## 1 Introduction  
Optical signals have certain levels of attrition when being transmitted along optical fibers, and the transmission distance is limited by the attrition. Therefore, optical signals must be intensified to transmit them over long distances. The traditional method of intensification involves using a regenerator. However, different bit rates and signal formats require different regenerators and each channel requires an individual regenerator, which results in a high network cost. Therefore, a method that does not involve the use of generators is required to intensify optical signals. Optical amplification is a light intensification method that does not require the use of generators. Semiconductor optical amplifiers (SOAs) can support any bit rate and signal format because they simply amplify the received signals. In addition, compared with optical fiber amplifiers, SOAs have smaller structures and are easier to integrate with other components. With the new wave of wearable devices and the development of the Internet of Things economy, small and low-power electronic components are receiving increasing attention. Thus, SOAs are a topic of interest in mainstream studies [1; 2; 3]. With the development of light communication technologies, the demand for achieving high speed and large capacity is increasing and SOAs have become a crucial research topic. According to the reports of the market research institution Technovio, the global optical amplifier market size will grow by USD 935.94 million during 2019-2023, Compound annual growth rate (CAGR) of close to 9% during the forecast period [4]. Therefore, the global SOA market will continue to grow [5]. Increasing studies are focusing on the development of SOA technology [1; 2; 3]. However, previous studies have mostly focused on the discussion of technical details [6; 7; 8] or market research [5; 9]. These studies have failed to indicate the critical technologies that drive the development of SOAs. Specifically, SOAs are used in multiple areas, including optical fibers, lasers, optical components, semiconductor manufacturing, and communication components. Because SOAs have infinite future business possibilities, the identification of critical technologies is important. For research universities or businesses, the most critical problem is resource allocation, that is, determining the number of personnel or amount of funds to be invested in different technology fields. This study attempts to solve this problem by using the network analysis method on critical technologies, explaining the statuses and positions of different technology fields in the technology network, and determining the critical technologies. The focus of this study was the technology network of SOAs, and the SOA technology network model was constructed through patent analysis. Patents are the most direct evidence for measuring innovation output and can be used as indicators to observe technology development tendencies [10; 11; 12]. Consequently, investigations of industry-academia technology collaboration outcomes [13; 14] or the related studies of the industrial technologies can be conducted using patents [15; 16]. Thus, patent information can be used as one of the most direct indicators of technology development. This study examined the SOA technology development tendencies and critical technology fields according to patent information. Overall, this study is different from previous studies that have discussed the technical and market aspects of SOAs in that it mainly discusses the critical technologies of SOAs. This study focused on the establishment of a technology network model and technology development tendencies. The study results are expected to provide references regarding technology development tendencies to the government, academia, and industry. The subsequent literature review focuses on research in SOAs and critical technology network analysis.