## 1 Introduction  
Internet backend users' demand for high bandwidth services, such as high quality images, high-speed and high-capacity data transmission, mobile broadband, big data, the Internet of Things (IoT), and cloud services, has been increasing rapidly. Transmission mediums based on coaxial cables and twisted pairs, including power-line communication, asymmetric digital subscriber line, and very-high-bitrate digital subscriber line, are no longer sufficient for supporting the high bandwidth services demanded by users. Because coaxial cables have a limited data-carrying capacity, with a maximum downstream capacity of only 200 Mbit/s, they cannot satisfy users' demand for bandwidth capacity. This renders optical networks crucial, and indeed, such networks have been receiving considerable academic attention from scholars [1; 2; 3]. Regarding the development of 5G and optical networks, 5G is a next-generation product in the domain of personal mobile communication; it will ultimately replace the 4G system currently in use. The key characteristics of 5G are a higher capacity and lower latency, which could support a higher density of mobile users as well as provide a comprehensive infrastructure for IoT and machine-to-machine (M2M) communication. According to a study conducted by Research and Markets, 5G will create considerable new opportunities for the optical networking industry through the 5G infrastructure--both backhaul and fronthaul [4]. In response to the transmission requirements for 5G (i.e., low latency, high transmission volume, and the transmission of big data), network infrastructures must be enhanced through upgrading optical networks. Accordingly, the United Kingdom published the Future Telecoms Infrastructure Review, in which it stated that the country aims to connect 15 million premises to full fiber broadband by 2025 and ultimately achieve the target of nationwide fiber broadband coverage [5]. The development of 5G requires more optical resources. For example, the radio access network (RAN) architecture for mobile 5G and beyond is undergoing a fundamental evolution, bringing optics into the world of radio [6]. Using optical networks could increase transmission distances and data bandwidths, while simultaneously reducing energy consumption. Optical fiber is a pathway that is thinner than human hair and can be used to transmit light. By contrast, metal wires are more maintenance-intensive and prone to signal degradation and electromagnetic interference. The use of optical fiber substantially increases the data transmission capacity. An optical fiber can achieve a bandwidth that requires several thousand copper wires, which is why most submarine and transcontinental communication cables are made of optical fiber. Therefore, optical fiber remains one of the most favorable options for data transmission. The high bandwidth and transmission volume of high-speed optical networks fulfill the requirements of device-to-device (D2D) transmission. Additionally, it could minimize paralysis and disconnections on 5G networks, enable the serial connection of more devices, and enable direct communication between devices--beyond interactions between people and devices. To date, studies have held preliminary discussions on the topic of 5G optical networks [4; 6; 7; 8]; however, such studies have mostly focused on discussions on the technical [6; 7; 8] or the market aspects [4; 9]. Furthermore, their findings have not indicated the key development domains for 5G optical networks. The development of 5G could create considerable business opportunities [10], and optical networks are an indispensable type of infrastructure for the development of 5G; therefore, the incorporation of wireless and optical communication into access networks is an inevitable future development trend. Additionally, patents are the manifestation of industrialized technologies; they are beneficial for businesses to achieve monopoly in the technology market, and could serve as indicators for technology development trends [11; 12; 13].