【作者】 周勇；

【导师】 许祖彦； 王安廷；

【作者基本信息】 中国科学技术大学 ， 光学， 2017， 博士

【摘要】 轴对称矢量光束是指光束电场分布在光束横截面上关于中心点对称,是全矢量电磁波方程的轴对称解,常见的两种轴对称矢量光束有径向和角向偏振光束两种分布。此类光束可以通过主动或被动方法得到,在保证轴对称特性的同时对其进行操控的技术也被逐渐开发出来。此类光束奇异的偏振特性导致了特殊的大数值孔径聚焦特性,在高精度成像、金属加工以及粒子操控领域内有很重要的应用。少模光纤中的LP11模式由四种简并模式(TM01,TE01,HE21even,HE21odd)组成,这四种模式均表现出轴对称偏振特性,其中TM01和TE01分别对应径向和角向偏振光束。利用错位耦合或者长周期布拉格光栅可以实现少模光纤内LP11模式的激发,但是很难在保证耦合效率的同时得到高纯度的模式输出,因此基于少模光纤光栅的模式选择方案被提出用于高纯度的轴对称矢量光束产生。在本文中,我们介绍了轴对称矢量光束的数学特性及各类产生方法,并基于光纤模式理论详细阐述了少模光纤光栅的反射及模式选择理论。本研究课题得到国家自然科学基金的支持,实现了轴对称矢量光束的锁模脉冲产生及谐振效率增强。本文主要的研究工作及成果如下:1.通过在光纤激光器内加入电光调制器结合错位耦合级联少模光纤光栅的方式实现了轴对称矢量光束的锁模脉冲输出。实验得到的脉冲轴对称矢量光束重频12.059MHz,脉宽2ns,并且激光器工作在极窄线宽的单波长范围内,30dB光谱宽度0.2nm,极大的保证了输出轴对称矢量光束的模式纯度。2.基于碳纳米管的可饱和吸收特性,实现了轴对称矢量光束的超短脉冲被动锁模输出。选择损伤阈值高、工作波长宽且偏振无关的碳纳米管作为腔内可饱和吸收体实现了自启动的被动锁模,通过调节腔内PC实现了径向偏振光束的超短脉冲输出,得到的锁模脉冲重频10.61MHz,脉宽22.73ps。锁模状态下激光器输出光谱被很好的限制在了少模光纤光栅的基模反射峰内,30dB输出谱宽0.3nm。通过模式积分方法计算得到的径向偏振光束的模式纯度为98.03%。3.在全少模双布拉格反射光纤激光器中利用双包层掺镱光纤中增益饱和以及横模纵向拍频引起的横场空间烧孔长周期光栅,实现了谐振效率极大增强的轴对称矢量光束输出。当泵浦达到某阈值时,激光器输出效率从13.36%突变为32.48%,同时我们利用此类光致长周期光栅机制结合速率方程通过数值模拟验证了其对于激光器谐振的影响。模拟结果得到了与实验结果一致的结论,并得到了 LP01及LP11模式不同泵浦状态下在激光器内的运转情况。4.为摆脱少模光纤光栅反射谱宽的限制,设计了两款宽谱的模式响应器件。基于模式耦合理论提出了模式选择耦合器的方案,并利用光纤熔融拉锥的方式初步验证了理论的合理性,此外还设计了基于PLC波导耦合的方案并提出了一种基于模式传输相干叠加的耦合理论解释。此外设计通过光纤拉锥镀膜形成的包层金属化光纤实现了宽谱的角向偏振选择输出,并通过数值模拟详细研究了锥区粗细、金属膜厚以及工作波长对不同模式损耗的影响。本论文的主要创新点有:1.利用电光调制器结合错位耦合级联少模光纤光栅的方式,在保证激光器的窄谱运转同时实现了轴对称矢量光束的主动锁模输出。2.选择损伤阈值高、工作波长宽且偏振无关的碳纳米管作为腔内可饱和吸收体,实现了光纤激光器的自启动窄谱锁模,并利用少模光纤光栅实现了轴对称矢量光束的超短脉冲输出。3.在全少模双布拉格反射光纤激光器中利用双包层掺镱光纤中增益饱和以及横模纵向拍频引起的横场空间烧孔长周期光栅,实现了谐振效率极大增强的轴对称矢量光束输出。并结合速率方程和光致长周期通过数值模拟验证了此模式畸变对于激光器谐振的影响,得到了与实验结果一致的结论。更多还原

【Abstract】 Cylindrical vector beams(CVBs)refers to the beams with an axially symmetric distribution in the cross section,which are the symmetrical beam solution to the full vector electromagnetic wave equation.Two common CVBs show the electrical distribution of radially and azimuthally polarization beam.These beams can be generated by active and passive methods.Techniques of CVB manipulations with the polarization maintainment have also been developed.Their special symmetry gives rise to unique high numerical aperture focusing properties that find potential applications in the areas of high accuracy imaging,metal processing and particle manipulation.The LP11 mode consists of four engine CVB modes(TMo1,TEo1,HE21even and odd),TM01 and TE01 are the radially and azimuthally polarization beams,respectively.The LP11 mode can be excited with an offset coupling or long period Bragg gratings,however it is hard to realize a high purity mode emitting with a high excitation ratio.Thus the scheme of using few mode fiber Bragg grating(FMFBG)as a mode filter have been proposed for high purity CVB generations.In this dissertation,we introduced the mathematical properties and generation methods of CVBs,and clarified the reflection and mode filter theory of FMFBG based on the fiber mode theory.Our research is funded by the National Nature and Science Foundation of Chin,realized the mode-locked CVBs output and enhanced resonance efficiency for CVBs.The primary results of this dissertation are as follows:1.We realized a stable actively mode-locking through the cascaded usage of an electro-optical modulator in a fiber laser,the pulsed CVBs output was obtained by cascade connection of offset coupling and FMFBG.The generated pulsing CVBs had a repletion rate of 12.059MHz with the pulse duration of 2ns,the fiber laser operated within a narrow spectrum of 0.2 nm at 30 dB depth under mode-locking state,it give rise to a high mode purity of the output CVBs.2.We realized a passively mode-locked CVB output with ultrafast pulses based on the saturable absorption of Carbon Nano-tube(CNT),the CNT integrated the properties of high damage threshold,wide operation waveband and polarization independence was chosen as the intra-cavity saturable absorber to achieve self-starting passively mode-locking.The pulsed CVBs with a repetition of 10.61 MHz and duration of 22.7 ps was obtained through adjusting the intra-cavity polarization controller.The output spectrum of the fiber laser was well limited within the fundamental mode reflection peak of the FMFBG with a spectra width of 0.3 nm at 30 dB depth,the mode purity of the generated radially polarization beam was calculated to be 98.03%with a mode integration method.3.We introduced an optically induced long period fiber grating(OLPG)into a DBR fiber laser which was caused by the gain saturation and beat frequency between different transversal modes,the OLPG realized a strong enhancement of the resonance efficiency for CVB output.The slope efficiency of the fiber laser had a sudden change from 13.36%arrives 32.48%when the pump power reach a threshold.We contributed the physical origin of this threshold property of the OLPG due to the experimental conditions and the exhaustive investigation.Meanwhile,we clarified the influence on laser resonance through numerical simulation.The simulation predicted the same phenomenon with the experimental results,the operation of LP01 and LP11 modes in the cavity under different pump power was also realized.4.To get rid of the narrow reflection peak of the FMFBG,we designed two kind of mode response elements which operated within a wide spectrum.Mode selective coupler was contributed based on the waveguide mode coupling theory,the feasibility has been demonstrated by the method of tapered fiber melting.In addition,we designed a scheme based on the planer lightwave coupler(PLC)and explained the coupled theory by mode superposition along with the coherent propagation.An azimuthally polarized selective element within wide spectrum was also designed by metal coated fiber taper,the influences on the loss of different modes of the tapered diameter,metal thickness and operation wavelength were demonstrated through numerical simulation.The innovative works and highlight results in this thesis are as follows:1.The actively mode-locked fiber laser with CVB output was realized by the cascaded usage of electro-optical modulator,offset coupling and FMFBG.2.The CNT with high damage threshold,wide operation waveband and polarization independence was chosen as the intra-cavity SA for the self-starting passively mode-locked fiber laser,ultrafast pulsed CVB was also obtained through the usage of FMFBG.3.The OLPG was introduced into a DBR fiber laser which was caused by the gain saturation and beat frequency between different transversal modes,which realized a remarkable efficiency enhancement for the CVB output.The numerical simulation combined of rate equations and OLPG clarified the influence on the laser resonance and predicted the same phenomenon with the experimental results.更多还原