• Current Optics and Photonics
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• v.6 no.3
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• pp.297-303
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• 2022

Multiple fiber Bragg gratings (FBGs) have been proposed and demonstrated for gas-flow measurements in a flow channel, using the temperature-difference method. This sensor consists of two FBG temperature sensors and two coil heaters. Coil heaters are used to heat the FBGs. The flow rate of the gas can be obtained by monitoring the difference in the Bragg-wavelength shifts of the two FBGs, which has features that exclude the effect of temperature fluctuations. In this study, experiments are conducted to measure the wavelength shift based on the flow rate, and to evaluate the gas-flow rate in a gas tube. Experimental results show that the sensor has a linear characteristic over a flow-rate range from 0 to 25 ℓ/min. The measured sensitivity of the sensor is 3.2 pm/(ℓ/min) at a coil current of 120 mA.

• Journal of Advanced Marine Engineering and Technology
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• v.33 no.8
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• pp.1220-1225
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• 2009

In this paper, a tunable fiber feedback laser based on the use of Sagnac loop filter and fiber Bragg grating (FBG) is proposed. The Sagnac loop filter using a high birefringence fiber provides precise 0.33-nm channel spacing as a multi-wavelength grid filter. Ni-Cr wire wound on the FBG is useful to induce the thermo-optic effect of the fiber. Two types of FBG structures, which have a different length of wire, are demonstrated to show the wavelength shift and separation. To tune FBG by resistant heat, some current is supplied into the wire. When the wavelength matched with one of the cavity modes of Sagnac loop filter, the mode-locked lasing is occurred. The electrical power sensitivity of the resonant wavelength is measured to 1.75pm/mW. This laser configuration can be applied the electrical power system for monitoring the power fluctuation.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.10
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• pp.1101-1109
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• 2012

Accelerometers play a key role in the structural assessment. However, the current electric type accelerometers have certain limitations to apply some structures such as heavy cabling labor, installed sea structure and sensitivity to electromagnetic fields. An optical Fiber Bragg Grating (FBG) accelerometer has many advantages over conventional electrical sensors since their immunity to electromagnetic interference and their capability to transmit signals over long distance without any additional amplifiers, and there is no corrosion from sea water. In this paper, we have developed a new FBG-based accelerometer. The accelerometer consists of two cantilevered type beams and a mass and two rollers. A bragg grating element is not directly glued to a cantilever to avoid possible non-uniform strain in the element. Instead, the bragg grating element will be attached to rotation part that rolled inducing vertical movement of the mass and support cantilever beams so that the bragg grating element is uniformly tensioned to achieve a constant strain distribution. After manufacturing, we will prove the performance and the natural frequency of the accelerometer through the experiment with a vibration shaker. The FBG-based accelerometer is developed for measuring the vibration not exceeding 50 Hz for the marine and civil structures.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.5
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• pp.489-494
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• 2004

Long Period Gratings (LPG) is currently receiving considerable attention because of their consistent measuring results fur pressure, temperature, strain and flow. LPG is easier to prepare and has a high sensitivity compared with Fiber Bragg Gratings (FBG). In addition, this kind of optical fiber sensors could be used for implementations in various structures. In this paper, LPG was used to monitor in situ the resin flow and the curing process in VARTM (Vacuum Assisted Resin Transfer. Molding). In order to demonstrate the effectiveness of the method, FBG is inserted into the glass mat to monitor the resin flow using optical spectrum analyzer (OSA). The curing reactions in VARTM are also observed using the same method. From the results, the attenuation wavelength shift and the loss change of attenuation band can be obtained from the status of the RTM (Resin Transfer Molding) sample owing to the internal variations of the .effective index, temperature, and pressure. It is shown that the proposed LPG is more effective in monitoring the curing reaction than FBG.

• Journal of the Korean Society for Railway
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• v.14 no.4
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• pp.364-369
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• 2011

For the safety management of tunnel, effective measurements should be continuously carried out. Most of sensors currently being applied for tunnels measure only the local status, thus many of them are needed to monitor an entire tunnel. For the railway tunnel where trains of same conditions are regularly operated, dynamic responses of tunnel structure to train operations can be a good index to estimate the deformation of tunnel structure in wide area. Meanwhile, the electromagnetic interference caused by overhead centenary in railway tunnel obstructs the use of electric-based sensors. In this study a brand new accelerometer using FBG optical fiber sensors is developed to solve these problems. Sensitivity and capacity of the accelerometer are enhanced with effective structural design of its components and verified with laboratory tests. A case history where the developed accelerometers were applied to a safety monitoring system of a high-speed train tunnel is presented. The performances of the developed accelerometers are validated from the measured acceleration data.

• Korean Journal of Optics and Photonics
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• v.30 no.6
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• pp.226-229
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• 2019

We investigated an interrogation system for fiber Bragg gratings by using a 50-GHz 96-channel array waveguide grating. Linearity of the sensitivity (the wavelength shift in response to the change in strain or temperature) is achieved for a Bragg grating of sufficiently wide bandwidth. The present wavelength-monitoring system could measure the change in Bragg wavelength with a resolution of 0.01 nm, at intervals of 10 seconds. When this interrogation system was used for a linear array of 12 acrylaterecoated fiber gratings, the wavelength sensitivity changed from 0.018 nm/℃ to 0.01 nm/℃ when the operating temperature changed from -25℃ to 85℃.

• Journal of the Optical Society of Korea
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• v.7 no.2
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• pp.84-88
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• 2003

A new method for temperature compensation of fiber Bragg grating (FBG) using hi-metal is proposed and experimentally demonstrated. Bi-metal bends toward the metal of low temperature expansion coefficient as the temperature increases, and this property is utilized to cancel the thermo-optic effect of the fiber. The optimum thickness of the high coefficient metal was empirically found by the trial-and-error method. The temperature sensitivities were 8.1 pm/$^{\circ}C$ and -0.018 pm/$^{\circ}C$ for the uncompensated and compensated FBGs, respectively, which indicates a reduction to a mere 0.22 % of the original sensitivity. No appreciable change in the spectral shape was observed. The packaging technique described in this paper is simple and compact, and it can be used for FBGs in WDM and DWDM communication systems that have stringent requirements on the temperature stability of the components.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.36 no.6
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• pp.594-597
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• 2023

We have proposed and demonstrated a fiber optic RH (relative humidity) sensor based on fiber Bragg grating covered with a polyimide film. As the polyimide film absolves the moisture in the air, its volume expands. As a result, the grating period of the FBG (fiber Bragg grating) covered with a polyimide film becomes wide and the Bragg wavelength is shifted. The sensor is implemented by fixing a 30 ㎛ thickness polyimide film on the surface of an optical fiber grating using an adhesive, and the characteristics of the device according to humidity are analyzed. The fabricated FBG RH sensor showed a high sensitivity of 0.0186 nm/RH% and a wide measurement range from 30% to 90%. The influence of environmental temperature on the characteristics of the RH sensor was also measured and analyzed. The feasibility of commercialization is presented.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.4C
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• pp.163-174
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• 2009

Mechanical device using the load cell or strain gage sensor can be influenced by tempearute changes because temperature change can cause a shift in the load cell or straing gage output at zero loading. In this paper, micro cone penetrometers with 1~7mm in diameter, are developed by using an optical fiber sensor (FBG: Fiber Bragg Grating) to compensate the continous temperature change during cone penetration test. Note the temperature compensated method using optical fiber sensor which has hair-size in diameter, and is not affected by environmental conditions because the measured data is the wavelength shifting of the light instead of the intensity of the electric voltage. Temperature effect test shows that the output voltage of strain gage changes and increases with an increase in the temperature. A developed FBG cone penetrometer, however, achieves excellent temperature compensation during penetration, and produces continuous change of underground temperature. In addition, the temperature compensated FBG cone shows the excellent sensitivity and detects the interface of the layered soils with higher resolution. This study demonstrates that the fiber optic sensor renders the possibility of the ultra small size cone and the new fiber optic cone may produce more reliable temperature compensated tip resistance.

• Journal of the Korean Society for Nondestructive Testing
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• v.29 no.4
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• pp.299-307
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• 2009

Emerging fiber optic sensor technologies have shown great potential to overcome the difficulties associated with conventional sensors. Fiber optic sensors are immune to EM noise and electric shock and thus can be used in explosion-prone areas. Several kinds of fiber optic sensors have been developed over the last two decades to take advantage of these merits. There have also been many field applications of fiber optic sensors for structural health monitoring as NDT/HDE. However, very few sensors, particularly a load cell have been successfully commercialized. This Paper Presents a load cell using fiber Bra99 gra1ing (FBG) sensors. The shape of the load cell is a link type, and three FBG sensors are used for measuring strains at three different points. Especially, these strains are processed with a differential method in order to exclude common mode noise such as temperature. Moreover, the sensitivity, the linearity and the resolution of the load cell were successfully verified from the experiment of tension test.