Optimum Design On The Waveguide Structure Of Large Mode Area Optical Fiber

With the development of fiber materials,fiber devices and integrated technology,the output power of fiber laser has reached the level of 20,000 watts.Fiber laser is widely used in advanced manufacturing,scientific research,military defense and other fields due to the advantages of good beam quality,high electro-optic efficiency,compact structure and convenient thermal management.At present,the nonlinear effect,mode instability and other problems limit the further improvement of output power of fiber lasers,and the stability and reliability of fiber lasers under high power conditions are seriously affected.Increasing the mode field area of fiber can reduce the laser power density in the core,raise the threshold of the nonlinear effect and the mode instability,and is considered as the most direct and effective way to break the bottleneck of the power raising of the fiber laser.Based on this,the design and optimization of large mode area fiber has been studied in this paper,the specific contents are as follows:1.The waveguide structure of large mode area fiber with comb-index core is optimized by adding a high refractive index ring in the outermost layer of fiber core.After optimizing the width and refractive index of the high refractive index ring layer,the effective mode field area of the fiber increases by 700?m²,and the bending loss of fundamental mode is reduced from 6 dB/m to 0.1 dB/m at the same bending radius.Moreover,the Gauss-like distribution,flatten distribution and hollow distribution can be realized by changing the parameters of the outermost high refractive index ring layer.2.A large mode area fiber with triangle-platform-index core fiber?TPIF?is proposed,the core consists of periodic alternating low refractive index ring layer and triangular high refractive index distribution ring layer,and the outermost layer of core is a high refractive index ring layer with adjustable width and refractive index.By changing the number of periods,the duty cycle of the high refractive index ring layer with triangular distribution and the difference between high and low refractive index,the maximum effective mode field area of TPIF can reach 3100?m²;when the bending radius is 17 cm,the bending loss of LP₀₁1 mode is 0.04 dB/m and the bending loss of LP₁₁1 mode is 3 dB/m,the fiber can realize single mode operation;different mode field distributions,such as Gauss-like distribution,flatten distribution and hollow distribution,can be realized by changing the parameters of the outermost high refractive index ring layer,which can meet the application needs of special laser processing industry.3.Based on the design of segmented cladding fiber,low refractive index internal radiation type fiber and low refractive index external radiation type fiber are optimized and analyzed,the distribution area of low refractive index in cladding of the two type fibers radiates inward and outwards,respectively.When the low refractive index internal radiation type fiber is not bent,the leakage loss of LP₀₁1 mode is 0.02 dB/m,and the leakage loss of LP₁₁1 mode is 1 dB/m,single mode operation can be realized;however,there is no leakage loss for low refractive index external radiation type fiber without bending,and when the bending radius is 16 cm,the LP₀₁mode leakage loss is 0.1 dB/m,and the LP₁₁1 mode leakage loss is 1 dB/m,and single mode operation can be realized.