What is a transponder?

Transponder is the integrated part of WDM systems use to transmit signal over a DWDM link. This module takes black and white or grey signals as input on 1310nm, 1550nm or 850nm and converts those signals into colored channels or certain frequencies in C or L band. This is achieved by using optical-electrical-optical conversion mechanism. Transponders along with Optical source also includes complex components that helps signal in serialization and deserialization of frames, control and monitoring capabilities etc.

There are two types of transponders: 

• optical – to – electrical – to – optical (O – E – O)
• optical – to – optical (O – O). 

The O – E – O transponder may also act as a 3R repeater; that is, it performs signal reshaping, retiming, and reconstitution or gain; O – E – Os are more complex and more expensive, Because the signal is converted to electronic, an O – E – O node allows for add – drop functionality, in addition to simple optical relay or transponder. 

The O – O transponder, or optical relay, is technologically more attractive because it performs direct optical – to – optical amplification using optical amplifiers (doped fiber – based (EDFA) or semiconductor optical amplifiers (SOA)) thus acting as an all – optical relay.