This paper reviews the advantages of the coherent optical fiber communication systems over the conventional systems. There are two kinds of detection schemes: heterodyne and homodyne detection

they provide higher receiver sensitivity than direct detection. On the other hand

there are two kinds of digital modulation schemes: FSK

CPFSK

and PSK.DPSK

their receiver sensitivity and linewidth requirements are dif- ferent from each other. Then the paper discusses the requirments to single frequency laser

including phase noise and linewidth.It shows that DFB structure is desirable. The dual-PIN balanced mixer used for heterodyne detection is mentioned. The progress of bitrate-distancc product of experimental system is discribed

up to now

the best records ars 400 Mb/s-290 km and 2 Gb/s-200km. Since the coherent optical fiber communication offers very close channel spacing

it allows densely packed FDM operation

theorectically up to 1000 channels of optical carriers. The use of phase diversity reception and polarization diversity reception to improve the system performance is briefly described. Finally the paper briefly mentions the possible application of the coherent fiber communication in local subscriber network. There are highly -coherent

medium-coherent and weakly-coherent systems

present applications are mostly of medium-coherent optical fiber communications

which will be widely used after the opto-electronic integration OEIC becomes successful.