the Bragg wavelength shift and flow speed for single-phase flow monitoring. When multiphase flows are taken in consideration, a sudden Bragg wavelength shift represents an abrupt change in the force applied onto the fiber, which means a transition between liquid and air. It is hard to localize turbulences in single phase flow or establish the bubble position for multiphase flow from the response of a single FBG. Therefore, the sensors in this study have been arranged forming an 8 × 8 grid, with a total of 16 different FBGs multiplexed in wavelength. FBG grid enables the detection of turbulences or air bubbles within the pipe by means of an adequate aggregation and processing of the response of the FBGs at each crossing point, with a total of 64 crossings (12 crossings are out of the cylindrical shape pipe). Different flow speeds and void conditions with distinct void fractions and flow rates have been studied. The optical fiber sensors performance agreed with that of a wire-mesh system, which is conventionally used as a reference high performance measurement tool for multiphase flow. Results showed the great potential of this technique that reduces in more than a half the costs, complexity and size of actual devices used for the same purpose.

Theoretical study of all-optical RZ-OOK to NRZ-OOK format conversion in uniform FBG for mixed line-rate DWDM systems

In this work we study all-optical multi-channel return-to-zero (RZ)–on-off keying (OOK) to nonreturn-to-zero (NRZ)–OOK format conversion in single uniform fiber Bragg grating (FBG) for mixed line-rate dense wavelength-division multiplexing systems using mathematical simulations. Forty and 20 Gbit/s RZ–OOK signals with 33% and 50% duty cycles are converted to NRZ–OOK signals in single uniform FBG with 21% reflectivity. Impact of amplitude noise from FBG contrast profile on modulation format conversion efficiency is also studied.

Optimally-designed single fiber Bragg grating filter scheme for RZ-OOK/DPSK/DQPSK to NRZ-OOK/DPSK/DQPSK format conversion.

We investigate return-to-zero (RZ) to non-return-to-zero (NRZ) format conversion by means of the linear time-invariant system theory. It is shown that the problem of converting random RZ stream to NRZ stream can be reduced to constructing an appropriate transfer function for the linear filter. This approach is then used to propose novel optimally-designed single fiber Bragg grating (FBG) filter scheme for RZ-OOK/DPSK/DQPSK to NRZ-OOK/DPSK/DQPSK format conversion. The spectral response of the FBG is designed according to the optical spectra of the algebraic difference between isolated NRZ and RZ pulses, and the filter order is optimized for the maximum Q-factor of the output NRZ signals. Experimental results as well as simulations show that such an optimally-designed FBG can successfully perform RZ-OOK/DPSK/DQPSK to NRZ-OOK/DPSK/DQPSK format conversion.

Carrier-suppressed return-to-zero to non-return-to-zero format conversion based on a single fiber Bragg grating with knife-shaped spectra.

We propose a novel carrier-suppressed return-to-zero (CSRZ) to non-return-to-zero (NRZ) conversion scheme based on a single custom-designed fiber Bragg grating (FBG) with a knife-shaped spectrum. The structure of the FBG is designed and synthesized using a discrete layer-peeling inverse scattering technique. It is shown that such an FBG can replace the combination of interferometer and the cascaded filter that is invariably employed in the reported schemes for CSRZ to NRZ format conversion. Simulation results show that conversion of 40Gbit/s CSRZ into NRZ signals results in a Q factor that is 4.5dB greater than the previously reported schemes.

Development of a double fiber probe with a single fiber Bragg grating for dimensional measurement of microholes with high aspect ratios.

A fiber Bragg grating (FBG)-based probe is developed by designing a novel double fiber structure to make the probe sensitive to both axial and radial contact displacements. A matched FBG pair interrogation system for the probe is modeled, and the best matched initial condition is found to make the probing system work in the linear and most sensitive mode. Actual measurements of ring gauges and fuel injection nozzles indicate that, for a microhole with an aspect ratio of greater than 14∶1, an axial resolution of 8nm and a radial resolution of 30nm can be achieved with the developed probe. It is therefore concluded that the double fiber probe with a single FBG developed can be used to measure high aspect ratio microholes.

RZ-OOK to NRZ-OOK format conversion based on a single fiber Bragg grating

We propose a return-to-zero on-off keying (RZ-OOK) to non-return-to-zero (NRZ) OOK conversion scheme based on a single custom-designed fiber Bragg grating (FBG). The custom-made FBG is designed and syn­thesized using discrete layer-peeling algorithm. It is shown that such a FBG can replace the combination of interferometer and the cascaded filter that are invariably employed together in the reported schemes for ­RZ-OOK to NRZ-OOK format conversion. Simulation results show that the input 20-Gb/s RZ-OOK signals with different duty cycles can be converted into NRZ-OOK signals with high Q-factor.

Absorbance properties of gold coated fiber Bragg grating sensor for aqueous ethanol

Optical Fiber Bragg Grating (FBG) is commonly deployed as a wavelength selective filter in telecommunication as well as to detect physical changes such as pressure, temperature and strain in sensing applications. This paper presents an investigation of FBG as a chemical sensor towards ethanol in aqueous solution. Telecommunication standard single mode FBGs were coated with different thicknesses of thin gold films via sputtering deposition method. The combination of Bragg gratings and gold film enhances the evanescent wave on the surface of the optical fiber. It was found that the FBG coated with 50 nm gold layer exhibits the strongest response towards water with varying concentrations of ethanol. The sensor shows 55% change in absorbance levels when the concentration of ethanol is increased from 0 to 99.7% in water.

Union self-compensated packaging of FBG strain sensor

To solve the cross-sensitivity to temperature of fiber Bragg grating (FBG) strain sensor, an effective method is firstly proposed for the discrimination of strain and temperature by using a single FBG adopting a special compensated structure, which is based on the thermal stress. The relationship of strain and temperature responses of FBG is analyzed theoretically. The experimental results are in agreement with the obtained theoretical analyses. This sensor structure has excellent characteristics, such as simple structure, easy manufacture and higher strain sensitivity.

Use of Spectral Conditions to Separate Strain and Temperature Effects in Fibre Bragg Grating Sensors Embedded in Load-Carrying Anisotropic Laminates

This work presents a further development of the methods of simultaneous determination of temperature and axial strain using a single fibre Bragg grating (FBG). The reflected spectrum of composite-embedded FBG sensors has been analyzed in order to separate temperature and load effects. We found out that during the curing process of the laminates, a superstructure has been introduced on the FBG. Analyzing this effect, a temperature and strain separation was implemented by simply calculating and comparing the integrated intensity of the spectral response signal. A mathematical four-parametric model has been developed to calculate the reflected spectrum of a superstructured fibre Bragg grating. The mathematically achieved data has been evaluated with experimentally determined data from fibre Bragg gratings embedded in glass-fibre reinforced composite materials.

Sideband‐suppressed narrow bandpass fibre Fabry‐Pérot filter composed of fibre Bragg grating and dielectric mirror

To suppress the undesirable sideband transmission occurring in the conventional fibre Bragg grating based Fabry-Perot filter, a fibre Fabry-Perot interferometer that is composed of a single fibre Bragg grating and a broadband high-reflectance dielectric mirror coated on a fibre end surface is considered and its very narrow transmission bandwidth of 8.8 pm and high sideband suppression ratio of more than 20 is experimentally demonstrated in good conformity with the numerical simulation.

Strain and temperature discrimination technique by use of A FBG written in erbium doped fiber

We propose and demonstrate strain and temperature discrimination technique using a single fiber Bragg grating (FBG) written in the core of an erbium doped fiber. We observed that amplified spontaneous emission power varying linearly from the erbium doped fiber with temperature which determines temperature changes and strain is estimated by subtracting the wavelength shift due to temperature change, from the measured shift corresponding to the dip in the transmission spectrum of the FBG. A simple and compact FBG sensor is presented with improved rms errors of 21.2μɛ and 1°C over ranges of 0–800μɛ and 40–95°C, respectively. The sensor is shown to have strain and temperature sensitivity of 0.8pm/μɛ and 12pm/°C.

New design for temperature–strain discrimination using fiber Bragg gratings embedded in laminated composites

A new smart structure based on fiber Bragg gratings (FBGs) embedded into composite laminates for temperature and strain simultaneous measurement has been designed and experimentally tested. Two holes have been drilled at preset locations in the composite plate to create different strain sensitivities at different locations. The proposed design has been compared to three reference sensing heads also based on embedding FBGs into composite materials. Experimental results agree remarkably well with mechanical simulations and validate all the tested designs for the temperature–strain discrimination. Based on the same principle, another sensing head with a long single FBG embedded has also been designed and experimentally tested, obtaining temperature independent strain measurement.