Supercontinuum Generation In Tapered And Cascaded Tapered Photonic Crystal Fiber

Supercontinuum (SC) generation in photonic crystal fibers (PCFs) is currently acutting-edge research field. SC offers an attractive optical source in diverse applications,such as wavelength division multiplexing, optical coherence tomography, ultrashortpulse generation, optical frequency metrology, etc. Recent advances in the postprocessing of PCFs, i.e., hole inflated and tapered PCFs possess manageable dispersionand enhanced nonlinearity, open new possibilities in SC generation. In this dissertation,the SC generation in tapered PCFs and cascaded PCF tapers is studied theoretically andexperimentally.Firstly, the theoretical foundation of SC generation in PCFs is described, and theimportance of dispersion and nonlinear effects in the SC generation process is analyzed.Meanwhile, the numerical model of SC generation in tapered PCFs is clarified, and SCgeneration in tapered PCF is numerically simulated by self-compiled program. Theimpacts of taper structure, pump wavelength and input pulse duration on SC evolutionare considered and examined.Secondly, since the importance of dispersion in determining the generated SCmakes the control of the dispersion profile a key factor in fiber design, the effectiveindex, dispersion characteristic and group velocity of tapered PCFs are studied.Accordingly, the concept of cascading tapered PCF is proposed for the first time, anddifferent approaches to achieve a blue-extended and flat SC are proposed. Using self-compiled program, the SC generation in cascaded same and different PCF tapers is alsonumerically simulated. Hence the function of cascading is illustrated. Via cascadingPCF tapers with same structural parameter, we can overcome the limitation of thetapering rig to enhance the nonlinear interaction length. Via cascading PCF tapers withdifferent structual parameter, we can not only enhance the interaction length, but alsoobtain higher nonlinearity and more manageable dispersion to increase the qualities ofSC, such as spectral range and flatness.Thirdly, using tapering rig, PCFs with various structal parameters are tapered whilekeeping d/unchanged, all the tapers possess low loss which is less than0.3dB inthe complete structure. Combining tapering and hole inflation techeniques, instead ofusing splicing techenique, cascading same and different PCF tapers in a singlemonolithic fiber device are realized, which can shape spectral amplitude and controlbandwidth of SC output. Fourthly, SCs are generated in tapered PCFs. Using1050nm femtosecond source,1064nm picosecond source and1064nm sub-nanosecond source, the SC outputs morethan an optical octave are obtained, which are broader and flatter than those fromuntapered PCFs. In the tapered PCF with30cm-long taper length, an8.7W SC pumpedby1064nm picosecond source is obtained. To our knowledge, it is the highest“white-light” SC powers based on short tapered PCFs in this pump regime.Fifthly, ultrabroad and ultraflat SCs are generated in cascaded PCF tapers pumpedby1064nm source. In cascading PCF tapers with same parameter, SC which containsmore than two optical octave output are demonstrated. In an optimal configuration, theoutput SC with sepctral range470nm~1670nm (-20dB) is demonstrated, with anultraflat (3dB) spectrum from500to1000nm. In cascading PCF tapers with differentparameter, extending the short wavelength edge of SC pumped by1064nm sources tobelow400nm is acheived. The widest SC extending from352nm to over1750nmwith5dB spectral range of598nm (from368to966nm) is achieved. To ourknowledge, it is the shortest wavelength of SC pumped by1064nm sources.