# Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited)
## 4 PIC for Next-Generation PON
### Integrated Photonics Packaging  
Photonic packaging technology has achieved a higher significance under optical communication systems' recent developments [82] by covering the optical and electronic connections in/out of the PIC. Packaging process developments are of great importance for the next generation of optical components [6]. Being one of the most complex segments of the integrated component viability, it can pose several challenges, such as limitations in terms of volume, cost, RF performance, power dissipation, and its high-volume manufacturing capability [83], which can highly impact the cost of the solution. Currently, packaging is seen as one of the most significant bottlenecks in the development of commercially relevant PIC devices [60; 84]. The packaging design flow is divided into three main areas: the optical design, the electrical design, and the thermal management of the module. A first open-access PIC assembly and packaging pilot line of Europe's leading industrial and research organization teams (i.e., PIXAPP) are dedicating efforts to the development of a photonic packaging platform to gather state-of-the-art technologies and procedures [62]. This highly interdisciplinary team provides single-point access to PIC assembly and packaging for users. The project key objectives include: (i) custom solutions through standard packaging technologies; (ii) training and education as a future workforce; and (iii) link the PIC ecosystem through the development of packaging standards and roadmaps [17]. Considering the typical wavelength gain band of an active device, ranging from 20 nm to 30 nm for the O- to L-bands, and the technology wavelength map presented in Figure 7, several approaches may be required to merge in a single device the upstream/downstream signals. There are several network standards, like XGSPON, which require upstream/downstream wavelengths spaced by more than 200 nm. A 200 nm spacing brings higher challenges and may require hybrid integration, which poses additional packaging challenges [7]. Figure 7: Diagram describing transmitter and receiver technologies with potential use in access networks. Typical gain and lasing media profiles are presented, with 20 nm to 60 nm bandwidths for common current materials, PINs with different doping materials, and APDs. (**A**): transmitter/(pre-) amplification side; (**B**): receiver side. In the same technological space, NGPON2 requires only 60 nm between the upstream and downstream, thus a fully monolithic approach may be achievable by changing slightly the doping or applying regrowth. Figure 7 provides a representation of the gain curve for a generic material doping, the abstraction of a laser for the transmitter side, and three types of available receiver techniques/devices. On the transmitter side tunability limitation of a laser is depicted and corroborated by the limited gain media in which the laser light is generated, see Figure 7A. This constraint results in 20 nm to 30 nm tunability range, considering laser operation with significant output power. Simple tuning based on thermal effects usually results in an available tuning range of 3 nm to 4 nm. A combination of a tunable device with an external cavity supported by a convenient control mechanism results in a wider tuning range. On the receiver side several techniques are described, such as a positive-intrinsic-negative photodiode (PIN), APD, and coherent [7; 8], see Figure 7B. The PIN is one of the cheapest and simplest detecting techniques presenting a wide light wavelength detection range. Its profile can be changed by the type of materials and doping used. PINs can be easily integrated into e.g., InP PIC, which is already available for bandwidths exceeding \(>\)35 GHz and operational wavelength ranges covering the common fiber telecommunication bands [33; 60; 85]. APDs are tougher to integrate, however, from the system point of view, they result in improved sensitivity due to their intrinsic avalanche gain.