# Optical Technology for NFV Converged Networks
## 3 Silicon Photonics Devices in Optical Transport: Challenges and Opportunities
### Thermal Management in Silicon Photonics  
Thermal management represents one of the main challenges of silicon photonics. This aspect has important impacts in the use of tunable optical components, since lasers and optical filters based on resonant optical elements are controlled via the thermo optic effect. Currently, the approach adopted in transceivers with a tunable laser source is the use of solid state thermo-electric coolers (TEC), also indicated as macro-TEC, together with resistive heating elements for fine-tuning each laser. The TEC acts to maintain a reference temperature of the photonic chip: a temperature sensor monitors the temperature at a given location in the chip and the measure is used in a feedback control, generally based on a Proportional-Integral-Derivative (PID) algorithm, that operates the TEC. The heatersthen are used to independently set the local temperature that corresponds to the desired frequency of operation of each laser in the chip. This method is not optimal from the point of view of power consumption and packaging, especially in the perspective of obtaining a higher integration level. A different approach that promises a more efficient temperature control and a higher potential for dense integration consists in the use of micro-TECs [40; 41]. These are micron-size coolers that are placed in proximity of the hot-spots, minimizing the parasitic thermal resistance between the heat source and the thermoelectric cold junction. The coefficient of performance (COP) of micro-TECs is estimated to be about 0.8 COP, against the 0.3 value of traditional macro-TECs. An alternative approach to the use of TECs is the use of a more complex feedback loop based on the monitoring of the local temperature and the adaptation of local heater's power. Temperature monitoring can be performed directly, with thermal photodiodes [42], or indirectly by inferring the thermal shift from the output power [43] or the data signal [44]. The tuning precision of the micro-heaters can be conveniently set to meet the maximum tolerated deviation from the laser nominal frequency. For example, an optical filter or switch based on a micro-ring resonator (MRR), having a typical radius size of 6 \(\upmu\)m, has a 0.01 nm precision (1.25 GHz at 1550 nm) on the central frequency, for a temperature control precision of 0.1 K. This calculation refers to a Silicon on Insulator platform (SOI), the value of the thermo-optic coefficient of silicon is CS = 1.86 \(\times\) 10\({}^{-4}\) K\({}^{-1}\). However, the actual frequency deviation depends on the speed of the feedback control mechanism, i.e., in the actuation of the heater power change following a monitoring measurement. The control speed must be able to follow the temperature fluctuations that, for various reasons, the chip can undergo. It is possible to achieve a stronger stability to temperature fluctuations with materials such silicon nitride, which has thermo optic coefficient CN = 2.51 \(\times\) 10\({}^{-5}\) K\({}^{-1}\)[45]. Alternatively, larger radius MRRs can be designed. Both these options allow to achieve a higher precision of the tuning mechanism, but at the expenses of a reduction in the tuning range. Is Silicon Photonics Promise of Cost Reduction and Ease of Integration, When All Issues Are Considered, Significant Enough to Make Silicon Photonics a Game Changer? In the previous sections, we have introduced silicon photonics as the photonic integration technology with the highest potential for reducing the cost of optical components in communications systems, ideally making costs and volumes comparable to those of ICs in consumer devices. We also discussed the performance gaps that remain to be filled before this goal can be achieved, in terms of coupling issues with optical fiber, polarization control issues, integration issues with semiconductor optical amplifiers, and issues in interfacing silicon photonics devices with laser sources.