Abstract
Coherent optical transmission systems have been researched energetically since the 1980's. Many
technologies and devices were developed such as heterodyne/homodyne detection, narrow linewidth
lasers, LiNbO3 modulators, and twin-PIN photodetectors. CPFSK optical heterodyne detection systems
for submarine communications were developed and tested for commercial use in 1990. However, the
developed systems were not in use because optical amplifiers were developed and used instead of
coherent systems. Recently, large capacity networks are required by increased internet traffic. Large
capacity transmission systems are required without increasing the symbol rates. Therefore, coherent
systems are attracting attention due to their potential for polarization multiplexing and multi-level
modulation by combining digital processing technologies. Digital coherent communication systems,
which have also been investigated, are indispensable for future networks. These technologies will play
an important role for multi-core and few-mode fiber transmission systems.
