节点文献
全正色散光纤激光器的锁模及调Q特性研究
Study on the Performance of Mode Locking and Q Switching of All-normal-dispersion Fiber Laser
【作者】 张丽强;
【导师】 卓壮;
【作者基本信息】 山东大学 , 光信息科学与技术, 2014, 博士
【摘要】 超短激光脉冲具有峰值功率高、脉冲宽度窄,光谱宽等一系列优点,在微观世界探测、高容量密钥通信、材料微加工、生物医学等领域有重要应用价值。被动锁模或调Q技术是获得超短脉冲的有效方法。与固体激光器相比,锁模光纤激光器具有结构紧凑、散热效果好、成本低等优点,但光纤中非线性相移的积累限制了被动锁模光纤激光器中脉冲能量的提高。提高脉冲能量的有效方法是使其工作在正色散区,通过展宽脉冲来降低峰值功率,从而控制积累的非线性相移。展宽脉冲在腔外压缩,可以获得飞秒量级的超短脉冲。对于超短脉冲技术的研究,在追求更高脉冲指标的同时,也注重新的饱和吸收材料的研究。石墨烯是一种新兴的锁模器件,具有饱和吸收性与波长无关、恢复时间快等优点,自2009年报道具有可饱和吸收性以来,其在光电领域的应用引起了国内外研究人员的广泛关注。本论文对基于非线性偏振旋转的全正色散锁模光纤激光器的工作特性进行了数值研究;实现了全正色散耗散孤子锁模光纤激光器可调谐输出;研究了全正色散耗散孤子锁模光纤激光器中的脉冲分裂现象;研究了全正色散锁模激光器输出啁啾脉冲的压缩行为,实现了瓦级飞秒激光输出;以石墨烯为饱和吸收体,研究了光纤激光器的调Q和锁模两种情况下激光的输出特性。具体内容如下:1.基于耦合的Ginzburg-Landau方程和各器件的琼斯矩阵,建立了基于非线性偏振旋转的全正色散锁模光纤激光器的数值模型,计算了腔内各点脉冲不同部分的偏振态。计算结果表明,当线性双折射较强时,光纤中脉冲的偏振态近似以拍长为周期变化,一个拍长内的演化过程为右旋椭圆偏振光-线偏振光-左旋椭圆偏振光-线偏振光-右旋椭圆偏振光。与一般的饱和吸收体不同,非线性偏振旋转等效饱和吸收体的调制深度随波片角度变化。计算了波片方位角改变时,调制深度的变化情况。相比于偏振分束器之前的二分之一波片及四分之一波片,偏振分束器之后的波片对调制深度的影响更大。2.计算了基于非线性偏振旋转的全正色散锁模光纤激光器中,激光器的稳定状态随滤光片带宽、小信号增益系数及波片角度的变化。在稳定锁模范围内,全正色散锁模激光器可输出圆顶形光谱、耗散孤子的陡峭边缘光谱、分裂光谱及自相似脉冲的抛物线形光谱。计算了不同状态下,腔内脉冲形状和光谱的演化过程。3.以双包层掺镱光纤为增益介质,双折射滤光片为波长选择器件,搭建了可调谐耗散孤子锁模光纤激光器。通过旋转滤光片,激光器的输出波长在1063.9nmn-1091.1Inm范围内连续可调,脉冲宽度及光谱带宽的变化范围分别为13.9ps16.8ps和15.8nm-18nm。中心波长1063.9nm时,激光器输出功率212mW,单脉冲能量8.5InJ。基于耦合的Ginzburg-Landau方程和双折射滤光片的透射曲线,建立了可调谐耗散孤子锁模激光器的数值模型,对激光器的可调谐性能进行模拟,模拟结果与实验结果一致。4.研究了全正色散耗散孤子锁模光纤激光器中的脉冲分裂现象。以非线性偏振旋转锁模器件的输出端口作为输出的耗散孤子锁模光纤激光器中,调节波片角度或泵浦功率,激光器输出的光谱和脉冲分为两部分。数值模拟和实验结果表明,脉冲分裂是由非线性偏振旋转本身的锁模原理引起的,当非线性偏振旋转的效应较强时,脉冲中心部分几乎无损耗地通过偏振分束器,偏振分束器过滤掉的脉冲只包含脉冲的边缘,非线性偏振旋转的排出口输出的脉冲分为两个子脉冲,由于脉冲是啁啾的,脉冲不同部分对应不同的频率,输出的光谱也分成了两部分。5.分析了光栅对色散补偿的原理,讨论了用光栅对进行脉冲压缩时光栅对参数的选择问题。用透射光栅对对正色散锁模光纤激光器输出的啁啾脉冲进行压缩,压缩前脉冲宽度6.73ps,平均功率2.36W。压缩后脉冲宽度161.5fs,平均功率1.22W,压缩总效率51.7%。压缩前后脉冲的峰值功率分别为6.09kW和131.4kW。对光栅对进行脉冲压缩时限制脉冲压缩效果的因素进行了分析。6.将化学气相沉积法生长的石墨烯转移到光纤头上制作成饱和吸收器件,在环形腔掺饵光纤激光器中获得了调Q脉冲。泵浦功率57.2mW至114.4mW变化时,脉冲重复频率的变化范围为34.72kHz至53.2kHz,输出功率的变化范围为0.504mW至0.926mW。获得的最短脉冲为3.2μs,对应的单脉冲能量为17.41nJ。7.以生长在SiC上的石墨烯为饱和吸收体,搭建了可调谐调Q光纤激光器。通过旋转双折射滤光片,激光器的中心波长在1038.54nm至1056.22nm之间可调谐。泵浦功率4.08W时,获得了0.65μJ的脉冲能量,对应的脉冲宽度、平均输出功率和峰值功率分别为1.60μs、35mW和406m W。8.将石墨烯转移到侧面抛光的光纤上,使光纤中的倏逝波与石墨烯相互作用,制作成可饱和吸收器件。研究了不同输出耦合比的情况下,倏逝波锁模光纤激光器的输出特性。使用30%的输出耦合器,泵浦功率582mW时,获得了8.34nJ的脉冲能量,脉冲的重复频率为8.4MHz,脉冲宽度为13.8ns。本研究论文主要创新点如下:1.详细计算了全正色散锁模光纤激光器中偏振态的演化过程,首次计算了波片方位角改变时非线性偏振旋转等效饱和吸收体调制深度的变化情况,研究了激光器的稳定状态随激光器参数的变化规律。2.在掺镱双包层光纤激光器中实现了可调谐耗散孤子输出,激光器平均输出功率212mW,对应的单脉冲能量8.5nJ,是当时报道的可调谐耗散孤子锁模光纤激光器的最高输出功率。将双折射滤光片的透过函数和脉冲在光纤中的传输方程相结合,建立了可调谐锁模激光器的数值模型,对激光器的性能进行了模拟,模拟结果与实验结果一致。3.首次报道了散耗散孤子光纤激光器中的脉冲分裂现象及产生的原因。脉冲分裂是由非线性偏振旋转本身的锁模原理引起的,当非线性偏振旋转效应较强时,脉冲中心部分无损耗地通过偏振分束器,脉冲前后沿则被输出腔外。4.首次搭建了基于SiC上生长的石墨烯的可调谐调Q光纤激光器。激光器的中心波长在1038.54nm至1056.22nm之间可调谐。泵浦功率4.08W时,单脉冲能量、脉冲宽度、输出功率和峰值功率分别为0.65μJ、1.60μs、35mW和406m W。
【Abstract】 Ultra-short laser pulses have the advantages of high peak power, narrow pulse width, broad spectrum and so on. They have been widely applied in such fields as probing the microworld, huge volume communication, micro-fabrication, biomedicine and so on. Passively mode-locking and Q-switching are the two effective methods to obtain ultra-short laser pulses.Mode-locked fiber lasers have several obvious advantages over their solid-state counterparts, such as minimal alignment, effectively thermal management, low cost and so on. However, the accumulation of excessive nonlinear phase shift presents a fundamental limitation to the increase of pulse energy in passively mode-locked fiber lasers. One effective way to increase the pulse energy is to make the fiber lasers operate in positive dispersion regime. Positive dispersion can broaden the pulse, thus effectively reduce the accumulated nonlinear phase shift. The broadened chirp pulse can be dechirped to near their transform-limited duration outside the cavity, thus femtosecond pulse is achieved.Except for pursuing high pulse energy, the researchers of ultrashort pulse technology also pay much attention to new saturable absorbers. Graphene has emerged as a new kind of potential saturable absorber with the advantages of broadband saturable absorption, ultrafast recovery time and so on. After the first report on graphene mode-locked fiber lasers in2009, its applications in field of photoelectric have attracted wide attention of researchers worldwide.In this thesis, the operating characteristics of an all-normal-dispersion fiber laser mode-locked by nonlinear polarization evolution (NPE) were numerically studied. A wavelength tunable, passively mode-locked Yb-doped double-clad fiber laser with a birefringent filter was demonstrated. The pulse splitting behaviour of a dissipative soliton fiber laser was investigated. We also studied the mode-locking and Q-switching characteristics of the fiber lasers with graphene as saturable absorber. The main contents of this thesis are as follows: 1. Based on the coupled Ginzburg-Landau equations and Jones matrix, a numerical model of an all-normal-dispersion fiber laser mode-locked by NPE was proposed. The polarization states of different points across the pulse were calculated along the cavity. It was found that when the linear birefringence of the fibers was strong, the evolution of polarization state in the fiber had round a period of one beat length. Over one beat length, the state of polarization changed from right-handed elliptic to linear, left-handed elliptic, linear, and then back to right-handed elliptic. Different from a common saturable absorber, the modulation depth of the NPE equivalent saturable absorber varied with the wave plates angles. The dependence of modulation depth on wave plates angles was calculated. The results show that compared with the half wave plate and quarter wave plate before polarization beam splitter, the one after polarization beam splitter had a more obvious effect on modulation depth.2. The dependence of the stability domain on filter bandwidth, small signal gain coefficient and the orientation of wave plates was calculated. The proposed all-normal-dispersion fiber laser could deliver dome-shaped spectrum,"cat-ear" spectrum, splitting spectrum and parabolic self-similar pulse. Evolution of the intra-cavity pulse and spectrum was calculated under different states.3. A wavelength tunable, passively mode-locked Yb-doped double-clad fiber laser with a birefringent filter was demonstrated. By carefully rotating the filter, the central wavelength of the mode-locked fiber laser could be continuously tuned from1063.9nm to1091.1nm. The pulse duration ranged from13.9ps to16.8ps, and spectrum width varied between15.8nm and1091.1nm. The maximum average output power of212mW was obtained at the central wavelength of1063.9nm. The corresponding pulse energy was8.5nJ. A theoretical model was established based on the coupled Ginzburg-Landau equations and the transmission function of the filter. Simulation results agreed with the experimental results.4. Pulse splitting in all-normal-dispersion dissipative soliton fiber lasers was investigated. By appropriately setting the orientation of wave plates and pump power, output pulse from mode-locked fiber laser taking NPE rejection port as the output split into two parts. Both simulation and experiment results show, pulse splitting phenomenon described here was a result of intrinsic NPE-mode-locking mechanism. When the strength of NPE effect was strong enough, the central region of the pulse passed through the polarization beam splitter (PBS) with almost no loss. In this case, the rejected pulse from PBS consists almost entirely of the pulse wings, which could be regarded as two sub-pulses. As the pulse was chirped, the spectrum split into two parts simultaneously.5. The principle of dispersion compensation was analyzed, and the characteristics and limitations of grating pair were discussed. The chirped pulse from the all-normal-dispersion fiber laser was compressed with transmission grating pair. The pulse duration before and after compensation were6.73ps and161.5fs, respectively. The average power before and after compensation were2.36W and1.22W, with the compensation efficiency of51.7%. The peak power before and after compression were6.09kW and131.4kW, respectively.6. A passively graphene Q-switched Erbium doped fiber laser around1.5μm was demonstrated. The graphene saturable absorber was fabricated by sandwiching a thin graphene film produced via chemical vapor deposition between two FC fiber connectors. Stable pulse trains were obtained with the pulse repetition rate varying between34.72and53.2kHz, and the average output power ranging from0.504mW to0.926mW. The achieved shortest pulse duration and highest pulse energy were3.2μs and17.41nJ, respectively.7. A passively Q-switched tunable Yb-doped double-clad fiber laser was demonstrated with graphene epitaxially grown on SiC. The spectral tuning of the Q-switched fiber laser was implemented by rotating a quartz plate filter inside the cavity. The central wavelength of the fiber laser can be continuously tuned from1038.54nm to1056.22nm. The maximum pulse energy of0.65μJ was obtained at the pump power of4.08 W, and the corresponding pulse duration, average output power and peak power were1.60μs,35mW and406mW respectively.8. By transferring graphene film onto side-polished D-shaped fiber, a mode-locked Erbium-doped fiber laser based on the evanescent field interaction between the propagating laser light and graphene layer was established. The operating performance of the mode-locked fiber laser was investigated with different output couplers. Under the output coupling ratio of30%, the laser generated maximum pulse energy of8.34nJ at the repetition rate of8.4MHz with the pulse duration of13.8ns. The main innovations of this thesis are as follows:1. A numerical model of an all-normal-dispersion fiber laser mode-locked by NPE was proposed. The polarization states of different points across the pulse were calculated along the cavity. The dependence of the modulation depth of the NPE equivalent saturable absorber on wave plates angles was calculated for the first time. We also investigated the changing of the laser stability domain with the cavity parameters.2. A wavelength tunable, Yb-doped double-clad dissipative soliton fiber laser was demonstrated. The average output power and pulse energy were212mW and8.5nJ, respectively, which was the highest result in tunable dissipative soliton fiber laser to our knowledge. A theoretical model was established based on the coupled Ginzburg-Landau equations and the transmission function of the filter. Simulation results agreed with the experimental results.3. Pulse splitting in a dissipative soliton fiber laser taking NPE rejection port as the output was investigated for the first time. Pulse splitting was a result of intrinsic NPE-mode-locking mechanism. When the strength of NPE effect was strong enough, the central region of the pulse passes through the PBS with almost no loss, while the leading and trailing edges were rejected.4. A tunable Yb-doped double-clad fiber laser passively Q-switched by graphene epitaxially grown on SiC was demonstrated for the first time. The central wavelength of the fiber laser could be continuously tuned from1038.54nm to1056.22nm. The maximum pulse energy of0.65μJ was obtained at the pump power of4.08W, and the corresponding pulse duration, average output power and peak power were1.60μs,35mW and406mW respectively.
【Key words】 fiber laser; mode-locking; nonlinear polarization evolution; all-normal-dispersion; dissipative soliton; graphene; Q-switching; tunable;