## 1 Introduction  
In optical links consisting of a standard single mode fiber (SSMF), chromatic dispersion and the nonlinear Kerr effect are the significant intrinsic barriers to increasing transmission distance and bit rate. Optical phase conjugation has proved to be an efficient technique to compensate for the signal impairment due to both phenomena. Principally, the optical phase conjugator (OPC) is placed in the middle of the total transmission length, and this scheme is referred to as a mid-span spectral inversion (MSSI). In this system, the entire signal is distorted through the former half of the total link preceding the midway OPC being phase conjugated. The dispersive and nonlinear Kerr effect of the phase-conjugated signal are reversed through the latter half of the total link, and finally the deteriorated signal can be recovered, if the dispersion-power profile is symmetric with respect to the OPC position [1, 2]. The symmetry of dispersion-power profile is difficult to establish in a real link because of loss of fibers and amplification of signal power by erbium-doped fiber amplifiers (EDFAs) in a lumped amplification system [3]. Several approaches to maximize the symmetry in MSSI have been proposed. Among of them, applying short span length to the transmission link contributes to increasing the symmetry [2]; however, this increases the required number of EDFAs, and consequently the nonlinear Kerr effect is further increased as optical signal power increases. Other approaches include the use of distributed Raman amplification [4, 5] and applying MSSI to the dispersion-managed link [6, 7]. Another limitation of optical phase conjugation in a reallink is the difficulty of implementing a flexible link configuration. That is, the transmission link parameters, such as the length of the SSMF, dispersion accumulated in each span, and more importantly the OPC position have to be determined to symmetrize the local dispersion distribution and power profile with respect to the OPC. For this reason, the OPC position should be fixed around midway of the total link, the length of each SSMF should be generally constant, and maximum launch power of signal is restricted. Thus, the variety of the link configuration is limited in the optical phase conjugation system for compensation of chromatic dispersion and the nonlinear Kerr effect. Dispersion management (DM) is a well-known approach that can mitigate fiber chromatic dispersion [8; 9; 10; 11]. The dispersion coefficient of the dispersion-compensating fiber (DCF) is opposite to the SSMF; thus, the dispersion accumulated in the SSMF can be eliminated or reduced by controlling the length and coefficient of the DCF. In the DM link, if Kerr nonlinearity is not present and input power is sufficient to overcome amplified spontaneous emission (ASE) [12] noise impairment, any bit rate can be transmitted for any distance. In the DM link configuration, the significant link parameter capable of impacting the dispersion compensation is residual dispersion per span (RDPS), which is defined as dispersion accumulated in each fiber span. The RDPS is determined by the lengths and dispersion coefficients of the SSMF and DCF. The simplest configuration in the DM link is where the same RDPS is applied to each fiber span of whole link. This configuration means the lengths and coefficients of fibers are fixed to uniform values in every span. However, this uniform distribution of RDPSs is also restricted to adaptive implementation of the DM link required by the demand of network design plan. One approach to supplement the incomplete compensation due to the difficulty of symmetry in MSSI is in combination of DM link configurations [6; 7]. Authors have also confirmed a compensation improvement of 960 Gb/s wavelength division multiplexed (WDM) transmission in an MSSI-based DM link [13; 14; 15].