Numerical Study Of Supercontinuum Generation Based On Highly Birefringent Optical Fiber With All-normal Dispersion

Supercontinuum spectra is widely employed in the optical communication system,spectroscopy,national defense security and biomedicine due to its broad spectral widths and high coherence.In this paper,three kinds of the normal-dispersion polarization maintaining(PM)fiber with high nonlienarities are proposed.The supercontinuum generation with two orthogonal polarized fundamental modes(FMs)in the PM fibers is numerically simulated and analyzed.The main works are as follows:(1)The theretical model of the pulse propagating along the optical fiber is introduced,including the definitions and calculation equations of the group velocity dispersion(GVD),nonlinear parameter,loss and birefringence.Additionally,the generalized nonlinear Schrodinger(GNLS)equations combined with its numerical solution are also discussed.(2)The PM single-mode fiber based on the highly nonlinear liquid CS₂ is proposed.The CS₂-filled silica fiber with two air holes in the cladding can offer high nonlinearity and birefringence,low loss and all-normal GVD profiles for both two orthogonal polarized FMs by an appropriate choice of the fiber parameters.Then,the influences of the peak power,the width and the polarization angle of the input pulse on the supercontinuum generation are studied.The numerical results show that when the1920-nm 0.1-ps pump pulse with the 25-k W peak power is coupled into the 0.1-m fiber and linearly polarized along the slow and fast axes,the output spectra can keep the linear polarizaiton along two principal axes across the entire supercontinuum(SC),covering one octave in the wavelength range of 1000–2500 nm at-20 d B.Furthermore,the pulse energy tranfer from the fast axis to the slow axis induced by the nonlinear coupling effect can result in the degradation of linear polarization,even though the input polarization angle is only 2^o from the fast axis.(3)Two kinds of the PM single-mode fibers based on the Ge-doped silica are proposed.The first one is the heavily Ge-doped silica fiber with two symmetrical air holes in the cladding.Both the orthogonal polarized FMs have high nonlinearity and birefringence,low loss and all-normal GVD profiles.The nonlinear parameters of the two FMs are about 0.01 W^-1/m at 1920 nm,and the birefringence is more than 10^-4.The numerical results indicate that when the 1920-nm 0.1-ps pump pulse with the 150-k W peak power is coupled into the 0.5-m fiber and polarized along the slow-and fast axes,the output spectra can cover over one octave in the wavelength range of around 900–2500 nm at-20 d B.At the same time,the output pulses have an excellent coherence in the whole wavelength range.The second one exhibits high birefringence for two orthogonal polarization FMs by introducing elliptical air holes in the heavily Ge-doped core.By optimizing the structure parameters of the fiber,a low and flat GVD profile is obtained in the wavelength range of 1000–3000 nm.The nonlinear parameter at 1920 nm is as high as 0.01 W^-1/m,and the birefringence reaches the order of 10^-3.The numerical results show that when the ultrashort pulse linearly polarized along the principal axes is coupled into the PM fiber with elliptical air holes,the linear polarization supercontinuum with exellent coherence can be obtained.