Theoretical And Experimental Study On Few-mode Fiber Optical Parametric Amplifier

With the single-mode fiber based communication system approaching its capacity limit,space-division multiplexing(SDM)attracts more and more attention because of its potential for providing orders of magnitude increase in per-fiber transmission capacity.Few-mode fiber amplifier is a key block in long-haul SDM system,and differential modal gain is a critical parameter to characterize its performance.However,simultaneously achieving a high gain and low noise with equalized gain for multiple modes is still a challenge for the few-mode fiber amplifier.To figure out this problem,few-mode fiber optical parametric amplifier(FM-FOPA)is studied in this thesis theoretically and experimentally.First of all,a two-mode fiber is designed according to the phase-matching condition of intramodal four-wave mixing(FWM)for achieving gain equalized parametric amplification at telecom band.The impact of random coupling between polarization and spatial mode on the parametric gain is also studied.Moreover,an elliptical-core four-mode fiber is designed to eliminate random coupling between degenerate spatial modes.Next,the performance of the FM-FOPAs,especially the gain dependent nature on the polarization and spatial mode,are experimentally studied.Furthermore,two techniques respectively based on the nonlinear phase shift and intramodal FWM are proposed to measure the equivalent nonlinear coefficient of higher-order modes in a few-mode fiber.Finally,considering the fact that more and more attention is being paid to the few-mode fiber based quantum communication,we experimentally produce telecom band correlate photon pairs in different spatial modes,which can be used for generating spatial mode entanglement.The main innovations are summarized as follows:1.According to the phase-matching condition of intramodal FWM,we propose two types of dispersion-shifted few-mode fiber with simple structures,which can be used for two-mode and coupling-free four-mode parametric amplification at telecom band with equalized gain,respectively.The impact of random coupling and polarization mode dispersion on FM-FOPA is also analyzed.2.We experimentally study the gain characteristic of intramodal and intermodal FWM,respectively.Specially,we investigate the polarization and spatial mode dependent nature of the FWM gain.Moreover,by utilizing intramodal FWM process,we firstly demonstrate the telecom band FM-FOPA,whose gains are 21.7 d B and 4.2d B for the LP₀₁ and LP₁₁ mode signal,respectively.3.We propose two technique methods,which are based on the nonlinear phase shift and intramodal FWM,respectively,to measure the equivalent nonlinear coefficient of higher-order modes in a few-mode fiber.We carry out the measurement and obtain the intramodal and intermodal nonlinear coefficient of higher-order modes.Furthermore,we deduce the corresponding reduction factors,which are in accordance with theoretical predictions.4.By utilizing intermodal FWM process,we experimentally generate telecom band correlated photon pairs in different modes,and simultaneously resolve the frequency,polarization and spatial mode of the photon pairs.We also analyze the polarization and spatial mode of the noise photons originating from Raman scattering.By performing the measurement of two-fold coincidences,we achieve a coincidence to accidental-coincidence ratio of 28,which clearly indicates the existence of quantum correlation.