Analysis of OEO Transponder Application in WDM Network

Anyone who has experiences of deploying WDM networks, either DWDM or CWDM networks, may be familiar with OEO transponder. Since in WDM network deployment, especially for long haul transmission, OEO transponder plays an important role. OEO transponder, also known as WDM transponder, means optical-to-electrical-to-optical. That is to say, it converts an optical signal to an electrical signal, and then recovers it to an optical signal. In some cases, OEO transponder serves as fiber mode converter or repeater for long distance transmission.

Functions of OEO Transponder
Wavelength Conversion
As we all know, when add a CWDM Mux/Demux or DWDM Mux/Demux into a WDM network, there is a requirement to convert the optical wavelengths like 850nm, 1310nm and 1550nm to CWDM or DWDM wavelengths. Then the OEO transponder comes to assist. The OEO transponder receives, amplifies and re-transmits the signal on a different wavelength without changing the signal content.

Fiber Mode Conversion
It’s know to us that multimode fiber optic cables (MMF) are often used in short distance transmission, while single mode fiber optic cables (SMF) are applied in long optical transmission. Therefore, in some network deployment, considering the transmission distances, MMF to SMF or SMF to MMF conversions are needed.

Signal Repeating
In long haul fiber optic transmission, OEO transponder also can work as repeater to extend network distance by converting wavelengths (1310nm to 1550nm) and amplifying optical power. The OEO converter converts the weak optical signals from the fiber into electrical signals, and regenerates or amplifies, then recovers them into strong optical signals for continuous transmission.

Analysis of OEO Transponder Application Case
Having known about the function of OEO transponder, here let me take some application cases as examples to illustrate its applications clearly.

Case One
The distance between site A and site B is about 165km, and there is a repeater station C. The distance between A and C is 90km. The client needs to build connection between A and B. Just like the following picture shows.

In this solution, three OEO transponders are used in this links according to the requirements of the client. The use of the first OEO converter at site A is to convert the signals from MMF to SMF, achieving the long distance transmission between site A and C. The second OEO transponder re-generates and amplifies the optical signal, then convert the it from dual fiber to single fiber. At site B, the OEO transponder re-amplifies the optical signal and recovers it to multimode transmission.

Advantages of this solution: use OEO transponder to achieve fiber mode conversion and long distance transmission; make full use of the OEO transponder (retime, regenerate and reshape) to realize high quality connections; save cost by using the OEO transponder.

Case Two
This solution is more complicated than the first one. There are three sites with fiber links between them. The distance between site A and B is 84km, and site B and C is 1km. Site A and C is 84km too. All the 10G connections are dual fiber transmission. Here is a simple picture of this solution.

As we can see in the figure, to build DWDM networks between these three sites, six OEO transponders are deployed. Each site uses two OEO transponders. The OEO transponder at site A converts the 10G-LR signals into 10G DWDM wavelengths, then the wavelengths are multiplexed by the DWDM Mux. At site B, the separated wavelengths are recovered to 10G-LR signals through the OEO transponder. The transmission between site B and C, site A and C are similar to the transmission between site A and B. In addition, there are two EDFAs in each two long distance transmissions.

Advantages of this solution: using OEO transponder for wavelength conversion. Converting common 10G signals into DWDM wavelengths and transmitting them with DWDM MUX/DEMUX increase the network capacity easily. At the same time, it also reduces the damage of optical transceivers.