Fiber Bragg Grating Reflection Spectrum Research Articles

Monitoring the cohesive damage of the adhesive layer in CFRP double-lapped bonding joint based on non-uniform strain profile reconstruction using dynamic particle swarm optimization algorithm

To well understand the failure mode and monitor the internal damage of the carbon fiber reinforced plastic (CFRP) bonding joint provides guarantee for the safety and reliability of the aerospace. This article obtained the mapping relationship model between the strain profiles change of the adherend and the damage evolution of adhesive layer in the CFRP double-lapped bonding joint by ABAQUS based on the cohesive zone model. Then the Fiber Bragg Grating (FBG) was embedded in CFRP adherend to calculate the reflection spectrum of FBG for reconstructing the adherend strain profile of FBG sensing area with the dynamic particle swarm optimization algorithm. The reconstructed results showed that the maximum error between the simulated strain profile and the reconstructed one under different loads was not exceeding 1.701, revealing good consistency and making it possible to monitor the cohesive damage of adhesive layer based on the non-uniform strain profile reconstruction.

Fault identification and localization for Ethernet Passive Optical Network using L-band ASE source and various types of fiber Bragg grating

Abstract This paper presents a centralized and fault localization technique for Ethernet Passive Optical Access Network. This technique employs L-band Amplified Spontaneous Emission (ASE) as the monitoring source and various fiber Bragg Gratings (FBGs) as the fiber's identifier. An FBG with a unique combination of Bragg wavelength, reflectivity and bandwidth is inserted at each distribution fiber. The FBG reflection spectrum will be analyzed using an optical spectrum analyzer (OSA) to monitor the condition of the distribution fiber. Various FBGs reflection spectra is employed to optimize the limited bandwidth of monitoring source, thus allows more fibers to be monitored. Basically, one Bragg wavelength is shared by two distinct FBGs with different reflectivity and bandwidth. The experimental result shows that the system is capable to monitor up to 32 customers with OSNR value of ∼1.2 dB and monitoring power received of −24 dBm. This centralized and simple monitoring technique demonstrates a low power, cost efficient and low bandwidth requirement system.

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.

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.

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.

Gaussian-LM算法在光纤布拉格光栅反射光谱寻峰中的应用

Gaussian-LM算法在光纤布拉格光栅反射光谱寻峰中的应用

Experimental investigation on the time-delay signature of chaotic output from a 1550 nm VCSEL subject to FBG feedback

The time-delay signature (TDS) of chaos output in a 1550 nm vertical-cavity surface-emitting laser (VCSEL) subject to fiber Bragg grating (FBG) feedback is investigated experimentally. Autocorrelation function (ACF) and mutual information (MI) are used for quantitatively identifying the TDS of chaos. For various bias currents, the TDS evolution with the feedback strength is different, as the FBG provides wavelength-selective feedback. Furthermore, based on the TDS map of the FBG feedback VCSEL (FBGF-VCSEL) in the parameter space of feedback strength and bias current, the optimal TDS suppression regions, where the dominant polarization mode of FBGF-VCSEL locates at the edge of the main lobe of FBG reflection spectrum, have been determined. Finally, for comparative purpose, the TDS of chaos in mirror feedback VCSEL (MF-VCSEL) also has been presented, and the results show that an FBGF-VCSEL possesses better TDS suppression performance than an MF-VCSEL.

Temperature-Insensitive Fiber-Optic Contact Force Sensor for Steerable Catheters

A steerable catheter with a compact fiber optic sensor for temperature-insensitive contact force measurement is demonstrated. A fiber Bragg grating (FBG) is used as the sensor, which is very small and can be fixed on the outer arc of a steerable catheter, such that it does not occupy any space inside the catheter. Contact force sensing performance under various curve diameters during steering is studied and the results show high repeatability. Since the curve diameter during steering is predetermined, accurate contact force measurement can be performed through calibrating the measurement with various steered catheter positions. Different from most conventional FBG contact force sensors, this approach does not require the analysis of FBG reflection spectra. Instead, contact force applied on the catheter tip is determined by the direct measurement of total FBG reflection power, enabling real time, low cost, and compact sensing. Furthermore, the proposed catheter is tested under different temperatures, and temperature-insensitive performance is also obtained.

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.

Theoretical and experimental analysis of interaction from acoustic emission on fiber Bragg grating

Fiber Bragg grating (FBG) sensors have been indicated to be ideal candidates for high frequency acoustic emission (AE) detection. This paper reports that the strain fields along FBG axis generated by acoustic emission waves contain Gaussian-sine strain field distribution. Propagation matrix method is adopted to evaluate the dynamic characteristics of FBG reflection spectrum under the action of the Gaussian-sine strain field. Then, intensity demodulation signal of FBG acoustic emission detection system using 3-dB bandwidth method is theoretically simulated based on the spectrum analysis. In order to obtain accurate measurement using this system, the maximum displacement amplitude of the strain field generated by acoustic emission is explored. Furthermore, in AE experiments, Gaussian-sine waves are extracted to confirm the accuracy of the novel presentation regarding strain field generated by acoustic emission waves along FBG axis. These studies provide useful tools for acoustic emission sources position and identification by time waveform analysis.

Spiral-structured fiber Bragg grating for contact force sensing through direct power measurement.

A high-sensitivity fiber Bragg grating (FBG) force sensor based on direct optical power measurement is presented. The approach utilizes a novel structure where the FBG is mounted on a thin tube-like fixture spirally. Contact force measurement is achieved through direct measurement of the FBG reflection power at a single wavelength using a power meter. The measuring system in our approach is simple and does not require processing of massive amount of spectral data, enabling real-time contact force monitoring. When force is applied to the FBG sensor, the unique spiral structure leads to FBG chirping and reflection spectrum broadening. A proportional relationship and linear fit are found between the force applied (up to 1.55 N) and the optical power reflected by the proposed FBG sensor. An average sensitivity of 11.16 dB/N is experimentally achieved. This design significantly reduces system complexity and improves data processing speed, which has great practical value in real-time FBG sensing applications.

Suppression of side lobes in the fiber Bragg grating reflection spectrum

A revision of works on the methods of writing and apodization of fiber Bragg gratings (FBG) is presented. The possibilities of apodizing FBGs recorded by holographic methods with phase masks and Lloyd interferometer using laser radiation with a Gaussian intensity profile are considered. FBG reflection spectra are numerically simulated for different parameters of the writing beam and configurations of the experimental setup. Side-lobe suppression and FBG spectrum smoothing are experimentally demonstrated. It is shown that this effect is due to equalization of the average value of the induced refractive index in the FBGs. FBG applications, in particular, as frequency selectors in fiber lasers (tunable, narrow-band, and multiwavelength) are described.

强度调制的光纤布拉格光栅磁场传感器

A novel magnetic field sensor based on a chirped effect of Fiber Bragg Grating(FBG)was proposed.The relationship between the magnetic induction intensity and the variable bandwidth of reflection spectrum of the FBG was derived.The working principle of the sensor was introduced.Firstly,the magnetic force from an energized spiral coil was applied to a disc-shaped soft iron in a magnetic field to make the rectangular cantilever beam deformed,which resulted in the bandwidth change of the reflection spectrum of FBG that was mounted at a side of the cantilever.As mentioned above,the magnetic induction intensity could be obtained by measuring the variable bandwidth of the reflection spectrum of the FBG with a spectral analyzer.The experiments show when the optical spectral analyzer is with a resolution of 0.001nm,the measuring range of the sensor is up to 6-70mT.Experimental results demonstrate that the variable bandwidth of the reflection spectrum of FBG is not sensitive to temperature change.The 3dB bandwidth change is less than 8nm when the temperature changes from 0to 45℃,which is consistent with theoretical value well.The sensor scheme is feasible and practical.

TESTING A REAL TIME MONITORING SYSTEM FOR PASSIVE OPTICAL NETWORKS USING AN ARRAY OF FIBER BRAGG GRATINGS

In this paper, we propose and design a passive optical network using an array of fiber Bragg gratings (FBGs) for centralized monitoring of the branched optical networks. Further, we exploit the linear properties, namely, reflection and transmission spectra of FBGs for identifying the fiber fault. Besides, we also verify the quality of the downstream signals (1490 nm and 1550 nm) through the well-known eye-diagrams through the spectrum analyzer. We find that the quality of downstream signals remains same even in the presence of monitoring signal and the passive element FBG. Thus, these numerical results corroborate that the monitoring signal and passive element (FBG) do not affect the quality of the downstream signals. Thus, we envisage that the results presented in this paper would turn to be an asset in the area of passive optical network (PON).

Coupled-mode analysis for linear fiber Bragg grating made of metamaterial

We derive the coupled-mode equations for a metamaterial fiber Bragg gratings (FBGs). The coupled-mode theory describing wave propagation in periodic structures such as FBG is generalized for studying the transmission and reflection of electromagnetic waves through a FBG containing metamaterial layers. This approach shows that the band gap becomes broader in comparison with a conventional FBG. It is observed that the central frequency of the band gap has a shift in regard of changing metamaterial thickness. We verify this investigation through attaining the transmission and reflection spectra of a conventional FBG while we are setting related parameters. Furthermore, this proposed structure can be applied as a tunable filter.

Recovering a fiber Bragg grating axial strain distribution from its reflection spectrum

A processing scheme able to obtain any arbitrary intragrating strain distribution of a fiber Bragg grating (FBG) is proposed and demonstrated. The processing method employs just the intensity of the FBG reflection spectrum to obtain its deformation profile by combining a geometrical processing scheme with the particle swarm optimization technique. The technique has been evaluated using several spectra generated from very heterogeneous strain distributions and with a real spectrum obtained from a 5 mm length FBG under an asymmetric perturbation. The achieved results exhibit a good spatial resolution (Δ≈0.5 mm) and repeatability.

Time-frequency analysis of long fiber Bragg gratings with low reflectivity

A new technique to investigate the spatial distribution of the reflection spectrum along fabricated long weak fiber Bragg gratings (FBG) is experimentally demonstrated, together with its potential applications for distributed fiber sensing and broadband signal processing. A short pulsed coherent light signal is launched into a FBG and the signal frequency is scanned through the FBG reflection spectrum. When the pulse duration is set much shorter than the transit time through the grating a time-resolved reflected signal can be obtained for each signal frequency. It informs about the distribution of the refractive index periodic perturbation along the entire FBG length, hence the uniformity or frequency chirp information of the fabricated FBG. This technique has been implemented to demonstrate a distributed temperature sensing system with high spatial resolution and to also realize a robust all-fiber tunable delay line for broadband signals.

Distributed Strain Reconstruction Based on a Fiber Bragg Grating Reflection Spectrum

Abstract In this paper, we present a synthesis of the parameters of the fiber Bragg grating (FBG) and the reconstruction of the distributed strain affecting the grating, performed by means of its reflection spectrum. For this purpose, we applied the transition matrix method and the Nelder-Mead nonlinear optimization method. Reconstruction results of the strain profile carried out on the basis of a simulated reflection spectrum as well as measured reflection spectrum of the FBG indicate good agreement with the original strain profile; the profile reconstruction errors are within the single digit percentage range. We can conclude that the Nelder-Mead optimization method combined with the transition matrix method can be used for distributed sensing problems.

Core-offset-based fiber Bragg grating sensor for refractive index and temperature measurement

We present a fiber-optic sensor based on a fiber Bragg grating (FBG) whose core has an offset to that of the lead-in fiber from splicing for simultaneous refractive index (RI) and temperature measurement. Due to the core offset, the core mode in the lead-in fiber is coupled to both the core mode and the cladding modes of the sensing fiber with the FBG. Then the two oppositely propagating code modes, the cladding modes, and the core modes are coupled by the FBG at particular wavelengths determined by the effective indices of the modes and the FBG structure, resulting in two peaks in the FBG reflection spectrum. The peak wavelength corresponding to the core-cladding mode coupling is sensitive to both RI and temperature, while the peak wavelength corresponding to the core-core mode coupling is only sensitive to temperature. Utilizing the difference of two reflection peaks of these coupling modes, both RI and temperature can be detected simultaneously. We fabricate and characterize a sensor. The preliminary experimental results show that it can detect the refractive index in a range of 1.33 to 1.4421, with a temperature sensitivity of 10 pm/°C .

Analysis of FBG Reflection Spectrum with PSO Algorithm

When the parameters were measured by using fiber Bragg grating (FBG) in practice, there were some parameters hard to measure, which would influenced the reflective spectral of FBG severely, and make the characteristic information harder to be extracted. Therefore, particle swarm optimization algorithm was proposed in analyzing the uniform force reflective spectral of FBG. Based on the uniform force sense theory of FBG and particle swarm optimization algorithm, the objective function were established, meanwhile the experiment and simulation were constructed. And the characteristic information in reflective spectrum of FBG was extracted. By using particle swarm optimization algorithm, experimental data showed that particle swarm optimization algorithm used in extracting the characteristic information not only was efficaciously and easily, but also had some advantages, such as high accuracy, stability and fast convergence rate. And it was useful in high precision measurement of FBG sensor.