ting GPON in the same ODN is removed, the situation changes completely. In this case, the use of CWDM is quite effective, allowing eight colored GPONs (16 wavelengths) to be superimposed on the same ODN SO creating a simple and powerful structure that takes advantage of the simplicity of the transmitter and receiver that are sold in great volumes for datacom and other applications. However, the main problem, in term of cost of this architecture, is the need for colored sources both at the OLT and at the ONU. As a matter of fact, not only colored (and tem- perature stabilized in the DWDM case) sources have a much higher cost with respect to the simple sources used in standard GPON, but the management of the spare parts that have to be available for fast maintenance in case of failure is more complex, increasing the OPEX related to these equipments. This problem can be removed in two ways. The first is to introduce a sufficiently cheap tunable laser suitable for this application. In this case, the management of spare parts is the same as in the gray case. Today, tunable lasers, existing only for DWDM applications, are still too costly, but the situation could change. As a matter of fact, on one side VCSELs seem to be almost ready for application in the third window, and tunable VCSELs have been obtained and tested in labs experi- ments [17]. On the other hand, the technology of lasers bars (see Chapter 5) could open the way to the realization of a CWDM tunable laser, which is an impossible task if designed with traditional tunable architectures. The second method to solving the problem of having colored interfaces at the ONU is more on the system side. At the OLT, 2N lasers (one for each different ODN terminating on the OLT) generate 2N wavelengths that are divided into upstream and downstream bandwidths. Downstream wavelengths are directly modulated and transmit the signal toward the end users. The New Access Network Systems and Enabling Technologies Unmodulated BLS Cyclical Downstream band Upstream band FIGURE 11