.9 Implementation of a colored ONU in a PON using WDM multiplexing (either a WDM overlay of GPONs or a real (WDM-PON) using the injection locking of a Fabry-Perot laser diode with a seed coming from the OLT at the correct wavelength. Upstream wavelengths are transmitted unmodulated up to the end user. Every ONU is equipped with an identical, colorless Fabry-Perot Laser. When an unmodulated external optical beam is injected into the cavity of an FP Laser, it will experience the internal gain from the semiconductor material. This process results in a reflected and amplified signal that contains the data modulation imparted by the pump current. Since the injected signal can be much larger than the internally generated spontaneous noise, the majority of the optical output power will be at the wavelength of the injected signal. The normal multiwavelength spectrum of the FP is transformed into a quasi sin- gle-wavelength spectrum similar to that of a DFB laser. This narrowband output signal can then be efficiently transmitted through a WDM communication channel. The external injected wavelength is called the "locking" or "seeding" wavelength. This mechanism is shown in the plot of Figure 11.9. It is to be noted that since the upstream signal is amplified by the FP gain, it is possible to design the system SO that it is not disturbed by small reflections of the seed arriving in the downstream direction. Last, but not least, a single AWG can be used in this system, pro- cessing the signal in two directions. This is possible with the so-called cyclical AWGs [18] that are devised to contemporary multiplex one bandwidth in one direction and demulti- plex the other in the other direction. In this way, a colorless FP is forced to emit a colored signal by the incoming wavelength containing the wanted modulation and constituting the upstream signal coming from the ONU. It can be noted that in this WDM-PON architecture also, every used wavelength is emitted by a colored source. However, all the sources are collected in the OLT, 