• Journal of the Korean Society for Precision Engineering
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• v.33 no.4
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• pp.303-307
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• 2016

The fiber optical temperature sensing device was developed by using a Fiber Bragg Grading (FBG) sensor and a linear transmittance optical filter. The temperature change causes change in the FBG sensor reflectance wavelength and the reflectance wavelength from FBG sensor is transmitted to a linear transmittance filter so that the optical signal value is determined by the wavelength. The temperature can be measured by the optical signal value by passing FBG reflectance wavelength to the linear transmittance filter. Using the developed system, temperature ranges from $10^{\circ}C$ to $50^{\circ}C$ were measured and the measured data were almost linear.

• Korean Journal of Optics and Photonics
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• v.15 no.1
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• pp.34-38
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• 2004

A fiber laser of which wavelength was scanned by an intra-cavity FP (Fabry-Perot) filter was used to interrogate a fiber Bragg grating strain sensor array. We calculated the wavelength variation of the fiber laser using quadrature signal processing with an unbalanced M/Z (Mach-Zehnder) interferometer and time-delayed sampling technique. The calculated wavelengths are mapped to corresponding temporal reflection peaks from the sensor array, which enables more accurate and stable interrogation without the problems caused by the FP filter's nonlinear characteristics. Wavelength resolution of ∼20 pm was obtained in our experimental setup, which could have been greatly enhanced with faster phase modulation.

• 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.

• Journal of The Institute of Information and Telecommunication Facilities Engineering
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• v.10 no.3
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• pp.110-114
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• 2011

The recent development of 4G mobile communication system has led to the rapid technology shift from conventional repeater systems using coaxial cable links towards new fiber optic repeater systems using fiber-optic links between the base station and the outdoor fiber optic repeater. The technical changes have brought up the increasing needs of robust optic links that can be used in harsh environments. Based on the most demanding requirements, a new outdoor optical connector employing 2-channel fiber optics, uplink and downlink, has been developed for the applications where the rugged environmental protection is essential. This paper describes the development of the new connector along with the design criteria and performance results. In summarizing, the prototype optical connectors have undergone extensive laboratory and field test, and they have shown exceptional optical and mechanical characteristics under extreme environmental conditions. The connectors have also exhibited capabilities of providing fast and easy installation while maintaining high performance fiber optic connections.

• Korean Journal of Optics and Photonics
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• v.29 no.4
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• pp.159-165
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• 2018

In this paper, we report on the high power amplification of a narrow-linewidth Yb-doped polarization-maintained (PM) fiber laser in a 3-stage, all-fiber master oscillator power amplifier (MOPA) system. The linearly polarized single-mode output power was 400 W with an 85% slope efficiency, with a linewidth of 2.5 GHz (full width at half maximum). Furthermore, mitigation of mode instability (MI) has been demonstrated by tightly coiling the gain fiber to a diameter of 11 cm. In addition, methods for higher power scaling are discussed.

• Korean Journal of Optics and Photonics
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• v.14 no.6
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• pp.600-605
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• 2003

We have experimentally demonstrated a Direct Optical Switching (DOS) CDMA for future wide-band mobile communication systems at the 40 ㎓ band by using orthogonal (crosscorrelation $\leq$2) unipolar type codes with code length of 16 and chip rate of 2.5 Gcps for radio-over-fiber (RoF) systems. Pulse-amplitude-equalized 40 ㎓ laser pulses were provided by rational-harmonically mode-locking a 10 ㎓ fiber ring laser.

• Korean Journal of Optics and Photonics
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• v.16 no.5
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• pp.411-416
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• 2005

We have designed an extended L-band Erbium-doped fiber amplifier to amplify 1625 nm optical time domain reflectometry signal for a long distance monitoring system. The proposed amplifier has a dual-stage structure without an isolator. Gain improvement of 5.1 dB has been achieved by adding a fiber Bragg grating and a narrow band pass filter. As a result, the 16.3 dB gain and 7.1 dB noise figure has been successfully accomplished.

• Korean Journal of Optics and Photonics
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• v.19 no.3
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• pp.175-181
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• 2008

We develop a fiber optic acceleration sensor with LED, PD, POF, and a cantilever beam, having film grating at the edge of the beam. Light is transmitted from LED to PD through the film grating. When the cantilever beam moves by external vibration, output light is modulated as sinusoidal signals. The characteristics of output signals are dominated by the spacing of the film grating and also by the size and the elasticity of the beam. Two output signals, having constant initial phase difference, are obtained by two gratings with 90 degree phase difference. Those two signals are used to determine phase angle, which is proportional to the displacement of the beam. Finally, the acceleration is determined from conversion equation between displacement and acceleration. This sensor is designed for monitoring the vibration of large and complex building in the low frequency range of below 7 Hz, and is particularly suitable to measure acceleration in electromagnetic environments.

• Korean Journal of Optics and Photonics
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• v.10 no.1
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• pp.58-63
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• 1999

A 10 GHz harmonically mode-locked semiconductor-fiber ring laser was implemented using a semiconductor optical amplifier at $1.5\mu\textrm{m}$ The laser pulse has 13~18 ps pulse duration, 0.4~0.6 nm spectral width and was positively chirped. The output pulse with an average power of 4 dBm was compressed to 6.8 ps using 2 km long standard single mode optical fiber.

• Korean Journal of Optics and Photonics
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• v.14 no.4
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• pp.365-368
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• 2003

We have demonstrated mechanically induced Mach-Zehnder interferometers (MZI) based on a long-period fiber grating pair. The transmission spectra, the interference fringes, and the fringe spacing of the long-period fiber grating pairs have also been investigated experimently and analytically. The fringe spacings of the fabricated LPG pairs agree well with the theoretical results.