# Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited)
## 2 PIC Evolution
Despite the high processing technology maturity of silicon photonics, as an indirect bandgap material native gain is difficult to realize under this platform. The platform features are in-between InP and ultra-low loss SiN technologies, with some active detection elements but no light source solutions, and medial optical losses (amid InP and SiN) [55]. Regarding wavelength properties, Si/SiO- and InP-based platforms are mainly used in near-infrared (NIR), while silicon nitride can operate from visible to NIR [48]. Ultra-low loss SiN planar lightwave circuits (PLC) with propagation loss below 0.1 dB/m have been demonstrated by University of California, Santa Barbara (UCSB) and LioniX(r) research [58], orders of magnitude lower than other reported platforms. As a result, high-performance passive components are enabled by this platform, though electrical pump for gain is unavailable due to the insulating nature of SiN/SiO glass WGs [55]. However, the demonstration of optical gain using erbium-doped WG distributed Bragg reflector (DBR) and distributed feedback (DFB) laser arrays integrated within ultra-low loss SiN [59] shows potential for the platform's active component integration. Silicon is \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Building Block (BB)** & **InP** & **Si** & **SiN** \\ \hline Passive components & ✓ & ✓ & ✓ \\ Lasers & ✓ & \(\bigcirc\) & H \\ Modulators & ✓ & ✓ & ✓ \\ Switches & ✓ & ✓ & ✓ \\ Optical amplifiers & ✓ & \(\bigcirc\) & H \\ Detectors & ✓ & ✓ & ✓ \\ \hline Footprint & ✓ & ✓ & ✓ \\ Chip cost & ✓ & ✓ & ✓ \\ CMOS compatibility & \(\bigcirc\) & ✓ & ✓ \\ Low-cost packaging & \(\bigcirc\) & \(\bigcirc\) & ✓ \\ \hline Performance: ✓ & very good; ✓ & good; ✓ & modest; & challenging; & very challenging. \\ H: hybrid/heterogeneous processes to overcome the design challenges associated with the monolithic integrated technology. 1 End-fire coupling (low reflection); 2 vertical coupling (medium reflection). & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Key features from three available photonic integration platforms, i.e., indium phosphide (InP), silicon (Si), and silicon nitride (SiN) monolithic technologies. among the most mature photonic integrated platforms, leveraging from existing technologies like silicon electronics and micro electro-mechanical systems (MEMS) industries and enabling foundry services robustness and product commercialization. InP platforms can be seen as one of the options that stem out from the alternatives since it integrates monolithically high-quality lasers, receivers, amplifiers, and passives [33; 35; 60]. Si and SiN are more prone to be used for passive and filtering components, with the need to recur to hybrid packaging [61] when incorporating lasing and gain. Some of the available foundries/players under the InP, Si, and SiN integrated technologies are described below:  
1. InP: Fraunhofer Heinrich-Hertz-Institut-Fraunhofer HHI (Berlin, Germany); Smart Photonics (Eindhoven, The Netherlands); Global Communication Semiconductors, LLC (Torrance, CA, USA); and fabless solution providers, e.g., the INTENGENT III/V Photonics Company (Ontario, Canada), VLC Photonics (Valencia, Spain), Bright Photonics (Eindhoven, The Netherlands), and PICadvanced (Aveiro, Portugal);
2. Si: Acacia Communications, Inc. (Maynard, MA, USA); Luxtera Inc. (Carlsbad, CA, USA); Intel Corporation (Mountain View, CA, USA); Cisco Systems Inc. (San Jose, CA, USA); Mellanox Technologies (Sunnyvale, CA, Israel/USA); Finisar Corporation (Sunnyvale, CA, USA); Hamamatsu Photonics K.K. (Shimokanzo, Iwata City, Japan); International Business Machines Corporation (IBM, Armonk, NY, USA); GlobalFoundries Inc. (Santa Clara, CA, USA); NeoPhotonics Corporation (San Jose, CA, USA); Oclaro Inc. (San Jose, CA, USA); CompoundTek (Singapore); and the American Institute for Manufacturing Integrated Photonics (AIM Photonics, Albany, NY, USA);
3.