FBG Interrogator Research Articles

Highly birefringent side-hole fiber Bragg grating for high-temperature pressure sensing.

We demonstrate a novel, to the best of our knowledge, high-temperature pressure sensor based on a highly birefringent fiber Bragg grating (Hi-Bi FBG) fabricated in a dual side-hole fiber (DSHF). The Hi-Bi FBG is generated by a femtosecond laser directly written sawtooth structure in the DSHF cladding along the fiber core through the slow axis (i.e., the direction perpendicular to the dual-hole axis). The sawtooth structure serves as an in-fiber stressor and also generates Bragg resonance due to its periodicity. The DSHF was etched by hydrofluoric acid to increase its pressure sensitivity, and the diameter of two air holes was enlarged from 38.2 to 49.6 µm. A Hi-Bi FBG with a birefringence of up to 1.8 × 10-3 was successfully created in the etched DSHF. Two distinct reflection peaks could be observed by using a commercial FBG interrogator. Moreover, pressure measurement from 0 to 3 MPa at a high temperature of 700°C was conducted by monitoring the birefringence-induced peak splits and achieved a high-pressure sensitivity of -21.2 pm/MPa. The discrimination of the temperature and pressure could be realized by simultaneously measuring the Bragg wavelength shifts and peak splits. Furthermore, a wavelength-division-multiplexed (WDM) Hi-Bi FBG array was also constructed in the DSHF and was used for quasi-distributed high-pressure sensing up to 3 MPa. As such, the proposed femtosecond laser-inscribed Hi-Bi FBG is a promising tool for high-temperature pressure sensing in harsh environments, such as aerospace vehicles, nuclear reactors, and petrochemical industries.

High Performance AWG-Based FBG Interrogator Using Cascaded Neural Network

High Performance AWG-Based FBG Interrogator Using Cascaded Neural Network

Fully integrated on-chip FBG interrogator for high-accuracy measurement of wavelengths.

We present the design and fabrication of an on-chip FBG interrogator based on arrayed waveguide grating (AWG) technology. The spectral overlap between adjacent channels in the integrated AWG is significantly enhanced through a combination approach involving the reduction of the output waveguide spacing and an increase in the input waveguide width. As a result of these design choices, our AWG demonstrates excellent spectral consistency, with spectral cross talk exceeding 30 dB. The interrogator seamlessly combining optical and circuitry components achieves full integration and enables a wide range of interrogation wavelengths, including C-band and L-band. With an interrogation range extending up to 80 nm, it theoretically has the capacity to simultaneously interrogate the wavelengths of 20 FBG sensors. Experimental findings demonstrate an absolute interrogation accuracy of less than 2 pm for the fully integrated interrogator. With its compact size, cost-effectiveness, exceptional precision, and ease of integration, the proposed interrogator holds a substantial promise for widespread application in the realm of FBG sensing.

Highly Variable FBG Interrogator With Electro-Optical Attenuation and Time Delay

Highly Variable FBG Interrogator With Electro-Optical Attenuation and Time Delay

Design and fabrication of AWG with large bandwidth applied in FBG interrogation system

Fiber Bragg Grating or FBG sensors are widely adopted in fields of structural health and temperature monitoring. The increasing demand for compact, high-performance FBG interrogators is highly desired. We have designed and fabricated high-performance arrayed waveguide gratings or AWGs using SOI technology with different parameters in FBG interrogator systems applications. Key parameters that affect FBG interrogating performance during AWG design are theoretically investigated and experimentally demonstrated. Using our proposed and optimized parameters, our fabricated AWGs have up to 2.88 dB of bandwidth, with low crosstalk and low on-chip loss. With custom supporting circuit, our proposed optimized AWG can keep the comparable interrogation accuracy and resolution performance to the previous best reported AWG based FBGI system while having four-time larger measuring dynamic range of 2 nm. This paper also provides guidance for optimizing the AWG-based FBG interrogation system during the AWG designing phase.

FBG-Based Inclinometer for Landslide Monitoring in Tailings Dams

Most of the dam's disasters that occurred in the world could be avoided if proper and preventive measures were taken, such as long-term surface or underground monitoring to detect small soil displacements. However, due to the complex operation necessary to monitor underground layer displacements, inclinometers are not used with suitable regularity. To circumvent the logistics of field campaigns necessary in conventional inclinometers, a real-time, remote monitoring system was developed. In this paper, a fiberoptic inclinometer system was designed, consisting of FBGs arrays fixed along a flexible tube to be inserted into conventional boreholes to monitor underground layers displacements. Tests on the bench consisted of applying lateral displacements to curve the tube with the Bragg wavelengths acquired by an FBG interrogator. Then, a specially developed algorithm, applying the Frenet-Serret equations, and the cubic spline interpolation method were used to reconstruct the tube profile. For the first validation of the proposed method, the algorithm was applied to real data obtained in-field by a conventional inclinometer. The results of these tests showed a good agreement between the curve reconstruction and the plotted data of field measurements. The final validation was performed by applying lateral displacements to the instrumented tube and comparing the real tube profile measured by a laser ruler with the calculated profile by the algorithm. The errors found in all tests were less than 1% attesting that the proposed system can be an alternative methodology to the conventional techniques of geotechnical instrumentation, through continuous monitoring with fast response and in real-time, with possible application to quantify the mass movement of soils; movements from earth dams, landfills, and slopes.

Fiber Bragg Grating Sensors for Dynamic Strain Measurements in Gasoline Direct Injectors

The strain in a gasoline direct injector was measured during a single injection process by using a fiber Bragg grating sensor installed along the axis of the injector stem in different operating pressure conditions. A fast FBG interrogator, based on linear transmission filters and direct detection, permits to measure dynamic strain up to 240 MHz frequency with a detection limit of 1.1 με and a dynamic strain resolution of 20.2 nε/√Hz. The FBG sensor was first calibrated for an internal pressure ranging from 50 to 250 bar and an energizing duration of 5ms; the FBG sensor allows to measure the static internal pressure of the injector inlet, which is linearly dependent on the FBG static strain, as well as the dynamic strain during the opening/spray/closing processes induced by pressure fluctuations, mechanical vibrations and the impact forces related to the injector needle bounces. The fast FBG interrogator, has been first validated by comparison with commercial interrogators, and then used to detect fast transients. The measured FBG strain variation provides important information on the dynamic behavior of the injector during the spray, confirmed by the analysis of the spray injection performed through a High Speed Video camera. The static and dynamic measurements demonstrate the effectiveness of the FBG sensors technology for monitoring the behavior of the injector and controlling the engine performance and emissions.

Temperature sensor based on multi-layer MoS2 coated etched fiber Bragg grating.

The development of an etched fiber Bragg grating (eFBG)-based temperature sensor with a uniform multi-layer molybdenum-di-sulphide (MoS2) coating is presented in this paper. Multi-layer MoS2 has been coated on clad etched FBG sensors by DC magnetron sputtering of molybdenum (Mo) and subsequent sulfurization. The dependence of temperature sensitivity on the coating thickness of MoS2 on eFBGs has been tested from room temperature to 100°C. It has been found that MoS2 coated clad etched FBG sensors with a coating thickness of around 10nm exhibit a maximum temperature sensitivity of ∼95 pm/°C (almost one order higher than that of bare fiber Bragg gratings), with a resolution of ∼0.01°C obtained using an FBG interrogator with 1pm resolution.

A cost-effective edge-filter based FBG interrogator using catastrophic fuse effect micro-cavity interferometers

This paper proposed an inexpensive high contrast in-line micro-cavity interferometer for edge-filter strain and temperature interrogation of a fiber Bragg grating sensor. The simulation model is presented in order to describe the sensor behavior which strongly agrees with the experimental results with a mean error lower than 0.04%. The results show evidences of high repeatability and stability achieved in measurements of strain and temperature from 0 to 1440 μStrain and 5 °C to 50 °C, respectively.

Impact localization on composite structure using FBG sensors and novel impact localization technique based on error outliers

Structural health monitoring of composite structures is essential for timely detection and localization of low velocity impacts. In this paper, a novel method for localizing impacts using an error outlier based impact localization algorithm is proposed. The outlier method for impact localization was developed and implemented by localizing several impact points on a quasi-isotropic composite structure. Multiplexed FBG sensors were attached to the composite structure to monitor low velocity impacts, sampled at a frequency of 100kHz using a high speed FBG interrogator. Localization results for the error outlier based localization method were compared with the RMS and correlation based impact localization results. The novel localization technique presented in this paper demonstrates an effective means of estimating impact locations. Furthermore, overall improvement in the localization performance was obtained using the outlier based impact localization algorithm: impact points on the composite plate were localized with an average error of 10.7mm.

High-Speed Interferometric FBG Interrogator With Dynamic and Absolute Wavelength Measurement Capability

A passive, interferometric wavelength demodulation technique has been extended to measure the absolute wavelengths of a multiplexed array of fiber Bragg grating sensors. The scheme retains its original strain resolution of 10 nε/√{Hz}. A proof-of-concept interrogation system was able to determine the absolute wavelength of Bragg peaks to within 20 pm (17 με). Static and dynamic Bragg grating strains were accurately demodulated in both absolute and relative wavelength measurement modes. This demonstration indicates that interferometric techniques are able to provide absolute, static and dynamic measurements of strain within a single platform.

Polarization-Switching FBG Interrogator for Distributed Point Measurement of Magnetic Field Strength and Temperature

We report on the design and implementation of a novel optical fiber system for dual point measurement of magnetic field strength and temperature. The system incorporates a polarization switching technique that enables ratiometric processing of the orthogonal circularly polarized reflections from low-birefringence fiber Bragg gratings. High-resolution magneto-optical measurement of either static or dynamic magnetic fields may be achieved while retaining the capability for serial multiplexing and temperature measurement. The system provides a practical solution to intrinsic fiber point measurement of magnetic field, leading to potential applications in adverse environmental conditions, where electronic or extrinsic optical technologies are unsuitable.

광섬유 브래그 격자 센서를 이용한 복합재 구조물의 충격 모니터링 기법 연구

복합재 구조물에서 발생하는 저속 충격에 의한 손상은 대부분 복합재의 내부나 충격을 받은 면의 반대 면에서 발생하기 때문에 검출이 쉽지 않아 시간이 지날수록 구조물이 위험에 처할 확률이 높아진다. 하지만 기존의 비파괴검사 방법은 일정한 주기에 따라 수행되기 때문에 즉각적으로 충격 손상을 감지할 수 없다는 단점이 있다. 따라서 최근에는 이러한 단점을 극복하고자 비파괴검사 장비를 구조물 내에 탑재하여 실시간으로 구조물의 건전성을 확인하는 개념인 구조 건전성 모니터링에 관한 연구가 활발히 진행되고 있다. 그 중의 하나인 충격 모니터링 시스템은 운용 중에 발생한 충격 이벤트를 감지하고 그 위치 및 손상 정도에 대한 정보를 제공해 주어야 한다. 이를 위한 첫 번째 단계로 본 연구에서는 복합재 평판 및 복잡한 복합재 시편 구조물에 FBG 센서를 부착하여 충격 위치 검출 시험을 수행하였고, 이와 같은 복합재 시편에 대해 충격 파손 시험을 수행하여 손상 발생 유무를 예측하는 시험을 수행하였다. 저속 충격에 의해 발생하는 음향 파는 (주)파어버프로에서 개발한 고속 FBG interrogator를 사용하여 4개의 다중화된 FBG 센서로부터 동시에 취득하였고, 신경회로망을 이용한 학습을 거쳐 충격 발생 위치를 검출하였다. 또한 충격 파손 시험으로부터 취득한 음향 파의 웨이블릿 변환을 통해 충격 손상의 발생 유무 예측 가능성을 확인하였다. Low-velocity impact can cause various damages which are mostly hidden inside the laminates or occur in the opposite side. Thus, these damages cannot be easily detected by visual inspection or conventional NDT systems. And if they occurred between the scheduled NDT periods, the possibilities of extensive damages or structural failure can be higher. Due to these reasons, the built-in NDT systems such as real-time impact monitoring system are required in the near future. In this paper, we studied the impact monitoring system consist of impact location detection and damage assessment techniques for composite flat and stiffened panel. In order to acquire the impact-induced acoustic signals, four multiplexed FBG sensors and high-speed FBG interrogator were used. And for development of the impact and damage occurrence detections, the neural networks and wavelet transforms were adopted. Finally, these algorithms were embodied using MATLAB and LabVIEW software for the user-friendly interface.

Detection of Impact Location for Composite Stiffened Panel Using FBG Sensors

We carried out experiments to detect impact locations on a composite plate using two types of composite plates, a composite flat plate with a constant thickness of 5 mm and a composite stiffened panel with stringers. Four multiplexed FBG sensors were attached to the bottom surface of the composite plates to acquire impact signals. The FBG sensor wavelength shift data were collected at a sampling frequency of 40 kHz using a high-speed FBG interrogator (SFI-710, Fiberpro Inc., Korea). The arrival times of the impact signals at each FBG sensor were obtained using a signal processing procedure. The arrival times were affected by noise level and signal-to-noise ratio. In order to overcome this weakness, signal processing techniques such as wavelet decomposition, normalization using each noise level and filtering with a moving average were adopted. To calculate the impact locations of the composite plate, a neural network algorithm was applied.