Coherent fiber-optic communications (review)

This review is devoted to heterodyne and homodyne methods for the transmission and reception of data along single-mode fiber waveguides. The advantages of the coherent data transmission methods over the currently used methods of modulation of light intensity and direct detection are demonstrated. The main problems are identified and the methods of overcoming them are suggested. These problems include the need to reduce the width of an injection laser line and to stabilize its frequency, temperature, and injection current, as well as the problems of fabrication of single-mode fiber waveguides which conserve the polarization of light and of the use of optical amplifiers in communication lines in place of regenerative repeaters. Heterodyne and homodyne reception systems are described.