Fiber Bragg Grating Interrogation Research Articles

Large bandwidth array waveguide grating design for FBG interrogation system

The array waveguide grating (AWG) demodulation method has been widely used in recent years. However, the resolution and total measurement range of AWG-based Fiber Bragg Grating (FBG) interrogation systems are limited by the output characteristics of AWGs. We designed and fabricated a multi-channel SiO2-based AWG as a key component of FBG Interrogation. To increase the dynamic range of demodulation, a multimode interference coupler (MMI) structure is introduced in the middle of the input waveguide and the input slab waveguide. From the simulation results, the 3-dB bandwidth of the AWG is increased from 1.04 nm to 1.86 nm. We test the performance of the interrogation system based on this AWG. The results demonstrate that the system can achieve continuous demodulation in the C-band, with an interrogation accuracy better than 20.22 pm and a wavelength resolution of 1 pm.

FBG Interrogator Using a Dispersive Waveguide Chip and a CMOS Camera.

Optical sensors using fiber Bragg gratings (FBGs) have become an alternative to traditional electronic sensors thanks to their immunity against electromagnetic interference, their applicability in harsh environments, and other advantages. However, the complexity and high cost of the FBG interrogation systems pose a challenge for the wide deployment of such sensors. Herein, we present a clean and cost-effective method for interrogating an FBG temperature sensor using a micro-chip called the waveguide spectral lens (WSL) and a standard CMOS camera. This interrogation system can project the FBG transmission spectrum onto the camera without any free-space optical components. Based on this system, an FBG temperature sensor is developed, and the results show good agreement with a commercial optical spectrum analyzer (OSA), with the respective wavelength-temperature sensitivity measured as 6.33 pm/°C for the WSL camera system and 6.32 pm/°C for the commercial OSA. Direct data processing on the WSL camera system translates this sensitivity to 0.44 μm/°C in relation to the absolute spatial shift of the FBG spectra on the camera. Furthermore, a deep neural network is developed to train the spectral dataset, achieving a temperature resolution of 0.1 °C from 60 °C to 120 °C, while direct processing on the valley/dark line detection yields a resolution of 7.84 °C. The proposed hardware and the data processing method may lead to the development of a compact, practical, and low-cost FBG interrogator.

The development of multiplexing capability for reflective matched fiber bragg gratings interrogation technique and its application in real-time micro cracks detection

Abstract Cracks detection in engineering structures is pivotal for ensuring structural reliability and preventing catastrophic failures. In this article we developed multiplexing capability of reflective matched fiber Bragg Gratings (RM-FBGs) interrogation technique and utilized it for crack detection in an assembly aluminum plates. Firstly, the proposed multiplexing capability is analyzed by applying axial strain on an array of five FBGs pasted on five separate aluminum assemblies. The strain in any FBG due to localized micro-crack resulted in variation its output itself by ascertaining no effect in the outputs of other FBGs, certifying the multiplexing capability of RM-FBGs scheme. The developed RM-FBGs scheme is practically applied micro-cracks detection in a V-shape crack produced in assembly of aluminum plates. The crack is monitored by an array of three multiplexed FBGs epoxied to the plate’s junction at three different positions P1, P2 and P3. Where P1 is at beginning, P2 is at middle and P3 is at end of the crack. Sensitivities of the FBGs at points P1, P2 and P3 were found to be 0.09 ± 0.0001 V μm−1, 0.10 ± 0.007 V μm−1 and 1.08 ± 0.09 V μm−1, respectively and resolutions were found to be 0.22 μm, 0.20 μm and 0.02 μm, respectively. Variation in gauge length of the optical fiber from 100 mm to 200 mm resulted in 76.36% increase in crack width sensing range but at the cost of compromise in sensitivity and resolution. The proposed multiplexed RM-FBG can be deployed for early micro-cracks detection in metallic and non-metallic structures with sub-micrometers resolution.

Length-extended 3D shape sensor using wavelength/space-division multiplexing grating arrays in a multicore fiber.

Limited by the multiplexing number of fiber Bragg grating (FBG), further improvement in the length of 3D shape sensing based on FBG technology is challenging. In this Letter, a wavelength-division and space-division multiplexing multicore fiber grating method is proposed, which extends the sensing length. Employing the femtosecond-laser point-by-point technology, we inscribed WDM grating arrays in six outer cores of a seven-core fiber, respectively. Three cores were utilized as a segment for shape sensing, and two such segments were offset by a specific length and combined to form a shape sensor. Utilizing an FBG interrogator, the proposed shape sensor achieved 2D and 3D shape sensing at a length of 967 mm and effectively mitigated the effects of temperature variations. In experiments, maximum shape reconstruction errors per unit lengths are 1.89%, 2.72%, and 1.47% for 2D shape, 3D shape, and an arbitrary shape under variable temperature conditions, respectively. The proposed method holds promise for further extending the shape sensing length by utilizing multicore fibers or fiber clusters containing more cores.

Fiber Bragg Grating Interrogation System Combining Wavelength-Swept Laser and Convolutional Neural Network

Fiber Bragg Grating Interrogation System Combining Wavelength-Swept Laser and Convolutional Neural Network

The Performance of a Passive Autoranging Method for a Photonic Current Transducer.

This paper reports on the testing and evaluation of a passive autoranging (AR) method designed to dynamically extend the measurement range of a photonic current transducer (PCT) to pave the way toward a realization of a combined metering- and protection-class current sensor. The PCT utilizes a current transformer (CT), a piezoelectric transducer (PZT), and a fiber Bragg grating (FBG) to enable current measurement at multiple points in an electrical power network whereby multiple sensors are deployed and interrogated serially using a single optical fiber. The autoranging technique relies on incorporating static MOSFET switches to instantaneously short individual serially connected CT burdens in response to a measured current magnitude exceeding pre-set thresholds. The AR circuit switching events produce distinctive signal features that are used by the proposed switching algorithm to apply appropriate scaling factors to reconstruct the measured current from the optical signal. It is shown through laboratory experiments that the AR circuit correctly reacts to pre-set burden current thresholds of 130% of the nominal value and 22 times the nominal value, signifying its "metering" and "protection" range boundaries. The circuit reaction time is below 4 ms, rendering it suitable for standard power system protection purposes. Moreover, the operation of the AR circuit is demonstrated for burden currents of up to 100 A for over 1 s, satisfying a test procedure for the secondary CT circuit, as required by some power system operators. It is demonstrated that the proposed switching algorithm allows for a correct reconstruction of the burden currents from the optical signal acquired by the FBG interrogator, offering the potential to realize a dual-class optical current sensor.

Monolithic integrated chip of AWG and PD for an FBG interrogation system

To advance the development of a compact and highly integrated fiber Bragg grating (FBG) interrogation system, to the best of our knowledge, this paper is the first to present the design and fabrication of a monolithic integration chip based on silicon-on-insulator (SOI), which is specifically intended for application in fiber grating sensing interrogation systems. By considering the impact of coupling structure dimensions on coupling efficiency as well as the effect of the photodetector (PD) parameters on the optical absorption efficiency of the device, we refine the structure of the monolithic integrated chip for arrayed waveguide grating (AWG) and PD. The test results reveal that the coupling loss between AWG and PD is -2.4 dB. The monolithic integrated interrogation chip achieves an interrogation accuracy of approximately 6.79 pm within a dynamic range of 1.56 nm, accompanied by a wavelength resolution of 1 pm. This exceptional performance highlights the potential of the monolithic integrated chip to enhance the integration of AWG-based fiber grating interrogation systems.

Study of hybrid integrated PLC-AWG chip for FBG demodulation

Fiber Bragg grating (FBG) sensors have been widely used in various fields. To develop a miniaturized, high-precision, and high-response FBG interrogation system, arrayed waveguide grating (AWG) has been widely used as a key device. In this paper, a 40-channel arrayed waveguide grating based on silica planar lightwave circuits (PLCs) is designed fabricated, coupled to a photodetector array using hybrid integration techniques. The hybrid-integrated demodulation chip enables precise demodulation of the FBG sensor wavelength in the C-band. The test results show that the AWG has a good transmission spectrum with a 3-dB bandwidth of 1.98 nm, insertion loss of about 3.5–4.1 dB, and adjacent channel crosstalk of about −7.84 dB. The established FBG interrogation system can realize continuous interrogation in the dynamic range of 1514.6–1578.6 nm, the wavelength resolution is 1 pm and the demodulation precision is 2.11 pm in the dynamic range of 1555.3–1556.1 nm.

Performance analysis of PLC-based 32-channel arrayed waveguide grating used for FBG interrogator

The Fiber Bragg grating (FBG) is an optical fiber component that reflects or diffracts optical signals of specific wavelengths through periodic refractive index changes inside it. Currently, the use of arrayed waveguide grating (AWG) to interrogate such optical signals is a popular research topic at present. Using the planar lightwave circuit, we designed and fabricated a 32-channel AWG. The AWG has an insertion loss of less than 4 dB, a 3 dB bandwidth of about 1.76 nm, and a crosstalk of non-adjacent channels of less than −28 dB. Additionally, we integrated the AWG with a photodetector (PD) array and built a corresponding wavelength interrogation system, which was tested using three different interrogation methods. The test results indicate that when using the linear fitting, this AWG provides the best results using the dual-channel edge filtering method with an interrogation range of 1.6 nm and an interrogation accuracy of 18.9–20.1 pm. The interrogation accuracies of the relative intensity method are 9.4 pm and 3.7 pm when Gaussian and Lorentzian fittings are used.

Ameliorated 3 × 3 coupler-based demodulation algorithm using iteratively reweighted ellipse specific fitting.

Passive demodulation scheme using 3 × 3 coupler has been widely used in phase-sensitive optical time-domain reflectometry (φ-OTDR), interrogation of fiber Bragg gratings or fiber optic interferometric sensors, and sensor multiplexing. However, the asymmetry of the 3 × 3 coupler in real applications affects the demodulation performance seriously. We proposed an ameliorated 3 × 3 coupler-based demodulation algorithm using iteratively reweighted ellipse specific fitting (IRESF) to overcome the drawback. IRESF combines iterative reweight technology with ellipse specific fitting, which decreases the weights of high noise points and always outputs ellipse solutions. Any two output signals from the 3 × 3 coupler-based interferometer are fitted by the IRESF and then corrected as a pair of quadrature signals. The stability of the fitting parameters is utilized to resolve the failures of IRESF under small signals. A real-time 1/4 ellipse arc judging module is designed, if the Lissajous figure is larger than 1/4 ellipse arc, IRESF is executed to offer ellipse correction parameters. Otherwise, the fixed parameters preset in the algorithm are used. The fixed parameters are mean values of the fitting parameters of IRESF under a large stimulus. The desired phase signal is finally extracted from the corrected quadrature signals. Experimental results show that the ameliorated algorithm does not require strict symmetry of the 3 × 3 coupler and can work under small signals. The noise floor of the proposed algorithm is -112 dB re rad/√Hz and the demodulated amplitude is 23.15 dB (14.37 rad) at 1 kHz when THD is 0.0488%. Moreover, the response linearity is as high as 99.999%. Compared to the algorithm using direct least squares, the proposed demodulation algorithm is more robust and precise, which has broad application prospects.

On-Chip Sub-Picometer Continuous Wavelength Fiber-Bragg-Grating Interrogator

Miniaturized fiber-Bragg-grating (FBG) interrogators are of interest for applications in the areas where weight and size controlling is important, e.g., airplanes and aerospace or in-situ monitoring. An ultra-compact high-precision on-chip interrogator is proposed based on a tailored arrayed waveguide grating (AWG) on a silicon-on-insulator (SOI) platform. The on-chip interrogator enables continuous wavelength interrogation from 1 544 nm to 1 568 nm with the wavelength accuracy of less than 1 pm [the root-mean-square error (RMSE) is 0.73 pm] over the whole wavelength range. The chip loss is less than 5 dB. The 1 × 16 AWG is optimized to achieve a large bandwidth and a low noise level at each channel, and the FBG reflection peaks can be detected by multiple output channels of the AWG. The fabricated AWG is utilized to interrogate FBG sensors through the center of gravity (CoG) algorithm. The validation of an on-chip FBG interrogator that works with sub-picometer wavelength accuracy in a broad wavelength range shows large potential for applications in miniaturized fiber optic sensing systems.

Intelligent Vibration Monitoring System for Smart Industry Utilizing Optical Fiber Sensor Combined with Machine Learning

In this paper, we proposed and experimentally demonstrated the association of a fiber Bragg Grating (FBG) sensing system with You Only Look Once V7 (YOLO V7) to identify the vibration signal of a faulty machine. In the experiment, the YOLO V7 network architecture consists of a backbone, three detection heads (Headx3), a path aggregation network (PAN), and a feature pyramid network (FPN). The proposed architecture has an FBG sensor and the FBG interrogator employed for collecting sensing vibration signals or vibration data when degradation or fault occurs. An FBG interrogator collects vibration data independently, and then the YOLO V7 object detection algorithm is the recognition architecture of the vibration pattern of the signal. Thus, the proposed vibration recognition or detection is an assurance for detecting vibration signals that can support monitoring the machine’s health. Moreover, this research is promising for ensuring a high accuracy detection of faulty signals rate in industrial equipment monitoring and offers a robust system, resulting in remarkable accuracy with an overall model accuracy of 99.7%. The result shows that the model can identify the faulty signal more accurately and effectively detect the faulty vibration signal using the detection algorithm.

Time- and wavelength-division multiplexed interrogation of fiber Bragg gratings based on dual-wavelength differential detection

The dual-wavelength differential detection technique is used to interrogate fiber Bragg grating sensors. A directly modulated distributed-feedback laser array acts as a multi-wavelength, frequency-scanning pulsed light source. Pulse pairs with frequency drift and identical intensities are generated at three wavelengths. By interrogating 10 serially connected Bragg gratings at the three wavelengths using these pulse trains, we realize a time- and wavelength-division multiplexed temperature sensor system based on dual-wavelength differential detection using a simple setup.

SOI-based 12 × 12 arrayed waveguide grating for fiber Bragg grating interrogation system

Fiber Bragg grating (FBG) interrogator is a scientific instrument that can indirectly monitor physical quantities such as temperature and pressure by detecting the center wavelength of FBG. The arrayed waveguide grating (AWG), one of the fundamental parts of the FBG interrogation system, is essential for driving the downsizing of the system and high-speed rate. In this paper, a 12-channel silicon on insulator based AWG with an average 3 dB bandwidth of 2.23 , insertion loss of less than 5 dB, and crosstalk of −16.64 dB is designed and fabricated for a photonic integrated FBG interrogation system. The wavelength resolution of the FBG interrogation system of this AWG is 8 , and the wavelength accuracy measured in the dynamic range of 0.8 is 10.59 . This AWG can be well used in FBG interrogation systems.

Multitasking FBG sensors for condition monitoring with a wideband (DC-MHz) interrogation system.

We demonstrate diagnosis of several machine-condition failures using wide-frequency-band interrogation of fiber Bragg grating (FBG) sensors. In collaboration with Israel's national water company Mekorot Ltd., a scaled-down version of a semi-submerged pumping system was constructed. By monitoring broadband signals from DC to ultrasound (>M H z), at different points of the engine and the submersed pump, the system was able to diagnose incipient cavitation, faulty bearings, and submerged dynamic water-level measurements. In addition, a metal embedded FBG sensor was investigated, revealing the potential of using FBGs in applications where bonding is problematic such as bearing housing. These results prove that wideband data acquisition, together with advanced analytics, could open a variety of new applications in the fields of structural health and machine-condition monitoring.

Augmentation of Sensitivity of FBG Sensor Using Microwave Photonics and Nonlinear Effect

A simple and unique scheme of high sensitive Fiber Bragg Grating(FBG) interrogation system based on microwave signal measurement utilizing the concept of microwave photonics and nonlinear Four wave mixing(FWM), is proposed. The concept of FWM process is combined with microwave photonics to enhance the sensitivity of FBG sensor. The strain induced wavelength shift of the sensing FBG is translated to higher wavelength shift of the corresponding higher order FWM signals on either side of FBG sensor wavelength in opposite directions. Thus the wavelength separation between the FWM signals increases with small increase in strain. Utilizing the FWM effect and microwave photonics technique, the microwave phase difference is altered, which ultimately is converted to significant change in RF intensity. A novel mathematical relationship relating FWM, applied strain and the microwave power of the proposed scheme is established. A high strain sensitivity of 36.52 μV/με (26.67 μW/με) is attained using the proposed FWM and microwave photonics technique against initial sensitivity of 4.9 μV/με (0.48 μW/με) using only microwave photonics technique. The proposed scheme automatically compensates the temperature variations and eliminates the requirement of high dispersive component to enhance the sensitivity.

Ultracompact Microinterferometer-Based Fiber Bragg Grating Interrogator on a Silicon Chip

We report an interferometer-based multiplexed fiber Bragg grating (FBG) interrogator using silicon photonic technology. The photonic-integrated system includes the grating coupler, active and passive interferometers, interferometers, a 12-channel wavelength-division-multiplexing (WDM) filter, and Ge photodiodes, all integrated on a 6x8 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> silicon chip. The system also includes optical and electric interfaces to a printed board, which is connected to a real-time electronic board that actively performs the phase demodulation processing using a multitone mixing (MTM) technique. The device with active demodulation, which uses thermally-based phase shifters, features a noise figure of σ = 0.13 pm at a bandwidth of 700 Hz, which corresponds to a dynamic spectral resolution of 4.9 fm/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . On the other hand, the passive version of the system, based on a 90º-hybrid coupler, features a noise figure of σ = 2.55 pm at a bandwidth of 10 kHz, also showing successful detection of a 42 kHz signal when setting the bandwidth to 50 kHz. These results demonstrate the advantage of integrated photonics, which allows the integration of several systems with different demodulation schemes in the same chip and guarantees easy scalability to a higher number of ports without increasing the dimensions or the cost.

Compact high-resolution FBG strain interrogator based on laser-written 3D scattering structure in flat optical fiber

We demonstrate a fiber Bragg grating (FBG) strain interrogator based on a scattering medium to generate stable and deterministic speckle patterns, calibrated with applied strain, which are highly dependent on the FBG back-reflection spectral components. The strong wavelength-dependency of speckle patterns was previously used for high resolution wavemeters where scattering effectively folds the optical path, but instability makes practical realization of such devices difficult. Here, a new approach is demonstrated by utilizing femtosecond laser-written scatterers inside flat optical fiber, to enhance mechanical stability. By inscribing 15 planes of pseudo-randomized nanovoids (714 times 500 voids per plane) as a 3D array in a 1 times 0.7 times 0.16 mm volume, the intrinsic stability and compactness of the device was improved. Operating as a wavemeter, it remained stable for at least 60 h with 45 pm resolution over the wavelength range of 1040–1056 nm. As a reflection mode FBG interrogator, after calibrating speckle patterns by applying tensile strain to the FBG, the device is capable of detecting microstrain changes in the range of 0–200 mu epsilon with a standard error of 4 mu epsilon, limited by the translation stage step size. All these characteristics make it an interesting technology for filling the niche of low-cost, high-resolution wavemeters and interrogators which offer the best available trade-off between resolution, compactness, price and stability.

SOI-based 15-channel arrayed waveguide grating design for fiber Bragg grating interrogator

Arrayed waveguide grating (AWG) is the core component of the photonic integrated interrogation system. Its spectral characteristics will affect the wavelength interrogation performance of the photonic integrated fiber grating interrogation system. We designed, simulated and fabricated a 15-channel 400 GHz AWG based on silicon on insulator (SOI) substrate. The test result shows that the AWG the insertion loss is about 2.5–5 dB, the crosstalk is about −20 dB and the 3 dB bandwidth is about 2 nm. A wavelength interrogation system based on this AWG is established. According to the test results, the system has a wavelength resolution of 4 pm and an interrogation accuracy of 31.4 pm in a dynamic range of 0.8 nm. Our test result confirms that the AWG can be used as a spectral device in the fiber Bragg grating (FBG) interrogation system.

Fiber Bragg Grating Interrogation System Based on the Laser Wavelength Modulation in a Digital Supermode -Distributed Bragg Reflector Laser

In this paper, we propose a Fiber Bragg Grating (FBG) interrogation system based on the modulation of the laser wavelength. Key component of the interrogation scheme is in a Digital Supermode -Distributed Bragg Reflector (DS-DBR) Laser. The DS-DBR features a large spectral range and a fast modulation capability in a low cost device. When the laser spans over the FBG spectral range, the power of the reflected signal changes over time according the FBG spectral shape. The information on the Bragg wavelength can be extracted from the modulated signal reflected by the FBG in the time domain. We implemented the proposed interrogation scheme by discrete components demonstrating its capability to retrieve the Bragg wavelength of one or more FBGs. The interrogation system features an intrinsic tradeoff between spectral resolution and interrogation speed. A Bragg wavelength resolution of 1.5 pm has been obtained using 1 kHz modulating frequency. The proposed interrogation system is particular suited in applications in which an adaptive interrogation strategy can be conveniently adopted.