What Is Optical Transponder (OEO)?
An optical transponder consists of a transmitter and a responder, which is similar to a transceiver that includes a transmitter and a receiver. The optical transponder extends the transmission distance by converting the wavelengths and amplifying the signal. It automatically receives, amplifies and then retransmits a signal on a different wavelength without changing the data/signal content. That is to say, an optical signal received by the transponder is converted to an electrical data stream which it then processes and regenerates. The transponder then converts the signal of standard optical wavelengths to an optical CWDM (coarse WDM) or DWDM (dense WDM) signal. This process is commonly referred to as OEO (optical to electrical to optical) conversion.
Figure 1: Schematic diagram of a fiber optic transponder.
Instead of simply regenerating the original signal during the process, modern WDM transponders are endowed with the 3R system (reshape, retime, reamplify), which is capable of cleaning up, monitoring and amplifying the signal accurately and closely.
Why Need Optical Transponder (OEO) in WDM System?
The optical transponder is needed in WDM system for several reasons. Firstly, in a case where equipment of different wavelengths need to communicate with each other, the optical transponder can solve the incompatible problem between them. Secondly, there is more than one fiber optic network provided by different providers and of different standards. We need WDM transponder to traverse from one fiber optic network to another. These requirements are divided into three converting types in practical applications.
Convert Multimode to Single-mode Fiber
It is known to us that multimode fiber (MMF) is often used in short-distance transmission and single-mode fiber (SMF) is for long-distance transmission. Mode conversion is needed when the transmission distance exceeds the limitation of MMF or when connectivity between a multimode device and a single-mode device is required. For instance, the following two switches located far from each other are connected via two optical transponders that convert MMF to SMF. Typical application of this function is for extending the distances of 10G Optical Transport Network (OTN) and Synchronous Optical Network (SONET) ring.
Convert Dual Fiber to Single Fiber
Conversion between dual fiber and single fiber is also required in networks. Dual fiber uses same wavelength over two different strands of fiber, while single fiber uses two different wavelengths over one strand of fiber, which is known as bidirectional (BiDi) transmission. In this case, two dual fiber switches of long distances are linked via two optical transponders. As the BiDi single fiber owns two different wavelengths in one strand of fiber, the transmit (Tx) at one end of the fiber matches the receive (Rx) from the other end, and vice versa.
Figure 3: Convert dual fiber to single fiber.
Convert Wavelengths
Wavelength conversion is the most common application of optical transponder in WDM system. Fiber network equipment with fixed fiber interfaces that operate over legacy wavelengths (850 nm, 1310 nm, 1550 nm) must be converted to CWDM or DWDM wavelengths via optical transponders, which are capable of wavelength conversion together with small-form pluggable (SFP) transceivers that transmit different wavelengths. In the figure below, a 10G switch with signal output at 1310 nm is needed to be connected to a CWDM Mux/Demux channel port with wavelength of 1530 nm. A transponder operating with a SMF SFP+ and a 1530nm CWDM SFP+ is used between the switch and the CWDM Mux/Demux to achieve wavelength conversion.
Figure 4: Convert wavelengths.
Optical transponder is versatile for converting different types of signal, including multimode to single-mode, dual fiber to single fiber and one wavelength to another wavelength. During the process, the signal is also re-amplified, monitored and cleaned up but will not be modified to be different from its original bit rate. So that the transmission distance will be extended.
