• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2017.10a
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• pp.711-713
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• 2017

Fiber Brag grating sensors were written in standard Ge-doped telecom optical fiber (Corning SMF-28) using an 800nm femtosecond laser and a phase mask. It were exposed to gamma-radiation up to a dose of 100 kGy to evaluate the radiation effect. The fs-FBG-2 sensor showed incomplete optical characteristics during irradiation, but the fs-FBG-1 sensor showed excellent radiation resistance with Bragg wavelength shift(BWS) of less than 10pm at a dose of 100 kGy.

• Journal of the Korea institute for structural maintenance and inspection
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• v.3 no.3
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• pp.237-243
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• 1999

Recently, there has been considerable interest in the development of fiber-optic sensors based on fiber Bragg gratings (FBGs), which can be made into Ge-doped fiber's core by UV phase mask or holographic methods. A good sensitivity and small size of this sensor make it an ideal candidate for distributed sensing in smart structures or other structural monitoring applications. In this study, fiber optic Bragg grating sensor, which could be applied to measure the absolute strains, was constructed and the strain sensitivity of this sensor was investigated in order to apply to the structural health monitoring. Fiber Fabry-Perot (FFP) filter has been used to detect the optical signals instead of optical spectrum analyzer. It has been convenient to determine the structural strains from the output signal of FBGs. The fiber optic Bragg grating sensor was attached on the aluminum beam near the electrical strain gage to measure the same strain. The relationship between strain and fiber signal was linearly fitted. The strain sensitivity of the fiber optic Bragg grating sensor was determined as $l.57{\mu}{\varepsilon}/{\mu}sec$ from the aluminum beam test.

• Journal of Sensor Science and Technology
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• v.25 no.5
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• pp.344-348
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• 2016

We developed a 2-channel fiber-optic temperature sensor (FOTS) using a temperature sensing probe, a fiber-optic coupler, transmitting optical fiber, and an optical time domain reflectometer (OTDR). The temperature sensing probe is divided into a sensing probe and a reference probe for accurate thermometry. A sensing probe is composed of a silicon oil, a FC terminator, a brass pipe, and a singlemode optical fiber and the structure of a reference probe is identical with that of the sensing probe excluding a silicon oil. In this study, we measured the modified optical powers of the light signals reflected from the temperature sensing probe placed inside of the water with a thermal variation from 5 to $70^{\circ}C$. Although the optical power of the reference probe was constant regardless of the temperature change, the optical power of the sensing probe decreased linearly as the temperature increased. As experimental results, the FOTS using a subtraction method showed a small difference (i.e., hysteresis) in its response due to heating and cooling. The reversibility and reproducibility of the FOTS were also evaluated.

• Journal of the Optical Society of Korea
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• v.19 no.4
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• pp.376-381
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• 2015

Transmission characteristics of long-period fiber gratings (LPFGs) fabricated by periodically etching a conventional single-mode fiber (SMF) are investigated. After coating the SMF with photoresist, the cladding of the SMF is symmetrically and periodically removed by using a wet etching technique resulting in the formation of the LPFG. Tensile strain reinforces the coupling strength between the core and the cladding mode based on the photoelastic effect. The extinction ratio of the SMF-based LPFG at a wavelength of 1550.8 nm is measured to be -15.1 dB when the applied strain is $600{\mu}{\varepsilon}$. The ascent of ambient index shifts the resonant wavelength to shorter wavelength because of the increase of the effective refractive index of the cladding mode. The extinction ratio is diminished by increase in the ambient index because of the induction of the optical attenuation of the cladding mode. The transmission characteristics of the proposed LPFG with variations in torsion are also measured. The photoelastic effect based on torsion changes the extinction ratio and the resonant wavelength of the proposed SMF-based LPFG. The polarization-dependent loss of the LPFG is also increased by torsion because of the torsion-induced birefringence. The polarization-dependent loss of the LPFG at torsion of 8.5 rad/m is measured to be 3.9 dB.

• Journal of the Korean Geophysical Society
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• v.7 no.4
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• pp.269-276
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• 2004

Optical fiber sensors have shown a potential to serve real time health monitoring of the structures. They can be easily embedded or attached to the structures and are not affected by the electro-magnetic field. Furthermore, they have the flexibility of the sensor size and very highly sensitive. In this study, we conducted several laboratory and field tests using a novel optical sensor based on Brillouin scattering. One of the advantages of this technique is that the bare fiber itself acts as sensing element without any special fiber processing or preparation. Test results have shown that BOTDR can be a great solution for sensor systems of Civil Engineering Smart Structures.

• Journal of Sensor Science and Technology
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• v.8 no.2
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• pp.133-138
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• 1999

In this pager, the authors have implemented a blood flow simulator and a LDF(laser Doppler flowmeter) probe using self-mixing effect of the laser diode. The purpose of the blood flow simulator is to simulate microvascular blood flow in tissue. It consists of melinex film (thickness = $123\;{\mu}m$) which has similar optical characteristics to epidermis and porous polyethylene filter (Vyon, porosity 35%, mean pore size $50\;{\mu}m$, thickness=1 mm) which has similar optical characteristics to dermis. The blood flowmeter probe consists of laser diode(5 mW, 780 nm wavelength), CD lens(focal length 12 mm). current-to-voltage converter, highpass filter, and premplifier. It doesn't need optical fiber, therefore, implementation of the probe is simpler than conventional probe using optical fiber.

• Journal of Sensor Science and Technology
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• v.15 no.4
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• pp.297-301
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• 2006

We have fabricated a multi-mode nonlinear plastic optical fiber (POF) using organic nonlinear optical materials and demonstrated the propagation of light. The refractive indices of core and cladding are 1.5240 and 1.5172. We made a POF preform by rod-in tube method. The core diameter of the fabricated POF is about $30{\mu}m$. We evaluated the temperature characteristics of POF. The sensitivity is $0.345{\;}mW/^{\circ}C$ and the linearity of sensor was good.

• Journal of the Korean GEO-environmental Society
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• v.19 no.9
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• pp.13-19
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• 2018

Recently, there have been various problems aroused on the domestic infrastructures as the domestic cities become old. Accordingly, the national concerns grow on the urban railway and the related structures, which brings the national interests are brought on the research on the maintenance and rehabilitation of the old infrastructures. The underground structure of urban railway are checked with the strain gages or fiber brag grating (FBG) sensors on the railway. However, these methods are known to have resolution limitations on the investigations of the specified abnormal section. Therefore, the applicability of the Brillouin Optical Correlation Domain Analysis (BOCDA) based distributed fiber optic sensor system on the railway was evaluated in this study. The constructed BOCDA fiber optic sensor system shows high resolution of 10, 20, 50, 100 cm and capability of continuous monitoring on overall or specified section within 2 km range. The applicability evaluation was performed on the 250 m distribution of fiber optic sensors abandoned railway for continuous monitoring. The applicability of the system on the specified area was evaluated with wheel load testing. As a result, data loss tends to increase with the reduction of spatial resolution from 1.0 m to 0.1 m. Even though the measuring speed is reduced with lower spatial resolution, data accuracy increases on the location and deformation. The system can be applicable to various structures if the proper distribution method is invented later.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.40 no.6
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• pp.547-553
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• 2020

As aging infrastructures increase along with time, the efficient maintenance becomes more significant and accurate responses from the sensors are pre-requisite. Among various responses, strain is commonly used to detect damage such as crack and fatigue. Optical fiber sensor is one of the promising sensing techniques to measure strains with high-durability, immunity for electrical noise, long transmission distance. Fiber Bragg Grating (FBG) is a point sensor to measure the strain based on reflected signals from the grating, while Brillouin Optic Correlation Domain Analysis (BOCDA) is a distributed sensor to measure the strain along with the optical fiber based on scattering signals. Although the FBG provides the signal with high accuracy and reproducibility, the number of sensing points is limited. On the other hand, the BOCDA can measure a quasi-continuous strain along with the optical fiber. However, the measured signals from BOCDA have low accuracy and reproducibility. This paper proposed a multi-fidelity data-fusion method based on Gaussian Process Regression to improve the fidelity of the strain distribution by fusing the advantages of both systems. The proposed method was evaluated by laboratory test. The result shows that the proposed method is promising to improve the fidelity of the strain.

• Journal of the Korea institute for structural maintenance and inspection
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• v.4 no.3
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• pp.185-195
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• 2000

Detection of cracks in concrete beams using optical fiber sensors is useful for monitoring of concrete structures. In this study, optical time domain reflectometry (OTDR) is used to detect cracks. Resolution of OTDR is the main contributor to detect cracks in concrete structures. The OTDR used in this study can detect cracks with high precision of 0.5 m. Two mortar beams, reinforced with a 19 mm diameter steel bar, are made with the dimensions of 140 mm (width) ${\times}$ 200 mm (depth) ${\times}$ 2.000 mm (length). Two fibers are embedded inside each beam and two fibers are attached under the beams. The application of measurement system which consists of fiber and FC/PC connecter is studied. For this, theory of optics, resolution, crack moment, and size of specimens are investigated. From the measured data, it is verified that fibers which are attached under the beam can detect the crack in beams effectively. However, fibers embedded inside the beam are unable to detect cracks in beams using the OTDR in this study.