Research On Phase-preserving Amplitude Regeneration Technology For High-order Modulation Signal

With the rapid increase of transmission capacity in optical transmission systems,high-order modulation format signals with high spectral efficiency are more and more applied to non-coherent or coherent optical communication systems.However,high-order modulation format signals in contrast to on-off keying(OOK)signal are more sensitive to amplitude and phase noise,and require higher optical signal-to-noise ratio.All-optical regeneration technology can reduce the accumulation of the amplified spontaneous emission(ASE)noise and nonlinear distortion in optical fiber transmission.In recent years,phase-preserving amplitude regeneration technology has attracted more attention,in particular appropriate for conherent communication system.This thesis mainly focuses on the research on phase-preserving amplitude regeneration technology based on nonlinear optical loop mirror(NOLM).The main content and innovations are summarized as follows:1.The condition of achieving phase-preserving amplitude regeneration for the universal NOLM structure is analyzed theoretically and a method of determining the optimal working point for amplitude reshaping is presented.A normalized amplitude-to-phase transfer parameter is introduced to measure the phase-preserving performance at the working points of multilevel regeneration.Furthermore,three new phase-preserving amplitude regenerators are proposed and investigated,those are nonlinear amplifying loop mirror based on optical attenuator(Att-NALM),nonlinear amplifying loop mirror based on nonreciprocal phase shifter(NP-NALM)and mach-zender interferometer based on bidirectional highly nonlinear fiber(Bi-HNLF-MZI).We also listed the conditions for phase-preserving amplitude regeneration of a variety of NOLM structures.2.Two sets of optimized parameters for Att-NALMs are respectively used for phase-preserving amplitude regeneration of QPSK/8PSK and two amplitude 8QAM signals.The optimal working point can respectively be designed on the first or second plateau of the corresponding optical power transfer function.The simulated power transfer curves are consistent with the theoretical calculation.Our simulation shows that,their noise ratio reductions(NRRs)can repectively be up to 5.44d B,4.7d B and 3.7d B for only amplitude noise,and 1.86d B,1.58d B and 1.39d B for simultaneous amplitude and phase noise.3.Based on the Att-NALM structure,a new nonlinear amplifying loop mirror(NALM)-based phase-preserving amplitude regenerator(so-called NP-NALM)by introducing a nonreciprocal phase shifter is proposed to further reduce the power of optical working point.The first working point power can be as low as 115 m W by optimizing the nonreciprocal phase shifter.The cascaded NP-NALM transmission system for QPSK signals with ASE noise is also simulated,and it is shown that the output EVM can reduce to 23%from the EVM limit of 30%(corresponding to BER=10^-3)after introducing NP-NALM regenerator.3.In the NOLM-based regenerators,the clockwise and counter-clockwise signals experience the same optical coupler.We put forward Bi-HNLF-MZI structure with different optical couplers,which preserves the advantage of NOLM structure in sharing the same section of highly nonlinear fiber and combines the flexibility of MZI structure.By using the similar theoretical analysis method,the splitting ratios of two optical couplers of Bi-HNLF-MZi are optimized.The optimal working point are respectively designed on the first or second plateau of the optical power transfer function for the simulation of phase-preserving amplitude regeneration performance of QPSK/8PSK or8QAM/16QAM degraded signals.Research results show that the maximal NRR are respectively 23.09d B,23.01d B,3.19d B,and 3.24d B for only amplitude noise.