###### Abstract  
In this paper, we review different designs of distributed Raman amplifiers which have been proposed to minimize the signal power profile asymmetry in mid-link optical phase conjugation systems. We demonstrate how the symmetrical signal power profiles along the fiber can be achieved using various distributed Raman amplification techniques in the single-span and more realistic multi-span circumstances. In addition, we show the theoretically predicted results of the Kerr nonlinear product reduction with different Raman techniques in mid-link optical phase conjugator systems, and then in-line/long-haul transmission performance using numerical simulations. Raman amplification; coherent fiber optic communications; optical phase conjugation 202  
and adjusted with fiber Bragg gratings (FBGs). A purposefully built DRA can also improve the power symmetry of the optical fiber link, enabling enhanced efficiency of nonlinearity compensation, and therefore the overall transmission performances [31; 32; 33]. Thus, the DRA not only provides an improved signal-to-noise ratio (SNR) without OPC, but also gives a large margin in the transmission performance (maximum reach, BER or data capacity) improvement when using an OPC to compensate a significant portion of nonlinear product in the transmission systems. In [12; 13], [PERSON] et al. investigated the signal power symmetry and transmission performance of the bidirectional pumping over dispersion flattened non-zero dispersion-shifted fiber (NZDSF) in a mid-link OPC system, but the selected Raman scheme was restricted to first-order bidirectional pumping with similar pump power from both directions. The conclusion was that the shortest fiber length (25 km) gave the highest signal power profile symmetry. This optimized fiber length of 25 km is too short for realistic fiber spans in current optical transmission systems. In this paper, we review various designs of distributed Raman amplification schemes aimed at improving the symmetry of the link for the transmission systems with mid-link optical phase conjugators. We show the optimized Raman amplification designs over single fiber span and multiple fiber spans (two) which can both demonstrate symmetry levels of above 93%. For the single-span link with a 50 km standard single mode fiber (SSMF) with backward Raman pumping only, the dual-order DRA can achieve 97% signal power profile symmetry and 39 dB nonlinear product compensation. This leads to more nonlinear product compensation, 12 dB higher, in comparison with conventional first-order DRA. For longer span links (e.g., 62 km), bidirectional Raman pumping is required to achieve good symmetry. A distributed Raman scheme based on the random distributed feedback laser architecture has been shown to maximize the signal power profile symmetry (97% symmetry) without introducing significant transmission performance penalty from relative intensity noise (RIN) of forward pumping. This proposed scheme can give 37.6 dB nonlinear product compensation, comparable to the 39 dB over 50 km dual-order backward pumping Raman scheme. Furthermore, for multi-fiber-span (i.e., 2 \(\times\) 50 km SSMF) links, the impact of loss between the spans is crucial when optimizing the symmetry. The conventional dual-order backward-pumped DRA can give only \(\sim\)66% signal power profile symmetry, achieving \(\sim\)17 dB nonlinear product compensation. An over-pumped first span using the net gain to compensate the loss between spans would help improve the symmetry to 81% leading to a 25 dB nonlinear product reduction. The best solution is that a 25 cm erbium-doped fiber (EDF) is embedded in the conventional dual-order DRA, but the Raman pumps are used to pump the EDF and so compensate the loss between the two spans, which enables the overall link symmetry to be optimized to 93.9% providing up to 32 dB nonlinear product compensation with 50 km per span.