Study On Tunable Erbium-doped Fiber Laser Based On Multimode Interference Effect

Fiber laser is widely used in mechanical precision processing,optical fiber sensing,laser radar and so on because of its small size,light weight,strong electromagnetic interference resistance,low cost and other advantages.As a branch of fiber lasers,tunable fiber lasers have also been used in many fields such as high speed wavelength division multiplexing/optical time division multiplexing(DWDM/OTDM).In this paper,in order to realize the tunable all-fiber erbium-doped fiber laser,the single-mode fiber-multimode fiber-single-mode fiber(SMS)fiber structure is selected as the filter,and the transmission characteristics of light transmission in the filter structure are analyzed from the theory,and based on this,the tunable erbium-doped fiber laser is designed and built.The main contents of the article include:(1)Through reading the literature related to multimode interference effect,the development process of tunable fiber laser is summarized,focusing on the filtering principle of SMS structure,the theory of structural mode and the research status of laser based on multimode interference effect.(2)Analyze the basic theory of self-imaging effect,derive the relevant equation of self-image peak,grasp the factors affecting self-image peak,use CAD-Rsoft software to build SMS filter structure in the beam propagation module based on finite difference method(Beamprop),and analyze the influence of multimode fiber length,core diameter,numerical aperture and external refractive index changes on self-image peak in the filter structure.The Optisystem software was used to simulate the entire ring cavity fiber laser and fiber amplifier by solving the erbium ion rate equation and the power transfer equation in the ring cavity.By analyzing the simulation results,the length of multimode fiber has the greatest impact on the self-imaging effect of the fiber,and wavelength tuning can be achieved by changing the length of the multimode fiber,and in the fiber amplifier part,the gain fiber with a length of 12 m can be selected to obtain the lowest noise and highest gain output.(3)Based on the theoretical simulation results,the annular cavity tunable erbiumdoped fiber laser and fiber amplifier were built respectively.The fiber laser uses the SMS structure as a filter,and by comparing multimode fibers with different core diameters,lengths and numerical apertures,multimode fibers with a length of 85 cm,a numerical aperture of 0.15 and a core diameter of 105μm are selected to access the fiber loop to achieve tuning of the output wavelength > 40 nm.In the fiber amplifier,an 11.4m long erbium-doped fiber is used as the gain fiber to amplify the previously built tunable seeder,and the output power can reach 50 m W at different wavelengths when the pump power is250 m W.In this paper,the tunable laser based on multimode interference effect is studied from three parts: theory,simulation and experiment.The construction of the tunable laser in the work team has a certain reference value.