轴对称矢量光束光纤激光器脉冲产生及效率增强

发布时间：2018-07-17 06:58

【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。
[Abstract]:Axisymmetric vector beam is an axisymmetric solution of the distribution of electric field on the center point on the cross section of the beam. It is an axisymmetric solution of the full vector electromagnetic wave equation. The common two axisymmetric vector beams have two kinds of distribution of radial and angular polarization beams. This kind of beam can be obtained by active or passive methods. The technology of its manipulation has also been developed. The singular polarization characteristics of such beams lead to special large numerical aperture focusing characteristics, which are very important in the field of high-precision imaging, metal processing and particle manipulation. The LP11 mode in the less mode fiber consists of four degenerate modes (TM01, TE01, HE21even, HE21odd), which are four. The model shows axisymmetric polarization, in which TM01 and TE01 correspond to radial and angular polarization respectively. Using dislocation coupling or long period Prague grating can realize the excitation of LP11 mode in the less mode fiber, but it is difficult to obtain the mode output of high purity while ensuring the coupling efficiency. Therefore, the optical fiber grating is based on the less mode fiber Bragg grating. The mode selection scheme is proposed for high-purity axisymmetric vector beam generation. In this paper, we introduce the mathematical characteristics and various generation methods of axisymmetric vector beams. Based on the optical fiber mode theory, the reflection and mode selection theory of the small mode fiber Bragg gratings are elaborated in detail. The research subject gets the National Natural Science Foundation. The main research work and results are as follows: 1. the pulse output of the axisymmetric vector beam is realized by adding an electro-optic modulator in a fiber laser with a staggered coupling cascaded less mode fiber Bragg grating. The experimental pulse is obtained. The impulse axisymmetric vector beam has a heavy frequency 12.059MHz and a pulse width of 2ns, and the laser is working in the single wavelength range of the very narrow line width and the 30dB spectrum width 0.2nm, which greatly guarantees the mode purity of the output axisymmetric vector beam,.2. is based on the saturated absorption characteristic of the carbon nanotube, and the ultra short pulse passive mode locking output of the axisymmetric vector beam is realized. The carbon nanotube with high damage threshold, wide working wavelength and independent polarization can realize self starting passive mode locking as an intracavity saturable absorber. The ultrashort pulse output of the radial polarization beam is realized by adjusting the PC of the cavity, and the mode locked pulse heavy frequency 10.61MHz is obtained, and the output spectrum of the laser is very good under the pulse width 22.73ps. mode. Limited to the base mode reflection peak of the less mode fiber Bragg grating, the mode purity of the radial polarization beam calculated by the 30dB output spectrum width 0.3nm. through the mode integration method is 98.03%.3. in the full mode double Prague reflective fiber laser using the gain saturation in the double cladding Ytterbium fiber and the transverse field space burning caused by the transverse mode longitudinal beat frequency. The output efficiency of the axisymmetric vector beam is greatly enhanced by the long period grating. When the pump reaches a threshold, the output efficiency of the laser is changed from 13.36% to 32.48%. At the same time, we use this kind of photoinduced long period grating mechanism and the rate equation to verify the effect of its effect on the laser resonance. According to the conclusion that is consistent with the experimental results, and the operation of the laser in the LP01 and LP11 modes is obtained in the laser, two wide spectrum pattern response devices are designed to get rid of the width of the reflection spectrum of the small mode fiber Bragg grating. Based on the mode coupling theory, the scheme of mode selection coupler is proposed and the fiber melting is used. The mode of the taper has preliminarily verified the rationality of the theory. In addition, a coupling theory based on the PLC waveguide coupling is designed and a coupling theory based on the coherent superposition of mode transmission is proposed. In addition, the wide spectrum angular polarization selection output is realized by the cladding metallized fiber formed by the optical fiber taper coating, and the numerical simulation is carried out by numerical simulation. The effects of cone thickness, metal film thickness and working wavelength on different modes loss are studied in detail. The main innovations of this paper are as follows: 1. using an electro-optical modulator to combine the dislocation coupled cascaded small mode fiber Bragg grating, the active mode locked output.2. selection loss of the axis to the vector beam is realized at the same time in the narrow spectrum operation of the laser. The carbon nanotube with high injury threshold and wide working wavelength and independent polarization is used as an intracavity saturable absorber, realizing the self starting narrow spectrum lock mode of the fiber laser, and using the small mode fiber Bragg grating to realize the ultra short pulse output.3. of the axisymmetric vector beam in the double cladding Ytterbium doped fiber laser with the full mode double Prague reflective fiber laser. The long period grating in the transverse field caused by the gain saturation and the transverse mode longitudinal beat frequency has realized the output of the axisymmetric vector beam with greatly enhanced resonant efficiency. The effect of the pattern distortion to the laser resonance is verified by the rate equation and the long period of light induced by the numerical simulation. The conclusion is consistent with the experimental results.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TN248

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