Fiber Bragg Grating Interrogation Research Articles

Machine Learning Methods for Discriminating Strain and Temperature Effects on FBG-Based Sensors

The biggest challenge of using fiber Bragg grating (FBG) based sensors is the cross-sensitivity between the strain and temperature effects on FBG. In this letter, we demonstrate the ability of machine learning (ML) methods to discriminate between the strain and temperature effects on FBG sensors on a single measurement of change in the Bragg wavelength. Spectral data are collected using an FBG interrogation system at various strain and temperature conditions and are applied to different ML methods to determine the strain and temperature effects. We further simulate FBG with the same strain and temperature conditions using VPIphotonics. For comparison, the same ML methods are applied to both simulated and experimentally collected data. The experimental results reveal that our proposed model can predict strain and temperature with 90% accuracy on a single measurement of Bragg wavelength. We also demonstrate the stability of the model by comparing the testing and training errors of the applied ML methods. Therefore, our proposed technique reduces the cost and complexity associated with the existing FBG-based sensor system.

Optoelectronic chromatic dispersion in germanium PN photodiodes: wavelength monitoring and FBG interrogation.

Optoelectronic chromatic dispersion (OED) has recently been shown to be a significant source of chromatic dispersion in photodiodes. We characterize the OED in a commercial germanium PN-type photodiode and determine the optimum conditions for maximum OED sensitivity and wavelength monitoring. A peak OED sensitivity of 1 deg/nm is measured in a spectral range of 1550-1558 nm with 4 MHz modulation. We also demonstrate an application of OED in fiber Bragg grating (FBG) interrogation. Quasi-static and vibration strains are monitored, with a spectral and strain sensitivity of 1.25pm/Hz and 1.08µε/Hz, respectively. Photodiode OED can form the basis of inexpensive chip-scale grating-less spectral analysis.

Investigation of Peak Detection Algorithms for Fiber Bragg Grating Interrogation based Sensing Systems for Temperature, Depth and Salinity Measurements

Fiber Bragg Grating (FBG) sensors are now one of the prominent and emerging technologies in in the field of optical sensing technology and are used for measurement of strain, depth, temperature and many other applications. This paper reviews the accuracy and stability parameters of four conventional peak seeking algorithms. These algorithms are used in for calculating the peak Bragg wavelength for three different scenarios in which the reflection spectrum of FBG sensors are calculated in our case we consider temperature at 25 C, depth at 6 cm and salinity at 25 PSU. The performance of each of the algorithms i.e., centroid method, Gaussian fitting, polynomial fitting and spline fitting are compared for the above parameters for the actual spectrum and the noisy spectrum on the basis of accuracy and stability and the results are reviewed..

SOI-Based Multi-Channel AWG with Fiber Bragg Grating Sensing Interrogation System

For the development of minimized and high-rate photonic-integrated fiber Bragg grating interrogation (FBGI) systems, arrayed waveguide grating (AWG) has been widely used as one of the critical components. In this paper, we present an 8-channel SOI-based AWG for a photonic integrated FBG interrogation microsystem. The channel spacing of the AWG is designed to be 3 nm to meet a high-dynamic-range demodulation requirement. The core size of the fabricated AWG is about 335 × 335 μm2. The simulation results and experimental results are in high agreement, showing that AWG has a fine transmission spectrum with crosstalk below −16 dB, nonuniformity below 0.4 dB, insertion loss below −6.35 dB, 3 dB bandwidth about 1.3 nm and 10 dB bandwidth of 2.3 nm. The proposed AWG can be applied perfectly to the SOI-based AWG demodulation microsystem, exhibiting a large dynamic range of 1.2 nm, the resolution for measurements is 1.27 pm and a high accuracy of 20.6 pm.

Performance analysis of fiber Bragg grating sensor interrogators based on arrayed waveguide gratings

A planar lightwave circuit-based multi-channel arrayed waveguide grating (AWG) is used as a critical part of the fiber Bragg grating (FBG) interrogation unit and designed to be integrated into the fiber grating sensing system. We designed and fabricated a high-performance, 40 channels AWG in the device, enabling whole C-band wavelength demodulation while maintaining high-measurement resolution. The AWG was designed to have 100-GHz channel spacing; the channel crosstalk was measured to be −45 dB. An AWG-based FBG interrogation system was built to analyze the characteristics of our unit. The testing result shows that our system has the capability of demodulating wavelength from 1526.438 to 1563.863 nm with wavelength accuracy within ±10 pm; the wavelength resolution is measured to be 1 pm. Our testing result confirms that an AWG-based FBG interrogator is an excellent choice for FBG sensing systems.

Distributed temperature sensing system based on a densely spaced FBG array for small fire recognition

A distributed temperature sensing (DTS) system based on a densely spaced fiber Bragg grating (FBG) array is proposed in this work for small fire recognition. A large capacity identical FBG array with centimeter-scale grating spacing is employed to build a high-density sensor network, increasing the probability that the small fire will affect at least one FBG. To retrieve the sensing information, we exploit an FBG interrogation system based on optical time-domain reflectometry and swept laser techniques, together with a fiber section (FS) spectrum interrogation method. The sensing technique consists in scanning the sensing array by means of a low-coherence wavelength-tunable pulsed laser and in reconstructing and analyzing the returned spectra signals. The sensing performance of the proposed DTS system is studied by theoretical analysis and experimental demonstration, verifying the feasibility and superiority of the densely-spaced FBG array based DTS system for small fire recognition and location.

Fiber-end antireflection method for ultra-weak fiber Bragg grating sensing systems

A novel fiber-end antireflection method for reducing the reflection noise in ultra-weak fiber Bragg grating (FBG) sensing systems is proposed in this study. The principle of fiber-end antireflection is analyzed and simulated theoretically, and then an antireflection structure is designed and fabricated. In this method, the fiber end is cut into an inclined surface and covered directly with an ellipsoid photosensitive ultraviolet (UV) glue drop, whose refractive index is similar to that of the fiber core. Under the online monitoring of the fiber-end reflection with the ultra-weak FBG interrogator, the refractive index of the glue drop is adjusted by controlling the UV curing time to achieve optimal reflection suppression. Results show that this method can reduce fiber-end reflection to less than −60 dB and effectively eliminate the influence of fiber-end reflection noise on the last sensor. The proposed method thus provides an excellent solution for the fiber-end processing of ultra-weak FBG sensing systems.

SOI-Based AWG with Extended Dynamic Range for Fiber Bragg Grating Interrogator

SOI-Based AWG with Extended Dynamic Range for Fiber Bragg Grating Interrogator

High-speed interrogation of multiplexed fiber Bragg grating sensors for monitoring dynamic strain responses during a flexible plate impact on a water surface

The investigation of fluid-structure interaction during the impact of a flexible plate on a water surface has received much attention. Measurement of highly transient, distributed strain of the plate during the slamming event is of great interest. Multiplexed fiber Bragg grating (FBG) strain sensors provide a promising solution for such measurement since these sensors are inherently waterproof and are immune to electromagnetic interference. However, in order to monitor the highly transient, distributed strain responses (up to 20 kHz), high-speed simultaneous interrogation of multiplexed FBG sensors is required, which is challenging by using commercial optical interrogators. We present a tunable-wavelength-filter-based optical interrogation system for high-speed simultaneous interrogation of multiplexed FBG strain sensors and demonstrate its application for structural monitoring of a flexible plate during the slamming event. The interrogation system employs a piezoelectric-transducer-controlled Fabry–Perot tunable filter. By operating the tunable filter at its resonant frequency and demodulating the sensor signal based on a peak tracing method, we demonstrated an interrogation speed of 100 kHz, an interrogation range of 98 nm, and an interrogation resolution of 5 pm. For proof-of-performance, the interrogation system was used to monitor the vibrational responses of a cantilever plate under impact loading and the measurement of vibration modes up to 6.785 kHz was demonstrated. Finally, the slamming experiments were carried out with six multiplexed FBG strain sensors mounted on a flexible plate. The dynamic strain measurement of the plate during the slamming event was successfully demonstrated by using the high-speed FBG interrogation system.

An edge-filter FBG interrogation approach based on tunable Fabry-Perot filter for strain measurement of planetary gearbox

• An improved TFP-based edge-filter FBG interrogation approach is proposed. • A feedback channel is adopted to control the creep of the piezoelectric actuator. • The TFP is driven by both the coarse voltage and fine voltage. • The FBG interrogation approach is applied to the strain measurement of ring gear. Fiber Bragg gratings (FBGs), with smaller size, are suitable to measure the tooth root strain of ring gear which is an ideal signal for fault diagnosis of planetary gearboxes. In order to identify the fault features in the strain signal, a high-speed and high-resolution FBG interrogator is needed. Compared with wavelength-scanning interrogation approach, the edge-filter approach can achieve such a performance more easily and economically. However, conventional edge-filter approaches based on fixed edge filters have low applicability. The adoption of a tunable Fabry-Perot filter (TFP) increases the flexibility of the edge-filter system, but the creep characteristics of the piezoelectric actuator in TFP may cause interrogation errors. This paper proposes an improved TFP-based edge-filter FBG interrogation approach with creep control ability. First, the interrogation principle of the proposed approach is analyzed by comparing with the conventional edge-filter approaches. The creep control method is presented by employing a feedback channel and two kinds of TFP driving voltage. Next, a FBG interrogator based on the proposed approach is established. The creep control performance, static interrogation performance and dynamic interrogation performance are all tested. The results show that the interrogator has the wavelength repeatability of ~ 0.1 pm at 1 kHz and ~ 0.2 pm at 5 kHz respectively. Finally, the proposed edge-filter approach is applied to measure the tooth root strain of ring gear in a planetary gearbox. The measured strain signal has a high quality, and the tooth fault feature can be identified obviously.

An Integrated Design of Ultra-High-Speed FBG Interrogation System-Based on FDML Laser

An integrated design of ultra-high-speed Fiber Bragg Grating (FBG) interrogation system is illustrated and experimentally demonstrated. A Fourier Domain Mode Locked (FDML) laser is employed as a high-speed wavelength-swept light source. The use of the Mach-Zehnder interferometer (MZI) and the single-peak filter (SPF) overcomes the nonlinear effect and the scanning wavelength drift problem. A signal acquisition and peak-searching method based on the hardware is designed, allowing a real-time and multichannel interrogation. The controlled experiment has verified the feasibilities of the MZI and the SPF. Dynamic and static FBG experiments show that the integrated system has an accuracy of 0.02 nm, a stability of 0.006 nm, a linearity of 0.9994 and a 40 kHz demodulation frequency with 4 simultaneous interrogation channels.

Silicon Waveguide Integrated with Germanium Photodetector for a Photonic-Integrated FBG Interrogator.

We report a vertically coupled germanium (Ge) waveguide detector integrated on silicon-on-insulator waveguides and an optimized device structure through the analysis of the optical field distribution and absorption efficiency of the device. The photodetector we designed is manufactured by IMEC, and the tests show that the device has good performance. This study theoretically and experimentally explains the structure of Ge PIN and the effect of the photodetector (PD) waveguide parameters on the performance of the device. Simulation and optimization of waveguide detectors with different structures are carried out. The device’s structure, quantum efficiency, spectral response, response current, changes with incident light strength, and dark current of PIN-type Ge waveguide detector are calculated. The test results show that approximately 90% of the light is absorbed by a Ge waveguide with 20 μm Ge length and 500 nm Ge thickness. The quantum efficiency of the PD can reach 90.63%. Under the reverse bias of 1 V, 2 V and 3 V, the detector’s average responsiveness in C-band reached 1.02 A/W, 1.09 A/W and 1.16 A/W and the response time is 200 ns. The dark current is only 3.7 nA at the reverse bias voltage of −1 V. The proposed silicon-based Ge PIN PD is beneficial to the integration of the detector array for photonic integrated arrayed waveguide grating (AWG)-based fiber Bragg grating (FBG) interrogators.

Wideband Fiber Bragg Grating Accelerometer Suitable for Health Monitoring of Electrical Machines

A fiber Bragg grating (FBG) accelerometer suitable for health monitoring of electrical machines has been proposed. A polymer composite material is used to design the cantilever of the accelerometer. Thus it is ensured that there are no conducting parts in the accelerometer. Detailed finite element analysis of the cantilever is done to optimize the sensitivity and the resonant frequency. The sensitivity is significantly improved compared to the conventional FBG accelerometer having similar bandwidth of operation. Two different configurations, one having sensitivity 14.4 pm/g with resonant frequency 444 Hz, and the other having sensitivity 7.5 pm/g with resonant frequency 940 Hz have been designed and fabricated. The wide band optical accelerometers will enable monitoring of vibration modes of critical components of electrical machines and more importantly using commercially available standard FBG interrogators, instead of using any complex interrogation techniques. A good match between the simulation and experimental results was observed.

Fast Demodulation of Fiber Bragg Grating Wavelength From Low-Resolution Spectral Measurements Using Buneman Frequency Estimation

This paper presents a fast demodulation algorithm to determine fiber Bragg grating (FBG) from spectra acquired by low wavelength resolution spectrometers. A modified and improved Buneman frequency estimation algorithm is employed in tracking FBG wavelength and eliminating FBG wavelength discontinuities in real-time wavelength demodulation calculations. The FBG demodulation algorithm was tested using a 512-element detector array spectrometer with a 156 pm/element resolution. At a 2-kHz spectrum acquisition rate, the demodulation algorithm achieved a 0.048 pm Bragg wavelength resolution. The computational efficiency and robustness of the new algorithm was compared with a Gaussian FBG peak fitting method. Our method provides a real-time demodulation scheme to track FBG wavelength using low-resolution spectrometers or tunable lasers. It has the potential to significantly reduce the cost of FBG interrogation systems that are employed in high-sensitivity and high-speed sensing applications.

High-Speed FBG Interrogation With Electro-Optically Tunable Sagnac Loops

We report high-speed Fiber Bragg Grating (FBG) interrogation based on fiber interferometry. An electro-optical modulator, used as a tunable retarder, is implemented in a fiber Sagnac loop, enabling fast tuning of the interference fringes. Applying an active modulation scheme, the wavelength of the FBG can be retrieved without any requirement of the spectral position of the FBG with respect to the fringes, with no need of tunable sources or spectral analysis. We report experimental results of wavelength measurements with a bandwidth of 100 kHz (only limited by our data acquisition rate) and a dynamic wavelength resolution of σ = 3.7 fm/√Hz (corresponding to a strain resolution of 3.1 nϵ/√Hz). We also show that the system can track a continuous tunable laser sweep of 40 nm along the C-band. The technique is polarization-independent and allows wavelength division multiplexing schemes using only one interferometer for all the sensing points. The system is fully based on low-cost off-the-shelf fiber-optical components, and the free-spectral-range of the interferometer can be easily modified by just changing the length of the polarization-maintaining fiber segment.

Low-Cost Interrogation of Long-Distance and Multipoint FBG Sensor Using Incoherent-FMCW Optical Ranging System

More than a decade, multipoint sensor systems involving fiber Bragg gratings (FBGs) have been widely used in various implementations, especially the structural health monitoring system (SHMS). In this paper, a three-points FBG sensor was successfully analyzed by combination of incoherent frequency modulated continuous wave (I-FMCW) optical ranging system and a vertical-cavity surface-emitting laser (VCSEL) as a wavelength-tunable laser source. This system is not only capable of reading out the FBG sensors, but also identifying the location of the installed FBGs. The I-FMCW can be conducted by intensity modulation of a low-cost commercial VCSEL. Furthermore, the wavelength of the VCSEL can also be tuned by the injection current change to match the Bragg wavelength of FBGs. The detected signal from the FBGs and the reference signal from the signal generator are electrically mixed to obtain the beat frequency. A thermo-electric controller (TEC) controls temperature of one of the FBGs in the range of 25° C ~ 45° C, and contrary the others are kept at room temperature. As well as measuring temperature, one of the FBGs is strained in the range of $369~\mu \varepsilon \sim ~2137~\mu \varepsilon $ . The measurement result shows that the system can clearly distinguish all FBGs installed in totally 6.6 km-long optical fiber. In addition, the Bragg wavelength shift of the FBGs according to temperature as well as strain changes was successfully measured. The system can be scaled up more than three FBGs and extended scale area, and is a candidate as a low-cost and simple FBG interrogator for a long-range SHMS.

Investigation of Low-Cost Interrogation Technique Based on Modulated Distributed Feedback Laser

In this paper a new low-cost fiber Bragg grating (FBG) interrogation technique for static applications is demonstrated. The idea is based on the multiplication between the FBG reflectivity profile and the emission of a sawtooth modulated distributed feedback (DFB) laser source. The variation of the FBG wavelength generates a modification in the shape, in time domain, of the reflected light signal. The concept has been proven, using simulation, to cover FBG wavelength shift up to 1.7 nm. Experimental validation was performed for FBG temperature ranging from 6 °C to 50 °C (equivalently 440 pm) with a modulation frequency of 1 kHz. Sensitivity higher than 3.5 mV/pm is achieved in all circumstances with an estimated resolution of 8 pm.

Suppression of sweeping fluctuation of Fabry-Perot filter in fiber Bragg grating interrogation using PSO-based self-adaptive sampling

Random fluctuations of the nonlinearity curves of Fabry–Perot (F–P) tunable filters cause instabilities in fiber Bragg grating (FBG) interrogation systems. FBG sensors are widely used in structural health monitoring (SHM), and suppressing random fluctuations can provide engineers with more precise information on the conditions of structures. While F–P etalon and acetylene (C2H2) are most commonly used to calibrate the wavelength of the F-P filter, in medium–high sweeping situations, they suffer from drastic fluctuations in the interrogation results. In addition, these two methods require more interrogation components and render the interrogation system more complex and expensive. This study investigates the fluctuation of interrogation errors caused by sampling frequency. A novel self-adaptive sampling approach based on particle swarm optimization (PSO) is proposed to automatically search the optimal sampling frequency with the smallest fluctuation. The experimental results show that the proposed approach successfully detected the optimal sampling frequency. The drastic fluctuations of micro strains caused by high sampling frequency are avoided completely, and the stability of interrogation system is guaranteed. The proposed approach is thus helpful for suppressing sweeping fluctuations by appropriately choosing the optimal sampling frequency.

Low-cost multiple FBG interrogation technique for static applications.

In this Letter, a low-cost interrogation technique for multiple fiber Bragg gratings (FBGs) is demonstrated. The design utilizes the major advantage of the distributed feedback lasers, which is tuning their emitted wavelength by current modulation and temperature variation. The sensitivity of this method and the influence of optical losses on the measurement have been carefully studied, and an accuracy of 5 pm is achieved.

Fast peak-tracking method for FBG reflection spectrum and nonlinear error compensation

A universal method is proposed for fast tracking the peak wavelength of a fiber Bragg grating (FBG) reflection spectrum, which is the essential procedure of most FBG interrogators. Assuming the FBG reflection spectrum is sampled uniformly, we decompose the Moore–Penrose of the coefficient matrix into a formula that only contains a division and a dozen additions and multiplications; thereby, the cost of computation can be greatly reduced. Based on the analysis of the characteristics of the error introduced by the spectrum’s nonlinearity, we found and defined an error associated with the distance between the sampled peak and actual peak, and the compensating method for the error is also given. After compensation, the proposed method can speed up the tracking process 10 times or more without sacrificing the accuracy. An economical FBG interrogator with a commercial field programmable gate array (FPGA) and a microspectrometer was built up to verify the method, which was able to achieve a measurement frequency of 17 kHz.