 to be done chip by chip. Moreover, the fabrication of chips on a certain surface of InP costs much more than the fabrication of chips on the same area of a silicon wafer due essentially to the different cost and dimensions of the reference wafer. The different cost trends, with volume offered by hybrid and integrated technologies, are an advantage that could be precious in the long run, when PONs volumes go up unavoidably. Telecommunication Networks On this ground, much slower that the optimistic forecasts of 10 years ago, a certain num- ber of GPON interfaces based on hybrid integration has been launched in the market in the last few years. This is an important event since it is the first time that integrated optics is used to reduce prices and acquire a scalable production model and not just to achieve better performances. 11.5.2 WDM-PON and XWDM-PON Interface Technology Differently from GPON, WDM-PON needs one optical interface per user not only in the ONU but also in the OLT as is shown in Figure 11.11. For this reason, optical interface tech- nology is a key in implementing a cost-effective WDM-PON. A trivial way of realizing a CWDM-PON is to use standard CWDM SFP transceivers. In this way, however, the cost of optical interfaces scales linearly with the number of users, being too high if a high number of users is considered. Moreover, colored ONU have to be used, thereby increasing operational costs due to the more complex management of spare parts. Both this problems could be solved by a new class of components, called Photonic Integrated Circuits (PIC), which we have introduced for a different application in Chapter 10 [31,32]. The functional scheme of a transceiver containing a transmitting and a receiving PIC is represented in Figure 11.17, which is functionally equal to Figure 10.1. In the transmitting PIC, a bar integrating a certain number of lasers is coupled with a PLC mux/demux (gener- ally an AWG) that launches the multiplexed channels into the output fiber. The scheme of Transceiver WDM PIC-RX 