Investigation Of All-optical Signal Processing Based On Highly Nonlinear Few-mode Fibers

In order to satisfy the rapidly growing demand of network capacity and overcome the bandwidth bottleneck of photonic-to-electronic conversion and digital signal processing,mode-division multiplexing(MDM)based on few-mode fiber(FMF)and all-optical signal processing based on four-wave mixing(FWM)have recently become hot research topics over the world.As a vital tool to achieve all-optical signal processing in wavelength division multiplexing(WDM)-MDM network nodes,inter-modal FWM arising in the FMF is ideally desired,due to its flexible phase-matching condition,capability for wavelength and mode conversion,?fs response time,and transparency to modulation formats and bit-rate.Most existing investigations of inter-modal FWM are focused on wavelength conversion.However,there are still some problems including relatively low inter-modal nonlinear coefficient,only use of a small number of modes,and parasitic wavelength conversion during the mode conversion,which need to be further solved.In this thesis,the study of all-optical signal processing based on FWM arising in highly nonlinear FMF is carried out.First,the theory of active mode-selective conversion is proposed,and two of highly nonlinear FMFs are presented to achieve inter-modal wavelength conversion and mode-selective conversion,respectively.The main research outcomes of this thesis are summarized as follows:(1)An active mode-selective conversion scheme based on wavelength-degenerate Bragg scattering(WD-BS)process and its phase-matching condition are presented,for the ease of achieving mode-selective conversion without parasitic wavelength conversion.For arbitrary input signal,complete mode-selective conversion can be realized by introducing a pair of pumps with specific power and wavelength into both initial mode and target mode.(2)A circular-core highly nonlinear few-mode fiber(HNL-FMF)is designed,fabricated and characterized.By increasing the doping concentration of germanium and reducing the core size,the HNL-FMF is able to support multiple inter-modal FWM processes among the fundamental mode HE₁₁ and three higher-order vector modes HE₂₁TE₀₁ and TM₀₁,with inter-modal nonlinear coefficients as high as 2.8(W·km)^-1 between any two of the supported modes.By using the 2-km-long HNL-FMF,simultaneous wavelength conversion among the fundamental mode and two higher-order modes,as well as wavelength conversion between two higher-order modes are experimentally demonstrated.(3)An optical cross-connect(OXC)scheme based on an elliptical-core highly nonlinear few-mode fiber(e-HNL-FMF)is presented,while the e-HNL-FMF can support three linearly polarized modes LP₀₁,LP_11a and LP_11b with their inter-modal nonlinear coefficients of?_01,11a=3.23 and?_01,11b=3.14(W·km)^-1,respectively.The differential mode group delay(DMGD)of each mode at 1548.45 nm is zero,which satisfies the phase-matching condition of the active mode-selective conversion.Based on the195-m-long e-HNL-FMF,the 3-d B bandwidth of active mode-selective conversion arising in the proposed OXC is more than 100 GHz over the C-band,and 3-wavelength to 3-mode superchannel conversion of 100 Gbaud 16QAM signals is accomplished.