Fiber Bragg Grating Interrogation Research Articles

Development and Testing of an Integrated Rotating Dynamometer Based on Fiber Bragg Grating for Four-Component Cutting Force Measurement.

Cutting force measurement is of great importance in machining processes. Hence, various methods of measuring the cutting force have been proposed by many researchers. In this work, a novel integrated rotating dynamometer based on fiber Bragg grating (FBG) was designed, constructed, and tested to measure four-component cutting force. The dynamometer consists of FBGs that are pasted on the newly designed elastic structure which is then mounted on the rotating spindle. The elastic structure is designed as two mutual-perpendicular semi-octagonal rings. The signals of the FBGs are transmitted to FBG interrogator via fiber optic rotary joints and optical fiber, and the wavelength values are displayed on a computer. In order to determine the static and dynamic characteristics, many tests have been done. The results show that it is suitable for measuring cutting force.

A Fiber Bragg Grating Interrogation System with Self-Adaption Threshold Peak Detection Algorithm.

A Fiber Bragg Grating (FBG) interrogation system with a self-adaption threshold peak detection algorithm is proposed and experimentally demonstrated in this study. This system is composed of a field programmable gate array (FPGA) and advanced RISC machine (ARM) platform, tunable Fabry–Perot (F–P) filter and optical switch. To improve system resolution, the F–P filter was employed. As this filter is non-linear, this causes the shifting of central wavelengths with the deviation compensated by the parts of the circuit. Time-division multiplexing (TDM) of FBG sensors is achieved by an optical switch, with the system able to realize the combination of 256 FBG sensors. The wavelength scanning speed of 800 Hz can be achieved by a FPGA+ARM platform. In addition, a peak detection algorithm based on a self-adaption threshold is designed and the peak recognition rate is 100%. Experiments with different temperatures were conducted to demonstrate the effectiveness of the system. Four FBG sensors were examined in the thermal chamber without stress. When the temperature changed from 0 °C to 100 °C, the degree of linearity between central wavelengths and temperature was about 0.999 with the temperature sensitivity being 10 pm/°C. The static interrogation precision was able to reach 0.5 pm. Through the comparison of different peak detection algorithms and interrogation approaches, the system was verified to have an optimum comprehensive performance in terms of precision, capacity and speed.

Strain detection and measurement using a matched fibre Bragg grating

We present and describe a method to demodulate the incoming strain signal from sensors based on fibre Bragg gratings (FBGs). This demodulation is achieved by using a reference FBG with similar characteristics to those of the FBG sensor as a band-pass filter to recover the total power in the detected signal from the shift in the Bragg wavelength caused by the strain experienced by the FBG sensor. From the experimental data and the numerical simulation, the system static transfer characteristic curves are obtained and compared. Using this method, a linear measurement range of 227 pm (183 με), along with a precision of 0.8 με and a resolution of 1.6 με were achieved. This simple and inexpensive method is a practical alternative to conventional FBG interrogation units because few optical elements are used in the set-up, and the Bragg wavelength shifts are determined by the changes in the detected optical intensity. These intensity changes are caused by the mechanical axial strain experienced by the FBG sensor.

Multichannel High-Speed Fiber Bragg Grating Interrogation System Utilizing a Field Programmable Gate Array

We demonstrate a multichannel high-speed fiber Bragg grating (FBG) interrogation system utilizing a field programmable gate array (FPGA). A temperature-controlled Fourier domain mode locking laser was operated with a sweep rate of 50.7kHz, sweep bands of 35nm, and an output power of 0.91mW. This laser enabled an FBG measurement speed of up to 9.9 s. Real-time processing could be achieved by incorporating a high-speed data acquisition system (DAQ) mounted on an analog to digital converter and an FPGA. The DAQ performed in-line processing with four 14-bit analog channels and a sampling frequency of 250MHz. The internals of the FPGA were implemented parallel processing for high-speed FBG detection to allow for the simultaneous measurement of the multiplexed FBG in three channels. We demonstrated that this system is capable of high-speed vibration measurements above several tens ofkHz with a multichannel.

High-Speed Interrogation for Large-Scale Fiber Bragg Grating Sensing.

A high-speed interrogation scheme for large-scale fiber Bragg grating (FBG) sensing arrays is presented. This technique employs parallel computing and pipeline control to modulate incident light and demodulate the reflected sensing signal. One Electro-optic modulator (EOM) and one semiconductor optical amplifier (SOA) were used to generate a phase delay to filter reflected spectrum form multiple candidate FBGs with the same optical path difference (OPD). Experimental results showed that the fastest interrogation delay time for the proposed method was only about 27.2 us for a single FBG interrogation, and the system scanning period was only limited by the optical transmission delay in the sensing fiber owing to the multiple simultaneous central wavelength calculations. Furthermore, the proposed FPGA-based technique had a verified FBG wavelength demodulation stability of ±1 pm without average processing.

Evaluation of fiber Bragg grating sensor interrogation using InGaAs linear detector arrays and Gaussian approximation on embedded hardware.

Fiber Bragg Grating (FBG) sensors have become popular for applications related to structural health monitoring, biomedical engineering, and robotics. However, for successful large scale adoption, FBG interrogation systems are as important as sensor characteristics. Apart from accuracy, the required number of FBG sensors per fiber and the distance between the device in which the sensors are used and the interrogation system also influence the selection of the interrogation technique. For several measurement devices developed for applications in biomedical engineering and robotics, only a few sensors per fiber are required and the device is close to the interrogation system. For these applications, interrogation systems based on InGaAs linear detector arrays provide a good choice. However, their resolution is dependent on the algorithms used for curve fitting. In this work, a detailed analysis of the choice of algorithm using the Gaussian approximation for the FBG spectrum and the number of pixels used for curve fitting on the errors is provided. The points where the maximum errors occur have been identified. All comparisons for wavelength shift detection have been made against another interrogation system based on the tunable swept laser. It has been shown that maximum errors occur when the wavelength shift is such that one new pixel is included for curve fitting. It has also been shown that an algorithm with lower computation cost compared to the more popular methods using iterative non-linear least squares estimation can be used without leading to the loss of accuracy. The algorithm has been implemented on embedded hardware, and a speed-up of approximately six times has been observed.

Pulse Compressed Time Domain Multiplexed Fiber Bragg Grating Sensor: A Comparative Study

This paper experimentally demonstrates the feasibility of improving the sensing performance of fiber Bragg grating (FBG)-based point sensors by deploying two pulse coding techniques, simplex coding (SSC), and Golay complementary codes (GCC). The two techniques are separately combined with the conventional time-domain multiplexed FBG (TDM-FBG) interrogation sensor, and their results are compared with respect to the conventional single pulse interrogation technique. Consistently with the theory, for both the coding techniques, we have confirmed that the signal-to-noise ratio improved proportionally to the code length. In details, for both the techniques, the signal-to-noise improvement ratio of two FBGs located after around 16.35 km of fiber has significantly increased up to about 6 dB. Furthermore, the decoded signals from both techniques conserve their original spatial properties, confirming the successfulness of Hadamard transform and Golay auto-correlation calculations. Our experimental analysis confirms the simplex codes with their significant noise reduction ability, which is translated into well formed and accurate mean amplitude calculations. Moreover, the analysis of TDM-FBG-based GCCs have confirmed the unique properties of the auto-correlation process to increase the FBG signals up to around 128 times compared with the single pulse case.

Current Status and Future Trends of Photonic-Integrated FBG Interrogators

In this paper, we present an overview of the current efforts toward integration on chip of fiber Bragg grating (FBG) sensor interrogators. Different photonic-integration platforms are discussed, including monolithic planar lightwave circuit technology, silicon on insulator (SOI), indium phosphide, and gallium arsenide material platforms. Furthermore, various possible techniques for wavelength metering and methods for FBG multiplexing are discussed and compared in terms of resolution, dynamic performance, multiplexing capabilities, and reliability. The use of linear filters, array waveguide gratings (AWG) as multiple linear filters, and AWG-based centroid signal processing techniques are presented as well as interrogation techniques based on tunable microring resonators and Mach-Zehnder interferometers for phase sensitive detection. FBG sensor interrogation based on SOI platform using active and passive phase sensitive detection is also described, demonstrating specifically the potential of passive phase demodulation for high-speed dynamic strain measurements. This paper finally presents the challenges and perspectives of photonic integration to address the increasing requirements of several industrial applications.

Integrated nano-opto-electro-mechanical sensor for spectrometry and nanometrology

Spectrometry is widely used for the characterization of materials, tissues, and gases, and the need for size and cost scaling is driving the development of mini and microspectrometers. While nanophotonic devices provide narrowband filtering that can be used for spectrometry, their practical application has been hampered by the difficulty of integrating tuning and read-out structures. Here, a nano-opto-electro-mechanical system is presented where the three functionalities of transduction, actuation, and detection are integrated, resulting in a high-resolution spectrometer with a micrometer-scale footprint. The system consists of an electromechanically tunable double-membrane photonic crystal cavity with an integrated quantum dot photodiode. Using this structure, we demonstrate a resonance modulation spectroscopy technique that provides subpicometer wavelength resolution. We show its application in the measurement of narrow gas absorption lines and in the interrogation of fiber Bragg gratings. We also explore its operation as displacement-to-photocurrent transducer, demonstrating optomechanical displacement sensing with integrated photocurrent read-out.

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.

Wavelength-shifted chirped FBGs for temperature compensated strain measurement

Fiber Bragg grating (FBG) is the most researched and implemented sensor of all the available fiber optic sensors till date. Despite being highly sensitive and accurate, the applications of FBG sensors are limited. The main reasons for their limited applications are the high cost and bulkiness of the interrogation system, optical spectrum analyzer (OSA). In this paper, a wavelength shifted chirped FBG (CFBG) interrogation system is presented. It is an optical intensity based FBG interrogation system which can measure positive/negative strain and temperature changes simultaneously. This system is simple, cost effective, and compact with a strain resolution of around 1με. A chirp bandwidth of 5nm can easily provide a strain measurement range of about ±4000με. Essentially, this sensor design abrogates the need for OSA and makes FBG more suitable for field applications.

Strain monitoring of reinforced concrete with OTDR-based FBG interrogation technique

An experimental study is presented of the application of fiber Bragg grating (FBG) interrogation method based on optical time-domain reflectometery (OTDR) to monitoring strain in bent reinforced concrete beams. The results obtained with the OTDR-based method are shown to agree well with the direct spectral measurements. Strain sensitivity, resolution and measurement range amounted to 0.0028 dB/strain; 30 strain; 4000 strain, correspondingly. Significant differences are observed in surface and inner deformations of the test beams which can be attributed to different mechanical properties of concrete and steel reinforcement. The prospects of using OTDR-based FBG interrogation technique in real-life applications are discussed.

An Advanced Multi-Sensor Acousto-Ultrasonic Structural Health Monitoring System: Development and Aerospace Demonstration.

A key longstanding objective of the Structural Health Monitoring (SHM) research community is to enable the embedment of SHM systems in high value assets like aircraft to provide on-demand damage detection and evaluation. As against traditional non-destructive inspection hardware, embedded SHM systems must be compact, lightweight, low-power and sufficiently robust to survive exposure to severe in-flight operating conditions. Typical Commercial-Off-The-Shelf (COTS) systems can be bulky, costly and are often inflexible in their configuration and/or scalability, which militates against in-service deployment. Advances in electronics have resulted in ever smaller, cheaper and more reliable components that facilitate the development of compact and robust embedded SHM systems, including for Acousto-Ultrasonics (AU), a guided plate-wave inspection modality that has attracted strong interest due mainly to its capacity to furnish wide-area diagnostic coverage with a relatively low sensor density. This article provides a detailed description of the development, testing and demonstration of a new AU interrogation system called the Acousto Ultrasonic Structural health monitoring Array Module+ (AUSAM+). This system provides independent actuation and sensing on four Piezoelectric Wafer Active Sensor (PWAS) elements with further sensing on four Positive Intrinsic Negative (PIN) photodiodes for intensity-based interrogation of Fiber Bragg Gratings (FBG). The paper details the development of a novel piezoelectric excitation amplifier, which, in conjunction with flexible acquisition-system architecture, seamlessly provides electromechanical impedance spectroscopy for PWAS diagnostics over the full instrument bandwidth of 50 KHz–5 MHz. The AUSAM+ functionality is accessed via a simple hardware object providing a myriad of custom software interfaces that can be adapted to suit the specific requirements of each individual application.

Chip-scale demonstration of hybrid III–V/silicon photonic integration for an FBG interrogator

Silicon photonic integration is a means to produce an integrated on-chip fiber Bragg grating (FBG) interrogator. The possibility of integrating the light source, couplers, grating couplers, de-multiplexers, photodetectors (PDs), and other optical elements of the FBG interrogator into one chip may result in game-changing performance advances, considerable energy savings, and significant cost reductions. To the best of our knowledge, this paper is the first to present a hybrid silicon photonic chip based on III–V/silicon-on-insulator photonic integration for an FBG interrogator. The hybrid silicon photonic chip consists of a multiwavelength vertical-cavity surface-emitting laser array and input grating couplers, a multimode interference coupler, an arrayed waveguide grating, output grating couplers, and a PD array. The chip can serve as an FBG interrogator on a chip and offer unprecedented opportunities. With a footprint of 5 mm×3 mm, the proposed hybrid silicon photonic chip achieves an interrogation wavelength resolution of approximately 1 pm and a wavelength accuracy of about ±10 pm. With the measured 1 pm wavelength resolution, the temperature measurement resolution of the proposed chip is approximately 0.1°C. The proposed hybrid silicon photonic chip possesses advantages in terms of cost, manufacturability, miniaturization, and performance. The chip supports applications that require extreme miniaturization down to the level of smart grains.

Performance Optimization Design for a High-Speed Weak FBG Interrogation System Based on DFB Laser.

A performance optimization design for a high-speed fiber Bragg grating (FBG) interrogation system based on a high-speed distributed feedback (DFB) swept laser is proposed. A time-division-multiplexing sensor network with identical weak FBGs is constituted to realize high-capacity sensing. In order to further improve the multiplexing capacity, a waveform repairing algorithm is designed to extend the dynamic demodulation range of FBG sensors. It is based on the fact that the spectrum of an FBG keeps stable over a long period of time. Compared with the pre-collected spectra, the distorted spectra waveform are identified and repaired. Experimental results show that all the identical weak FBGs are distinguished and demodulated at the speed of 100 kHz with a linearity of above 0.99, and the range of dynamic demodulation is extended by 40%.

Compact multichannel high-resolution micro-electro-mechanical systems-based interrogator for Fiber Bragg grating sensing

We propose a novel type of compact high-resolution multichannel micro-electro-mechanical systems (MEMS)-based interrogator, where we replace the linear detector with a digital micromirror device (DMD). The DMD is typically cheaper and has better pixel sampling than an InGaAs detector used in the 1550nm range, which leads to cost reduction and better performance. Moreover, the DMD is a 2D array, which means that multichannel systems can be implemented without any additional optical components in the interrogator. This makes the proposed interrogator highly cost-effective, particularly for multichannel systems. The digital nature of the DMD also provides opportunities for advanced programmable Hadamard spectroscopy, which, without significant penalties, can greatly improve the wavelength fit resolution. Our results show that DMDs can be used in high-resolution spectroscopy and for Fiber Bragg grating (FBG) interrogation.

Ultrafast Fiber Bragg Grating Interrogation for Sensing in Detonation and Shock Wave Experiments

Chirped fiber Bragg grating (CFBG) sensors coupled to high speed interrogation systems are described as robust diagnostic approaches to monitoring shock wave and detonation front propagation tracking events for use in high energy density shock physics applications. Taking advantage of the linear distributed spatial encoding of the spectral band in single-mode CFBGs, embedded fiber systems and associated photonic interrogation methodologies are shown as an effective approach to sensing shock and detonation-driven loading processes along the CFBG length. Two approaches, one that detects spectral changes in the integrated spectrum of the CFBG and another coherent pulse interrogation approach that fully resolves its spectral response, shows that 100-MHz–1-GHz interrogation rates are possible with spatial resolution along the CFBG in the 50 m to sub-millimeter range depending on the combination of CFBG parameters (i.e., length, chirp rate, spectrum) and interrogator design specifics. Results from several dynamic tests are used to demonstrate the performance of these high speed systems for shock and detonation propagation tracking under strong and weak shock pressure loading: (1) linear detonation front tracking in the plastic bonded explosive (PBX) PBX-9501; (2) tracking of radial decaying shock with crossover to non-destructive CFBG response; (3) shock wave tracking along an aluminum cylinder wall under weak loading accompanied by dynamic strain effects in the CFBG sensor.

Acoustic emission localization based on FBG sensing network and SVR algorithm

In practical application, carbon fiber reinforced plastics (CFRP) structures are easy to appear all sorts of invisible damages. So the damages should be timely located and detected for the safety of CFPR structures. In this paper, an acoustic emission (AE) localization system based on fiber Bragg grating (FBG) sensing network and support vector regression (SVR) is proposed for damage localization. AE signals, which are caused by damage, are acquired by high speed FBG interrogation. According to the Shannon wavelet transform, time differences between AE signals are extracted for localization algorithm based on SVR. According to the SVR model, the coordinate of AE source can be accurately predicted without wave velocity. The FBG system and localization algorithm are verified on a 500 mm×500 mm×2 mm CFRP plate. The experimental results show that the average error of localization system is 2.8 mm and the training time is 0.07 s.

A New Fiber Bragg Grating Interrogation and Multiplexing Schema Using an F-P Laser Light Source

While the interrogation and multiplexing are important to the application of fiber Bragg grating (FBG), the issues of high cost, complexity, and instability existing in demodulation system have been obstacles to the further development of grating. To overcome these issues, a new FBG interrogation and multiplexing approach has been proposed through taking advantage of a Fabry–Perot (F-P) laser light source. The proposed multiplexing schema is realized by the multi-longitudinal modes of the F-P laser together with four broad bandwidth FBGs, and the sharp peak of each mode from laser can be used for the signal demodulation. In our demonstration experiments of temperature sensing by using an FBG, the results show that the temperature resolution based on our proposed schema reaches 0.02 °C with the effective dynamic measurement range of 24 °C. The range has also been experimentally demonstrated to be extended dynamically. The interrogation and multiplexing system based on F-P laser will well satisfy the demand for the dynamic range, such as biological and medical system.

Improved KLT Algorithm for High-Precision Wavelength Tracking of Optical Fiber Bragg Grating Sensors

Fiber Bragg Gratings (FBGs) are among the most popular optical fiber sensors. FBGs are well suited for direct detection of temperature and strain and can be functionalized for pressure, humidity, and refractive index sensing. Commercial setups for FBG interrogation are based on white-light sources and spectrometer detectors, which are capable of decoding the spectrum of an FBG array. Low-cost spectrometers record the spectrum on a coarse wavelength grid (typically 78–156 pm), whereas wavelength shifts of 1 pm or lower are required by most of the applications. Several algorithms have been presented for detection of small wavelength shift, even with coarse wavelength sampling; most notably, the Karhunen-Loeve Transform (KLT) was demonstrated. In this paper, an improved algorithm based on KLT is proposed, which is capable of further expanding the performances. Simulations show that, reproducing a commercial spectrometer with 156 pm grid, the algorithm estimates wavelength shift with accuracy well below 1 pm. In typical signal-to-noise ratio (SNR) conditions, the root mean square error is 22–220 fm, while the accuracy is 0.22 pm, despite the coarse sampling. Results have been also validated through experimental characterization. The proposed method allows achieving exceptional accuracy in wavelength tracking, beating the picometer level resolution proposed in most commercial and research software, and, due to fast operation (>5 kHz), is compatible also with structural health monitoring and acoustics.