A low-crosstalk design of 1.25 Gbps optical triplexer module for FTTH systems

In this paper, we analyzed and measured the electrical crosstalk characteristics of a 1.25 Gbps triplexer module for Ethernet passive optical networks to realize fiber-to-the-home services. Electrical crosstatk characteristic of the 1.25 Gbps optical triplexer module on a resistive silicon substrate should be more serious than on a dielectric substrate. Consequently, using the finite element method, we analyze the electrical crosstalk phenomena and propose a silicon substrate structure with a dummy ground line that is the simplest low-crosstalk layout configuration in the 1.25 Gbps optical triplexer module. The triplexer module consists of a laser diode as a transmitter, a digital photodetector as a digital data receiver, and an analog photodetector as a cable television signal receiver. According to IEEE 802.3ah and ITU-T G.9833, the digital receiver and analog receiver sensitivities have to meet -24 dBm at BER=10(-12) and -7.7 dBm at 44 dB SNR. The electrical crosstalk levels have to maintain less than -86 dB from DC to 3 GM. From analysis and measurement results, the proposed silicon substrate structure that contains the dummy line with 100 mu m space from the signal fines and 4 mm separations among the devices satisfies the electrical crosstalk level compared to a simple structure. This proposed structure can be easily implemented with design convenience and greatly reduce the silicon substrate size by about 50 %.