With 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 utilized widely for large capacity and high-speed data transmission. However, signals in such modulation formats are easily degraded by channel crosstalk and amplified spontaneous emission (ASE) noise relative to binary signals. In this case, all-optical regeneration technology is helpful to improving optical signal-to-noise ratio (OSNR) directly in the optical domain. All-optical amplitude or phase regeneration can be usually achieved by some optical configurations with nonlinear effects, such as Mach-Zehnder interferometer (MZI), nonlinear optical loop mirror (NOLM), phase sensitive amplifier (PSA) and semiconductor optical amplifier (SOA). In the process of all-optical amplitude regeneration, the conversion of amplitude noise to phase disturbance is always introduced to certain extent. For this reason, phase-preserving amplitude regeneration (PPAR) schemes for QPSK or QAM signals are put forward. Unfortunately, there still exists phase disturbance (larger than 3.8°). The objective of our work is to present a perfect PPAR scheme with very low phase disturbance (close to 0°), especially by silicon-based MZI regeneration chips.

Most of amplitude regeneration schemes are able to be modeled by a MZI configuration. In this paper, we propose a phase-preserving regeneration scheme of MZI cascading optical phase conjugator (OPC-MZI) to preserve the optical phase of input signals with very low phase disturbance. By analyzing the transmission characteristics of the input signals in the whole cascade system, the silicon-based MZI regeneration chip is designed and optimized as a regeneration unit. Then, the power and phase transfer characteristics of three different regeneration schemes based on the same designed silicon-based MZI regeneration chip are compared. Further, the feasibility of OPC-MZI phase-preserving regeneration scheme is verified by simulation on the QPSK modulation signals. Finally, the optical light field output from the OPC-MZI phase-preserving regeneration scheme is derived, and is utilized to explain the phase preserving mechanism from two aspects of amplitude and phase. In addition, we discuss the influence of the loss coefficient of silicon wire waveguides on phase-preserving amplitude regeneration.

For OPC-MZI phase-preserving regeneration scheme (Fig. 1), we optimize the structural parameters of silicon-based MZI chip by the power transfer function (PTF) and amplitude-to-phase conversion curves. The phase preserving performance of OPC-MZI phase-preserving regeneration scheme is analyzed from both amplitude and phase. It is shown that the OPC-MZI phase-preserving regeneration system has a phase disturbance close to 0° at every working point, that is, the so-called perfect phase preserving can be well achieved (Fig. 5). Further, the feasibility of OPC-MZI phase-preserving regeneration scheme is verified by simulation on the QPSK modulation signal. The parameter of noise reduction ratio (NRR) is defined as the ratio of the input to output error vector magnitudes (EVMs). The simulation results show that, in comparison with the single MZI regeneration chip (Figs. 7 and 8), when the input signal-to-noise ratio (SNR) is 16 dB, the OPC-MZI phase-preserving regeneration scheme has a larger NRR by 1 dB and the phase disturbance is also reduced to 0.07°. Finally, we discuss the applicability of the OPC-MZI phase-preserving regeneration scheme when the loss coefficient of silicon wire waveguide increases. It should be pointed out that, when the working points between two MZI regeneration units perfectly match with each other, the perfect phase preserving performance can still be achieved even if the waveguide loss coefficient is increased (Fig. 10).

To further eliminate the residual phase disturbance of PPAR schemes available now, we propose a phase-preserving regeneration scheme of OPC-MZI, capable of perfectly preserving the optical phase of input signals. In order to analyze the phase preserving performance of OPC-MZI regeneration scheme, a silicon-based MZI regeneration chip is designed and optimized as the cascade regeneration unit, and the input power at the first working point is set to 0.564 W. Among three regenerators considered here, the OPC-MZI phase-preserving regeneration scheme has a minimum phase disturbance of 0.1° at the first working point, much better than the single-MZI chip scheme and the MZI cascading optical amplifier (OA) scheme. Then, we simulate the OPC-MZI phase-preserving regeneration scheme for QPSK modulation signals. The simulation results show that, at the input SNR of 16 dB, the OPC-MZI phase-preserving scheme has a larger NRR and the phase disturbance is also reduced to 0.07° compared with the single MZI regeneration chip. At the end of this paper, the phase preserving mechanism of OPC-MZI regeneration scheme is analyzed, and the universality of cascade OPC regeneration scheme is also pointed out.