| In order to meet the large capacity transmission requirements of new services such as mobile Internet,big data and the Internet of Things,modern optical fiber communication has begun to evolve from non-coherent systems to coherent systems.High order modulation format signals such as pulse amplitude modulation(PAM)and quadrature amplitude modulation(QAM)have been applied in optical networks.However,higher-order modulated signals are more sensitive to the degradation caused by fiber nonlinearity or amplified spontaneous emission noise(ASE),which requires the channel to have higher optical signal-to-noise ratio performance.The optical-electricaloptical relay and all-optical regeneration technology can effectively improve the signal performance and extend the transmission distance of the optical fiber communication system.Among them,all-optical regeneration directly processes optical signals in the optical domain,can replace the traditional relay type,bypasses the so-called "electronic bottleneck",and has the advantages of transparent data rate and modulation format.So,it can meet the requirements of high-capacity and high-speed transmission system.All optical regeneration can be classified into amplitude regeneration,phase regeneration,phase preserving amplitude regeneration(PPAR),amplitude phase simultaneous regeneration and other functions.The realization of these functions depends on nonlinear optical structures,such as nonlinear optical loop mirror(NOLM),Mach Zehnder interferometer(MZI),phase sensitive amplifier(PSA),semiconductor optical amplifier(SOA),etc.At the same time,with the development of photonic integrated chip technology,it is possible to realize the chip of all-optical regenerator.High performance all-optical regeneration chip with advantages of integration also becomes one of the important means to realize all-optical regeneration technology.Therefore,all-optical regenerator based on optical fiber and chip is the main research direction of all-optical regeneration technology.This thesis focuses on the all-optical NOLM and MZI regenerators,including PAM optical signal amplitude regeneration and QPSK/QAM optical signal phase preserving amplitude regeneration,which involve two material systems: nonlinear optical fiber and silicon-on-insulator(SOI)chip.The main research contents and innovations of this thesis are as follows.1.For PAM modulation format signals in the incoherent communication system,three all-optical amplitude regeneration schemes,based on cascade NOLM regenerators,polarization orthogonal continuous wave auxiliary and polarization maintaining coupler,are studied to improve the number of regeneration levels,enhance the uniformity of power transfer function(PTF)and expand the width of regeneration area by improving the NOLM structure and optimizing the parameters of related structures,respectively.(1)Using normalized power transfer function(NPTF)and normalized differential gain(NDG)curves,the optimal working point(WP)of the NOLM regenerator based on self-phase modulation(SPM)is analyzed,and the design rules of cascaded NOLM schemes are summarized.Simulation results show that four-level and eight-level PAM signals can be regenerated by optimizing the coupling ratio of the optical coupler and the gain coefficient of the inter-stage matching amplifier.(2)A polarized orthogonal continuous wave assisted NOLM(PC-NOLM)all-optical amplitude regeneration scheme is proposed.Theoretical analysis and experiments verify the step uniformity of its PTF curve.This scheme is based on cross phase modulation(XPM)and theoretically supports amplitude regeneration of any number of levels.The experiment shows that,when the average input power of PAM-4 optical signal is 29.7d Bm,the noise reduction ratio(NRR)reaches 2.33 d B,and the noise in four amplitudes is suppressed simultaneously.(3)A special polarization maintaining coupler is made and uses to build up a polarization-maintaining NOLM(PM-NOLM)regenerator.The experiment obtains a wider regeneration range than the traditional NOLM scheme.The regeneration area nearly doubles,which could improve the signal-to-noise ratio of PAM-4 by 4.5 d B.The cooperation mechanism between Kerr nonlinearity and polarization effect in the regenerator is theoretically analyzed.2.According to the phase characteristics of QPSK/QAM optical quadrature modulation signals in coherent communication systems,the regeneration mechanism of phase preservation is revealed theoretically from both amplitude and phase aspects,that is,the contribution of the linear phase shift component to the amplitude of the regenerated optical field must be much greater than the nonlinear phase shift component,and the optical phase conjugator(OPC)can be used to compensate the residual phase disturbance.Accordingly,three phase preserving all-optical amplitude regeneration schemes are proposed:(1)An MZI regenerator with amplifier module(G-MZI)is proposed,and the phase-preserving amplitude regeneration of 256 QAM signals is simulated;(2)A PPAR scheme of MZI-nested NOLM(MZI-NOLM)is proposed.The experimental results show that the average phase perturbation during QPSK signal regeneration is 4.37 °,close to the theoretical value of 3.8 °;(3)An optical phase conjugator-assisted NOLM(OPCNOLM)scheme is proposed to achieve perfect phase preserving with almost zero phase distoration,which solves the problem of residual phase perturbation in existing PPAR schemes.Simulation results show that the proposed scheme can further improve the noise rejection ratio of 16 QAM signal by 3.8 d B when the input signal to noise ratio(SNR)is15 d B compared with the traditional single NOLM case.3.In order to promote the integration of all-optical regenerative devices,PPAR experiments of 16 QAM signals are carried out using 2.31 cm SOI silicon wire waveguide chips,which verify the feasibility of the MZI regenerative chip scheme.The experiment shows that when the input SNR is 17.3d B,the all-optical regenerative chip can achieve a noise rejection ratio of 2.4d B.The influence of two-photon absorption(TPA)effect in SOI waveguide on regenerator performance is analyzed theoretically. |
