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Essential technologies for developing coherent optical fiber transmission systems are reviewed from the viewpoint of regenerative repeater spacing and transmission data rate improvements. After describing the system features behind these advantages, recent progress in individual device and system technologies is delineated. Such progress includes frequency stabilization and linewidth reduction of semiconductor lasers, optical phase or frequency modulation, AM and FM noise suppression in coherent receivers, optical polarization stabilization and control in signal transmission, and optical direct amplification. Also described is progress in coherent optical transmission experiments carried out in many laboratories. Finally, future problems are discussed on the basis of the current state of the art.