Coherence properties of supercontinuum generated in highly nonlinear photonic crystal fibers.

摘要

In this dissertation, experimentally measured spectral and coherence evolution of supercontinuum (SC) is presented. Highly nonlinear soft-glass photonic crystal fibers (PCF) were used for SC generation, including lead-silicate (Schott SF6) PCFs of a few different lengths: 10.5 cm, 4.7 mm, and 3.9 mm, and a tellurite PCF of 2.7 cm. The pump is an optical parametric oscillator (OPO) at 1550 nm with pulse energy in the order of nanojoule (nJ) and pulse duration of 105 femtosecond (fs). The coherence of SC was measured using the delayed-pulse method, where the interferometric signal was sent into an optical spectrum analyzer (OSA) and spectral fringes were recorded. By tuning the pump power, power-dependent evolution of spectrum and coherence was obtained.;Numerical simulations based on the generalized nonlinear Schrodinger equation (GNLSE) were performed. To match the measured data, the simulated spectral evolution was optimized by iteratively tuning parameters and comparing features. To further match the simulated coherence evolution with the measurement, shot noise and pulse-to-pulse power fluctuation were added in the pump, and the standard deviation of the fluctuation was tuned.;Good agreement was obtained between the simulated and the measured spectral evolution, in spite of the unavailability of some physical parameters for simulation. It is demonstrated in principle that, given a measured spectral evolution, the fiber length, and the average power of SC, all other parameters can be determined unambiguously, and the spectral evolution can be reproduced in the simulations. Most importantly, the soliton fission length can be simulated accurately.;The spectral evolution using the 4.7- and the 3.9-mm SF6 PCFs shows a pattern dominated by self phase modulation (SPM). This indicates that, these fiber lengths are close to the soliton fission length at the maximum power. The spectral evolution using the 10.5-cm SF6 PCF and the 2.7-cm tellurite PCF shows a soliton-fission-dominated pattern, indicating these lengths are much longer than the soliton fission length at the maximum power.;For the coherence evolution using the SF6 PCFs, the simulations and the measurements show qualitative agreement, confirming the association between coherence degradation and soliton fission. For the case of the tellurite PCF, nearly quantitative agreement is shown, and it is shown that the solitonic coherence degrades slower than the overall coherence.;Fluctuation of coherence occurs at the regime where the coherence starts to degrade, in the measurement and the simulations of the SF6-PCF case. It is shown that the cause is the pulse-to-pulse power fluctuation in the pump.;The pulse-to-pulse stability of spectral intensity is another characterization of SC stability, other than the coherence. It is shown by simulations that these two exhibit different dynamics, and have low correlation.