Design And Research Of Novel Large Mode Area Fiber

With the rapid increase in output power of thulium-doped fiber lasers,nonlinear effects and transverse mode instability have become the main obstacles limiting its development.At the same time,the lack of efficient transmission waveguides in the 2μm band also limits the expansion of the application field of high-power miniaturized thulium-doped fiber lasers.Bend-resistant large mode area fibers can reduce adverse effects of nonlinear effects,maintain good single-mode transmission and excellent bending resistance,and thus they have become the key to support the development of fiber lasers with high power,high beam quality,high stability and application expansion.In order to meet the urgent demand of high power miniaturized thulium-doped fiber laser for high performance bend-resistant large mode area fiber,this thesis focuses on the 2μm waveband and carries out the design and research of bend-resistant large mode area fibers with stable single-mode characteristics and compact bending radius.The main research contents and innovative achievements of this thesis are as follows:1.The basic principle of finite element numerical analysis method and the main performance parameters of optical fiber are introduced.The bending equivalent refractive index model is introduced,which is combined with the finite element method to establish the characteristic analysis model of bend-resistant fiber.Then,the characteristic analysis model is used to analyze the characteristics of multi-trench fiber and segmented cladding fiber.Numerical results verify the accuracy and validity of characteristic analysis model.2.A novel large mode area fiber with multilayer core and hole assisted is proposed.In the structural design,a three-layer step-index core is used to modulate the mold field distribution and restrain the bending deformation.At the same time,air holes are introduced into the cladding to limit mode leakage and improve the bending performance of the fiber.As a result,a robust single-mode operation is obtained at an operating wavelength of 2μm and a bending radius of 10 cm.The mode field area can reach up to 2044.8μm~2,which greatly improves the bending resistance of the fiber.3.In order to further improve the single-mode performance in the bending state,an improved structure is proposed based on this structure.Two layers of air holes are introduced into the cladding and thus a new fiber structure is formed. the higher order mode loss ratio(HOMLR)is increased to 3.6×10~3 at a wavelength of 2μm and a bending radius of 10 cm.By enlarging the core size, the mode field area can be extended to 2106.2μm~2.In addition,the fiber is capable of maintaining single-mode operation over a broad wavelength range of 1.6μm to 2.1μm.At the same time,the fiber is not sensitive to the bending direction,and the HOMLR exceeds 100 in any bending direction,ensuring the robust single-mode performance of the fiber.The error tolerance characteristics are discussed from the perspective of fabrication,which provides guidance for practical fabrication.4.An all-solid bend-resistant large mode area fiber with segmented high-refractive-index doped rods cladding is proposed.Doping rods with high refractive index are introduced in the cladding to increase the loss of higher-order modes.At a wavelength of 2μm and a bending radius of 10 cm,the high-order mode bending loss is as high as 37 d B/m,and the corresponding HOMLR can reach 10~4.Thus,this fiber has excellent single-mode performance.It is suitable for small bend radius applications with a usable bend radius as low as 9 cm.The fiber structure can ensure the robust single-mode operation in the bending direction angular of±26°.In addition,by optimizing the core size,the mode field area of the fiber can be extended to 1547μm~2.The optical fiber has the advantages of simple structure,high feasibility of preparation process and good tolerance characteristics.