【作者】 张斌；

【导师】 李平； 陈晓寒；

【作者基本信息】 山东大学 ， 光学工程， 2023， 博士

【摘要】 超快激光技术的研究对推动众多基础学科及应用领域的发展具有重要意义,稀土掺杂光纤较大的增益带宽为超快锁模激光脉冲的产生提供了可能。钕掺杂光纤的发射光谱包括0.9、1.06和1.34 μm三个波段,其中0.9 μm波段的超快激光在双光子显微技术等领域有重大应用价值,该波段尚难以用其他离子直接跃迁产生。然而,实现工作在0.9μm波段的掺钕光纤锁模激光器面临一些挑战,必须找到能够有效抑制1.06 μm波段强烈的模式竞争的方案。目前0.9 μm波段的掺钕光纤锁模激光器除掺钕光纤外,均采用空间器件,且均为特殊设计的钕掺杂光纤,不仅不利于激光器的集成化,且存在光纤拉制困难、成本高昂等问题。相比之下,钕掺杂普通单模光纤制备工艺已相当成熟,造价低廉,因此实现工作在0.9 μm波段的基于普通商用钕掺杂单模光纤作为增益介质的锁模光纤激光器将具有重大意义。本文采用商用钕掺杂单模光纤作为增益介质,基于非线性偏振旋转(NPR)技术分别搭建了工作在1.06和0.9 μm波段的光纤锁模激光器,并重点围绕0.9 μm波段掺钕光纤锁模激光器展开了详细研究。首先简单介绍了被动锁模光纤激光器的基础理论及脉冲在光纤中的传输方程,建立了 NPR锁模光纤激光器的数值模型并实现了模拟仿真,为实验研究提供了重要的理论指导。实验中首先对所用钕掺杂单模光纤的性能进行了表征,然后搭建了工作在1.06 μm的掺钕全光纤锁模激光器,获得了类噪声脉冲(NLP)、7脉冲相互作用形成的束缚态孤子脉冲、拉曼调Q锁模脉冲输出,并研究了光纤耦合器分光比和腔内滤波器带宽对输出性能的影响。然后,创新性地提出了利用单模光纤的弯曲损耗抑制掺钕光纤在1.06 μm的激光发射,成功实现了工作在0.9 μm波段的掺钕全光纤锁模激光器,并通过对腔内色散和非线性效应的控制,分别获得了耗散孤子共振(DSR)、3脉冲簇、12阶谐波锁模(HML)脉冲输出,极大地丰富了 0.9 μm掺钕全光纤锁模激光器的研究。最后提出利用光栅对对0.9 μm掺钕光纤锁模激光器进行色散管理以改善激光器的输出性能并获得更加丰富的脉冲类型。当以反射式光栅对作为色散管理元件时,成功获得了 NLP、DSR及色散管理孤子脉冲输出,并研究了波长调谐特性及色散对输出性能的影响。为了提供更大的负色散量并降低损耗,采用透射式光栅对作为色散管理元件,成功实现了 NLP、自相似脉冲、7阶HML、传统孤子及束缚态孤子锁模脉冲输出,并研究了各种脉冲的波长调谐特性及色散对输出性能的影响。本文具体研究内容如下:1.简单介绍了脉冲在光纤中的传输方程及数值解法,建立了 NPR被动锁模光纤激光器的数值模型,实现了 NPR锁模光纤激光器的数值模拟,仿真中研究了耗散孤子及双脉冲束缚态孤子的建立过程,并分别分析了腔内泵浦强度、光纤色散及非线性效应对仿真结果的影响,为接下来掺钕光纤NPR锁模光纤激光器的实验研究奠定了良好的理论基础。2.基于NPR技术,首先搭建了一台工作在1.06 μm波段的掺钕全光纤锁模激光器,并通过更换光纤耦合器的输出光比例和滤波器的带宽等参数,对所获得的各种类型的锁模脉冲输出展开了详细研究。当输出光比例为90%时,1.06 μm掺钕全光纤锁模激光器的输出性能最佳。通过调节腔内偏振态,分别获得了 NLP脉冲、7脉冲束缚态孤子及拉曼调Q锁模脉冲。3.创新性地提出了利用单模光纤(SMF)的弯曲损耗抑制1.06μm波段的激光发射,找到了最佳弯曲半径,成功实现了工作在0.9 μm波段的掺钕全光纤NPR锁模激光器。当腔内插入～110 μm的SMF时,获得了 DSR、3脉冲簇输出。为了进一步增强腔内拉曼增益及非线性效应,将腔内SMF长度增加至～400 m,获得了 12阶HML脉冲输出。4.采用反射式光栅对作为色散补偿器件,并通过光栅对的波长选择性有效抑制了掺钕光纤在1.06 μm波段的发射,实现了工作在0.9μm波段的掺钕光纤色散管理锁模激光器。分别采用600和1200 lines/mm反射式光栅对在长腔和短腔色散管理激光器内获得了 NLP、DSR等锁模脉冲输出,并详细研究了泵浦功率对输出性能的影响、波长调谐特性及色散对光谱和脉冲的影响等。然后通过设计1200 lines/mm反射式光栅对的间距,使长度较短的谐振腔净色散位于微负色散区,成功获得了色散管理孤子脉冲输出。5.为了提供更大的负色散量并降低腔内损耗,将色散管理元件更改为闪耀波长为890 nm的1841.6 lines/mm的透射式光栅对,实现了性能优化的0.9 μm掺钕光纤色散管理锁模激光器,分别获得了 NLP、自相似脉冲、7阶HML脉冲输出,并仔细研究了泵浦功率对输出特性的影响、波长调谐特性及色散对光谱和脉冲的影响等。保持光栅间距不变,剪短谐振腔长度,使激光器处于大的净负色散区,获得了传统孤子锁模输出。更多还原

【Abstract】 The research of ultra-fast laser is of great significance to promote the development of many basic disciplines and application areas.The large gain bandwidth of rare-earth(RE)doped fibers provides the possibility of ultra-fast pulses generation.The emission spectra of Nd-doped fibers include three bands of 0.9,1.06 and 1.34 μm,where ultra-fast lasers operating in the 0.9μm band have significant applications in fields such as two-photon microscopy.So far,the 0.9μm band is difficult to generate by direct leap of other ions.There are some challenges to realize the Nd-doped fiber mode-locked lasers operating at 0.9μm band,and a solution must be found to effectively suppress the strong mode competition from 1.06 μm band.Currently,most of the 0.9 μm Nd-doped fiber mode-locked lasers are based on spacial devices in addition to the Nddoped fiber.And the structure of Nd-doped fiber is specially designed,which is not only unfavorable to the integration of laser,but also has the problems of difficult fiber pulling and high cost.In contrast,the preparation process of Nd-doped commercial common single-mode fiber has been quite mature and the cost is low.Therefore,it is of great significance to realize a mode-locked fiber laser operating at 0.9 μm band based on the Nd-doped single-mode fiber(SMF)as gain medium.In this thesis,the mode-locked fiber lasers operating at 1.06 and 0.9 μm wavelengths were built based on the nonlinear polarization rotation(NPR)technique using a commercial Nddoped SMF as the gain medium,and the 0.9 μm Nd-doped fiber mode-locked lasers were studied in detail.Firstly,the basic theory of the passive mode-locked fiber laser and the pulse transmission equation in fiber were briefly introduced.The numerical model of NPR modelocked fiber laser has been established and the simulation has been achieved,which provides important theoretical guidance for the experimental study.The performance of the Nd-doped SMF was first characterized,and then a Nd-doped all-fiber mode-locked laser operating at 1.06μm was built.In which,noise-like pulses(NLP),7-pulse bound-state solitons,and Raman Qswitched mode-locked pulses were obtained,and the effects of fiber coupler splitting ratio and filter bandwidth on the output performance were investigated.Then,the Nd-doped all-fiber mode-locked laser operating at 0.9 μm band was successfully realized by using the bending loss of SMF to suppress the laser emission at 1.06μm.The dissipative soliton resonance(DSR),3pulse burst,and 12th-order harmonic mode-locked(HML)pulses were obtained by controlling the dispersion and nonlinear effects in the resonant cavity,respectively.The study of 0.9 μm Nd-doped all-fiber mode-locked lasers is greatly enriched.Finally,the dispersion management of 0.9 μm Nd-doped fiber mode-locked laser using the gratings pair as dispersion management element is proposed to obtain a wider variety of pulse types.When the reflective gratings pair is employed,NLP,DSR and dispersion-managed soliton pulses are successfully obtained,and wavelength tuning characteristics and the effects of dispersion on output performance are investigated.In order to provide greater negative dispersion and reduce losses,the transmission gratings pair was used as the dispersion management element.The NLP,self-similar pulses,7th order HML pulses,conventional solitons and bound-state solitons pulses are successfully realized,and wavelength tuning characteristics and the effects of dispersion on the output performance of various pulses are investigated.The details of this thesis are as follows:1.The transmission equations of pulses in optical fiber and numerical solution are briefly introduced,and the numerical model of NPR passively mode-locked fiber laser is established.The numerical simulation of the NPR mode-locked fiber laser is realized,and the establishment process of dissipative solitons and double-pulse bound-state solitons is studied in the simulation.The effects of intracavity pumping intensity,fiber dispersion and nonlinear effects on the simulation results are also analyzed,which lay a good theoretical foundation for the next experimental study of Nd-doped fiber mode-locked laser based on the NPR.2.Based on the NPR,a Nd-doped all-fiber mode-locked laser operating at the 1.06 μm waveband was built.Various types of mode-locked pulses were investigated in detail by changing the output optical ratio of the fiber coupler and the bandwidth of filter.The best output performance of the 1.06 μm Nd-doped all-fiber mode-locked laser was obtained when the output optical ratio was 90%.By adjusting the polarization state in the cavity,NLP pulses,7-pulse bound-state solitons and Raman Q-switched mode-locked pulses,respectively.3.The innovative scheme of using the bending loss of SMF to suppress the laser emission in the 1.06 μm band was proposed,and the optimal bending radius was found.A Nd-doped all-fiber NPR mode-locked laser operating at the 0.9 μm was successfully realized.DSR,3.pulses burst were obtained when～110 m SMF was inserted in the cavity.In order to further enhance the intracavity Raman gain and nonlinear effect,the length of SMF was increased to～400 m in the cavity.12th-order HML pulses were obtained.4.A Nd-doped fiber dispersion-managed mode-locked laser operating at the 0.9 μm was realized by using the reflective gratings pair as dispersion compensation element.The emission of the Nd-doped fiber at the 1.06 μm was completely suppressed by the wavelength selectivity of the gratings pair.The mode-locked pulses of NLP,DSR were obtained in long-cavity and short-cavity dispersion-managed lasers using 600 and 1200 lines/mm reflective grating pairs as dispersion compensation element,respectively.The wavelength tuning characteristics and the effects of dispersion on the spectrum and pulses were studied in detail.Then,the dispersion-managed solitons were successfully obtained by designing the spacing of the 1200 lines/mm reflective gratings pair so that the net dispersion of the short-length resonant cavity lies in the slightly negative dispersion region.5.In order to provide a larger negative dispersion and reduce the intracavity loss,the dispersion management element was changed to a transmission grating pair of 1841.6 lines/mm with a blaze wavelength of 890 nm.A 0.9 μm Nd-doped fiber dispersion-managed mode-locked laser with optimized performance was realized.The NLP,self-similar pulse,7th-order HML pulses were obtained,and wavelength tuning characteristics and the effects of dispersion on the spectra and pulses were carefully studied.Finally,keeping the grating pitch constant,the conventional soliton pulse was obtained by cutting the resonant cavity length so that the net dispersion of the resonant cavity was in the large net negative dispersion region.更多还原