Evaluation method of propagated optical wave for coherent optical transmission

In coherent optical transmission systems using heterodyne detection for long‐distance and high bit rate transmission, the quality coefficient (product of polarization coefficient and degree of coherence) was introduced to evaluate the propagated optical signal (POS). Relating this coefficient to the bit error rate in the ASK, FSK, and PSK optical transmission methods, the minimum received optical power needed to realize the coherent optical transmission system has been obtained as a function of the quality coefficient and the transmission bit rate. Next, analyzing the phase difference between the POS and local optical signal (LOS) caused by the length of the optical fiber, the chromatic dispersion, and the thermal characteristic of the optical fiber, it was found that the thermal characteristic of the optical fiber is an important factor in determining the degradation of coherence. Additionally, considering the allowable quality coefficient and the degree of coherence in the presence of thermal variation, the range of the polarization state, which the POS must satisfy, has been clarified. For example, when using a light source with a relative spectral width of f/f= 10^‐9 and an optical fiber with length of 300 km, the FSK and PSK transmission with a bit error rate less than 10^‐9 at 1 Gb/s is carried out under the temperature variation of 40°C; the polarization state which the POS should satisfy is that {degree of polarization × cos(2 × azimuth)} °0.69. The polarization state should be controlled to stay in this range. Actually, the condition that {polarization degree × cos(2 × azimuth)} =1 represents the limit of the transmission distance.