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.