Siwoong Park

This study presents a beam divergence optimization strategy to minimize rain-induced signal attenuation in free-space optical communication (FSOC) systems. FSOC technology holds considerable potential for providing high-data-transmission speeds and reliability in the communication era beyond 5G and 6G. However, it is highly vulnerable to extreme atmospheric conditions, such as rainfalls. The impact of beam divergence adjustment on FSOC system performance at varying rainfall rates was experimentally analyzed at a transmission distance of 100 m. Additionally, communication performance was evaluated based on simulations at various beam divergence angles and transmission distance scenarios. Furthermore, an adaptive beam divergence control algorithm based on reinforcement learning is proposed to enhance link stability by optimizing beam divergence in response to real-time weather changes. The experimental results and simulation analyses highlight the need for a balance between the optimization of the beam divergence angle to the mitigation of signal attenuation, ensuring the performance of the pointing, acquisition, and tracking system. The findings provide valuable insights for enabling stable communication under extreme weather conditions and contribute to the durability and scalability of next-generation wireless communication networks.

Free-space optical communication (FSOC) provides secure, high-speed connectivity essential for modern networks, but is highly susceptible to severe weather-induced attenuation. This study evaluates a full-duplex mobile FSOC system under controlled heavy rainfall and thick fog using the advanced facilities at the Yeoncheon SOC Demonstration Research Center. Experimental results confirm stable 2.3 Gbps data transmission at 35 mm/h rainfall and 10 m visibility, demonstrating system resilience. Comparative analysis with existing weather attenuation models reveals their significant limitations, especially under extreme conditions, highlighting the need for model refinement. These findings offer valuable insights for advancing FSOC performance modeling and support the deployment of FSOC in next-generation communication infrastructures, including mobile platforms, smart cities, and disaster recovery networks.

우주기술과 응용(Journal of Space Technology and Applications; J. Space Technol. Appl. 이하 JSTA )은 한국의 우주과학 및 우주 응용 분야에서 가장 권위 있는 학술단체인 한국우주과학회에서 Journal of Astronomy and Space Sciences 와 더불어 발행되고 있는 공식적인 정기 간행물이다. JSTA 는 2021년 5월 창간호를 시작으로 연 4회 분기별로 발간되고 있으며, JSTA 의 논문과 기고문 은 한국어 또는 영어로 게재 가능하며, 각 논문과 기고문에는 고유의 DOI가 부여된다. JSTA 에 게재 되는 모든 논문과 기고문은 Open Access 정책에 따라 JSTA 홈페이지(https://www.jstna.org)를 통해 누구나 전문을 무료로 열람할 수 있다.

This study proposes a liquid-crystal-based active wavelength filter for phase-sensitive optical time domain reflectometry to mitigate the amplified spontaneous emission (ASE) noise and accurately match the passband with the light source. The validity of the proposed system was verified using comparative experiments with conventional passive optical filters. The experiment showed an increase in signal-to-noise ratio (SNR) of up to 2.21 dB compared with passive filters. Additionally, the proposed system can effectively eliminate ASE noise, resulting in an SNR of 12.99 dB.

Free space optical (FSO) communication can support various unmanned aerial vehicles’ (UAVs) applications that require large capacity data transmission. In order to perform FSO communication between two terminals, it is essential to employ a pointing, acquisition, and tracking (PAT) system with an efficient and optimal performance. We report on the development of a common optical-path-based FSO communication system, tailored for applications in UAVs. The proposed system is equipped with a quadrant photodiode (QPD)-based PAT system without an additional beacon beam subsystem. The presented approach reduced the structural complexity and improved the tracking efficiency for the same size, weight, and power (SWaP). To achieve a robust FSO link in a dynamic UAV environment, the observability and controllability were obtained based on the linearized control according to the incident beam size on the QPD, which was verified by optical simulation and experiments. As a result, the QPD-based PAT system for implementing FSO links demonstrated an up to 4.25 times faster tracking performance. Moreover, the FSO link experimentally confirmed the 1.25 Gbps full-duplex error-free communication at a 50 m distance.

Thermal effects of power cables are critical factors for determining the remaining lifetime and efficiency of power grids. Herein, we propose a fiber Bragg grating (FBG) sensor package with high-temperature sensitivity. Using polyacetal, which has a higher coefficient of thermal expansion than typical optical fibers, we constructed the proposed sensor structure. The developed sensor exhibited about 14.1 times higher temperature sensitivity than a typical bare FBG sensor. To measure the temperature variation of power cable joints caused by partial discharge, we constructed a mock system similar to a manhole, and we installed the developed sensor at a 22.9-kV crosslinked polyethylene cable joint. The experimental result reveals that even a slight temperature difference of 0.2°C can be measured using PD (300 pC at 20 kV).

In this paper, we present a sensing device with the optical temperature sensors-based quad receiver (Quad-RX) module and a security module. In addition, in order to prevent cyberattacks on critical national infrastructures and key facilities, we implemented symmetric-key and secure hash algorithm-based hardware security modules in the key elements of the sensing device. A preliminary test was conducted prior to a field trial to verify the performance of the developed sensing device. The accuracy and stability of the sensing device were then verified for 1 month in a field test at facilities for energy storage systems and photovoltaic converters in sewage treatment plants.

We present a full-duplex mobile FSO system that provides a 2.5 Gbps single channel based on the 2.5 GBASE-T standard. We successfully confirm that the data transmitted 2.3 Gbps even in heavy rain and thick smoke.

For monitoring of the underground power cables, we proposed an optical sensor network. When the surface temperature of the power cable has increased from about 40 °C we confirmed partial discharge amplitude was significantly reduced.