Optical Transponders

The first commercial 10 - Gb/ s transponders, deployed in the mid 1990s, were based on a very simple modulation technique, i. e., a binary light intensity modulation with envelope detection by a single photodiode. To extend the fiber capacity, bandwidth-efficient modulation techniques such as duobinary line coding and multilevel intensity-modulation formats gained popularity in optical communications in the late 1990s. In the following years, the use of differential phase modulation in combination with interferometric detection allowed the transponder data rates to be increased up to 40Gb/s. However, despite all improvements, the system performance of these 40 - Gb/ s solutions was still not on par with state-of-the art 10 - Gb/ s systems at that time. With the advent of coherent detection, things suddenly changed and transmission rates of 100Gb/s and beyond could soon be achieved, thanks to the use of high-order modulation formats and advanced digital signal-processing techniques. In this chapter, the configuration and performance of the most common transmitter and receiver combinations that are currently used in optical transmission systems will be described, including an overview of transponder types and their hardware architectures. Finally, relevant standards will be discussed and pluggable optical transceiver modules used in modern transponder implementations will be explained.