# Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited)
## 4 PIC for Next-Generation PON
### Band and Wavelength Selection Criteria  
To improve reach, several techniques are available and many stem from the laser (e.g., low dynamic chirp or frequency modulation efficiency), laser driving (pre-chirping or equalization), and wavelength selection (e.g., O-band instead of C and L bands) [2; 70]. O-Band has been used for supporting upstream and was adopted by some of the standards like GPON and XGSPON. O-band, besides presenting low or null dispersion, only introduces slightly higher attenuation than C-band. These facts greatly reduce the pressure on the O-band components industry, impacting positively on the cost and yield. The current spectral allocation from O to L-band is depicted in Figure 5.  
If a certain technology is deployed in greenfield, a viable approach is to implement standards that are not back-compatible, i.e., does not support legacy technologies. In this scenario, the use of O-band is an option for future PON. The drawback of this option is that the operator has to guarantee that the PON evolution is on top of the new technology deployed. To cope with the maturity of the technology and promote an easier and simplified entry into the market, initial PON technologies, like GPON, were set in large bandwidths for better yield in laser choice with the upstream in the O-band [7; 67]. The wavelength range of 20 nm to 30 nm per traffic direction was reserved for this technology, which allowed it to easily mature while still being competitive. This strategy proved to be solid since it resulted in the current GPON BOSA to sit in the few dollars range (as of today). Furthermore, PON technology evolution can be achieved by data rate multiplication and more efficient use of the bands to promote further network evolution scenarios. Following this path, XGSPON  
Figure 5: Spectrum allocation in ITU-T based standards for PON and trends for channel spacing. standard was conceived with slightly smaller bandwidths for both upstream and downstream, which resulted in a four-times higher bitrate than GPON [70; 78]. To further exploit the available bands (see Figure 5), dense wavelength division multiplexing (DWDM) can be a route to explore. The possibility of including tunability allows network active transformation and management. Even considering the associated price to pay, it is an important solution under the requirement of tightly controlled lasers. Furthermore, with this capability, a projected OPEX reduction is expected, since clients can seamlessly migrate to any of the available wavelengths while the network grows smoothly and systematically. The most evident barrier for the spread of this technology is the laser and receiver requirements, which should be attained under a potential low cost and simultaneously have a high grade of stability [7; 8]. Figure 5 presents the bands for the different steps of the network evolution, i.e., from the legacy technologies wide bands to the next generation tighter bandwidths. A representation of the typical distribution of lasers, stemming from a single wafer run, is depicted in Figure 6A. Each of the lasers collected from the wafer will have its emission wavelength and thus they can be used in a more specific/tighter wavelength range. To accomplish this feature, a tuning process is required, e.g., temperature, current, etc. Each wafer run may present different statistics due to material deposition doping and processing, see Figure 6B. Nonetheless, the wafer uniformity wavelength map is in general scarcely reported by the foundries, some works suggest these claims [79; 80]. Tight control of the doping process may reduce this variation and greatly improve the yield and temperature control mechanisms. The finer the required tuning, the more complex the laser selection process. The qualification of a laser to operate in a certain wavelength range results from setting its maximum allowed wavelength at initial temperatures.