Gigabit Passive Optical Network (GPON) is a high bandwidth shared fiber access technology
The following diagram shows the architecture of a basic two wavelength GPON network, which is probably the most common implementation. The downstream wavelength is 1490nm and transmits data at 2.488 Gbps. The upstream wavelength is 1310nm and transmits data at 1.244 Gbps.
The GPON Optical Line Terminal (OLT) is typically installed in a Central Office (CO), though it could be installed elsewhere. The optical splitter is installed somewhere between the CO and the subscribers. And a GPON Optical Network Terminal (ONT) is installed at each subscriber’s home. Voice, video, and data traffic must all be delivered across the single GPON downstream wavelength. A nice facet of GPON for IP video support is that its downstream is naturally a broadcast medium, and it is very efficient for delivering multicast traffic.
The optics in the GPON ONT for a two wavelength implementation is called a diplexer. See the diplexer diagram below. Diplexers can be implemented with a three dimension bulk optic design (discrete components aligned and welded together manually) or with a Planar Lightwave Circuit (PLC) design (link to a good article by Enablence explaining bulk optics and PLCs). A PLC puts all its optical components on a silicon substrate for a two dimension design, eliminates all the complexity of dealing with a third dimension, and allows for low-cost automated manufacturing
Read more at http://www.fttxtra.com/ftth/gpon/gpon-tutorial/