# Optical Technology for NFV Converged Networks
## 3 Silicon Photonics Devices in Optical Transport: Challenges and Opportunities
### Silicon Photonics in Optical Transceivers and Laser Integration Issues  
An area where silicon photonics is already successful in the market is that of optical transceivers. However, even with a silicon photonic implementation, significant advances are required to meet the needs of 5G deployments that will require a throughput in the order of Terabit/s and power consumption of few pJ/bit/s [1,31]. This will probably call for co-packaged solutions where photonic transceivers are integrated in a multi-chip module with digital Integrated Circuits (IC). Integration of silicon photonics with III-V lasers is crucial also here: in [33], a co-packaged 1.6 Tb/s (4 \(\times\) 400 Gb/s) silicon photonic engine is described. In [34], a co-packaged engine based on silicon photonics integration with III-V quantum dot lasers on a large-scale wafer is proposed. The throughput ranges from 800 Gb/s to 3.2 Tb/s, using 100 Gb/s Non-Return to Zero (NRZ) or 200 Gb/s Order--4 Pulse Amplitude Modulation (PAM4). Silicon photonics can also be helpful to decrease the cost of DWDM high-speed TRX. For example, a cost-effective coherent receiver based on a silicon photonics 120\({}^{\circ}\) hybrid has been demonstrated in [35]. As a second example, in [36] 224 Gb/s were transmitted on a single wavelength over 10 km of fiber using a Stokes receiver and an integrated silicon nitride optical dispersion compensator.  
To support computationally insensitive applications, such as baseband processing in massive multi-input multi-output (MIMO) systems and Terabit/s packet switching, the density of Integrated circuits (IC) per surface units is expected to remarkably increase in modern Printed Circuit Boards (PCB). This leads to an increase in the devices operating temperature, especially in applications where active cooling is problematic for cost, size, or power consumption constraints, such as outdoor radio units. One known advantage of silicon photonics is its capability of operating at high temperatures [37]. However, lasers are needed anyway in optical transceivers and they are notoriously very temperature sensitive. Quantum dot lasers can in principle operate at high temperatures and can be integrated with silicon photonics devices [38] but in field operation and high yield need to be demonstrated yet. Using Remote Laser Sources (RLS), where the laser is placed far from the PCB thermal hotspots and is externally coupled to an optical transceiver hosting a silicon photonics modulator, is an alternative to quantum dot lasers. Separating the laser from the rest of the transceiver leads to negligible additional cost over short link distances (in the order of 1 m). The main cost contribution is due to the Polarization Maintaining Fiber (PMF) needed to connect laser and modulator. Over longer distances (in the order of 10 m or higher), using PMF is instead unpractical, due to cost. In those cases, special RLS arrangements [39] can be used to face the changes of the light polarization state during the propagation over a Standard Single Mode Fiber (SSMF) link. They unavoidably lead to an increase in cost that anyway remains lower compared to alternatives (e.g., active cooling), at least until quantum dot lasers will become available in large scale.