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是周期性补偿的，因此在不使用再生光纤的情况下，可以实现超过1000公里的传输距离。然而，在这样的系统中，光功率沿整个系统长度保持在一个较高的水平，因此光纤的克尔非线性不能再被忽视。克尔非线性又与光纤的群速度色散相互作用，造成严重的波形畸变。为了解决这一问题，我们提出了一种采用中间相位共轭的光传输系统，并对该系统的可行性进行了研究。本课题的主要研究成果如下：(1)发现波形畸变补偿效率不足主要是由于周期性光功率分布引起的边带调制不稳定造成的，并得出了通过减小边带调制不稳定影响来实现效率最大化的系统设计规则。(2)设计了色散位移光纤的光相位共轭器。根据设计制作的器件，实现了几个百分点的转换效率。(3)论证了利用半导体光放大器实现的光相位共轭器。当泵浦-探头失谐为200GHz时，实现了大于OdB的转换效率。在传输系统中引入该装置可以有效地消除光纤中产生的四波混频。

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.