Progress towards heterodyne-type single-mode fiber communication systems

The unique performances attached to the heterodyne (or coherent) optical detection process can be fully exploited with single-mode fiber transmission. Strong improvement over direct detection is expected in terms of bit error rate (BER) or in terms of repeater spacing, especially for 1.6 μm operation. The requirements of such a system are reviewed and interpreted in terms of individual components design. The emission frequency stability of laser diodes, the ability of single-mode fiber cables to maintain polarization, and the availability of direct light amplifiers are found to be the most critical points, and recent theoretical and experimental results are reported. It is concluded from these very promising results that a heterodyne-type communication system using single-mode fibers at 1.6 μm may offer repeater spacings of over 200 km in the near future.