What is an optical transponder (OEO)?
An optical transponder consists of a transmitter and a receiver, similar to a transceiver that includes a transmitter and a receiver. An optical transponder extends the transmission distance by converting wavelengths and amplifying signals. It automatically receives, amplifies and then re-transmits signals of different wavelengths without changing the data/signal content. That is, the optical signal received by the transponder is converted into an electrical data stream, which is then processed and regenerated. The transponder then converts the signal from standard optical wavelengths into an optical CWDM (coarse wavelength division multiplexing) or DWDM (dense wavelength division multiplexing) signal. This process is commonly referred to as OEO (optical – electrical- optical) conversion.
Modern WDM transponders are equipped with a 3R system (reshaping, re-timing, and re-amplifying) that goes beyond simple signal regeneration in the process. This system allows for accurate and precise cleaning, monitoring, and amplification of the signal.
Why do I need an optical transponder (OEO) in a WDM system?
There are several reasons why optical transponders are needed in WDM systems. Firstly, optical transponders can solve the problem of incompatibility between devices operating at different wavelengths when they need to communicate with each other. Secondly, there are multiple fiber optic network provided by different providers and operating under different standards. We need WDM transponders to facilitate conversions between these diverse fiber optic networks, these requirements can be divided into three types of conversions in practical applications.
Converting multi–mode fiber to single–mode fiber
It is well known that multi-mode fiber (MMF) is commonly used for short-distance transmission, while single-mode fiber (SMF) is used for long-distance transmission. Mode conversion is required when the transmission distance exceeds the limits of MMF or when connectivity is required between multi-mode devices and single-mode devices. For example, the following two switches, which are located far apart, are connected by two optical transponders that convert MMF to SMF. A typical application for this feature is commonly used to extend the distance between 10G optical transport network (OTN) and synchronous optical network (SONET) rings.
Converting Dual Fiber to Single Fiber
Dual and single fiber conversion is also required in networks. Dual fiber transmission uses the same wavelength on two different fibers, whereas single fiber transmission uses two different wavelengths on a single fiber, which is known as bi-directional (BiDi) transmission. In this case, two long-haul dual-fiber switches are connected using two optical transponders. With bi-directional BiDi single fibers, where two different wavelengths are transmitted on a single fiber, the transmission (Tx) at one end of the fiber matches the reception (Rx) at the other end, and vice versa.
Wavelength conversion is one of the most common applications of optical transponders in wavelength division multiplexing(WDM) systems. Fiber optic network equipment with fixed fiber interfaces operating at legacy wavelengths (850 nm, 1310 nm, 1550 nm) need to be converted to CWDM or DWDM wavelengths using optical transponders which are capable of transmitting different wavelengths with small form-factor pluggable (SFP) transceivers for wavelength conversion. In the figure below, a 10G switch with a signal output of 1310nm needs to be connected to a CWDM Mux/Demux channel port with a wavelength of 1530nm. An optical transponder operating with SMF SFP+ and 1530nm CWDM SFP+ is used between the switch and CWDM Mux/Demux for wavelength conversion.
Optical transponders can be used to convert different types of signals, including multi-mode to single-mode, dual-fiber to single-fiber, and one wavelength to another. During this process, signals are also re-amplified, monitored and cleaned up, without modifying their original bit rate. This enables longer transmission distances.