Introduction

Coherent detection, homodyne or heterodyne, was the focus of extensive research and developments during the 1980s and early 1990s [11,14-18], and was the main detection technique in the first three generations of lightwave transmission systems. At that time, the main motivation for the development of coherent optical systems was to improve the receiver sensitivity, commonly by 3-6 dB [14,17] and the boosting of the received optical channel by the mixing with a high-power local oscillation laser to overcome the limitation of transmission distance by fiber attenuation. This was thought possible due to the fabrication and manufacturing of the single-mode fiber (SMF). The

SMF has a very small index difference between the core and the cladding (about 0.3% difference), thus very weak tightness of the guided mode in the core of the fiber; only 70% of optical power of the guided mode is confined within the core region. This weak tightness of the mode allows the long-distance guidance and low scattering loss at the boundary between the core and the cladding. The guided mode has been termed as the weakly guided mode or the linearly polarized mode. Thence, the two-polarization mode has two very similar modes, the vertical and horizontal polarized modes, that propagate at a very close phase velocity. At that time, the operating speed of the system was still in the MHz region, and no attention was paid to the difference in these polarized guided modes till the last decade of the twentieth century.