Numerical Study Of Supercontinuum Generation Based On Normal-Dispersion Er-doped Optical Fiber

The supercontinuum?SC?fiber laser sources have many advantages such as wide spectra,high brightness,small light spot,high average power,good heat dissipation,high beam quality,compact structure,easy maintenance and low cost.The method of supercontinuum generation based on the erbium-doped highly nonlinear optical fiber can provide pulse spectrum broadening and energy enhancement at the same time.In this paper,two kinds of the normal-dispersion erbium-doped fiber structures are designed,and the numerical researches on the supercontinuum generation based on the designed fiber are further discussed.The works are as follows:Firstly,two types of erbium-doped step-index single-mode fibers are designed.The core of the first one consists of a CS₂-filled hollow core and an erbium-doped ring core based on the germanium-doped silica.The second kind of the fiber has an erbium-doped core based on the germanium-doped silica.Additionally,the relationships between the dispersion,nonlinear parameter,loss and gain characteristics of the two kinds of the erbium-doped fibers and the fiber parameters are analyzed.The curves of the dispersion and nonlinear parameters of the first kind of the fiber can be designed by tuning the hollow-core radius,ring-core radius and germanium-doped concentration.When the hollow-core radius r_a=0.24?m,the core radius r_b=1.2?m,the concentration of germanium X=50 mol%,the flat normal-dispersion curves in the near-infrared wavelength range from 1500 nm to 2500 nm can be obtained.For the second case,when the core radius r_b=1.4?m and the germanium-doped concentration X=40 mol%,the flat normal-dispersion curve in the wavelength range from 1240 nm to 2550 nm can be obtained.Secondly,the evolution of the pulse in the gain fiber is numerically analyzed by solving the generalized pulse propagation equation with fiber gain.In the normal-dispersion regions of two kinds of the erbium-doped fibers with different core structures,the influences of input pulse width,fiber length,gain coefficient,and dispersion on the supercontinuum generation are numerically analyzed.The results show that when the input pulse width is 1 ps,the peak power is 10 k W,the small gain coefficient is 9 m^-1,and the fiber length is 1 m,the 20-d B bandwidth of the generated supercontinuum based on the CS₂-filled erbium-doped fiber can reach up to 750 nm,the corresponding pulse energy is 11.1 n J.The supercontinuum based on the erbium-doped fiber without the CS₂-filled core has a 436 nm?at 20-d B level?bandwidth and a pulse energy of 14.7 n J under the same parameters of the input pulses.Finally,spectral filtering and temporal compression of the supercontinuum based on the CS₂-filled erbium-doped fiber with the 0.1 ps input pulse are numerically analyzed.The pulse spectrum of the output supercontinuum has oscillation structures with a decrease of the propagation length and an increase of g₀.For example,when the g₀are 9,7,and 5 m^-1,the propagation lengths in the fibers where the spectral oscillations occur are about 2,3 and 4 m,respectively.Consequently,a proper decrease of g₀is helpful to improve the compression quality when the output pulse from the erbium-doped fiber is directly compressed temporally.The cases of pulse compressions based on the gain fibers with g₀=3 and 9 m^-1are compared.The results show that when the values of g₀are 3 m^-1and 9 m^-1,the quality factors of pulse compressions from the4-m gain fiber are 26%and 23%,while the temporal widths of the compressed pulses in the two cases are around 0.1 ps.