The Research Of Bend Resistant Large Mode Area Fiber

Bend resistant large mode area fibers can restrain the nonlinear effect effectively,improve the damage threshold value and power capacity of the optical fiber.It provides a good solution to the discrepancy between large mode area and single mode operation in the bend state,and it have greatly contribute to the development of miniaturization and integration of high-power fiber lasers and amplifiers.The development of Tm-doped fiber laser starts late.Thus,few bend resistant large mode area fibers with 2?m operation wavelength was reported so far,and they still have problems such as high bend loss,large bend radius and sensitivity to bend direction.Considering bend orientation insensitive,small bend radius and stable single-mode operation characteristic,three bend resistant LMA fibers with step-index core are proposed and studied by uses Comsol software to simulate in this thesis.The proposed fibers can stably operate with a good single mode characteristics at the operating wavelength of 2 ?m under a bent radius of 10 cm.The main results are summarized as follows:1.The finite element method combined with the fiber bending equivalent refractive index model was developed to establish the analysis model of large mode field area fiber.By analyzing the characteristics of traditional step-index fibers with different core radius,the limitations of the traditional step-index fiber are pointed out.It provides theoretical guidance for the design of bend resistant large mode area fiber.2.Considering bend orientation insensitive,small bend radius and stable single-mode operation characteristic,a large mode field area fiber with a step-index core and a high-index doped rod cladding is proposed.By comparing it with a fiber of non-stepped core,it is clearly found that the step-index core has the advantage of adjusting the bend loss.Numerical results demonstrate that the proposed fiber with a mode field area of 1410 ?m can be single mode operation under a bent radius of 10 cm at the wavelength of 2 ?m.Furthermore,it is insensitive to the bend orientation.3.By reducing the number of cladding doped rods to further improve the fiber characteristics,the bend loss of fundamental mode reduces and the loss ratio of the high-order mode to the fundamental mode increases.The single mode property of the fiber in the bent state is improved and the structure of the optical fiber is simplified.The proposed fiber with a high loss ratio of the high-order mode to the fundamental mode of 767 can be single mode operation under a bent radius of 10 cm at the wavelength of 2 ?m.And it is insensitive to the bend orientation.Especially,the loss ratio of the high-order mode to the fundamental mode can even reach up to 1590 when the bending angle is ± 150,which means the proposed fiber has superior single-mode characteristic.In addition,fabrication tolerance of the optical fiber structure was analyzed to guide the actual fabrication of the fiber.4.By replacing the doped rods in the cladding with high-index coupling rings,the fiber structure is simplified,which makes the fiber can be fabricated with the MCVD method on one fiber preform.When the working wavelength is 2 ?m andthe bending radius is 10 cm,the proposed fiber with a mode field area of 1508 ?m2 and a high loss ratio of the high-order mode to the fundamental mode of 887 can be single mode operation under a core radius of 41 cm.Due to the simple fiber structure,the mode field area can be expanded by increasing the core radius and optimizing the parameters reasonably.The optical fibers satisfy the single-mode condition and the maximum mode-field area can exceed 2450 ?m2 when the fiber core radius is in the range of 40 ?m-67 ?m.