Research On Supercontinuum Generation In Cascade Tapered Photonic Crystal Fiber

Supercontinuum(SC)has the advantages of high energy,good directivity,and wide frequency spectrum.As a kind of important light source,it has been extensively applied in different fields,such as wavelength division multiplexing,optical coherence tomography,ultra-short pulse generation,optical frequency comb,etc.The SC can be generated through pumping in the normal or anomalous dispersion regions of the nonlinear medium.When the pump pulses work in the anomalous dispersion region,a wider SC can be generated,but its coherence is poor.When the pump pulses work in the normal dispersion region,the coherence of the SC is good,but its bandwidth is usually narrow.The dispersion of photonic crystal fiber(PCF)can be flexibly adjusted.Tapering the PCF is an effective method to adjust its dispersion.Therefore,PCF taper provides a new way for the generation of highly-coherent SC.In this paper,the nonlinearity and coherence of the SC in the cascaded-tapered PCF are investigated.The main research contents and achievements are summarized as follows:1.Based on the investigation on the propagation mechanism of the pulse in the PCF,a cascade-tapered silica PCF model is designed.The light pulses with different peak powers and widths are launched into the PCF,and the corresponding SC evolution process and coherence are compared by the simulation.The simulation results show the differences in the nonlinear evolution and coherence of the SC in the cascaded-tapered PCF when the pump pulses with peak powers of 5,10,and 15 kW and pulse widths of 60,100,and 140 fs are used.It is concluded that when the pump pulses with width of less than 100 fs and peak power of less than 10 kW are propagated in the cascaded-tapered silica PCF,the SC with good coherence can be generated.2.When the pump pulses with center wavelength of 800 nm,width of 80 fs,and peak power of 10 kW are launched into the designed cascaded-tapered PCF,the nonlinear phenomena in the evolution process of the SC are systematically analyzed.The SC broadening is greatly affected by the self-phase modulation and stimulated Raman scattering effects of the first tapered fiber.3.A cascaded-tapered telluride PCF is designed.The nonlinear evolution and coherence of the SC in the cascaded-tapered PCF are investigated when pump pulses with widths of 50,100,and 150 fs and peak powers of 5,10,and 15 kW are used.It is concluded that as the pulse width and peak power increase,the SC is broadened significantly,but the coherence is degraded.According to the simulation results,the SC is generated when the optimized pump pulses with center wavelength 1500 nm,width 80 fs,and peak power 10 kW are used.The nonlinear phenomena in the evolution process of the SC are systematically analyzed.The broadening of the SC is mainly affected by the stimulated Raman scattering effect in the first tapered fiber.For the two PCF tapers,the soliton splitting effect has significant effect on the coherence of the SC.The research works of this paper on the evolution and coherence of the SC in cascaded-tapered PCF have demonstrated the nonlinear effects of the SC generated in the cascaded-tapered silica and telluride PCF and the influences of the pump pulse parameters on the coherence of the SC.The related research achievements have enriched the generation and optimization technology of the SC source,and actively promoted the application of the PCF post-processing technology.