SOA - Reach Extension 100G LR4 Metro Network

SOA - Simple Reach Extension for 100G LR4 Based Metro Network

Introduction


Before the introduction of optical amplifiers, reach extension was achieved via repeaters or regenerators. A regenerator is also called optical-electrical-optical (OEO) device since it converts the optical signal to an electrical signal, processes this signal (re-amplify, reshape and retime) and then converts back to an optical signal so that the signal can then cover longer distances. This process is not only expensive but also restricts the useable optical bandwidth due to the limitations of the electronics. 

The introduction of optical amplifiers in the 1990s conquered the regenerator technology and opened doors to the WDM technology. There are various types of optical amplifiers depending on the technique of amplifying, namely SOA (semiconductor optical amplifier), EDFA and Raman amplifier.
In this paper we delve in deeper into the SOA technology and look into its form-factor independent niche application with 100G LR4 Ethernet for metro networks. 

100G LR4 Amplification via SOA


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 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 Figure 3, 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. The 100G LR4 transceiver is available in different form-factors as depicted in Figure 3 (lower portion). It can be seen that LR4 transceivers are also available in smaller packages of CFP4 and QSFP28.

SOA presents itself as an easy plug and play alternative and because of its unique property of high integration; it can be developed in high-density line-cards. Since the 100G LR4 transceivers in different form-factors actually have subtle differences in the actual reach, the pump current of the amplifier needs to be changed to adjust the gain to satisfy the same application. With a simple addition of the SOA, the reach of the 100G LR4 CFP can be easily extended to approximately 40km. 

Conclusion


A semiconductor optical amplifier (SOA) is a simple plug-and-play reach extension solution for applications where 100GBASE LR4 Ethernet has been deployed and requires transmission reach up to 40km. 

External amplification using a SOA is independent of the 100G LR4 transceiver form-factors and supports pay-as-you-grow model to augment the existing 10Gbps DWDM services with additional 100Gbps service running over metro distances.
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