Fiber Bragg Grating Spectrum Research Articles

High-Speed Interrogation of Multiplexed Fiber Bragg Gratings With Spectral Distortion

Fiber Bragg grating (FBG) sensors can be multiplexed in large numbers to monitor the performance of large structures. This paper addresses the collection of FBG reflection spectra from wavelength division multiplexed sensors at fast acquisition rates. The spectral and temporal resolution is first derived as a function of the tunable filter and measurement system properties. The method is applied to impact loading investigations of a stiffened composite skin panel. The reflected spectrum of each FBG in an array, embedded in the panel, is collected at 100 kHz during the impact events with a spectral resolution down to 40 pm. Visualization of the FBG responses to these impact events, including the presence of spectral distortion in some FBG spectra, is presented. Future analyses based on the full-spectral data sets can enable the assessment of the localized progression of internal damage in such structures.

Review and Analysis of Peak Tracking Techniques for Fiber Bragg Grating Sensors.

Fiber Bragg Grating (FBG) sensors are among the most popular elements for fiber optic sensor networks used for the direct measurement of temperature and strain. Modern FBG interrogation setups measure the FBG spectrum in real-time, and determine the shift of the Bragg wavelength of the FBG in order to estimate the physical parameters. The problem of determining the peak wavelength of the FBG from a spectral measurement limited in resolution and noise, is referred as the peak-tracking problem. In this work, the several peak-tracking approaches are reviewed and classified, outlining their algorithmic implementations: the methods based on direct estimation, interpolation, correlation, resampling, transforms, and optimization are discussed in all their proposed implementations. Then, a simulation based on coupled-mode theory compares the performance of the main peak-tracking methods, in terms of accuracy and signal to noise ratio resilience.

Femtosecond Laser Inscription of Fiber Bragg Grating in Twin-Core Few-Mode Fiber for Directional Bend Sensing

We demonstrated femtosecond laser inscription of fiber Bragg gratings (FBGs) in a twin-core few-mode fiber (TC-FMF) for directional bend sensing. An FBG was selectively inscribed in one core of the TC-FMF by using an 800 nm femtosecond laser through a phase mask. Three resonance peaks at the wavelengths of 1549.05, 1547.65, and 1546.08 nm were observed in the reflection spectrum of the TC-FM FBG, and were generated by the LP01 mode resonance, LP01–LP11 mode cross-coupling resonance, and LP11 mode resonance, respectively. Moreover, the TC-FM FBG exhibited the capability of directional bend sensing and achieved a maximum bend sensitivity of −37.41 pm/m−1. Hence, the proposed TC-FM FBG directional bend sensors could further be developed as promising solutions for detecting vectorial seismic or acoustic waves and 3-D shape sensing.

Investigation on Characteristic Variation of the FBG Spectrum with Crack Propagation in Aluminum Plate Structures.

In order to monitor the crack tip propagation of aluminum alloy, this study investigates the variation of the spectrum characteristics of a fiber Bragg grating (FBG), combined with an analysis of the spectrum simulation. The results identify the location of the subordinate peak as significantly associated with the strain distribution along the grating, corresponding to the different plastic zones ahead of the crack tip with various crack lengths. FBG sensors could observe monotonic and cyclic plastic zones ahead of the crack tip, with the quadratic strain distribution along the grating at the crack tip-FBG distance of 1.2 and 0.7 mm, respectively. FBG sensors could examine the process zones ahead of the crack tip with the cubic strain distribution along the grating at the crack tip-FBG distance of 0.5 mm. The spectrum oscillation occurs as the crack approaches the FBG where the highly heterogeneous strain is distributed. Another idea is to use a finite element method (FEM), together with a T-matrix method, to analyze the reflection intensity spectra of FBG sensors for various crack sizes. The described crack propagation detection system may apply in structural health monitoring.

An EMD-SG Algorithm for Spectral Noise Reduction of FBG-FP Static Strain Sensor

Random noises in the reflection spectra of fiber Bragg grating (FBG) typically limit the resolution of static-strain sensor based on FBG. We use the empirical mode decomposition (EMD) and the Savitzky–Golay (SG) filter to denoise reflection spectra from a pair of FBG-based Fabry–Perot interferometers (FBG-FPs) for achieving high static-strain resolution. The EMD is proposed for extracting adaptive basis functions from two FBG-FPs’ reflection spectra. The SG filter is proposed for smoothing and denoising basis functions. These basis functions are used to reconstruct the reflection spectra, which can be used to calculate the relative wavelength difference. A simulation and two experiments are conducted to verify the performance of the proposed technique. The results of experiment conducted in the noisy laboratory and in the quiet basement are consistent with the simulation result. When SNR is less than 10 dB, the resolution of the EMD-SG method is approximately ten times the resolution of the traditional cross correlation. A minimumstatic strain resolution of 0.68 $\text{n}\varepsilon $ is achieved.

Fiber Bragg grating interrogation using wavelength modulated tunable distributed feedback lasers and a fiber-optic Mach-Zehnder interferometer.

This paper demonstrates a technique of high-resolution interrogation of two fiber Bragg gratings (FBGs) with flat-topped reflection spectra centered on 1649.55nm and 1530.182nm with narrow line width tunable semiconductor lasers emitting at 1651.93nm and 1531.52nm, respectively. The spectral shift of the reflection spectrum in response to temperature and strain is accurately measured with a fiber-optic Mach-Zehnder interferometer that has a free spectral range of 0.0523GHz and a broadband photodetector. Laser wavelength modulation and harmonic detection techniques are used to transform the gentle edges of the flat-topped FBG into prominent leading and trailing peaks that are up to five times narrower than the FBG spectrum. Either of these peaks can be used to accurately measure spectral shifts of the FBG reflection spectrum with a resolution down to a value of 0.47pm. A digital signal processing board is used to measure the temperature-induced spectral shifts over the range of 30°C-80°C and strain-induced spectral shifts from 0 μϵ to 12,000 μϵ. The shift is linear in both cases with a temperature sensitivity of 12.8pm/°C and strain sensitivity of 0.12 pm/μϵ. The distinctive feature of this technique is that it does not use an optical spectrum analyzer at any stage of its design or operation. It can be readily extended to all types of tunable diode lasers and is ideally suited for compact field instruments and for biomedical applications in stroke rehabilitation monitoring.

Magnetic field sensor of enhanced sensitivity and temperature self-calibration based on silica fiber Fabry-Perot resonator with silicone cavity.

In this paper, a magnetic field sensor with enhanced sensitivity based on a fiber Fabry-Perot (F-P) cavity formed by a pair of identical fiber Bragg gratings (FBGs) is demonstrated. The F-P cavity which was filled with silicone rubber was bonded to a magnetic alloy at two positions such that when longitudinal strain is applied, the cavity is lengthened while the FBGs was virtually strain-free, effectively magnified the magnetic-field induced strain of the magnetic alloy. The FBGs could also be used for temperature-compensation because the FBG spectrum change is negligible compared to the F-P spectrum.

EMD self-adaptive selecting relevant modes algorithm for FBG spectrum signal

Abstract Noise may reduce the demodulation accuracy of fiber Bragg grating (FBG) sensing signal so as to affect the quality of sensing detection. Thus, the recovery of a signal from observed noisy data is necessary. In this paper, a precise self-adaptive algorithm of selecting relevant modes is proposed to remove the noise of signal. Empirical mode decomposition (EMD) is first used to decompose a signal into a set of modes. The pseudo modes cancellation is introduced to identify and eliminate false modes, and then the Mutual Information (MI) of partial modes is calculated. MI is used to estimate the critical point of high and low frequency components. Simulation results show that the proposed algorithm estimates the critical point more accurately than the traditional algorithms for FBG spectral signal. While, compared to the similar algorithms, the signal noise ratio of the signal can be improved more than 10 dB after processing by the proposed algorithm, and correlation coefficient can be increased by 0.5, so it demonstrates better de-noising effect.

Research on the Phase-Shifted Fiber Bragg Grating Spectra under Dynamic Strain Fields

Fiber optic sensing technology has been widely used for acoustic emission (AE) measurement in aerospace and geotechnical engineering due to the advantages of immunity to electro-magnetic interference, high accuracy, and multiplexing capacity. As for the fiber Bragg grating based on AE measurement system, traditional efforts have been made to study the spectra of Fiber Bragg Grating under AE signal. While this paper focused on phase-shifted fiber Bragg grating (PS-FBG) which is a new type fiber Bragg grating, and investigated the spectra of under dynamic strain field generating with an AE signal. A dynamic strain sensing model of PS-FBG was built based on the transfer matrix theory, and a cosine exponential attenuation function was utilized to simulate the dynamic strain field. Then the effects of amplitude, sampling time, exponential attenuation coefficient, frequency and wavelength of AE signal on the spectra of PS-FBG were studied in detail using numerical simulation. The result demonstrate that the spectra of PS-FBG changes periodically with frequency and different sampling time, especially the peak wavelength of the transmission window in the PS-FBG spectrum shifts periodically; and the increase of amplitude mainly contribute to more harmonic peaks in the reflected spectrum. The attenuation coefficient affects the spectrum of PS-FBG within certain range. And the AE wavelength affect the spectrum of PS-FBG when it is between 0.1 to 2 L, beyond the range, the spectrum of PS-FBG has minor variation with the AE wavelength. Finally, dynamic strain experiments were conducted, and the spectrum of PS-FBG under continuously vibration signal with different amplitude and frequency were logged for analysis. The experimental results agree well with simulation, which indicate that under different dynamic strain fields generated by AE signal, the spectra of PS-FBG are different, while the variation of spectra follows certain laws. This paper provides theoretical support for the AE measurement system on the basis of PS-FBG.

Demodulation method of sensor based on tunable distributed feedback laser diode

A fiber Bragg grating (FBG) wavelength shift demodulation method based on tunable distributed feedback laser diode (DFB-LD) is proposed. A highly effective algorithm is proposed to demodulate the strain of the sensing FBG, which is scanned by the tunable DFB-LD. The system is unique in that accurate demodulation can be achieved by scanning only a 0.4-nm bandwidth FBG spectrum instead of relying on scanning the FBG’s principal power peak and the scanned spectrum can be any part of the FBG. Thus, the demodulation system is much more simplified in comparison to a traditional demodulation system, which enables using wavelength-division multiplexing for the FBG sensing network. With the strain experiment of the FBG, the results match the theory of the proposed demodulation method very well, and the errors of the strain experiment are within 2 pm. The tunable DFB-LD is based on reconstruction-equivalent chirp (REC) technology. The REC technology potentially offers a low-price, compact, and high-performance solution for the demodulation.

Agarose Filled Fabry–Perot Cavity for Temperature Self-Calibration Humidity Sensing

An agarose-based fiber-optic Fabry-Perot (F-P) sensor is presented for relative humidity (RH) measurement. The reflectors are formed by two identical fiber Bragg gratings (FBGs), and the cavity is filled with the agarose gel. Because of the hydrophilic characteristics of the agarose, the humidity variation will induce the expansion and refraction index change of the agarose gel, which will lead to the shift of the F-P resonance peak. The FBG spectrum can be recovered by applying a fitting algorithm to the spectrum of the F-P cavity, and is used for temperature self-calibration to resolve the temperature cross-sensitivity problem. The average sensitivity is 22.5 pm/%RH. Benefiting from the tiny size of the RH sensitive element, the rise time and the fall time are tested to be 5.11 and 10.35 s, respectively.

Theoretical analysis of spectrum flattening in fiber laser oscillator

The flatness of laser spectrum is important in many applications. In this manuscript, a method of acquiring flattened spectrum directly from a fiber oscillator by optimizing the reflective spectrum of Fiber Bragg Gratings (FBG) was demonstrated and optimized result at wavelength around 1064nm and 1080nm was presented. An optimization path to alter the reflectivity of FBGs using greedy algorithm was interpreted by analyzing the single-trip gain inside the resonant cavity. Our method has a guiding significance of controlling the output spectrum of laser oscillator using FBGs.

Hot Cavity Optical Fiber Fabry–Perot Interferometer as a Flow Sensor With Temperature Self-Calibrated

An optical fiber flow sensor is developed based on a hot cavity fiber Fabry–Perot (F-P) interferometer formed by a pair of fiber Bragg gratings (FBG). Part of the F-P cavity is made up by a short (2 mm) cobalt-doped fiber (CDF). The CDF can be heated up by a heating laser so that the effective optical path length of the cavity increases. The fluid flow can take some heat away depending on the flow rate and also changes the resonant wavelength of the F-P cavity. The FBG spectrum is recovered by fitting the envelope of the F-P spectrum. The generated heat is centralized inside the cavity and could hardly influence the FBG. As the F-P and FBG are with the same sensitivity to the environment temperature, the recovered Bragg wavelength can be used as a perfect reference for the self-calibration of the temperature.

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.

V-groove-based compact FBG package for thermal tuning and mechanical stability

We demonstrated a V-groove-based fiber Bragg grating (FBG) package that has been glue-filled and cured to make it a bulky component with much improved mechanical stability. The V-groove can be executed with many types of materials including plastics, ceramics, semiconductors, and metals, providing an easy method for redesigning the thermal tuning performance of FBGs by selecting among a wide variety of materials and processes. We achieved more than 10-nm thermal wavelength tuning and thermal sensitivity ranging from 15 to 160 pm/K. The original FBG spectrum can be maintained without any degradation because the fiber is buried in the V-groove. The compact package does not increase the original grating length and turns the FBG into a planar waveguide grating, improving FBG applications in telecommunications, external cavity lasers, and sensing areas.

Dynamic fiber Bragg grating sensing method

The measurement of high frequency vibrations is important in many scientific and engineering problems. This paper presents a novel, cost effective method using fiber optic fiber Bragg gratings (FBGs) for the measurement of high frequency vibrations. The method uses wavelength matched FBG sensors, with the first sensor acting as a transmission filter and the second sensor acting as the sensing portion. Energy fluctuations in the reflection spectrum of the second FBG due to wavelength mismatch between the sensors are captured by a photodiode. An in-depth analysis of the optical circuit is provided to predict the behavior of the method as well as identify ways to optimize the method. Simple demonstrations of the method were performed with the FBG sensing system installed on a piezoelectric transducer and on a wind turbine blade. Vibrations were measured with sampling frequencies up to 1 MHz for demonstrative purposes. The sensing method can be multiplexed for use with multiple sensors, and with care, can be retrofitted to work with FBG sensors already installed on a structure.

KLT-Based Algorithm for Sub-Picometer Accurate FBG Tracking With Coarse Wavelength Sampling

An algorithm based on Karhunen–Loeve transform (KLT) for accurate tracking of fiber Bragg grating (FBG) sensors is presented. The routine is based on main eigenvalue analysis of the KLT applied to the Fourier transform of the FBG spectrum. The algorithm achieves sub-picometer accuracy, down to 4.9 fm, when applied to coarse (156 pm) wavelength sampling, providing accuracy superior to correlation-based algorithms. Experimental validation has been carried out in a climatic chamber. The proposed algorithm can be applied to any off-the-shelf white-light setup for FBG interrogation, and is compatible with 100–1-kHz operation.

Fiber Bragg gratings in the radiation environment: Change under the influence of radiolytic hydrogen

The change of the transmission spectra of fiber Bragg gratings written in the optical fibers, whose silica cores are doped with either germanium or nitrogen, is studied experimentally under the influence of gamma-radiation. The transmission spectra in the neighborhood of the resonance (Bragg) wavelengths were regularly recorded “in-situ” in the course of irradiation during 24 days. For this purpose, uncoated gratings were placed in a pool near the spent fuel rods of a nuclear reactor. The fibers with the gratings written in them were in immediate contact with water. The estimated total absorbed radiation dose of the fibers is approximately 5 MGy. Molecular hydrogen, which is produced by radiolysis of water and penetrates into the core of silica fiber, is found to interact with the defects of Ge-doped silica induced by gamma-radiation, thereby causing a strong impact on the parameters of the spectrum of the Bragg gratings. On the contrary, in the case of gratings inscribed in N-doped silica fibers, the hydrogen molecules interact with defects induced in the course of laser UV exposure during the grating writing only. The possible subsequent formation of additional defects in N-doped silica under the influence of gamma-radiation has no substantial impact on the transmission spectra of Bragg gratings, which remained stable. The obtained results suggest that a small amount of molecular hydrogen resided in the fiber core is the main source of radiation instability of Ge-doped fiber Bragg grating sensors in radiation environments. These hydrogen molecules can remain in the Bragg gratings, in particular, after the inscription process in the hydrogen-loaded fibers.

In-Situ Cure Monitoring of Wind Turbine Blades by Using Fiber Bragg Grating Sensors and Fresnel Reflection Measurement.

A fiber-optic cure monitoring system is proposed to measure curing status of composite structure such as a large scale wind turbine blade. The monitoring is based on the measurement of Fresnel reflectivity at the optical fiber/epoxy resin interface. The refractive index of epoxy resin varies throughout curing stages, changing the Fresnel reflectivity. The curing status is decided by monitoring the reflected intensity variation. The usage of fiber Bragg grating (FBG) sensor helps to separate the temperature-induced cross effects. A Gaussian curve fitting algorithm was applied to FBG spectra which were distorted in curing procedure. The substantial measurement errors could be minimized by locating the centroids of the Gaussian curve-fitted spectra. From the experiments performed in various isothermal conditions, the proposed system successfully identified the onset of gelation and the completion of curing of epoxy resins.

Time-dependent variation of fiber Bragg grating reflectivity in PMMA-based polymer optical fibers.

In this Letter, we investigate the effects of viscoelasticity on both the strength and resonance wavelength of two fiber Bragg gratings (FBGs) inscribed in microstructured polymer optical fiber (mPOF) made of undoped PMMA. Both FBGs were inscribed under a strain of 1% in order to increase the material photosensitivity. After the inscription, the strain was released, and the FBGs spectra were monitored. We initially observed a decrease of the reflection down to zero after which it began to increase. After that, strain tests were carried out to confirm the results, and finally the gratings were monitored for a further 120 days, with a stable reflection response being observed beyond 50 days.