| Abstract [eng] |
Photonic crystal fibers (PCFs) are a unique type of optical fibers, containing a periodic microstructure area, thus allowing to enhance some of the fiber’s main characteristics. The engineering ability to modify the periodically orientated air hole diameter and distance between them, allowed to manufacture a nonlinear medium with enhanced nonlinearity. Material nonlinearity will determine the efficiency of nonlinear effects caused by third order nonlinear optical susceptibility. Nonlinear processes appearing in PCF can be qualitatively evaluated only if materials nonlinear refractive index and its dependency on pump wavelengths, i.e., n2 dispersion in known. The main task was to investigate continuum generation in photonic crystal fiber and evaluate nonlinear refractive index dispersion using tunable frequency femtosecond pulses. Latter pulses were obtained by constructing synchronically pumped optical parametric oscillator (644 nm - 1030 nm), which was seeded with amplified pulses, generated from Yb:KGW laser oscillator operating at 76 MHz repetition rate. For analysis of spectral continuum generation and nonlinear refractive index evaluation measurement of light spectrum dependency on pump power were performed, meanwhile temporal domain characteristics were obtained by implementing crosscorrelation frequency resolved optical gating (XFROG) technique, where XFROG traces were measured at various pump power values. Lastly, for qualitative analysis numerical simulation was applied and obtained resulted were compared with experimentally measured data. After investigating continuum generation in photonic crystal fiber using tunable frequency femtosecond pulses, not only PCF nonlinear refractive index dispersion, but also pulse spectral change dynamics were evaluated. It was observed, that the pulse broadening in normal dispersion regime is mainly influenced by self-phase modulation and also fiber‘s dispersion value. Furthermore, continuum spectrum formation also depends on the alignment of input light polarization with one of the fibers birefringence axes. Finally, after evaluating the photonic crystal fiber’s nonlinear refractive index at various pump pulse wavelengths, n2 dispersion was evaluated: n2 = 3·10−20 m2/W (646 nm); n2 = 2,8·10−20 m2/W (756 nm); n2 = 2,5·10−20 m2/W (866 nm); n2 = 1,6·10−20 m2/W (955 nm); n2 = 2·10−20 m2/W (1006 nm). From obtained results a significant dispersion of nonlinear refractive index can be observed. Furthermore, from obtained results it was noticed, that pulse spectral broadening is not only determined by dispersion and self-phase modulation effects, but also asymmetrical pulse broadening is determined by inertial material response, due to pulse intensity change in time. |
