| Abstract [eng] |
Measurement of Nonlinear Refractive Index Dispersion in Optical Fiber Photonic crystal fibers (PCFs) are different from conventional optical fibers, because PCFs have a periodic microstructure region inside them where light is propagating. Solid core PCFs are perfect nonlinear media, in which it is possible for nonlinear processes to occur due to third order nonlinearity [1 - 4]. Since the nonlinear refractive index is a significant parameter that describes the efficiency of phenomena caused by χ^(3) nonlineraty, it is very impotant to know its accurate value. The research aims to determine the nonlinear refractive index of PCF at a non-zero dispersion wavelength, using a new measurement method, that combines measurements of spectrum spreading caused by phase modulation, using femtosecond pulses and theoretical modeling. First of all, PCF group velocity dispersion (GVD) was measured using a supercontinuum generation and XFROG method. In order to estimate nonlinear refractive index, the optical setup for the measurements was constructed where a 1036 nm center wavelength (289,51 THz center frequency), 76 MHz repetition rate and 90 fs duration pulses, generated by a Yb:KGW laser oscillator, were introduced into PCF. Measurements of the spectrum emitted from the PCF and spread due to phase modulation were performed by varying the power of the pumping pulses with an attenuator composed of half-wavelength plate and Brewster polarizer. Then, theoretical modeling was compiled until the obtained results qualitatively coincided with the results of the experiment. After theoretical modeling, which produced results qualitatively corresponding to the practical experiment, nonlinear refractive index was estimated. This method enabled the evaluation of nonlinear absorption coefficient, nonlinear refractive index dispersion, and nonlinear absorption coefficient dispersion. According to the results of the analysis, it was determined that despite the fact that the length of PCF used (3 cm), which is shorter than fiber dispersion length (30, 2 cm), its dispersion still influences the form of the spectrum and the frequency of peak maximum. Furthermore, it became evident that in order to get qualitatively corresponding theoretical data and the results of the experiment, it is necessary to include nonlinear refractive index dispersion and nonlinear absorption dispersion, that take in to account theoretical model units, describing nonlinear group velocity and nonlinear second order dispersion. |
