# Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited)
## 3 Passive Optical Networks  
From the 80s onwards, PONs [64] have evolved greatly with the simplification of network management, pushing network traffic multiplexing closer to the user with the minimization of stranded capacity [65]. Since then, PON architecture remained one of the most predominant access network options. PONs have been under mass deployment, with more than 900 million broadband subscribers reported in 2017 [66]. This infrastructure has subsidized PONs establishment relevance for a long time [65]. PON basic technology includes: (i) the equipment at the edge of Metro/Access, i.e., the optical line terminal (OLT); (ii) the user end terminal, i.e., the optical networks unit (ONU); and (iii) the optical distribution network (ODN) that connects the OLT to the ONUs (see optical access network branch in Figure 1). PON emerging technical requirements are very unique, being the surviving solutions particularly sensitive to price and related costs, both capital expense (CAPEX) and operating expense (OPEX) [67]. Nevertheless, PON poses several advantages, such as the simplification of infrastructure management, as it is based on a fully passive ODN [65]. A completely passive ODN made of buried glass with cabinets to ensure connections and splitting anticipates long-term expectation of 20 years or even more. Its architecture derives from the technical approach to be deployed at the PON ends. Several technologies are competing to be implemented in the access space, with the most common based on a passive splitter tree topology. The OLT (interface from the operator side) feeds one trunk fiber that splits into several arms, each arm is divided into another set of arms, and so on, generating a tree structure, which, depending on the technology, can range from low (8 to 16) to high (64 to 256) terminal counts. Optical access standardization has evolved rapidly, with IEEE 802.3 and _International Telecommunication Union - Telecommunication Standardization Sector_ (ITU-T) Q2 groups playing dominant roles in the process [68]. The most common technologies are Gbit-capable PON (GPON) and Ethernet PON (EPON) [69], typically supporting up to 64 users in the same OLT trunk fiber. At the end users lays an ONU (the user terminal for the fiber). For each end technology, a different ONU is typically required. Currently, it is very common to have combined interfaces that allow multiple technologies to be handled within the same device, the so-called \"combo\" solutions [70]. Figure 2 illustrates some of the main existing characteristics of the ITU-T related technologies. Some operators are engaging in the 10 G-capable symmetric PON (XGSPON) and 10 G-capable EPON (10 GEPON). Moreover, a number of more aggressive and disrupting operators are investing in deploying NGPON2, merging the traditional concepts of time-multiplexed single-channel per direction PON combined with wavelength division multiplexing (WDM) [71]. This approach allows for further developing network flexibility concepts and the smooth evolution of offered data rates [70], i.e., up to 40 or 80 Gbit/s aggregated rates. An example of a PON with coexisting technologies supported by the current ITU-T standard series [71] is depicted in Figure 3.  
Figure 2: Recent _International Telecommunication Union-Telecommunication Standardization Sector_ (ITU-T) based optical technologies standards evolution. New trends are emerging and triggering discussions in the fora, mostly focused on either an architecture innovation, like the Super-PON, which is on the standardization path in IEEE and ITU-T, or on the efforts to move to high-speed PON (HSP) towards higher data rate technologies, such as 25 Gbit/s and 50 Gbit/s [72]. Current low-cost laser and receiver technologies face some challenges to deliver higher data rates without increasing packaging or electronic costs. Namely, the current methods used for interconnecting devices with drivers and boards are typically based on wire bonding and flex printed circuit board (f-PCB).