Hyunjin Kim

Bio: Hyunjin Kim is an academic researcher from Chonbuk National University. The author has contributed to research in topics: Fiber Bragg grating & Fiber optic sensor. The author has an hindex of 6, co-authored 28 publications receiving 118 citations.

Polymer-coated FBG sensor for simultaneous temperature and strain monitoring in composite materials under cryogenic conditions.

Abstract: A polymer-coated fiber Bragg grating (PCFBG) is examined for real-time temperature and strain monitoring in composite materials at cryogenic temperatures. The proposed sensor enables the simultaneous measurement of temperature and strain at extremely low temperatures by tracking the changes in the reflected center wavelengths from a pair of PCFBGs embedded in a composite material. The cryogenic temperature sensing was realized by introducing polymer coatings onto bare FBGs, which resulted in high temperature sensitivity under cryogenic conditions. A comparison of wavelength responses of the Bragg grating with and without a polymer coating toward temperatures ranging from 25°C to −180°C was performed. The polymer-coated FBG exhibited a sensitivity of 48 pm/°C, which is 10 times greater than that of the bare FBGs. In addition, the encapsulation of the FBG in a capillary tube made it possible to evaluate the strain accumulated within the composite during operation under cryogenic conditions.

In-Situ Cure Monitoring of Wind Turbine Blades by Using Fiber Bragg Grating Sensors and Fresnel Reflection Measurement.

Abstract: A fiber-optic cure monitoring system is proposed to measure curing status of composite structure such as a large scale wind turbine blade. The monitoring is based on the measurement of Fresnel reflectivity at the optical fiber/epoxy resin interface. The refractive index of epoxy resin varies throughout curing stages, changing the Fresnel reflectivity. The curing status is decided by monitoring the reflected intensity variation. The usage of fiber Bragg grating (FBG) sensor helps to separate the temperature-induced cross effects. A Gaussian curve fitting algorithm was applied to FBG spectra which were distorted in curing procedure. The substantial measurement errors could be minimized by locating the centroids of the Gaussian curve-fitted spectra. From the experiments performed in various isothermal conditions, the proposed system successfully identified the onset of gelation and the completion of curing of epoxy resins.

Cryogenic Temperature Sensor Based on Fresnel Reflection From a Polymer-Coated Facet of Optical Fiber

Abstract: We report a fiber-optic sensor based on Fresnel reflection for real-time monitoring of cryogenic temperatures. Coating highly thermo-sensitive polymers on the facet of optical fiber, we obtain average measurement sensitivity 0.04 dB/K in temperature range of 90–298 K. We show theoretical background of polymer’s thermo-optic response and the experimental results performed with two kinds of coating polymers, epoxy resin, and polymethyl methacrylate. The experimental results confirm the suitability for cryogenic temperature sensing applications, such as superconducting lines and magnets with simple and cost-effective sensor structure and signal processing.

A Fiber Laser Spectrometer Demodulation of Fiber Bragg Grating Sensors for Measurement Linearity Enhancement

Abstract: A novel fiber-optic sensor system is suggested in which fiber Bragg grating sensors are demodulated by a wavelength-sweeping fiber laser source and a spectrometer. The spectrometer consists of a diffraction grating and a 512-pixel photo-diode array. The reflected Bragg wavelength information is transformed into spatial intensity distribution on the photo-diode array. The peak locations linearly correspond to the Bragg wavelengths, regardless of the nonlinearities in the wavelength tuning mechanism of the fiber laser. The high power density of the fiber laser enables obtaining high signal-to-noise ratio outputs. The improved demodulation characteristics were experimentally demonstrated with a fiber Bragg grating sensor array with 5 gratings. The sensor outputs were in much more linear fashion compared with the conventional tunable band-pass filter demodulation. Also it showed advantages in signal processing, due to the high level of photo-diode array signals, over the broadband light source system, especially in measurement of fast varying dynamic physical quantities.

Multi-Stress Monitoring System with Fiber-Optic Mandrels and Fiber Bragg Grating Sensors in a Sagnac Loop

Abstract: Fiber Bragg grating sensors are placed in a fiber-optic Sagnac loop to combine the grating temperature sensors and the fiber-optic mandrel acoustic emission sensors in single optical circuit. A wavelength-scanning fiber-optic laser is used as a common light source for both sensors. A fiber-optic attenuator is placed at a specific position in the Sagnac loop in order to separate buried Bragg wavelengths from the Sagnac interferometer output. The Bragg wavelength shifts are measured with scanning band-pass filter demodulation and the mandrel output is analyzed by applying a fast Fourier transform to the interference signal. This hybrid-scheme could greatly reduce the size and the complexity of optical circuitry and signal processing unit, making it suitable for low cost multi-stress monitoring of large scale power systems.

High Sensitivity and Tunable Nanoscale Sensor Based on Plasmon-Induced Transparency in Plasmonic Metasurface

Abstract: A nanoscale sensor which comprises dielectric-metal-dielectric waveguide and plasmonic metasurface resonators is proposed to realize plasmon-induced transparency responses. The properties of the device are numerically and analytically investigated with different physical parameters. The effect of the incident polarization, geometrical parameters, plasmonic metasurface materials, prism dielectric constant, and the shape of metasurface in near infrared region are then studied to enhance the performance parameters of the structure including the sensitivity, figure-of-merit, and tunability. Our calculations show that the proposed devices are able to operate as a high-sensitivity and tunable sensor with maximum figure-of-merit of 480, and sensitivity of 497.8 nm/refractive index unit for slight change of $\Delta n = 0.24$ , in the refractive index, which stems from its ultra-narrow transparency window and long coupling length between resonators. Furthermore, the structure can be utilized for harnessing light propagation. It is shown that by using silver plasmonic metasurface and appropriate dielectric, a slow up factor as high as 800 is achievable in the proposed structure. We believe that the proposed sensor can be used as a promising platform for future sensing applications such as nanostructure biosensors.

Temperature and vibration insensitive fiber-optic current sensor

Abstract: We present a robust, temperature and vibration insensitive fiber-optic current sensor. The sensor has been integrated into a high-voltage circuit breaker. One year field experience has been gained in two substations of the Italian railways.

Design, fabrication and embedding of microscale interdigital sensors for real-time cure monitoring during composite manufacturing

Abstract: In this study, microscale interdigital capacitive sensors are designed, fabricated and embedded in glass fiber composite for real-time cure monitoring of resin. The microscale interdigital capacitive sensor offers great advantages due to its miniaturized size, high flexibility and high temperature stability. Most importantly, after integration, due to the small footprint and only 5 μm thickness, the embedded sensor minimally downgrades the composite mechanical properties. Apart from all these merits, the substrate foil is perforated to provide an opportunity for the resin to go through the substrate and bridge it. The sensor consists of two interdigitated arrays. There are two different sensor designs, which offer 900 and 450 electrodes in arrays. The electrodes are made out of tantalum and are fully insulated with about 50 nm tantalum oxide. Therefore, it is possible to embed the sensor even in conductive fibers, e.g. carbon, and to use the sensor out of the Cleanroom without getting contaminated with conductive particles to shortcut the arrays. Polyimide is chosen for the substrate of the sensor. This specific polymer has excellent flexibility and its geometry stays unchanged even at high temperature. The Dielectric Analysis (DEA) measurement proves the effective and real-time tracking of resin polymerization during laminate production.