Results and Discussions According to the PTF and amplitude-to-phase conversion characteristics, the phase-preserving mechanism of the OPC-NOLM regenerator is analyzed from the two aspects of amplitude and phase. The results show that the proportion of nonlinear components in the OPC-NOLM regenerator has been kept as small as possible to achieve intact PPAR. The OPC compensates for the phase perturbation at the transmission end by using the phase of the reflecting end to reduce the phase perturbation of the final output to almost zero. The structural parameters of the OPC-NOLM regenerator are optimized, including those of the MZI-nested NOLM, the gain of the OPC, and the coupling ratios of the transmission and reflecting ends of the NOLM. The detailed steps are summarized in the paper. The optimized results show that the OPC-NOLM regenerator introduces a phase perturbation of 0.002° (Fig. 3), which is smaller than that of the NOLM structure (at least 4.4°) and the references (at least 3.8°). Regarding the OPC-NOLM regenerator, the intact phase preservation can be attributed to the suppression of the amplitude noise and the compensation for phase perturbation.Then, a 16QAM coherent communication system is built to simulate and verify the PPAR performance of the OPC-NOLM regenerator. The parameter of noise reduction ratio (NRR) is defined as the ratio of the input error vector magnitude (EVM) of the OPC-NOLM regenerator to the output EVM of the regenerator. The variation in the NRR with the input SNR indicates that when the input SNR is 15 dB, the NRR of the OPC-NOLM regenerator is 3.8 dB higher than that of the NOLM structure (Fig. 6).Finally, some relevant regeneration structures are listed in Table 1 for a comparison of their numbers of voltage levels available for amplitude regeneration related to the PTF and their phase-preserving performance. This table reveals that the OPC-NOLM structure proposed in the paper not only supports multi-level amplitude regeneration but also gains an advantage over other structures in phase preservation.

Amid the rapid development of modern communication networks, high-order modulation formats, such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), have been used widely for large capacity and high-speed data transmission. However, compared with binary signals, high-order modulated signals are easily degraded by channel crosstalk noise and amplified spontaneous emission (ASE) noise. In this case, all-optical regeneration technology can help improve the optical signal-to-noise ratio (OSNR) directly in the optical domain. All-optical amplitude or phase regeneration can usually be achieved by some optical structures with nonlinear effects, such as the nonlinear optical loop mirror (NOLM), the Mach-Zehnder interferometer (MZI), the phase-sensitive amplifier (PSA), and the semiconductor optical amplifier (SOA). In the process of all-optical amplitude regeneration, the conversion of amplitude noise to phase perturbation is always adopted to a certain extent. Therefore, phase-preserving amplitude regeneration (PPAR) schemes have been put forward for QPSK and QAM signals. Nevertheless, phase perturbation (larger than 3.8°) remains. The objective of the paper is to present an intact PPAR scheme without phase perturbation.

This paper proposes an optical phase conjugator (OPC)-assisted NOLM (OPC-NOLM) PPAR scheme, in which the reflected signal from the NOLM unit is used to achieve a stepwise power transfer function (PTF) and the OPC is utilized to compensate for the phase perturbation. The optical field output from the OPC-NOLM regenerator is derived and used to analyze the phase-preserving mechanism of the regenerator from the two aspects of amplitude and phase. The structural parameters of the OPC-NOLM regenerator are optimized by the PTF and phase perturbation curves. Then, an OPC-NOLM regenerator simulation platform for optical 16QAM signals is built to verify the intact PPAR performance of the proposed scheme by comparison with the NOLM scheme.

To further eliminate the residual phase perturbation of the currently available PPAR schemes, this paper proposes a novel OPC-NOLM scheme capable of intact phase preservation for input signals. The optical field output from the OPC-NOLM regenerator is derived and then used to explain the phase-preserving mechanism of the regenerator from the two aspects of amplitude and phase. According to the PTF and phase transfer curves of the OPC-NOLM regenerator, this paper optimizes the structural parameters of the regenerator and calculates its phase perturbation (0.002°). With 16QAM signals as an example, the NRR performance of the OPC-NOLM regeneration scheme is simulated. According to the simulation results, the proposed scheme achieves an NRR 3.8 dB higher than that of the NOLM structure without the OPC under an input SNR of 15 dB.