## 2 Literature Review  
### Current Development of Optical Networks  
Because of the rapid development of mobile devices and the Internet of things, users have growing demands for network transmission. Hence, the provision of efficient transmission has become a major concern in network development, where optical networks play an essential role. Optical transmission technology has developed rapidly over the past thirty years through three main technological innovations: time division multiplexing (TDM) technology based on electrical multiplexing, optical amplification technology combined with wavelength division multiplexing (WDM) technology, and digital coherent technology [14]. Literature has used photonic technologies [15,16] and semiconductor optical amplifiers [17,18] to discuss 5G optical communication. However, at present, the technology development of 6G is gradually attracting attention. The development of optical communication technology should not be restricted to photonics or semiconductor optical amplifiers. Currently, the main focus in the development of optical networks is on optical fiber cables, optical devices (including chips), optical communication equipment,and optical modules. Optical fiber cables are the transmission channels for optical communication. Optical devices are the functional devices developed based on photoelectric effects, with functions such as sending and receiving information, WDM, amplifying, switching, and system management. They form the core of optical transmission systems and can be categorized into active optical devices and passive optical devices. Optical communication equipment is composed of optical devices, including an optical terminal transceiver and a switch. Optical modules are crucial devices that perform the conversion between optical signals and electrical signals in optical communication systems and play a major role in data centers and mobile broadband. The production process is as follows: first, optical chips and other components are combined into an optical device, and then the optical device is encapsuled as an optical module (e.g., a light source, an optical detector, or an amplifier). Optical networks have vast future business potential in various application fields (e.g., 5G, artificial intelligence, data centers, and photonic integrated circuits.) How to combine optical network technologies with the development and application of 5G and artificial intelligence has attracted increasing attention in academia and the professional industry [19; 20]. Currently, the focus is on the high transmission rate, low latency, and high bandwidth of 5G wireless communication, which can be applied to popular industries including artificial intelligence and unmanned self-driving automobiles. Furthermore, because of the immense growth of data, data centers are no longer limited to a few computer rooms; instead, they increasingly comprise groups of data center clusters. Such data centers function collaboratively and exchange tremendous amounts of data instantly, which creates a demand for data center interconnect. Data center interconnect equipment has unique requirements such as small size, low power consumption, large capacity, and high speed; optical interconnects have been introduced to provide said characteristics because of their high bandwidth, low power consumption, and reconfigurability [21]. Photonic integrated circuits have the potential to transmit vast amounts of data in a low-cost manner and have had a revolutionary influence on medical technology, photoelectric sensing, solar battery components, far-infrared light source, and light display. Photonic integrated circuit technologies combine optics, complementary metal-oxide-semiconductor technology, and advanced encapsulation techniques. If they are compatible with the semiconductor manufacturing process, the cost of optical integrated circuit chips can be reduced even further.