The Research About Mid-infrared Supercontinuum Generation In ZBLAN Fibers

The supercontinuum(SC) sources have been extensively applied in defense, medicine and scientific research because of their broad spectral range and strong spatial coherence. Particularly, the mid-infrared supercontinuum(mid-IR SC) with a wave band covering large number of molecular absorption spectrum can be used in environmental monitoring, gas molecular detection and so on. This situation has further attacted considerable attention of numerous researchers in the area of mid-IR SC. Therefore, the current major research should be focused on further broadening the spectral range and improving the flatness of mid-IR SC. Currently, the mid-IR SC generation has been reported generally based on the nonlinearity of soft-glass fibers. There are many researches about mid-IR SC generation in ZBLAN fibers. However, there is still no theoretical investigation on optimization simulations of fibers and pump sources to obtain ultra-broadband and ultra-flat supercontinuum yet. Therefore, it is particularly important to optimize the design of SC generation in ZBLAN fibers. In this dissertation, the theoretical simulations of mid-IR SC generation in ZBLAN fibers were conducted, and the whole dissertation can be divided into the following three parts:(1) The general nonlinear Schrodinger equation(GNLSE) based on supercontinuum was eatablished. Based on the split-step Fourier method, the matlab software was used to numerically solve the GNLSE. Since the GNLSE is a partial differential equation, the fourth-fifth order Runge-Kutta algorithm was used to solve the equation, thus establishing the theoretical model of supercontinuum. Subsequently, the frequency and temporal evolutions of pulse propagation in fibers were discussed and investigated.(2) Based on the theoretical model, the numerical simulations were used to investigate mid-IR SC generation in normal and anomalous dispersion regime. Considering the physical mechanisms of SC generation in different dispersion regimes, the effects of fiber length, peak power and input pulse duration on output pulse properties were discussed. Thus the optimized parameters were obtained respectively, such as the fiber length of 1~1.9 m and 2.9~3.4 m, the peak power of 660~1000 kW and 50~80 kW, the input pulse duration of 200 fs and 750~1000 fs, the spectral bandwidth of 1.727 μm and 3.249 μm. Contrasting the spectral broadening of different dispersion regimes, the simulation results showed that it is easier to achieve the ultra-broadband and ultra-flat supercontinuum for pumping in the anomalous dispersion regimes on the condition of controlling the input pulse duration effectively.(3) Based on the current situation that mid-IR SC is commonly generated pumping by 2 μm ultra-short pulse in the anomalous dispersion regime, the 2 μm passively mode-locked Tm3+ doped fiber laser based on the figure-of-eight nonlinear optical loop mirror(NOLM) was demonstrated. The experimental results showed that both stable solitary and noise-like mode-locked pulses were observed at all-anomalous-dispersion regime. The oscillator produced soliton pulse with pulse width of 2.8 ps and energy of 83.8 nJ centered at 2017.33 nm. The noise-like pulse was measured at 2017.24 nm with pulse width of 341 fs and energy of 249 nJ, which was the highest reported pulse energy from 2 μm mode-locked fiber lasers without amplification. The noise-like pulse with higher SNR(60 dB) and energy in this dissertation was a great potential used to pump mid-IR SC generation.