Characterization of fiber Bragg grating based on grating profile and apodization for sensor applications

In this paper, modelling, simulation and characterization of optical fibre Bragg grating (FBG) for maximum reflectivity for oil and gas sensing applications are presented. The fibre grating length and the refractive index profile are key parameters for effective and high performance optical Bragg grating. The spectral reflectivity, bandwidth and side lobes were analysed with changes in grating length and refractive index. Modelling and simulation of the Bragg grating which are based on solving the Coupled Mode Theory (CMT) equation by using the transfer matrix method were carried out using MATLAB and the results show that the changes in grating length and refractive index profile affect the bandwidth as the demand for bandwidth and high-speed transmission grows in oil and gas sensing applications.

Fiber Bragg Grating (FBG) sensors are very much applicable for measuring various parameters like temperature, strain, pressure etc. among other optical sensor due to its various advantage particularly in harsh environment. FBG can also be used for monitoring of various civil structures by measuring the sensitivity of physical parameters for different building materials. In this work the reflectivity and peak power value of several building materials with respect to the grating length has been discussed by using a gaussian apodized FBG. Effect of bandwidth has also been discussed with the change of grating length. The entire research work has been carried out in Opti-grating software for designing the apodized FBG. In some applications related to sensing, sidelobe suppression is much more important along with high reflectivity and narrow bandwidth. We have considered a Gaussian apodized FBG which removes all the side lobes of the reflection spectrum. Grating length can be represented as the most important parameter to improve the reflectivity and peak power of the spectrum. It has been observed that reflectivity increases from 31.28% to 97.46% and peak power value also increases from −5.0541 dB to −0.1115 dB if we increase the grating length of the FBG from 10 mm to 40 mm. Bandwidth is found to be decreased from 0.65 nm to 0.12268 nm with increasing grating length value from 10 mm to 40 mm for all the building materials.

Fiber Bragg gratings (FBG) sensor has attracted considerable attention for structural health monitoring (SHM) in aerospace aluminum alloy structures in recent years. The experimental FBG signals used to detect flaws contains noise. Therefore, in this paper, an algorithm that combines variational mode decomposition (VMD) with discrete wavelet transform (DWT) was proposed to deal with the signal denoising problem. Then, it is a challenge problem for structural damage detection to extract the features that sensitive to damage and robust to noise from the FBG signal response. This paper presents several damage features extraction algorithms, such as the multi-peaks central wavelength algorithm, full width at half Maximum (FWHM) algorithm, the spectral difference algorithm and the spectral area algorithm. Then, the denoised structure signal can be used to test the damage features extraction algorithm effectiveness and for future damage detection.

Theoretical approach to optimize fiber Bragg grating sensor performance using an automated new code

In This work small signal intensity (IM) response of hybrid soliton pulse source utilizing Gaussian apodized fiber Bragg grating (FBG) is presented. It has been shown in previous studies that some parameters lead to increase resonance peak spectral splitting (RPSS) in the IM spectra. However, simulation results of this study show that type of external cavity is important effect on RPSS in IM response as well as effect of laser and FBG parameters. The RPSS can be suppressed by using linearly chirped Gaussian apodized FBG as an external cavity instead of Gaussian apodized FBG.

We present a novel method for the synthesis of complex fiber Bragg gratings (FBGs) which is based on an impedance reconstruction layer aggregation technique. The method takes as a starting point the accurate approximation of the sinusoidal perturbation (uniform or nonuniform) of the refractive index by local step functions by means of an inverse Fourier series relationship. This reduces the otherwise continuous scattering to only two points per local period and thus, the maximum spatial resolution as limited by the Nyquist criterion is achieved and there is no need to lump various periods of the FBG into an equivalent mirror. The main advantage brought by the method is the possibility of synthesizing structures containing defects or discontinuities of the size of the local period, a feature which yields different results with priorly reported methods to which the one reported here can be considered as complementary. In addition, this enhanced spatial resolution allows the synthesis of very strong FBG devices providing convergent solutions. We illustrate this fact by demonstrating the synthesis of a flat-top filter with a maximum reflectivity of 99.999%. The method renders directly the refractive index profile n(z) as it does not rely on the coupled mode theory since the grating is reconstructed using a V-I matrix formulation. The method is simple and its complexity is equivalent to those previously reported in the literature. Examples are given which prove that perfect reconstruction is achieved in both simple and complex grating profiles. Although a layer aggregation is presented here, the layer peeling (i.e., moving time frame) version of this algorithm is straightforward.

In this paper we perform a numerical analysis of fiber Bragg grating (FBG) bandwidth characteristics for oil and gas sensing applications. It is shown that the grating length (Lg) and the change in the refractive index profile (δn) represent a key parameters in contributing to high FBG sensor performance. The analysis based on solving coupled mode equations were carried out using MATHCAD software. Results show that the changes in the Lg and δn affect the FBG bandwidth significantly at a time when demand for bandwidth is growing in oil and gas sensing applications. The obtained results are very important for FBG sensor applications.

본 논문은 광섬유 브래그 격자의 방사선 민감도에 대한 수소로딩 공정의 영향에 대해서 연구하였다. 게르마늄이 함유된 광섬유에 수소로딩 기간을 달리하여 광섬유 브래그 격자를 제작하였다. 광섬유 브래그 격자는 총 18 kGy 감마선에 조사하여 방사선에 의한 브래그 파장의 변화를 측정하였고, 방사선 조사 후에는 반치대역폭과 온도 감도 계수의 변화를 평가하였다. 수소로딩 조건에 따라서 방사선에 의한 브래그 파장의 변화는 약 2 배 이상의 차이를 보였다. This paper investigates the influence of hydrogen loading process on the radiation sensitivity of fiber Bragg gratings (FBG). We made the FBG inscribed in the same commercial Ge-doped fiber with different hydrogen loading periods. We measured the Bragg wavelength shift (BWS) of the FBG exposed to gamma-radiation up to a dose of 18 kGy, and evaluated the change of full width at half maximum (FWHM) and the FBG temperature sensitivity coefficient after irradiation. Varying hydrogen loading parameter led to BWS differences up to nearly a factor of two.

Superposed WDM fiber Bragg gratings (FBGs) provide a novel method to realize a large number of very short time-delay steps in applications such as photonics-based beam-forming for multi-beam phased arrays. In this paper, a systematic design method for superposd FBGs is presented. This provides informaiton for the key requirements of grating length determination, grating apodiziation function selection and channel spacing control, and the selection of a suitable method to obtain a linear time-delay profile of the designed superposed FBG. Grating length versus maximum number of superposed gratings for ps time-delays under different apodizations is presented. The effects of grating apodization on wavelength channel spacing of the superposed gratings is also given, and an optimization is described to minimize the wavelength step so as to maximize the nubmer of channels in a given optical spectral width. Finally, a technique for time-delay profile linearization of WDM superposed FBGs with a large number of ps delay steps is presented. Using these design procedures, results are given for a 32-superposed WDM FBG unit which shows 1 ps delay step and 1.7% standard deviation from linearity onthe time-delay profile, while maintaining >99% reflectivity for all grating channels and keeping the total refractive index chagne inteh fiber below 0.01.

A new method is proposed and demonstrated for fabricating phase-shifted fiber Bragg gratings (FBGs) using a variable-velocity scanning UV laser beam in combination with a shielded phase mask. The transmission properties of phase-shifted FBGs were analyzed based on coupled-mode theory and a transfer matrix method. The grating is divided into three parts to allow for easier analysis of FBG properties. These segments included a uniform FBG1 and FBG2 which were separated by a shielded section. A novel phase-shifted FBG was fabricated using this method, in which the refractive indices of FBG1 and FBG2 were different. Transmission properties of these phase-shifted FBGs were simulated numerically using MATLAB, and the experimental results and simulated results are in good agreement. In addition to the length and effective refractive index of the shielded section, the phase shift value of a phase-shifted FBG was also found to be dependent on the lengths and effective refractive indices of FBG1 and FBG2. Moreover, we predicted that changing the scanning velocity for fabricating FBG2 would adjust phase shift value, which exhibits a positive linear relationship with the scanning velocity. These results can provide guidelines for fabricating any phase shift value FBGs. This technique is simple, convenient, and may be developed further for use in fabricating novel tunable fiber filters or DFB fiber lasers.

The apodization function plays an important role in the behavior of the mean dispersion, group delay and significant reduction of sidelobe level in reflection spectrum. On the other hand, the grating length represents as one of the critical parameters in contributing to a high performance fiber Bragg grating compensator. In this paper, we have investigated and compared the performances of various apodization functions in linearly chirped apodized fiber Bragg grating. Six different apodization profiles have been studied including their effects on the performances of the dispersion compensator. Comparison reveals that the sinc profile results best performance for chromatic dispersion compensation. Simulations have been performed for sinc profile with different grating lengths. The simulated results were then analyzed by calculating the reflection spectrum. It is observed that the reflection spectrum becomes constant at a grating length of 10 cm.

Multi-Functional Measurement Using a Single FBG Sensor

The sensing applications of fiber Bragg gratings (FBGs) offer a high sensitivity, as well as other important advantages, such as real-time processing, long-term stability, immunity to electromagnetic interference, and convenient multiplexing capabilities. The reflection spectrum of fiber Bragg gratings with a uniform modulation of the refractive index is accompanied by series of sidelobes at the adjacent wavelength. It is very important to minimize and, if possible, eliminate the reflectivity of these sidelobes. The design of fiber Bragg gratings used Coupled Mode Theory. The coupled mode equations were solved by the Transfer Matrix Method since it was considered as good approximation to calculate the spectral response of fiber Bragg gratings. Apodization techniques are used to get optimized reflection spectrum. The uniform FBGs with several types of apodization were modeled in this project It was proven that apodization profile could be reduced the sidelobes. In addition, each profile also has the bandwidth and the maximum reflectivity better than others.

Composites are gradually replacing traditional metal materials in aeronautical manufacturing. Despite of excellent properties, composite materials suffer from barely visible impact damage which can be caused by low velocity objects. In this paper, an impact localization algorithm is proposed based on dynamic characteristic calibration and reference database to improve the accuracy and robustness. The installation parameters of fiber Bragg grating (FBG) sensor are determined by the dynamic characteristic calibration. The impact positions are obtained by calculating the similarity degree between the random impact signal and the signals in the reference database. The algorithm is tested over a carbon fiber reinforced polymer (CFRP) plate attached with FBG sensors. Impacts are stimulated by using a hammer. Noise signals are added to part of the FBG sensor signals during experiments. The results show the proposed algorithm can accurately locate the impacts and deal with the signal interference.

Fiber Bragg grating (FBG) sensors are excellent transducers for ultrasonic signal detection in structural health monitoring (SHM) application. While the FBG sensors are typically bonded directly on the surface of a structure to collect signals, one of the major challenges arises from demodulating relevant information from the low amplitude signal. The authors have experimentally demonstrated that the ultrasonic wave detection sensitivity of FBG sensors can be increased by bonding optical fiber away from the FBG location. This configuration is referred to here as remote bonding. However the mechanism causing this phenomenon has not been explored. In this work, we simulate the previous experimental work through a transient analysis based on the finite element method, and the output FBG response is calculated through the transfer matrix method. We first model an optical fiber bonded on the surface of an aluminum plate with an adhesive. The consistent input signal is excited to the plate, which is detected by the directly and remotely-bonded FBGs. The effect of the presence of the adhesive around the FBG is investigated by analyzing strain and displacement along the length of the FBGs at the locations of direct and remote bonding cases, and the consequent output FBG responses. The result demonstrates that the sensitivity difference between the direct and remote bonding cases is originated from shear lag effect due to adhesive.