# Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited)
## 2 PIC Evolution
State-of-the-art PIC-based consortia, such as JePPIX [35], TriPIeX [36], and ePIX [37], pioneered the effort in Europe and globally, followed by private and public-private arrangements, like AIM photonics [38], Compoundtek silicon photonics, and others [39]. They are working to replicate in optics what happened in the 80s-90s with electronics, e.g., the Marconi GaAs MPW runs and others [40]. The traction gathered by these consortia allowed a fast development of integrated optics simulators and of the process design software (PDS) market, supported by software suites running process design kits (PDK) from almost all available fabs. The development of a generic common language for the photonic design was an important breakthrough that allowed integrated photonics to be regarded as a promising sort-term technology. During these developments, many integrated and hybrid solutions emerged [41; 42; 43]; nonetheless, the majority were bulky, expensive, and typically non-replicable, making these approaches unfeasible to be integrated into further complex systems and subsystems. For instance, devices such as optical amplifiers [44], lasers [16; 45], and modulators [46], are not supplied by the different integrated platforms due to limitations of semiconductor integration technology. Thus, it restricted the investment of the different labs and research institutes in the development of their concepts/testing to validate their contributions/solutions while waiting for the necessary developments of the technology. In the 90s, few basic building blocks (BB) were made available in a packaged version, e.g., the semiconductor optical amplifier (SOA) based Mach-Zehnder interferometer (MZI) [47], though with an expensive associated cost of several tens of thousands of euros. Different materials are suitable to produce PICs, nonetheless, only a few have been selected by actors in the ecosystem and, therefore, have gained traction into foundries, software suppliers, design houses, and fabless companies [48]. Current main photonic integration platform technologiesoffering access to generic processes and manufacturing through multi-project wafer (MPW) runs are silicon (Si) [49; 50], indium phosphide (InP) [51; 52; 53], and silicon nitride (SiN) [48; 54]. The selection of the integrated photonic platform to realize PICs is driven by the functional requirements of the devices to be developed and the underlying available components (active/passive) for the different platforms [55; 33; 56]. An overview of the key characteristics between the different available integrated platform technologies is provided in Table 1[33; 56; 57]. BB components, such as lasers, optical amplifiers, and detectors, pose design challenges for their implementation in a SiN-based monolithic integrated platform; this constrain can be overcome with hybrid and/or heterogeneous processes, as presented in Table 1. Similarly, hybrid/heterogeneous processes are employed for lasers and optical amplifiers BBs in Si-based technology. Several advantages and/or limitations can be highlighted from the referred technologies. High gain, speed, and low loss are three of the main attributes of photonic devices' improved performance. As InP is a direct bandgap material, it presents the best available gain performance, however, with higher associated losses due to electrical doping from current injection which requires the inclusion of optical amplifiers [55]. Regarding systems-on-chip (SOC) complexity, InP platform has a good maturity with chips comprising up to hundreds of components. Silicon-based photonic processes are complementary metal-oxide-semiconductor (CMOS)-compatible, allowing its integration to be run in a CMOS foundry which provides well-controlled and more rapid scalability to the fabrication environment [42]. Nevertheless, as components in silicon substrates usually have higher dimensions than in InP and since silicon photonics lack light sources and amplifiers, InP-based platforms provide a higher potential for volume scaling and integration [25].