• Proceedings of the KIEE Conference
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• 1998.07b
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• pp.766-768
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• 1998

Optical Temperature Distribution measurement System (OTDS) is completely different from conventional electric point sensor in that it uses the optical fiber itself as the sensor. This new concept in temperature measuring system requires only one fiber to be laid. The use of optical fiber also gives the advantage of small diameter, light weight, explosion resistance, and electromagnetic noise resistance. The OTDS is a sensor which is capable of making a precise measurement over a wide range of areas using only a single optical fiber. Since current temperature sensors, such as the thermocouple, are only used to measure temperaturea of point, they are almost impractical for measuring a wider range because of the extremely high cost. In comparision with current sensors, the optical fiber distributed temperature sensor can make much quicker and more precise measurements at a comparatively low cost.

• Proceedings of the Korea Concrete Institute Conference
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• 2000.10a
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• pp.517-522
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• 2000

In the past few years, the nondestructive inspection technology has greatly developed due to the increased necessity to gain a complete understanding of the bridge behavior. Especially, the deformations of bridges contain a lot of informations about its health state. By measuring these deformations it is possible to analyze the loading and aging behavior of the structure. However, the current methods (such as LVDT, dial gage, optical displacement tranceducer, etc) are often of changeable application on site and have the limitations of installation. In this paper, the classical beam theory was reviewed and the deflections of structure are estimated using measured strain which is easy to acquire. The applicability of this algorithm is verified by a preliminary steel beam test and two types of concrete beam tests. Also fiber optic sensors as well as resistive strain gages were installed in the concrete beams to establish the applicability of fiber optic sensors in the field of civil engineering.

• Journal of the Korean Society for Nondestructive Testing
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• v.27 no.6
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• pp.556-562
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• 2007

This paper presents the use of a novel fiber optic accelerometer system to monitor ambient vibration (both wind-induced one and vehicle-induced) of a real bridge structure. This sensor system integrates the $Moir\'{e}$ fringe phenomenon with fiber optics to achieve accurate and reliable measurements. A low-cost signal processing unit implements unique algorithms to further enhance the resolution and increase the dynamic bandwidth of the sensors. The fiber optic accelerometer has two major benefits in using this fiber optic accelerometer system for monitoring civil engineering structures. One is its immunity to electromagnetic (EM) interference making it suitable for difficult applications in such environments involving strong EM fields, electrical spark-induced explosion risks, and cabling problems, prohibiting the use of conventional electromagnetic accelerometers. The other is its ability to measure both low- and high-amplitude vibrations with a constantly high resolution without pre-setting a gain level, as usually required in a conventional accelerometer. The second benefit makes the sensor system particularly useful for real-time measurement of both ambient vibration (that is often used for structural health monitoring) and strong motion such as earthquake. Especially, the semi-strong motion and the small ambient one are successfully simulated and measured by using the new fiber optic accelerometer in the experiment of the structural health monitoring of a real bridge.

• Journal of Sensor Science and Technology
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• v.8 no.3
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• pp.232-238
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• 1999

Single mode fiber optic interferometers using the Fabry-Perot configuration were embedded in a reinforced concrete structure. These interferometers investigated the character of phase shift and strain for internal loads. The 10 mm length of FFPI in the continuous length of single mode fiber (SMF) were produced with two pieces of SMF coated were $TiO_2$ dielectric film utilizing the fusion splicing technique. The fabricated fiber optic Fabry-Perot interferometer(FFPI) and the 6 mm length of steel bar were buried with specimen ($100{\times}100{\times}50\;mm^3$) which was made of concrete structure. The resin protects FFPI and fiber leads from squeezed concrete. Sensors at different point in the structure were multiplexed by TDM (Time Division Multiplexing) method and the deformation to the external loads at each point could be monitored simultaneously. The output signals were proportional to the external loads applied to the structure and the sensitivity of the sensors were $1.03^{\circ}/kg$ and $0.76^{\circ}/kg$ respectively.

• Journal of the Optical Society of Korea
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• v.7 no.2
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• pp.97-101
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• 2003

The transmission characteristics of multi-channel long period fiber gratings (LPFGs) in terms of the physical parameters like the separation distance, grating length and number of gratings will be discussed. Their transmission characteristics such as channel spacing, number of channels, loss peak depth, and channel bandwidth can be easily controlled by physical parameters. Based on the experimental results, their applications to optical multiwavelength Raman lasers and optical sensors will be investigated. A multiwavelength Raman fiber ring laser with 9 WDM channels with 100 ㎓ spacing and 19 channels with 50 ㎓ spacing using tunable multi-channel LPFGs will be experimentally demonstrated. The fiber-optic sensing applications with high resolution and sensitivity based on multi-channel LPFGs will be also presented.

• Smart Structures and Systems
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• v.20 no.3
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• pp.385-396
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• 2017

The recent advancements in sensing technologies allow us to record measurements from target structures at multiple locations and with relatively high spatial resolution. Such measurements can be used to develop data-driven methodologies for condition assessment, control, and health monitoring of target structures. One of the state-of-the-art technologies, Fiber Optic Strain Sensors (FOSS), is developed at NASA Armstrong Flight Research Center, and is based on Fiber Bragg Grating (FBG) sensors. These strain sensors are accurate, lightweight, and can provide almost continuous strain-field measurements along the length of the fiber. The strain measurements can then be used for real-time shape-sensing and operational load-estimation of complex structural systems. While several works have demonstrated the successful implementation of FOSS on large-scale real-life aerospace structures (i.e., airplane wings), there is paucity of studies in the literature that have investigated the potential of extending the application of FOSS into civil structures (e.g., tall buildings, bridges, etc.). This work assesses the feasibility of using FOSS to predict operational loads (e.g., wind loads) on chain-like structures. A thorough investigation is performed using analytical, computational, and experimental models of a 4-story steel building test specimen, developed at the University of Southern California. This study provides guidelines on the implementation of the FOSS technology on building-like structures, addresses the associated technical challenges, and suggests potential modifications to a load-estimation algorithm, to achieve a robust methodology for predicting operational loads using strain-field measurements.

• Journal of the Earthquake Engineering Society of Korea
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• v.10 no.6 s.52
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• pp.137-145
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• 2006

Recently, the interests in structural monitoring of civil infrastructures are increased. Especially, as the civil infrastructures such as bridges, tunnels and buildings become large-scale, it is necessary to monitor and maintain the safety state of the structures, which requires smart systems that can supply long-term monitoring during the service time of the structures. In this paper, we investigated the possibilities of fiber optic sensor application to the various structures. We investigate the possibility of using fiber optic Bragg grating sensors to joint structure. The sensors show good response to the structural behavior of the joint while electric gauges lack of sensitivity, durability and long term stability for continuous monitoring. We also apply fiber optic structural monitoring to the composite repaired concrete beam structure. Peel-out effects is detected with optical fiber Bragg grating sensors and the strain difference between main structure and repaired carbon sheets is observed when they separate each other. The real field test was performed to verify the behaviors of fiber Bragg grating sensors attached to the containment structure in Uljin nuclear power plant in Korea as a part of structural integrity test which demonstrates that the structural response of the non-prototype primary containment structures. The optical fiber Bragg grating sensor smart system which is the probable means for long term assessments can be applicable to monitoring of structural members in various civil infrastructures.

• Journal of Sensor Science and Technology
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• v.16 no.3
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• pp.192-196
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• 2007

A fiber-optic interferometric temperature sensor is fabricated using a hollow optical fiber with 8 um air hole. This interferometric sensor for measuring temperature consists of 13 mm long hollow optical fiber whose one end is attached to the single mode fiber and the other end is cleaved. After the sensor is put in a furnace, the phase change of the sensor output signal is measured as the temperature of the furnace increases from $28^{\circ}C$ to $100^{\circ}C$. The phase change of the fiber sensor is proportional to the change of temperature and the relationship between the change of phase and temperature is approximately linear. The sensitivity of this sensor is $2.7{\;}radians/^{\circ}C$.

• 대한공업교육학회지
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• v.34 no.1
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• pp.274-286
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• 2009

Fiber optic sensor is widely used in measuring acoustic and vibration. Especially interferometric sensors are more suitable to measure the acoustic signal. In this paper, a Fabry-Perot interferometric fiber optic sensor was used to measure flow induced vibration. This vibration also measured using an accelerometer, and the data was compared to one other. The venture, nozzle, drop barrel, and rapid expansion in the pipeline are the measuring objects. The flow rate is changed from 50 L/min to 150 L/min and the average flow velocity was about 7 m/s. Based on the experimental results the suggested fiber optic sensor detects flow induced vibration effectively. Therefore, this kind of fiber optic sensor can be applied to the monitoring the flow induced noise and vibration such as pipelines, cables, buildings.

• Journal of Sensor Science and Technology
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• v.15 no.1
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• pp.40-46
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• 2006

This paper presents a novel fiber optic accelerometer system for monitoring vibration of large-size structures. The system is composed of one (or multiple) sensor head, a light control unit and a signal processing unit. The sensing mechanism of the sensor head is based on a novel integration of the moire fringe phenomenon with fiber optics to achieve a robust performance in addition to its immunity to EM interference, easy cabling, and low cost. In this paper, a prototype of the fiber optic accelerometer system has been developed successfully. A low-cost light control unit has been developed to drive the system's optic and electronic components. A unique algorithm has also been developed to derive the sensor's acceleration from the raw signals of the light control unit; it is implemented via a separate signal processing unit. Finally, the shaking table tests successfully demonstrate the performance and the potential of the moire fringe fiber optic sensor system to monitor the health of civil infrastructures.