## 1 Introduction  
Optical networks are a means of communication that applies signals encoded onto light to transmit information in various telecommunication networks. According to a report by MarketsandMarkets, the optical interconnect market is projected to grow from USD 9.0 billion in 2020 to USD 17.1 billion by 2025; it is expected to grow at a compound average growth rate (CAGR) of 13.7% from 2020 to 2025 [1]. Optical networks have had revolutionary effects on the telecommunication industry and play an essential role in the digital age. They have large transmission capacity and provide excellent confidentiality; optical networks have become the main communication method in cloud computing and Internet of things' applications [2, 3, 4]. Optical networks have been extensively applied and incorporated in multiple industries, and this technology has progressed considerably. For example, it has been applied in quantum and optical communication [5,6], including quantum optics, a research field that discusses the quantum behavior of light. Photons are used to transmit wave functions to reduce the problem of information distortion caused by environmental interference. Another example is the application of SiO\({}_{2}\) to the field of optical materials [7,8]. The advancement of 5G, mobile devices, and broadband have also promoted the development of optical network technologies. The use of optical networks increases data bandwidth and reduces energy consumption per unit, which limits network latency and the number of disconnections, enables the connection of more networking devices, and improves the interaction and connection between them. Accordingly, optical networks are widely applied in multiple technical fields, such as optics, communication, semiconductors, materials, artificial intelligence, and computing. However, relevant studies have mostly focused on technical discussions [5, 6, 7, 8] or market research concerning optical networks [1,9] rather than on their technology distribution and development trends from a macroscopic perspective. Moreover, few studies have explored the technical trends in optical networks from the aspect of standard essential patents (SEPs). Given that technologies disclosed by SEPs are the foundation and also the key technologies that determine standards in relevant industries, we observed the development trends of these technologies from the perspective of SEPs. In this study, we focused on the SEPs of optical networks and constructed a technology network of SEPs. Many studies have predicted technology development by using patent data [10,11] and applied them to conduct patent landscape analyses [12,13] because patent data are one of the most direct indicators for observing technology trends. Specifically, SEPs are where standards and patents overlap. The technologies disclosed are the indispensable standards in the industry; they are cited by subsequent technologies and have influence on the industry's technological development. For example, the telecommunications technology standards in optical communication are mostly established by the third generation partnership project (3 GPP) members. The standard technologies approved by 3 GPP members are submitted to the standard setting organizations (SSOs), which publish them as telecommunications technology standards. As such, we attempted to address two research gaps. First, we examined the key technologies that determined the standards in these industries. Second, unlike the literature that focuses on discussions from a technical or market perspective, we explored technology distribution and trends in optical networks and constructed a technology network of SEPs to more effectively elucidate the key technologies protected by SEPs. In brief, we analyzed the technology trends and key technologies of optical network patents according to a technology network of SEPs and discussed the key technologies in optical networks as standards in the relevant industries.