【作者】 孙青；

【导师】 明海； 毛庆和；

【作者基本信息】 中国科学技术大学 ， 光学， 2009， 博士

【摘要】 空芯光子晶体光纤(HC-PCF)可显著增强光与填充介质间的相互作用,利用HC-PCF可有效降低气体的受激拉曼散射(SRS)阈值。当采用近红外稀土掺杂光纤激光器泵浦,基于HC-PCF的气体SRS效应在非线性光学、激光技术、激光光谱学以及高精度痕量气体检测等领域具有重要应用前景。本论文开展了基于HC-PCF的气体SRS效应的研究,对HC-PCF全光纤高压气体腔的制作与气压控制、SRS泵浦光源的研制以及基于HC-PCF的气体SRS实现方法等关键理论技术问题进行了深入的研究,主要包括以下几个方面:给出了SRS的经典电磁场描述,由此得出了稳态条件下的SRS增益系数,并分析了瞬态过程导致的SRS增益下降问题,以及泵浦光的线宽对前后向SRS过程的影响。在此基础上,建立HC-PCF中的SRS理论模型,从而揭示出HC-PCF损耗与模式特性对SRS过程的影响。利用平面波展开法对HC-PCF的模式特性进行了分析,并实验测量了HC-PCF的模场分布。通过HC-PCF气体填充置换和泄漏实验,结合微管管流理论,研究了HC-PCF气体填充过程中的动力学行为和气体泄漏特性,结果表明,HC-PCF的气体填充过程极为缓慢,减小出口密封腔体积可有效缩短充气时间。通过对熔接参数的优化,利用商用光纤熔接机实现了HC-PCF与传统SMF之间的低损耗熔接。在上述两项工作的基础上,提出一种全光纤型HC-PCF高压气体腔制作方案,并通过测量单端气体泄漏速度随时间变化关系评估了所研制的高压气体腔的最终腔压。研制了一台非线性偏振旋转被动锁模掺镱光纤激光器,通过光纤耦合器进行脉冲堆积实现了脉冲宽度的加宽,通过滤波将光谱变窄后再经级联掺镱光纤放大器进行功率提升,最终获得重复频率11.68MHz,光谱宽度0.5nm,脉宽约800ps,峰值功率11.8W激光脉冲序列。通过研究不同结构调Q光纤激光器的脉冲输出特性发现,放大的自发辐射(ASE)对调O掺镱光纤激光器的输出脉冲有影响,无论是环形腔还是线形腔,宽带ASE均会消耗掉大量反转粒子数,不利于调Q激光脉冲振荡的形成;通过在腔内加入选频器件以抑制ASE,可将腔内光子数在Q开关打开时保持在较低的水平,使得增益远大于腔损耗,从而利于形成高峰值功率的调Q激光脉冲;此外,由于线形腔结构可利用掺杂光纤的双程增益,调Q运转时,阈值更低,输出脉冲宽度更窄、峰值功率更高。在对调Q光纤激光器结构的研究基础上,设计并研制了一种波长可在1064±5nm范围内调谐的调Q掺镱光纤激光器,获得了重复频率为5kHz、脉宽约30ns、最大峰值功率达85W的激光脉冲序列。利用研制的高峰值功率调Q掺镱光纤激光器作为拉曼泵浦光源,首次实现了单模光纤激光器泵浦的氢气SRS效应,避免了光源耦合的问题,大大提高了泵浦效率,实现了全光纤化的气体拉曼激光波长变换,有利于促进气体拉曼激光器件的小型紧凑化。以上研究结果对研制实用化的气体拉曼激光器件以满足特殊波段激光应用需求具有重要的参考价值。更多还原

【Abstract】 Hollow-core photonic crystal fiber(HC-PCF) can be used for enhancing the nonlinear interaction between the guided light and the hydro-medium filled in it,in particular,the gases.It has been shown that the threshold pump power of stimulated Raman scattering(SRS) with gas-filled HC-PCF may be greatly reduced.This implies that the gas SRS effect based on such a HC-PCF technique may find applications in nonlinear optics,laser technology and spectroscopy,and high-resolution trace gas detection,when rare earth doped fiber lasers are used as the pump.In this dissertation,the technical issues for the SRS effect based on gas-filled HC-PCF,including all-fiber high-pressure gas cell based on the HC-PCF,the SRS pump sources,and the demonstration of the SRS effect of hydrogen-filled HC-PCF, are studied in details.The main research works are as follows.The gain coefficient of stable state SRS is deduced form the classical electromagnetic field theory and the gain decline caused by the transient process is analysed.The different influences of pump linewidth on forward and backward SRS process are discussed.Furthermore,the theoretical model of SRS in HC-PCF is established,and the influences of transmittion loss and mode characteristic on SRS process are opened out.Finally,the mode characteristic of HC-PCF is analysed using the plane wave expansion method and the mode filed distribution is tested experimentally.Based on gas filling and leaking experiments,combined with the micro-pipe flow theory,the gas-filling and leaking characteristics for HC-PCF are studied.The results show that,the gas-filling speed of the HC-PCF is very slow,and a very long time is needed to build pressure equilibrium between the inlet and the outlet of the HC-PCF.The time required for building the pressure equilibrium in the HC-PCF may be efficiently shortened by reducing the volume of the outlet cavity.The HC-PCF has been successfully spliced to the standard single-mode fibers with low splicing losses by just using a normal commercial fiber splicing machine with the splicing parameters suitably optimized.Moreover,a new method to build all-fiber high-pressure gas cell based on the HC-PCF is present,and such an all-fiber high-pressure gas cell is also made with its final equilibrious gas pressure valued by the measured leaking speed evolution behavior. A passively mode-locked ytterbium-doped fiber laser(YDFL) is prepared with the nonlinear polarization rotation mechanism.By broadening the pulse duration with the pulse stacking technique based on a fiber ring,narrowing the pulse linewidth with a filter,and enhancing the pulse peak power with cascaded YDF amplifiers,a pulse train with repetition of 11.68MHz,3dB linewidth of 0.5nm,duration of about 800ps and peak power of 11.8W is obtained when the mode-locked YDFL is used as the seed source.On the other hand,the experimental investigations of the output characteristics for actively Q-switched fiber lasers with different configurations show that,the amplified stimulated emissions(ASEs) affect the generation process of the Q-switched laser pulses.Since the broadband ASE depletes most of the inversion population,it may be an adverse effect for the formation of the Q-switched laser pulses for either ring or linear cavity fiber lasers.When the ASE is suppressed with a narrow filter inserted in the cavity,the photon number may be limited at a low level right after the Q-switch is opened,which results in the gain much higher than the cavity loss at that time,and thus,benefitting the formation of the high-power Q-switched pulses.Moreover,the experimental results show that the Q-switched fiber lasers with linear cavity may be better than that with ring cavity because the former uses double-pass gain of the doped fiber.Based on the experimental results,a wavelength-tunable Q-switched YDFL with linear cavity is designed and built,and the pulse train with repetition of 5 kHz,duration of 30ns,peak power of 85W,and wavelength tuning range between 1060 and 1069nm is obtained finally.With the all-fiber high-pressure gas cell based on the HC-PCF described above, the SRS phenomenon for hydrogen gas,for the first time to our best knowledge,has been observed with such a Q-swithced ytterbium-doped single mode fiber laser as the pump source.For the compatibility of the pump laser with the single mode fibers, such SRS effects may be complemented with all-fiber structure,and thus may be useful for developing compact Raman devices,for instance,such as gas Raman lasers.The investigation results described above may be very helpful in developing gas Raman laser devices for satisfying the application requirements of new spectral region.更多还原