# Optical Technology for NFV Converged Networks
## 3 Silicon Photonics Devices in Optical Transport: Challenges and Opportunities
### System Applications Enabled by SOAs Integrated in Silicon Photonics Devices  
Since SOAs can be integrated in a silicon photonic circuits, they allow to replace bulky and expensive Erbium Doped Fiber Amplifiers (EDFA) in optical networks. In an ideal implementation, optical amplifiers will be no longer present as separate blades or boxes, but they will be fully integrated in multi-functional silicon photonics subsystems including optical switches, wavelength multiplexers, optical receivers, and so on. Examples of applications enabled by silicon photonics circuits containing SOAs are optical receivers with integrated SOAs, to operate at low input powers in a Passive Optical Network (PON) [27; 28], and Dense WDM (DWDM) systems with SOAs used as line optical amplifiers in ROADMs [29]. However, this comes at a cost. It is known that SOAs typically have fast, sub-nanosecond, response times, which, combined with their non-linearity, may cause signal distortions [30]. For similar reasons, the use of SOAs is problematic in DWDM systems, where they introduce significant Cross-Gain Modulation (XGM) penalty [31]. Due to XGM, the gain experienced by a DWDM channel depends on amplitude and bit pattern of the other channels. Multi-channel non-linear effects, such as cross-phase modulation (XPM) and Four-Wave-Mixing (FWM), can further impair the crosstalk between channels [32]. For all these reasons, SOAs are typically used in single channel systems, at the transmitter output or the receiver input of a single channel transceiver, to increase the transmitted optical power or improve the receiver sensitivity, respectively. In DWDM systems, the XGM can be avoided operating in the small signal gain regime, i.e., with low input powers, where the SOA gain is insensitive to fluctuations of the input power. However, with low values of input power, the Amplified Spontaneous Emission (ASE) noise generated by the SOA is significant, reducing optical signal to noise ratio (OSNR) and system power budget. An example of SOA operating in small signal gain regime is its use at the receiver input in a PON, where the received optical power is limited by the presence of lossy passive splitters. This case will be analyzed in more detail in Section 4. Designing an amplified silicon photonic circuit requires to carefully model how the SOA behavior is affected by the silicon cavity characteristics, i.e., cavity size, facet reflectivity, maximum pumping current that is possible to use, etc. This poses additional challenges with respect to stand-alone SOAs. Design guidelines for integrating SOAs in silicon photonic circuits are provided in [30], for a SOI chip with the flip chip bonded SOAs. The numerical analysis used a multi section cavity model and was then experimentally validated in a 32 channels DWDM system. In [28], it is shown that the usual travelling-wave assumption for the SOA is no longer valid, due to unavoidable non-perfect mode matching at the interface between the spot-size converter and the SOA, and even small reflections need to be taken into account. So, a Nearly Traveling Wave (NTW) model was developed. Polarization insensitiveness was achieved by means of dual a polarization design with two SOAs placed at the two output of a polarization splitting (GC). The impact of the ASE noise was estimated in the worst-case polarization condition, i.e., when the signal is all in one of the input polarizations and only noise was present in the orthogonal one. The results in [29] indicate that a maximum residual facet reflectivity of \(4\times 10^{-4}\) for a chip-bonded SOA leads to a power penalty below 2 dB in a polarization-diversity twin SOAs receiver. Working in small signal gain regime is not always possible when SOAs are used as line amplifiers and in this case special care must be paid to limit the XGM. Design guidelines are given in [27], using as an example a 12 channels DWDM system with five ROADMs node in a ring network.