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


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.  

Fundamentals of SOA

A semiconductor optical amplifier (SOA) has been used in an arrayed of applications such as wavelength conversion, signal regeneration, pulse reshaping and power limiting. Because it is capable of high integration and volume manufacturing, it has been very popular when operation demand space and power efficiency. SOA has been deployed in wide spectrum of high-speed applications from long-haul to metro and access optical networks.  SOA is commercially available in different formfactors according to the application and requirements.

Figure 1, Stimulated Emission

The basic working principle of a SOA is the same as a semiconductor optical laser based on stimulated emission except that it does not have any feedback from reflection. Figure 1 explains the process of stimulated emission. The transmission medium is first excited with an external current such that the electron jumps to its higher energy level. An incoming photon of a specific frequency then interacts with the excited electron forcing it to drop back to its natural lower energy level. The energy lost during this process creates a new photon with identical phase, frequency, polarization, and direction of the incident photon.

Figure 2, Working Principle of SOA

Similarly, Figure 2 depicts the working principle of SOA. An electrical pump current is used to excite the electrons in the active region of the SOA. When the optical signal travels through the active region, it causes these electrons to lose energy in the form of photons and get back to the ground state. The stimulated photons have the same wavelength as the optical signal, thus amplifying the optical signal.
The key parameters used to characterize a SOA are gain, gain bandwidth, saturation output power and noise. Gain is the factor by which the input signal is amplified and is measured as the ratio of output power to input power (in dB). The optical gain of the unit is controlled by the SOA current. A higher gain results in higher output optical signal. Gain bandwidth defines the range of bandwidth where the amplification functions. A wide gain bandwidth is desirable to amplify a wide range of signal wavelengths.

Saturation output power is the maximum output power attainable after amplification beyond which no amplification is reached. It is important that the SOA has a high power saturation level to remain in the linear working region and to have higher dynamic range. Noise defines the undesired signal within the signal bandwidth which arises due to physical processing in the amplifier. A parameter called noise figure is used to measure the impact of noise which is typically around 5dB.

100G LR4 Amplification via SOA

The SOA is especially vital for 1310 nm wavelength optical transmission systems. The attenuation in 1310nm range of wavelength is 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. The high speed 100GBASE-LR4 Ethernet transceiver also 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.

Figure 3, SOA as a Pre-amplifier (up), LR4 Transceiver Form-Factors (down)

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.  

Figure 4 illustrates the application of SOA with 100GBASE-LR4 for special application where existing 10Gbps DWDM network in metro distance needs to be upgraded with an additional 100Gbps service. The SOA-enhanced LR4 signal can be multiplexed together with 40 times 10Gbps DWDM signals using a special 1310nm+DWDM multiplexer to increase the capacity to 500Gbps over metro distances. In this scenario the transmission distance would be >10km and depending on the multiplexer loss, fiber loss and transceiver form-factor, the maximum transmission distance can be evaluated. One also needs to bear in mind that in contrast to EDFA, SOA amplifies the 1310nm spectrum but vice versa blocks all 1550nm signals.

Figure 4, Overlaying 10G DWDM with SOA-enhanced 100GLR4 service over >10km distance


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.