Research On Multi-wavelength Erbium-doped Fiber Laser And Few-mode Amplifier

The vigorous development of information technology makes the demand for optical network capacity grow rapidly.Optical fiber communication based on multiplexing technologies such as wavelength division multiplexing and space division multiplexing has become a significant way to improve network capacity.Among them,optical devices are very important.Fiber laser and fiber amplifier are widely used in aviation industry,military defense and other fields.At the same time,they have attracted a lot of attention as light source and optical relay device in fiber communication.With the development of optical fiber communication system based on multiplexing technology,multi-wavelength erbium-doped fiber laser（EDFL）with more flexible and accurate output has become the new demand,while few-mode erbium-doped fiber amplifier（FM-EDFA）used to achieve mode gain equalization has become an urgent need in the industry.Therefore,under the support of the National Key R&D Program of China and the National Natural Science Foundation of China,this dissertation has carried out theoretical analysis,simulation and experiment on EDFL and EDFA in the wavelength and mode dimensions respectively according to the requirements of optical fiber communication system for high-performance optical devices.The main innovations of the dissertation are as follows:1.The influence of nonlinear effects on the output characteristics of multi-wavelength EDFL,such as the number of wavelengths and stability,is studied.First,a switchable multi-wavelength EDFL based on four-wave mixing effect（FWM）is proposed.By comparing the experimental results without and with FWM effect in the resonator,it is found that FWM effect has a significant impact on the improvement of wavelength number and flatness.Then,two switchable multi-wavelength EDFLs based on nonlinear optical loop mirror（NOLM）and nonlinear amplifying loop mirror（NALM）are constructed respectively.Both of them achieve suppression of wavelength competition in EDFL through the intensity-dependent loss mechanism.Comparing the experimental results under the same other conditions,it is found that the wavelength tuning and switching are more flexible when using NOLM,and the stability of multi-wavelength output is better when using NALM.2.The tunable multi-wavelength EDFLs based on cascaded filter structure are built,and the output can be tuned in quantity,range and wavelength interval at the same time.First,a multi-wavelength EDFL based on the cascade of Lyot filter and eccentric few-mode fiber filter is proposed,and the output of 1～7 wavelengths can be obtained in the experiment.When 1～3 wavelengths are output,wavelength tuning in the range of 30nm can be achieved,and the wavelength interval can be tuned in a specific polarization state.Thereafter,in order to reduce the randomness of wavelength interval tuning,the daul-stage Sagnac filter with two wavelength intervals is used to replace the eccentric few-mode fiber filter.When the wavelength interval of the filter spectrum is 2nm,the output of the laser can be tuned in the range of 30nm;when the wavelength interval of the filter spectrum is 0.5nm,the laser output can obtain wavelength intervals of 0.5nm multiples.Furthermore,to achieve more precise wavelength interval tuning,a multi-wavelength EDFL based on the tapered-fiber MZI filter and Sagnac filter cascaded is constructed.By changing the taper length of the MZI filter,a variety of accurate and controllable wavelength intervals can be obtained.The number of output wavelengths can be switched flexibly,and the wavelength tuning range is～15nm under the action of Sagnac filter.3.The FM-EDFAs based on the layered-doping few-mode erbium-doped fiber are studied,which reduce the differential mode gain（DMG）of the amplified signal and realize the optimization of the FM-EDFAs.On the basis of the layered-doping,the FM-EDFA based on step-index few-mode erbium-doped fiber is proposed.When the mode gain is 20d B,the corresponding DMG is 0.28d B.The influences of the errors of the doping radius and doping concentration on the performance of the amplifier are analyzed during the fabrication of the fiber.Subsequently,the FM-EDFA based on a ring-core few-mode erbium-doped fiber is studied.While reducing DMG,the mode excitation order is adjusted by the method of fiber refractive index center depression.The mode gain and DMG of FM-EDFA based on the gain fiber are discussed.In order to further reduce the DMG,the trench is added while the core refractive index of the fiber remains unchanged.When the mode gain is 20d B,the DMG can be reduced to0.03d B theoretically.4.The FM-EDFA for mode gain equalization is constructed.The FM-EDFA is an all-fiber structure,which is beneficial to the improvement of system stability.The few-mode erbium-doped fiber used is layered-doping structure with a cladding auxiliary trench.The outputs of the signal source are converted by using photonic lantern,and three modes of LP₀₁,LP_11aand LP_11bare obtained.The cladding-pumped coupler is fabricated by fusion taper system.In the experiment,the mode gain and DMG of FM-EDFA are measured and calculated with the change of pump power.At the same time,the output characteristics of the FM-EDFA in the C-band range are discussed.