Stimulated Brillouin Scattering And Supercontinuum In Mid-infrared Chalcogenide Fiber With Uniaxial Crystal Cladding

The mid-infrared spectrum has potential applications in remote sensing communications,environmental monitoring,laser surgery,and many industries because it covers two atmospheric transmission windows and a unique molecular fingerprint region.However,the loss of typical quartz fiber exhibits a significant increase at wavelengths larger than 2.4μm,which restricts its application in the mid-infrared wave.In the mid-infrared region,chalcogenide glass exhibits high nonlinearity,excellent transparency,and easy manufacturing.In recent years,sulfur-based optical fibers have been widely used in sensors,lasers,optical speed control,supercontinuum spectrum light sources,etc.Up to now,the reported chalcogenide fiber used different components of sulfur-based glass as the core and cladding material,which makes it difficult to achieve dynamic modification of the fiber performance.In this paper,a chalcogenide fiber mode less fiber with an electro-optic crystal cladding is proposed and the effect of an out electric field running along the axis of the fiber on the effective refractive index,dispersion,power confinement factor,and stimulated Brillouin scattering properties of the fiber are investigated,so that continuous tuning of the fiber performance can be realized by varying the electric field amplitude.In this study,an all-solid sulfur-based photonic crystal fiber is proposed and the effect of the out electric field on the supercontinuum spectrum’s spectral width and coherence is investigated.The main research contents and conclusions are as follows:1.A mid-infrared sulfur-based few-mode fiber with the cladding of LiNbO₃ crystal whose optical axis runs along the fiber axis is proposed.The effects of the electric field along the fiber axis on the refractive index ratios of the main axes of the LiNbO₃ crystal and the transmission properties of HE₁₁,HE₂₁,TE₀₁,TM₀₁,EH₁₁,HE₃₁,and HE₁₂ modes were estimated by using the finite element method.The results reveal that the applied axial electric field enhances each intra-mode dispersion while suppressing the differential mode delay between optical modes,and it also enhances the energy limitation for each mode.The influence of the electric field gets more evident with the optical mode order increasing.The difference dispersion values of the fundamental and highest order HE₁₂modes are 20.6413（ps/km/nm）and 260.1578（ps/km/nm）respectively,the power confinement factor varied by 1.369%and 14.603%between adding an electric field of 40×10⁸ V/m and no electric field at the wavelength of 2.8μm.The differential mode delay between HE₁₁ and HE₁₂ modes decreases by 156.8945（ps/km）at this wavelength.The effect of the high-intensity electric field on dispersion induces varying the zero dispersion wavelength of each optical mode,which of HE₁₁,HE₂₁,TE₀₁,and TM₀₁ modes blue shifts0.5695μm,0.3915μm,0.3862μm,and 0.5594μm in the short wavelengths,respectively.2.The effects of electric field intensity on the Brillouin gain spectrum,Brillouin frequency shift,Brillouin threshold,and slow light delays of inter-and intra-mode stimulated Brillouin scattering in LiNbO₃ crystal-clad chalcogenide few-mode fiber was examined.The results show that as the electric field strength increases,the contribution of higher-order acoustic modes to the Brillouin gain spectrum increases,and the Brillouin frequency shift can also be shifted,while the amount of slow light delay and the threshold of all mode pairs except TM₀₁-HE₂₁ are reduced.The peak value of the stimulated Brillouin scattering gain spectrum in HE₁₁ mode reduces by 1.94810^-11 m/W as the electric field intensity increases from 0 V/m to 40×10⁸ V/m and the Brillouin frequency shift offsets of 0.043 GHz.The Brillouin threshold of the TM₀₁-TM₀₁ mode pair hits 1.133 W when the effective length of optical fiber is 1 m.The maximum slow light delay simulated in this work is 194.799 ns when the electric field intensity is 0 V/m with 1.2 W pump light.The maximum slow light delay is 129.813 ns when the electric field strength is increased to 40×10⁸ V/m.3.A photonic crystal fiber filled with LiNbO₃ crystal rods is proposed.The effects of electric field strength on the dispersion,nonlinear coefficient,spectral width of the supercontinuum spectrum,and the coherence of the fiber with different filling factors were investigated.The results show that at the wavelength of 3.5μm,the filling factor increases from0.5 to 0.8 and the nonlinear coefficient increases from 163.79 W^-1m^-1 to 298.28 W^-1m^-1 with the electric field of 0 V/m.The nonlinear coefficient increases from 200.39 W^-1m^-1 to 345.32 W^-1m^-1 when the electric field strength increases to 40×10⁸ V/m.At the same pump power,the fiber with a filling factor of 0.6 produced a supercontinuum spectrum with not only the greatest spectral breadth but also a greater SC tuning range than the other structures.The maximum spectral width of 2.2420μm was obtained at a peak power of 5 k W for the pump pulse,an increase of 176.30 nm compared to that without the electric field,after subjecting the fiber with a filling factor of 0.6 to an electric field of 40×10⁸ V/m.The high intensity of the electric field also leads to the supercontinuum spectrum’s coherence factor converging to 1 over a wider wavelength range.