Novel coherent supercontinuum light sources based on all-normal dispersion fibers

[摘要] ENGLISH ABSTRACT: The concept of broadband coherent supercontinuum (SC) generation in all-normal dispersion(ANDi) fibers in the near-infrared, visible and ultraviolet (UV) spectral regionsis introduced and investigated in detail. In numerical studies, explicit design criteria areestablished for ANDi photonic crystal fiber (PCF) designs that allow the generation offlat and smooth ultrabroad spectral profiles without significant fine structure and withexcellent stability and coherence properties. The key benefit of SC generation in ANDifibers is the conservation of a single ultrashort pulse in the time domain with smooth andrecompressible phase distribution. In the numerical investigation of the SC generationdynamics self-phase modulation and optical wave breaking are identified as the dominantnonlinear effects responsible for the nonlinear spectral broadening. It is further demonstratedthat coherence properties, spectral bandwidth and temporal compressibility areindependent of input pulse duration for constant peak power. The numerical predictionsare in excellent agreement with experimental results obtained in two realizations of ANDiPCF optimized for the near-infrared and visible spectral region. In these experiments,the broadest SC spectrum generated in the normal dispersion regime of an optical fiberto date is achieved. The exceptional temporal properties of the generated SC pulses areverified experimentally and their applicability for the time-resolved study of moleculardynamics in ultrafast transient absorption spectroscopy is demonstrated. In an additionalnonlinear pulse compression experiment, the SC pulses obtained in a short piece ofANDi PCF could be temporally recompressed to sub-two cycle durations by linear chirpcompensation. Numerical simulations show that even shorter pulse durations with excellentquality can be achieved by full phase compensation. The concept is further extendedinto the UV spectral regime by considering tapered optical fibers with submicron waistdiameter. It is shown that coherent SC spectra with considerable spectral power densitiesin the usually hard to reach wavelength region below 300 nm can be generated usingthese freestanding photonic nanowires. Although technological difficulties currently preventthe fabrication of adequate nanofibers, the concept could be experimentally verifiedby coherent visible octave-spanning SC generation in tapered suspended core fibers withANDi profile. The work contained in this thesis therefore makes important contributionsto the availability and applicability of fiber-based broadband coherent SC sources withnumerous high-impact applications in fundamental science and modern technology.

[发布机构] Stellenbosch University