Multiplexed Fiber Bragg Grating Research Articles

Practical method for localizing low-velocity impact on a UAV composite wingbox structure under loading conditions

A low-velocity impact event can produce undetectable damages inside laminated composite structures. Such damages have been introduced as one of the main causes of catastrophic failure of whole structure, thus impact event detection and localization should be essential for many kinds of composite structures. In this study, low-velocity impact localization method was proposed using six multiplexed fiber Bragg grating (FBG) sensors on the composite wingbox structure. The principle is to find the location which has the most similar set of reference signals from the database. Firstly, the required reference signals were measured to construct the database for the test section which is located on the upper skin of the wingbox. Then, the localization was implemented with the process of detecting the time of arrival (TOA) and finding the impact location. The similarities between the impact-induced signals and reference signals are assessed by calculating the root mean squared (RMS) values. Through the impact localizations on the non-reference locations, this method was validated. Also, this method was applied to the impact localizations when the wingbox structure was under static loads. From the acceptable results of such experiments under static loading conditions, the practicality of this method was successfully verified.

Anomaly detection method for spacecraft propulsion system using frequency response functions of multiplexed FBG data

Anomaly detection and section estimation methods using multiplexed fiber Bragg gratings (FBGs) have been developed to identify propellant supply anomalies rapidly and autonomously in spacecraft propulsion systems. FBGs were used to measure the external strain of the pipe due to internal pressure fluctuations in the pipe during the pressure response caused by a water hammer. The measured strain was substituted into the constructed conversion formula to obtain the differential pressure through the FBGs. As a result, compared with the differential pressure measured by a conventional pressure sensor at the same position, the error was 2% for the maximum pressure of the water hammer and 1% for the natural frequency, indicating high applicability as a differential pressure sensor in piping with FBG simply wrapped around the outside of the pipe. Since the peak intensity and frequency of the measured values in the frequency domain change during flow anomaly and bubble localization, the frequency response function (FRF) between response data of the section to be detected was calculated, and the least-squares error of the FRF was used as the degree of anomaly. As a result, flow anomalies and bubble localization became detectable and sectional estimation was possible.

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.

Simultaneous Implementation of the High Reliability and Accuracy in a Fiber Bragg Grating Sensing System for Space Application

The complex space environment puts forward more stringent requirements for the reliability and accuracy of sensors and instrumentation. In this paper, a dual light sources multiplexing fiber Bragg grating (FBG) sensing system with high reliability and accuracy for space application is proposed. The proposed FBG sensing system has two independently driven light sources, which emit scanning laser light into the FBG sensors. The proposed system eliminates single-point failures (SPOF) compared with other system configurations. The five-year single channel reliability and the five-year all channels reliability are 0.9802 and 0.9349 respectively, which are the highest among the five systems. The reliability of the proposed system is improved by 14.1% compared with the common commercial FBG sensing system. Moreover, the experimental systems were established and two FBGs were tested six times on a constant temperature heating stage. The average coefficient of determination (COD) R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of FBG1 and FBG2 are 0.9112 and 0.9058 in the proposed system, which are 4.9% and 8.4% higher than those in the common commercial FBG sensing system owing to higher wavelength resolution. Therefore, the proposed dual light sources multiplexing FBG sensing system will provide higher reliability and accuracy performance in space applications.

Frequency Division Multiplexing Fiber Bragg Grating Sensing System Based on a Dual-Loop Optoelectronic Oscillator

Frequency Division Multiplexing Fiber Bragg Grating Sensing System Based on a Dual-Loop Optoelectronic Oscillator

Quasi-distributed FBG interrogation based on coherent wavelength-to-time mapping using mode-locked laser and pseudo-random barcode

A microwave photonic demodulation scheme for multiplexing fiber Bragg grating (FBG) sensing system based on coherent wavelength-to-time (WTT) mapping is proposed and demonstrated experimentally in this paper. With the help of mode-locked laser (MLL) source and dispersion element, the broadband pseudo-random barcode obtained from fiber random grating can be mapped into time domain to fulfill high-speed interrogation for FBGs. The shifts of FBG center wavelength will be transformed into the variations of filtered pseudo-random barcode, and finally converted into the time location changes of the cross-correlation peaks with narrow full-width half-maximum (FWHM), which is helpful to realize high-accuracy and high-resolution FBG demodulation utilizing oscilloscope. The experiment results exhibit good demodulation linearity and a strain interrogation sensitivity of 0.0302 ns/με in cascaded sensing network with a repetition rate of 20 MHz, corresponding to a strain measurement resolution of 2.05 με. Besides, the impacts of system noises and light source line-shapes on the demodulation performance are studied numerically, and the relevant solutions are proposed and verified respectively.

A flexible multiplexing method of FBG sensors based on OFDR

A flexible multiplexing method of fiber Bragg grating (FBG) sensors based on optical frequency domain reflectometer (OFDR) is proposed in this paper to reduce the influence of spectral shadow effect on measurement and improve the reliability and measurement flexibility of FBG sensing network. In this method, the interrogation principle of FBG position and wavelength shift based on OFDR is analyzed, and as a guide, the flexible multiplexing FBG array is designed, in which multi-channel identical weak reflection FBGs are interrogated using a photoelectric detector. Meanwhile, the crosstalk beat noise generated by the flexible multiplexing FBG array is avoided so as to improve the performance of the measurement. The experiment results show that the method presented in this paper can achieve the flexible multiplexing of multi-channel FBGs with a single acquisition channel, while the measurement accuracy can be guaranteed.

Dense Temporally Multiplexed Fiber Bragg Grating Sensing Based on Single-Photon Detection

Temporal multiplexed fiber Bragg grating (FBG) sensing system for detecting physical parameters with a high spatial resolution is desirable for applications such as structure health monitoring, oil and gas exploration, geophysical research, etc. However, due to the trade-off between bandwidth and sensitivity of the linear photodetection, the wavelength resolution of dense temporally multiplexed FBG sensing system is severely affected under a spatial resolution of centimeters. In this paper, we propose and demonstrate a dense temporal multiplexed FBG sensing scheme with single-photon detection at 1.5 μm. A proof-of-principle demonstration is implemented by using three FBG sensors spliced along a piece of fiber. The spatial resolution can reach 5.0 cm which is determined by a time jitter of superconducting nanowire single-photon detector and a pulse width of laser. In our experiment, by using the dual-wavelength differential detection method, a wavelength resolution of 0.5 pm over a range of 550 pm is obtained with a corresponding temperature resolution of 0.05 °C, which could be further improved to 0.01 °C by increasing the repetition rate of laser pulse.

Multiplexed Fiber Bragg Grating Optrodes in Photonic Crystal Fiber for Plasmonic Biosensing of Her2 Protein

Multiplexed Fiber Bragg Grating Optrodes in Photonic Crystal Fiber for Plasmonic Biosensing of Her2 Protein

Robust low-velocity impact localization algorithm on composite plate to dynamic operating conditions using fiber optic sensors

This paper introduces the impact localization algorithm on a composite stiffened plate using an array of four multiplexed fiber Bragg grating (FBG) sensors. For this algorithm, the impact signals are required as the reference data on the baseline points. In order to localize the impact event, the similarities between the obtained and each reference signal have to be evaluated. In this study, such similarities were assessed by calculating the root mean squared (RMS) values between the signals. In the first step, four RMS matrices from each sensor are obtained using the RMS values about each reference location on the test section as the elements. Then, the elements of each matrix are processed to extremely low value excepting a few number of elements with values near the minimum RMS values. From the element-by-element multiplications of these processed matrices, the final matrix could be obtained. Finally, the impact location could be predicted from the location of the high valued element in that matrix. This principle was validated from the localizations of non-baseline points. Also, to check the robustness of this algorithm to dynamic loading conditions, experimental investigations were performed. As a result, this suggested algorithm showed a reasonable localization performance.

High-Speed Interrogation System for Fiber Bragg Gratings With Buffered Fourier Domain Mode-Locked Laser

We develop a high-speed interrogation system with a wavelength-swept laser for sensing multiplexed fiber Bragg gratings (FBGs). High-speed and multipoint sensing is achieved by means of a buffered Fourier domain mode-locked (FDML) laser, a recently proposed high-speed wavelength-swept laser. This laser increases the measurement rate several times by tailoring the laser output with a buffer stage. The developed buffered FDML laser successfully achieves a measurement rate of 202.8 kHz using a general FDML laser driven at a sweep rate of 50.7 kHz. In order to detect reflection signals of FBGs at the high measurement rate, the measurement system introduces digital signal processing using a field programmable gate array (FPGA). However, FBG measurements with wavelength-swept lasers are affected by the propagation time (delay) in the optical fiber connecting from the laser to the FBG sensor, which reduces measurement accuracy. To overcome this limitation, the developed system uses a delay correction method with bidirectional sweep of the buffered FDML laser. The interrogation system with the buffered FDML laser achieves a time resolution of 4.9 μs without being affected by the delay.

Silicon-microring-based interrogator for TDM-FBG sensors enabled by pulse compression.

We propose a silicon microring resonator (MRR)-based high-resolution interrogator enabled by novel slope-assisted filtering and pulse compression technology for time division multiplexed fiber Bragg grating (TDM-FBG) sensors. Golay coded optical pulses are launched into an FBG sensor array to produce a series of reflected pulses with different time delays due to various transmission distances. A tunable MRR filter is used for interrogation of all FBG sensors by direct detection and subsequent signal processing. As a proof of concept, a strain sensing experiment is conducted using a 2×1 TDM-FBG sensor array. The performance of the proposed interrogation system is evaluated experimentally, and the dynamic strain resolution can be as high as 20nε/Hz.

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.

Shape reconstruction of woven fabrics using fiber bragg grating strain sensors

In this paper we develop a methodology that uses in-plane strain measurements to determine out-of-plane deflection for woven fabrics using an optical fiber based sensor network. A multiplexed fiber Bragg grating (FBG) network is used to collect strain at discrete locations in the fabric. To simplify the problem a circularly shaped two-dimensional woven fabric material under a spherical indenter load is studied. A finite element (FE) model of the fabric behavior, derived from benchmark testing, was used to help develop reconstruction algorithms, some of which account for slipping of the fabric at the clamped boundaries. Due to the large deflections, complex material behavior of the fabric and slipping of the fabric at the outer boundaries, a modified empirical approximation approach was found to be the optimal choice for the deflection reconstruction. Experiments are performed to evaluate one of the algorithms on strain data from FBG sensors for two test cases: bonded to and woven into the fabric. Despite the complex strain on the FBGs bonded to the fabric, the empirical approach well predicts the out-of-plane deflection, except in the region under the indentor, where the fabric deformation was different than that modeled in the FE simulations. This result is promising for structural applications were direct observations of the out-of-plane deflections are not possible. To increase the maximum deflection of the fabric that could be measured, weaving of the FBGs into the fabric is also attempted. This method was less successful, due to the large amount of relative slipping between the optical fiber and the fabric, drastically reducing the strain measured by the FBGs.

Monolithic multicore fiber based multi-parameter measurement based on spatial-division-multiplex sensing mechanisms

A monolithic all-solid multicore fiber based smart sensing system has been proposed and demonstrated in this paper. This technology enables the monitoring of multiple parameters of interest simultaneously, by spatially multiplexing specific sensing mechanisms seamlessly through different cores. The Raman distributed temperature sensor (RDTS), polarization detection based vibration sensor (PVS) and fiber Bragg grating (FBG) sensor are successfully integrated via a 7-core fiber for vibration frequency and discriminated strain and temperature sensing. The experimental results show that the performance of PVS based vibration frequency sensing is unacted on the influence of ambient strain and temperature. In addition, the discriminated strain and temperature measurements have also been experimentally investigated. All the results demonstrated that the proposed scheme of spatially multiplexing FBG with RDTS and PVS can readily achieve simultaneous measurement of vibration and discriminated strain and temperature, which provides a promising way of multi-dimension diagnosis for structural health monitoring.

Simulative Study of Wavelength Division Multiplexing Fiber Bragg Grating in Nuclear Reactors Monitoring

The technologies of wavelength division multiplexing (WDM) have been theoretically studied and analyzed for multiplexing fiber Bragg grating (FBG) in a single optical fiber. This method allows a single fiber to carry many of identical FBGs, making this sensor more appropriate in the nuclear reactors. The analysis demonstrates that the multiplexing capacity can be incredibly enhance small data rates and high channel spacing. The interference effect among FBGs multi-reflections channels must be taken into account. This paper simulate WDM based FBG for a channel spacing of 0.1, 0.3, 0.5, 0.8, 1 nm Gaussian apodized FBGs at data rates of 2.5, 10, 40,100,160, 250 Gb/s respectively for nuclear applications. All simulations were performed in Optisystem software.

Field-Programmable Gate Array-Based Multichannel Measurement System for Interrogating Fiber Bragg Grating Sensors

We develop a field-programmable gate array (FPGA)-based multichannel measurement system for sensing multiplexed fiber Bragg gratings (FBGs), using a high-speed wavelength-swept laser. The wavelength-swept laser, operated with Fourier-domain mode locking (FDML), exhibits a sweep frequency of 50.7 kHz and sweep band of ~60 nm. A breakthrough in multichannel and real-time measurement is achieved by implementing an FPGA with unique parallel-processing circuits. This FPGA enables the implementation of a high-speed centroid-peak–detection circuit for FBG detection, which can be operated with a sampling frequency of 250 MHz. In multichannel measurement systems, the light propagating through each sensor path produces a delay, which reduces the measurement accuracy. This can be solved by implementing a delay-correction technique, which utilizes the time difference between two spectra obtained by a bidirectional scan of the FDML laser. The system is demonstrated to exhibit multichannel sensing and high measurement time resolution of $9.9~\mu \text{s}$ , without being affected by the delay.

Integrated Dynamic Wavelength Division Multiplexed FBG Sensor Interrogator on a Silicon Photonic Chip

A wavelength division multiplexed fiber Bragg grating (FBG) sensor interrogator using an integrated unbalanced Mach–Zehnder interferometer followed by an arrayed waveguide grating on silicon-on-insulator platform is presented. The Bragg wavelength shift is determined through the phase-generated carrier demodulation technique, allowing accurate dynamic FBG interrogation. The performance of the proposed device was validated by comparing the measurements with a commercial FBG readout unit based on a spectrometer, serving as a reference. Experimental results demonstrate a dynamic strain resolution of 4.56 nϵ/√Hz and minimum detectable strain down to ∼0.49 μ ϵ at 1-kHz bandwidth.

Improving the Signal-to-Noise Ratio of Time Domain Fiber Bragg Grating Sensor Based on Hybrid Simplex and Golay Coding Technique

We report on the experimental results of the combination of unique auto-correlation properties of Golay complementary code and Hadamard matrix properties of standard simplex code. The combination of the two coding techniques results in improvement of the signal-to-noise ratio (SNR) of time domain multiplexed fiber Bragg grating (TDM-FBG) sensor for temperature measurement. Previously, we have analyzed the properties of both coding techniques when deployed separately in the TDM-FBG sensor. In this case, both coding techniques result in the same amount of SNR improvement for code length longer than 31 bits. In this paper, we demonstrate the combination and simultaneous deployment of the two techniques to measure multiple FBGs under room condition (25 °C) and 50 °C temperature. The two schemes combination results in remarkable improvement of SNR and still retains the original spatial property of the decoded FBG signals, confirming the successful deployment of the hybrid codes. From the measurement of two FBG sensors located after 16 km of fiber, the combination of 31 bits of simplex- and 64 bits of Golay codes has resulted in a total of 10.5 dB improvement of SNR.

Intensity and Wavelength Division Multiplexing FBG Sensor System Using a Raman Amplifier and Extreme Learning Machine

A fiber Bragg grating (FBG) sensor is a favorable sensor in measuring strain, pressure, vibration, and temperature in different applications, such as in smart structures, wind turbines, aerospace, industry, military, medical centers, and civil engineering. FBG sensors have the following advantages: immune to electromagnetic interference, light weight, small size, flexible, stretchable, highly accurate, longer stability, and capable in measuring ultra-high-speed events. In this paper, we propose and demonstrate an intensity and wavelength division multiplexing (IWDM) FBG sensor system using a Raman amplifier and extreme learning machine (ELM). We use an IWDM technique to increase the number of FBG sensors. As the number of FBG sensors increases and the spectra of two or more FBGs are overlapped, a conventional peak detection (CPD) method is unappropriate to detect the central Bragg wavelength of each FBG sensor. To solve this problem, we use ELM techniques. An ELM is used to accurately detect the central Bragg wavelength of each FBG sensor even when the spectra of FBGs are partially or fully overlapped. Moreover, a Raman amplifier is added to a fiber span to generate a gain medium within the transmission fiber, which amplifies the signal and compensates for the signal losses. The transmission distance and the sensing signal quality increase when the Raman pump power increases. The experimental results revealed that a Raman amplifier compensates for the signal losses and provides a stable sensing output even beyond a 45 km transmission distance. We achieve a remote sensing of strain measurement using a 45 km single-mode fiber (SMF). Furthermore, the well-trained ELM wavelength detection methods accurately detect the central Bragg wavelengths of FBG sensors when the two FBG spectra are fully overlapped.