Ultra long distance and high capacity optical fiber transmission system using midway optical phase conjugation

在光放大器链中，由于光纤中的光损耗被周期性地补偿，因此在不使用再生中继器的情况下，传输距离可以达到1,000公里以上。然而，在这样的系统中，光功率沿整个系统长度沿着保持在高水平，因此光纤的克尔非线性不能再被忽略。克尔非线性又与光纤的群速度色散相互交织，造成严重的波形失真。为了科普这个问题，我们提出了一种利用中间相位共轭的光传输系统，并研究了该系统的可行性。本研究项目取得的主要成果如下。(1)通过分析发现，波形失真补偿效率不足的主要原因是光功率周期性分布引起的边带调制不稳定性，并得出了通过减小边带调制不稳定性的影响来最大化补偿效率的系统设计准则。(2)设计了色散位移光纤相位共轭器。根据设计制作的器件，获得了几个百分点的转换效率. (3)介绍了利用半导体光放大器实现的光相位共轭器。当抽运-探测失谐量为200 GHz时，转换效率大于0 dB。将该器件引入传输系统，对消除光纤中产生的四波混频是有效的。

In an optical amplifier chain, transmission distance beyond 1,000km is attained without using regenerating rapeaters, because optical loss in fibers is periodically compensated. However, in such a system, the optical power is maintained at a high level along the entire system length and hence the Kerr nonlinearity of optical fibers can no longer be ignored. The Kerr nonlinearity, in turn, interplys with the group velocity dispersion of fibers, causing serious waveform distortion. To cope with this problem, we propose an optical transmission system using midway optical phase conjugtion, and investigate the feasibility of the system. The main results obtained in this research project are shown in the following.(1) We found that the insufficient efficiency of waveform distortion compensation was mainly caused by the sideband modulation instability originated from the periodic optical power distribution, and obtained the design rule of the system to maximize the efficiency by reducing the effiect of sideband modulation instability.(2) The optical phase conjugator using dispersion shifted optical fibers was designed. We achieved the conversion efficiency of several percent by the device fabricated according to the design.(3) The optical phase conjugator using semiconductor optical amplifiers was demonstrated. When the pump-probe detuning was 200GHz, the conversion efficiency larger than OdB was achieved. The introduction of this device into the transmission system was effective for eliminating the four wave mixing generated in optical fibers.