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The high speed 100GBASE-LR4 Ethernet transceiver uses four 28Gbps wavelength division multiplexed (WDM) co-propagating signals in the 1310nm region separated by 800GHz spacing.The reach of this transceiver is specified up to 10km. The attenuation in 1310nm range of wavelength is however higher than in the 1550nm band and therefore amplification becomes an important requirement. A normal Erbium doped fiber amplifier (EDFA) would not work as it amplifies 1550nm range of signals but blocks the 1310nm signal.
For applications beyond 10km, 100GBASE-ER4 Ethernet was proposed which has transmission reach specified up to 40km. The ER4 transceiver can be manufactured by either using a semiconductor amplifier (SOA) or an avalanche photo-detector (APD) at the receiver subassembly inside the transceiver to achieve better receiver sensitivity and increase the power budget. Commercial ER4 transceiver is available in CFP and CFP2 form-factors. However, because of the power and space limitation of smaller form-factors, 100G ER4 transceiver is not possible and therefore not available in CFP4 or QSFP28 form-factors. Additionally, the switches and routers have started to transition to high-density QSFP28 ports to maximize the capacity and minimize space, power dissipation and finally cost. This effectively makes ER4 transceivers uninteresting for such a system.
As depicted in the Figure, with a simple addition of the SOA, the reach of the 100G LR4 CFP can be easily extended to approximately 40km. The SOA is used as an optical pre-amplifier, i.e., it is used in front of the receiver to boost the incoming weak signal. For such a scenario, external amplification of the 100G LR4 signal presents as an optimal solution when longer distances (>10km) is desired. This would not only serve the purpose of amplification, but would also inherently enjoy the independence on the transceiver form-factor and switch port type.