I. Introduction
Fiber to the home (FTTH) has been developing rapidly in recent years and will become a major technology for next generation broad-band access networks. An FTTH system consists of an optical line termination (OLT), located in an operator's central office, and optical network units (ONUs), located at customers' premises. The OLT is connected by a single fiber to an optical power splitter, which supplies the optical signal to several ONUs. The triplexer is one of the key components in FTTH systems that employ an analog overlay channel for video broadcasting in addition to bidirectional digital transmissions. At each of the ONUs, a triplexer is used to demultiplex two downstream signals from a single fiber to a digital receiver and an analog receiver and at the same time to couple the upstream signal from a digital transmitter to the same fiber. Similarly, at the OLT, a triplexer is used to combine a digital channel and an analog channel for downstream transmission while receiving an upstream digital signal from the ONUs. Simultaneous transmission on the same fiber is enabled by using different wavelengths for each direction and for each of the digital and analog channels. According to the ITU G.983 standard, the three commonly used wavelengths in passive optical networks (PONs) are 1310, 1490, and 1550 nm, for upstream digital, downstream digital, and downstream analog channels, respectively.