Research On The Coherence Properties Of Supercontinuum In Photonic Crystal Fibers

Compared with the conventional fibers, photonic crystal fibers have manyadvantages, such as high nonlinearity, no cut-off single-mode, controllable dispersionand so on, which provide photonic crystal fibers with a wide range of applications inthe field of optical communication. When the pulse transmits through an optical fiber,the dispersion and nonlinear effect will greatly broaden the spectrum, and we call theprocess as the generation of the supercontinuum. However, due to the influence of thenoise in the process, the coherence properties of the resulted supercontinuum will bebadly affected. If the supercontinuum is meant to be used in various areas such asmedicine, metrology and optical pulse compression as broadband light source, it musthave greater coherence properties, thus it is of great importance to measure thecoherence qualities of supercontinuum, which is also highly emphasized in my thesis.Based on this, the main parts of my thesis are constructed as follows:Firstly, discussing how the coherence property of the generated supercontinuum isaffected under different pump pulse parameters such as pulse width and energy. Thispaper concretely analyzes the process of supercontinuum generation, which is mainlyaffected by the noise including input pump pulse noise and spontaneous Raman noiseduring the transmission of photonic crystal fiber. Later, starting from the nonlinearSchrodinger equation and using the optimized integration process of the step by stepFourier method to make numerical simulations. Simulation results show that under thesame conditions, when the pump pulse duration is short enough, the self-phasemodulation will play a major role in the process of the spectrum broadening while themodulation instability will become relatively weak. Meanwhile, when the pump pulsepeak power is lower, the nonlinear effect will play a weak role in the process of thespectrum broadening, consequently, the resulted coherence properties of generatedsupercontinuum will be greatly improved.Secondly, a photonic crystal fiber which is suitable for generating high coherencemid-infrared supercontinuum is designed. The paper selects bismuth-based glass asmaterials and adapts the multi-pole method, finally, the simulation results showed thatthe obtained optical fiber in mid-infrared band has flat dispersion curve. Throughsimulate the generation of the supercontinuum in the designed photonic crystal fiber,the resulted range of the supercontinuum is from1220nm to3300nm, and the entire range of the spectrum is well coherent. Self-phase modulation and wave splitting arethe main physical mechanisms in the process of the supercontinuum generation.