Design Of Thulium Doped Large Mode Area Optical Fiber And Analysis Of Its Amplification Characteristics

Because of the wider tuning spectrum,thulium doped fiber laser in the 2 ?m band has more advantages in narrow linewidth and high-power output,and plays an important role in laser welding and cutting of plastic materials.It is difficult to obtain high power and narrow line width of fiber laser now because of stimulated Brillouin scattering(SBS),stimulated Raman scattering(SRS),cross-phase modulation and other nonlinear effect in fiber lasers.Since the nonlinear effect of optical fiber is inversely proportional to the mode area of the optical fiber,the larger the mode area,the higher the nonlinear threshold.Therefore,the large-mode-area thulium doped fiber can effectively improve the output power of the fiber laser and the conversion efficiency of the fiber amplifier.In this paper,a thulium doped fiber with large mode area is designed and applied to fiber amplifier,the characteristics of the amplifier are theoretically simulated and analyzed.The main work of this paper is as follows:(1)For the structure of irregular Bragg fiber(IBF),we analyze the mode field distribution theoretically and study the six structures of IBF.By simulating the optical fiber,the influences of different optical fiber structures on the mode area are studied from the perspectives of the radius and the refractive index of the optical fiber so that the optical fiber structure with the optimal mode field area is selected.(2)According to the characteristics of the designed fiber structure,the influences of fiber parameters,mainly including the core radius,the refractive index difference and the radius thickness ratio of each layer on the mode area are analyzed in detail and the parameters are optimized,achieving mode area of 719 ?m2 large mode area doped fiber.Based on the fiber parameters,the bending loss and stability of the designed fiber are analyzed.The results show that the designed fiber has practical value.(3)According to the characteristics of TM3+ energy level structure,a theoretical model of large mode thulium doped fiber amplifier under 793 nm wavelength pumping mode is established.Considering the output power,amplifier efficiency and SBS effect of thulium doped fiber amplifier,the simulation model is obtained by using thulium doped large mode area fiber combined with theoretical model.(4)By comparing with the ordinary step fiber,the relationship between the length of the fiber,the pump power of the amplifier and the output power of the two kinds of mode area are studied respectively.When the pump wavelength is 793 nm,the pump power is 100 W and the concentration of TM3+ is 4.0 × 1025 m-3,the designed large mode area fiber can increase the conversion efficiency by 5%,reach 40%,and the output power is 41.08 W.The results show that the thulium doped fiber in large mode area can effectively improve the output efficiency of thulium doped fiber amplifier,and the whole amplifier system has high stability.