*Review* **Distributed Raman Amplification for Fiber Nonlinearity Compensation in a Mid-Link Optical Phase Conjugation System**

**Mingming Tan 1,\*, Paweł Rosa 2,\*, Tu T. Nguyen 1,†, Mohammad A. Z. Al-Khateeb 1,†, Md. Asif Iqbal 1,‡, Tianhua Xu 3, Feng Wen 4, Juan D. Ania-Castañón 5 and Andrew D. Ellis 1**


**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 multispan 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.

**Keywords:** Raman amplification; coherent fiber optic communications; optical phase conjugation
