Research And Development Of Testing System Of Burst-Mode Optical Transceiver Modules For EPON

The use of burst mode transmission is increasing in digital-communication systems. These systems realize point-to-multipoint connections over optical, wireless, and coaxial medias. Ethernet Passive Optical Network (EPON), a broad optical fibre access, that combined with proven technology of Ethernet and passive optical networks, resolves the bottleneck problem well. And EPON becomes one of the best solutions of broadband access, which provides the high efficiency and broad bandwidth for user terminals.Owing to star topology in EPON system, the upstream data multiplexed with TDMA (Time Division Multiplexed Access) mode in the Optical Network Units (ONU) are transmitted in burst mode. And the Optical line Terminal transmits the downstream data in broadcast mode, but it should receive data coming from the ONUs in burst mode, too. In practical EPON system, the signals arriving at the OLT have different amplitudes and phases because the distances between ONUs and OLT are different, and the attenuations are also different during the course of transport. So the burst-mode optical receiver module must establish the threshold to adapt to the rapid change of optical signals.In order to test the performances of the designed burst-mode optical transceiver modules, special testing devices or systems are necessary. But it is different between burst mode optical modules and continuous ones because of the speciality of burst optical signals and the high rate of 1.25Gbps. What's more, the normal error detector also cannot test Bit Error Rate (BER), so this context introduces a special testing system and offers the detailed testing methods. The testing system is consisted of the simulative generating system of burst optical signals and synchronous control circuits. The simulative generating system can generate two channel signals to simulate the upstream TDMA signal in practical EPON system. And the synchronous control circuits include the circuits of attracting framing head, counting packet length and detecting BER, etc. The multiplexed data can be divided through these circuits, and then all the indexes of the designed modules can be tested.The important testing indexes include launch optical power and extinction ratio for the burst-mode transmitter module, and sensitivity, saturated optical power and dynamic range for the receiver module. The extinction ratio is up to 6dB for the transmitter module. And for the receiver module, the saturated optical power is -8dBm, and the sensitivity is up to -30dBm at 1.25Gbps. In order to enhance the saturated optical power, the transimpedance of receiver module is replaced by the one with DC cancellation circuits, and then it is up to -1dBm.