Length Of Fiber Bragg Grating Research Articles

Monolithic 2-µm single-frequency linearly-polarized gain-switched distributed feedback fiber laser by femtosecond laser direct-writing.

We report a single-frequency, linearly polarized gain-switched, distributed feedback (DFB), 2-µm thulium doped silica fiber laser (TDFL), with an effective cavity length of 2.5 mm. The cavity is based on a heavily thulium doped non-polarization-maintaining silica fiber and composed of a π-phase-shifted fiber Bragg grating (FBG) with a total FBG length of 35 mm. The DFB FBG was written by femtosecond-laser point-by-point (PbP) method. In-band pumping scheme is chosen with a 1550 nm nanosecond pulsed erbium-doped silica fiber laser pump. Single-longitudinal, linearly polarized, gain-switched TDFL at 2002 nm, with a recorded shortest pulse duration of 4.7 ns, a repetition rate of 20 kHz, a maximum peak power of 170 W, and single pulse energy of 0.8 µJ, has been obtained, benefitting from the ultrashort DFB cavity made by the femtosecond laser direct-writing method.

Fiber Bragg Gratings for Temperature Measurements Under Thermal Gradients: Comparison Between Two Different Lengths

In recent decades, fiber Bragg grating (FBG) sensors found widespread acceptance in several biomedical applications thanks to their indisputable advantages. Thermal ablation treatments (TATs) account for one of the fields where FBGs have gained large applicability for temperature measurements. The sensing element length can affect the accuracy of the FBG measurement in this application. A longer length provides a more prominent peak in the FBG reflected spectrum, resulting in a higher signal-to-noise ratio (SNR). A shorter FBG length implies a lower SNR but offers the benefits of better spatial resolution. Therefore, the choice of an adequate length can be crucial to minimize measurement errors, but the literature lacks exhaustive investigations on this parameter. The aim of our study is to supply additional knowledge about the effect of two different FBG lengths on temperature estimation in the presence of linear thermal gradients and with gradients mimicking the condition caused by TATs. In both cases, we compared the output of a 10-mm FBG with those of four FBGs 1 mm long, using a thermal camera as a reference instrument. Results suggest the better suitability of shorter sensors to retrieve thermal gradient information along their position instead of the longer FBG, which could lead to unacceptable measurement errors during TATs.

Mach-Zehnder interferometer based fiber-optic nitrate sensor.

A biocompatible, reliable and quick responsive fiber-optic sensor based on Mach-Zehnder interferometer (MZI) is demonstrated for nitrate analytes tracing. The sensor was constructed by collapsing the air holes of a short length photonic crystal fiber (PCF) with the single-mode fibers (SMFs) on both ways. The proposed sensor has been coated with a graphene-PVA (polyvinyl alcohol) membrane using the thermal coating technique to make the sensor attractive to the nitrate ions in the aqueous solution. The maximum response is found to be 0.15 pm/ppm on the nitrate measurement scale of 0 ppm to 100 ppm with an average reaction time of ∼10 s. Also, a short length of FBG (fiber Bragg grating) is implanted with SMF to improve the sensing accuracy of the presented sensor.

Ultrasonic frequency response of fiber Bragg grating under direct and remote adhesive bonding configurations

Ultrasonic inspection based structural health monitoring is a powerful technique for damage detection in a structure. Ultrasonic waves are often excited at different frequencies to detect damages of different sizes, therefore understanding the frequency response of the sensor can be used to optimize the sensor performance. Fiber Bragg gratings (FBGs) are widely used for this ultrasound collection. The sensitivity of FBGs to a particular ultrasonic frequency is a function of the FBG length and the ultrasonic wavelength. Recently the authors demonstrated that its ultrasound sensitivity is improved for some conditions when the FBG is bonded at a distance away from the adhesive bond, referred to as remote bonding. However, the frequency response of this configuration has not been studied. Therefore, in this paper we measure and compare the ultrasonic frequency responses between a conventional directly bonded FBG and remotely bonded FBG. In theory, the FBG sensitivity varies as a function of ultrasound wavelength-to-grating length (λ/L) ratio. Therefore, for this experimental study, we maintain L constant and vary λ by changing the frequency of the input ultrasonic waves. We demonstrate that there is a region, below a cut-off values of λ/L, for which the remotely bonded FBG output has a higher sensitivity to the Lamb wave amplitude than the directly bonded FBG. The exact value of this λ/L cut-off depends on the mechanical properties of the structure, the windowing of the input Lamb wave, and the FBG properties. We also demonstrate that windowing the Lamb wave excitation signal has a similar affect to apodizing the FBG sensor in modifying the sensitivity response curve.

Fiber Bragg grating-electromagnetically induced transparent fast optical switch

Here, an all-optical switch is modeled in the form of fiber Bragg grating (FBG) based on electromagnetically induced transparency (EIT) phenomena dealing with a three-level EIT silicon (Si) nanocrystalline medium. The EIT consists of two fields, namely a strong control and weak probe fields that interact with the medium. Fast-optical switching of the probe is achieved by applying a pulsed control field, which relies on both steady-state and transient density matrices. The effects of four significant parameters of interest, i.e., Rabi frequency of the control field, EIT nanocrystal density, FBG length, and spontaneous decay rate, have been extensively investigated to optimize the probe transmission. Subsequently, the time-dependent density matrix is solved to obtain the response time of the FBG-EIT switch. Eventually, FBG is modeled such that low index layers contain EIT material and the high index layers do not. The FBG-EIT switch satisfies the criteria of maximum transmission and minimum rise time to give out the optimal operational condition. Despite the optical switching is in OFF mode at Ωc=0 THz demonstrating a high FBG reflectance, the reflectance suddenly vanishes at a certain Ω c s to satisfy the equivalence of low and high refractive indices (switching ON). Therefore, a fast-optical switch is envisaged operating as large as 100 GHz in theory. The future applications of the FBG- EIT include the fast Q-switching of fiber lasers, ultra-fast modulation, optical quantum communication, and high speed optical processing and computation.

Spectral Verification of the Mechanisms behind FBG-Based Ultrasonic Guided Wave Detection.

Ultrasonic guided wave (UGW) detection with fiber Bragg grating (FBG)-based sensors has received increasing attention in the last decades due to the ability to perform non-destructive inspection (NDI) of large plate-like surfaces with a network of lightweight and multiplexed sensors. For accurate UGW measurements, several studies concluded that the ratio between the wavelength of the UGW and the length of the FBG should be above 7. However, shorter FBGs suffer from a lower FBG reflectivity and less steep slopes in the reflection spectrum. In this work we experimentally verified the effect of a passing UGW on the Bragg peak of FBG sensors of different lengths. By performing edge-filtering interrogation throughout the FBG’s reflection spectrum, we were able to reconstruct the FBG’s spectral response to a UGW in function of time. Our experimental findings are partially in line with those in the literature considering the UGW wavelength to FBG length ratio and the corresponding Bragg peak changes. We experimentally show for the first time that for shorter FBG sensors, the strain modulation is translated mostly into Bragg peak shifting, while for longer FBG sensors, Bragg peak deformation takes over as main mechanism. Despite the different mechanism for the latter, the UGW can still be detected by edge-filtering on the steepest slope, and with a much higher sensitivity.

Wear measurement based on the length variation of a sacrificial FBG.

A high-precision wear measurement method with temperature stability achieved by measuring the length variation of a fiber Bragg grating (FBG) is proposed. The adoption of the optical frequency-domain reflectometry (OFDR) technology makes the spatial resolution of this measurement method reach 15.13 µm, and the offline and online measurement accuracies are 30 µm and 100 µm, respectively. The systematic error of the FBG length measuring system is within 30 µm. Because the length measurement is done with a short FBG instead of a much longer fiber, the measurement error induced by the time-varying temperature or strain is significantly reduced in the proposed method. The spatial resolution and accuracy of this method is suitable for wear measurements of various parts in the mechanical field, such as bearings, gears, and pistons.

High-precision fiber Bragg gratings inscription by infrared femtosecond laser direct-writing method assisted with image recognition.

For achieving high efficiency fiber Bragg gratings (FBGs) utilizing infrared femtosecond laser point-by-point inscription method, it is crucial to make the inscribed periodic structure perfectly in phase. It requires a perfect alignment between the micrometer-sized laser spot with the fiber along the length. Here we report the highly precise fabrication of FBGs by infrared femtosecond laser point-by-point direct-writing method. Image recognition technique is applied to for automatically aligning the trace of the laser spot with the referenced central axis of the fiber along the whole FBG length. FBGs inscription with high spatial precision is confirmed by multiple approaches, including microscopic photographing and FBG spectroscopic measurement. 50 mm-long uniform FBGs with high reflectivity have been successfully demonstrated in a small-core single-mode silica fiber using auto-aligning technique.

An enhanced WDM optical communication system using a cascaded fiber Bragg grating

In this paper, a cascaded fiber Bragg grating (FBG) system is proposed to reduce the dispersion in the optical signal in single mode optical fibers. This consequently enhances the system performance, which is evaluated by the bit error rate (BER) and quality factor (Q-factor). The proposed model consists of four uniform cascaded FBGs connected at the transmitter to get narrow linewidth, Δλ, of the optical signal, which is a major cause of the delay. The Optisystem7 is used to simulate the proposed model in a WDM system with and without the model for distance 200 km. The system parameters are investigated showing an enhanced performance with 12%, including eye diagram, Q-factor and BER. A 10−6–10−10 BER is achieved with a quality factor in the range 7–14, including the effects of fiber length, input power and FBG length.

Assessment of Optimal Bragg Grating Length for Optical performance using Two Apodization Functions

Fiber Bragg gratings (FBGs) are one of the most effective technologies because of their suitability in many applications of the fiber techniques. Moreover, it can be utilized in sensing elements and estimating the physical parameters of the optical fiber. In the current research, various fiber bragg lengths are considered and a comparative investagtion is made for the performance of the refractive index modulation at two different apodization profiles namely: Gaussian index profiles and uniform index profiles. Also, a comparative study of the optical communication system performance has been carried out of refractive indexes with two apodization functions. A comprehensive comparison in terms of gain, noise figure, OSNR, quality factor, bit error rate and power has been performed where the performance of the optical system is studied under various optical channel lengths (from 25 km to 100 km) with various FBG lengths, which are ranged from 4 mm to 14 mm in both apodization profiles, i.e. uniform profile and Gaussian profile. In order to carry out the simulation of the performance RZ, the optisystem version 9 is used for single channel based on over single mode fiber. The results showed that the maximum Q.F. as the performance parameter is obtained at the FBG length of 8 mm at all optical lengths. Moreover, the varation effect of optical fiber channel length was greater than the varation effect of refractive index for all the studied parameters (gain, noise figure, OSNR, quality factor, BER, and power) for two apodization functions.

Optical accelerometer for seismic measurement

In this work we present an optical accelerometer based in a cantilever beam attached to a fiber Bragg grating (FBG), which can be applied to the seismic and geological fields. This device main characteristics are a high strain along the FBG and a wide operational frequency range. The accelerometer can be easily modeled as a mass block and a fiber optic attached at the cantilever beam tip, the FBG length varies due to the movement of the mass caused by the vertical vibrations. The frequency response range has been enhanced and it was achieved a natural frequency of 242.4 Hz.

A IR-Femtosecond Laser Hybrid Sensor to Measure the Thermal Expansion and Thermo-Optical Coefficient of Silica-Based FBG at High Temperatures.

In this paper, a hybrid sensor was fabricated using a IR-femtosecond laser to measure the thermal expansion and thermo-optical coefficient of silica-based fiber Bragg gratings (FBGs). The hybrid sensor was composed of an inline fiber Fabry-Perot interferometer (FFPI) cavity and a type-II FBG. Experiment results showed that the type-II FBG had three high reflectivity resonances in the wavelength ranging from 1100 to 1600 nm, showing the peaks in 1.1, 1.3 and 1.5 μm, respectively. The thermal expansion and thermo-optical coefficient (1.3 μm, 1.5 μm) of silica-based FBG, under temperatures ranging from 30 to 1100 °C, had been simultaneously calculated by measuring the wavelength of the type-II FBG and FFPI cavity length.

Analysis of the performance of strain magnification using uniform rectangular cantilever beam with fiber Bragg gratings

A thorough analysis of the performance of different strain magnification structures with fiber Bragg gratings (FBG) is demonstrated in this paper. Four kinds of cantilever beams with different shapes have been designed by using a laser cutting technique. Simulation results show that the strain magnification structures of the different cantilever beams are different. There is a positive and linear correlation between the displacement of the free end of the cantilever beam and the average strain at the pasting position of the FBG. The correlation is used to calculate the displacement sensitivity and analyze the efficacy of the different strain magnification structures. Sensitivity enhancement factors (compared to a reference beam) of 2.67, 2.14 and 2.57 were measured experimentally for the different beam shapes studied, while the theoretical prediction factors were 2.67, 2.24 and 2.55, respectively. There is a reasonable agreement between simulation and experiment, which shows that the efficacy is related to the increasing rate of the average strain over the FBG length. The proposed method can be used to enhance the sensitivity of the FBG sensor and further improve the sensor structure design.

Reproducible ultra-long FBGs in phase corrected non-uniform fibers

Ultra-long fiber Bragg gratings (FBGs), i.e., FBGs of several tens of cm in length, have attracted much attention in the last few decades for their potential applications in advanced devices. Although numerous fabrication methods as well as ultra-long functionalized FBGs have been proposed and demonstrated successfully, such devices are difficult to reproduce. We have recently found that specialty optical fibers of the type required for these applications are highly non-uniform on a short length scale, severely affecting the characteristics of ultra-long FBGs. We propose here a new production technique that can be adapted to any non-uniform fiber for ultra-long FBG fabrication. This technique involves a fiber characterization prior to FBG inscription followed by the writing of a phase corrected ultra-long FBG. This technique has no limitations in terms of correction amplitude or FBG length. The results are quite astonishing, as near-perfect 1-m-length-scale FBGs are possible in fibers in which it was impossible to write uniform period gratings prior to phase correction.

Simulating increased Lamb wave detection sensitivity of surface bonded fiber Bragg grating

Fiber Bragg grating (FBG) sensors are excellent transducers for collecting ultrasonic wave signals for structural health monitoring (SHM). Typically, FBG sensors are directly bonded to the surface of a structure to detect signals. Unfortunately, demodulating relevant information from the collected signal demands a high signal-to-noise ratio because the structural ultrasonic waves have low amplitudes. Our previous experimental work demonstrated that the optical fiber could be bonded at a distance away from the FBG location, referred to here as remote bonding. This remote bonding technique increased the output signal amplitude compared to the direct bonding case, however the mechanism causing the increase was not explored. In this work, we simulate the previous experimental work through transient analysis based on the finite element method, and the output FBG response is calculated through the transfer matrix method. The model is first constructed without an adhesive to assume an ideal bonding condition, investigating the difference in excitation signal coherence along the FBG length between the two bonding configurations. A second model is constructed with an adhesive to investigate the effect of the presence of the adhesive around the FBG. The results demonstrate that the amplitude increase is originated not from the excitation signal coherence, but from the shear lag effect which causes immature signal amplitude development in the direct bonding case compared to the remote bonding case. The results also indicate that depending on the adhesive properties the surface-bonded optical fiber manifests varying resonant frequency, therefore resulting in a peak amplitude response when the input excitation frequency is matched. This work provides beneficial reference for selecting adhesive and calibrating sensing system for maximum ultrasonic detection sensitivity using the FBG sensor.

Three-straight-line approximation for phase delay on fiber Bragg grating reflection and its application to the mode analysis of a distributed Bragg reflector laser for single-frequency operation

A new mathematical algorithm for representing the phase delay on uniform fiber Bragg grating (FBG) reflection by a three-straight-line approximation has been developed to facilitate the mathematical treatment of the FBG response. The validity of the approximation is numerically demonstrated. By means of the developed algorithm, the mode equation in a distributed feedback reflector (DBR) linear cavity has been analytically solved, to the best of the author’s knowledge, and thereby the single-mode operation of the cavity could be theoretically analyzed by tractable mathematics. It is found that the mode selectivity is dependent on the ratio of cavity spacing to FBG length, rather than the spacing itself. The developed algorithm numerically has revealed the mode selectivities that remain unknown in single-frequency DBR fiber laser studies to date.

Spectra power and bandwidth of fiber Bragg grating under influence of gradient strain

The reflective spectrum power and the bandwidth of the fiber Bragg grating (FBG) under gradient strain are researched and experimentally demonstrated. The gradient strain is applied on the FBG, which can induce FBG bandwidth broadening, resulting in the variation of reflective power. Based on the coupled-mode theory and transfer matrix method, the segmental linear relationship between the gradient strain, the reflective power, and the bandwidth is simulated and analyzed, and the influence of the FBG length on the reflective spectrum is analyzed. In the experiment, the strict gradient stain device is designed; the experimental results indicate that the reflective optic power and the bandwidth of the FBG under gradient stain are concerned with the length of the FBG. Experimental results are well consistent with the theoretical analysis, which have important guiding significance in the FBG dynamic sensing.

Double Cantilever Beams Accelerometer Using Short Fiber Bragg Grating for Eliminating Chirp

An accelerometer based on the shorter fiber Bragg grating (FBG) is demonstrated and experimentally researched. The reflective optic spectra of FBG under inhomogeneous strain are researched by using the coupled-mode theory; the theoretical analysis demonstrates that the features of the reflective optic spectra are closely related with the chirping coefficient and the length of FBG under inhomogeneous strain. On these basics, an FBG accelerometer based on the double cantilever beams with small strain gradient is pointedly proposed. Experimental results show that the accelerometer has good response characteristic, and it provides broad flat frequency range from 4 to 30 Hz, corresponding sensitivity range from 7.76 to 10.8 pm/ms−2, while the flatness is less than 3 dB, the cross-axis sensitivity is less than 3.89% of the main-axis sensitivity, and the temperature sensitivity can be effectively eliminated by the self-symmetrical structure.

Multi-parameter Simultaneously Sensing Based on an Optimized Concatenated FBG with LPG

AbstractWe analyze an optical fiber sensor based on a concatenated fiber Bragg grating (FBG) with long period grating (LPG) with transfer matrix method and experiment. First, we optimize parameters of concatenated structure in the method of simulation and discuss the influence that the lengths, periods and dc induced index changes of FBG and LPG and length of optical fiber between LPG and FBG has to influence reflection spectrum. Second, based on the optimized parameters for concatenated structure, fabricate such structure and use it to monitor concentration and temperature simultaneously. Simulation result proves that increasing the lengths of FBG and LPG properly, and length of optical fiber between LPG and FBG fixed in 20 mm, more clearly reflection spectrum can be obtained. Experiment result shows a novel optical fiber sensor based on a concatenated strong fiber Bragg grating with a weak long period grating (CLBG) device can be used to monitor concentration and temperature simultaneously. The result of research has reference value in multi-parameter simultaneously sensing.

All optical self signal processing using chalcogenide nonlinear fiber Bragg grating

In this study, we simulate numerically quasi pulse propagation in a uniform nonlinear fiber Bragg grating (FBG) made of chalcogenide glasses. Because of bistability and nonlinearity behavior of FBG, the shapes of output pulse vary abruptly and strongly according to the input peak intensity. We take Gaussian pulse as input and produce saw-tooth wave and square wave in the output by suitable input amplitude and FBG length also we could reach pulse chopping and pulse compression. These all optical signal processing achieved for a few length, i.e. 6.6mm and few intensity, i.e. 35W/m2 because of high nonlinearity of chalcogenide glasses.