| Raman fiber lasers possess the advantages of the simplicity, good beam quality, and high conversion efficiency, moreover, the raman fiber laser with large output power laser can be achieved at any wavelength by using the suitable pump source. In the other hand, several kinds of mode-locked lasers based on different principles have been exploited owing to the vast potential for future development. And it’s worth noting that the3-5μm mid-infrared lasers have attracted growing attention and became the hot topic in the laser field, because the important applications in national defence, medicine, free space optical communication and materials processing. In order to achieve high-performance ultrafast mid-infrared laser with the characteristics of compact structure and low-cost, a convenient way is combining the advantages of raman fiber laser, mode-locked laser and mid-infrared laser. In this graduate academic paper, the research about the raman fiber laer was made from three aspects:1) the performance of intra-cavity and extra-cavity pumped1240nm raman fiber laser was analysed experimently,2) a harmonic mode-locked raman fiber laser based on intermodal-beating mode-locking (IBML) mechanism was achieved at1240nm,3) theoretical research of the3-5μm mid-infrared raman laser by using ZBLAN fiber was proposed. The main results are summarized as follows:1. By changing the reflectivity of the fiber loop mirror (FLM) used in our experiment, the characteristics of intra-cavity and extra-cavity pumped1240nm raman fiber lasers have been recored and analysed, respectively. Our results clearly show that the slope efficiency and the threshold decrease with increasing the reflectivity. Moreover, a interesting phenomenon was observed that only continues wave (CW) outputed in intra-cavity pumped raman laser, in contrast, stable pulses can be obtained in the extra-avity pumped raman laser, the repetition rate and pulse duration were match to conventional mode locked lasers.2. To further investigate the pulsed raman fiber laser obtained in (1), we optimized the cavity length of our fiber laser, and the mode locked pulse with repetition rate of47.178MHz and74.38MHz was obtained, respectively. To conform the obtained pulse was mode-locked, the IBML mechanism was introduced to explain the phenomenon we observed. Our results suggest that there is obvious influence on the IBML by changing the cavity length of the raman fiber laser and1064nm pump source. In addition, the tolerant of cavity length changing in IBML is0.1-0.9m.3. The3-5μm mid-infrared ZBLAN fiber lst/2nd-order Raman lasers are numerically analyzed and optimized. The theoretical optimization is mainly focused on the effects of ZBLAN fiber length and output mirror reflectivity on the Raman laser threshold, output power and conversion efficiency. The numerical results show:1) the raman laser threshold significantly reduces with the increase of the output-mirror reflectivity, and there always exists the optimized ZBLAN fiber length for the lowest Raman threshold (4.15W);2) to achieve greater output power as possible, the optimizing reflectivity of output mirror are84-98%(1st-order) and42-60%(2nd-order), the optimizing ZBLAN fiber lengths are6.7-8.9m (1st-order) and1.5-2.4m (2nd-order), respectively. The optimizing slope conversion efficiencies can be as high as72.36%(1st-order) and34.06%(2nd-order). These results could provide a significant guidance for experiment of mid-infrared fiber Raman lasers. |
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