【作者】 张晓磊；

【导师】 张行愚； Gang-ding Peng；

【作者基本信息】 山东大学 ， 光学工程， 2011， 博士

【摘要】 本论文研究内容包括两部分,一部分是多芯光纤激光器的理论研究；另一部分是掺钕钆镓石榴石(Nd:GGG)晶体激光器在新波长运转的实验研究。随着高功率光纤激光器的应用范围逐渐从光通信领域扩展到机械加工、材料处理、激光医疗、热核聚变、航空和军事等行业,百瓦乃至千瓦量级高功率光纤激光器成为当今光电子技术研究领域中最炙手可热的研究课题。但是由于非线性效应、放大自发辐射及热损伤阈值等因素的限制,传统单根光纤激光器的输出功率受到限制。因此科学家们采用双包层泵浦技术、光子晶体光纤、光纤激光器相干组束等多种方案,来提高光纤激光器的输出功率。而多芯光纤是最近提出的一种新的相干合成解决方案,同时满足高激光功率输出、稳定的位相锁定和高的光束质量,是实现高功率光纤激光器的最佳方案。开拓新波段以适应各种应用需求一直是晶体激光器的重要研究方向。掺钕钆镓石榴石(Nd:GGG)晶体与其他掺钕晶体相比,具有优良的激光性能和物理化学性能,已经在大功率热容激光器和LD泵浦激光器中得到广泛应用。目前已报道的Nd:GGG晶体激光器输出波长一般为1062nm,1331 nm和938 nm。目前1110nm激光主要由掺镱光纤激光器获得,应用于通信、机械加工等领域。1110nm激光可以作为产生谐波的光源,倍频产生555 nm的黄绿光,位于人眼最敏感波长,在医疗、生物检测分析等方面有重要应用。本论文研究内容可概括为两个方面：一方面是关于多芯光纤中超模传导特性,超模选择,及多芯光纤激光器的理论研究,另一方面是掺钕钆镓石榴石(Nd:GGG)晶体激光性能的研究,对其在新波长1110 nm附近的输出特性进行实验研究和分析。本论文主要研究内容包括：1.理论推导获得多芯光纤中的耦合模方程,并计算和分析三芯、四芯、六芯和七芯光纤中的超模特性,获得各个超模的近场和远场强度分布。计算各个超模的衍射及传输特性时,分别采用M2因子和桶中功率(PIB)来衡量其光束质量。通过比较可以得出在任意多芯光纤中,同相位超模总是具有最大的传播常数和最好的光束质量,其远场强度分布为准高斯分布。桶中功率(PIB)的计算在非连续模场分析中得到更好的应用。解析的耦合模理论适用于任何符合弱耦合的多芯光纤,与数值法相比更加直观。2.以三芯、四芯、六芯和七芯光纤为例,分别对环型、有心型多芯光纤中各个超模与单模光纤中的基模的耦合效率进行讨论。在单模光纤纤芯半径和两种光纤之间的间隙距离均为变量的情况下,对不同类型多芯光纤中的各个超模和单模光纤基模之间的耦合效率进行分析和优化,理论结果表明由于环型和有心型光纤中超模分布的不同特性,其与单模光纤中基模的耦合呈现出不同特点。为两种光纤的耦合连接提供了理论依据。3.基于三芯、四芯、六芯和七芯光纤中耦合效率的分析,提出一种新型选模机制,即利用单模光纤进行选模。提出利用耦合效率差来衡量不同机制中同相位超模的选择效率,优化后与传统塔尔伯特腔进行比较,证明单模光纤选模机制对环型和有心型多芯光纤中同相位超模的选择均具有优越性和灵活性,在实际应用中具有很大潜力。4.建立速率方程,以环形六芯光纤和有心型七芯光纤为例,对基于单模光纤选模机制的多芯光纤激光器进行理论模拟,获得信号光的功率传输和输出特性。单模光纤选模为求解速率方程提供边值条件,因此系统中耦合效率的变化将引起信号光输出功率的变化。模拟结果显示信号光输出和耦合效率成正比,同相位超模输出功率在总输出功率中所占比例与耦合效率差几乎成正比,因此验证了采用耦合效率差来衡量同相位超模选择效率的合理性。理论模拟显示基于单模光纤选模机制的六芯和七芯光纤激光器可以获得较高的光-光转换效率,优化后均可以实现同相位超模的单模输出,该研究为高功率高亮度的多芯光纤激光器的实现提供了理论基础。5.对掺钕钆镓石榴石(Nd:GGG)晶体的激光性能进行研究,获得1110 nm激光输出。实验过程中获得LD侧泵Nd:GGG晶体激光器1110nm和1105nm双波长连续运转,两种波长之间存在竞争。采取在谐振腔中插入另外一片平面镜的方法,可以成功地将1110nm选择出来。该平面镜对1110nm,1105nm及808nm均高透,通过微调其相对激光方向的角度,可以改变不同波长处的腔内损耗,从而抑制1105 nm的振荡,实现1110nm单波长运转。本论文主要创新点如下：1.以耦合模理论和基尔霍夫衍射理论为基础,首次对环型三芯、六芯和有心型四芯、七芯光纤中的各个超模的近场及远场特性做了系统分析和比较,并采用M2因子和桶中功率(PIB)两种不同参数理论计算各个超模的光束质量因子,证明同相位超模的光束质量明显高于其他超模。所建立的完整理论模型,可以用来对任何满足弱耦合条件的多芯光纤进行完善的模场分析。2.首次对单模光纤和多芯光纤之间的模式耦合效率进行系统的分析和优化。通过调节单模光纤纤芯尺寸和两者之间的间隙距离,对耦合效率和耦合效率差进行理论优化,并展示了不同类型多芯光纤与单模光纤之间耦合的不同特点。理论模拟结果为实际实验操作过程中光纤之间的耦合提供详细可靠的参考依据。3.首次提出利用单模光纤对多芯光纤进行选模,在单模光纤选模机制下对两类光纤与单模光纤之间的耦合特性进行优化和比较,证明单模光纤选模可以完全抑制环型多芯光纤中的高阶超模,同时可以抑制有心型多芯光纤中除同相位超模和反相位超模以外的高阶超模。提出使用耦合效率差来衡量同相位超模的选择效率,通过与塔尔伯特腔比较,单模光纤对同相位超模的选择效率可以提高一倍以上,从而证明单模光纤选模机制的有效性和优越性。4.首次从理论上获得单模输出的基于单模光纤选模机制的全光纤多芯光纤激光器。基于速率方程理论,对单模光纤选模环型六芯光纤和有心型七芯光纤激光器进行理论模拟,证明此种机制下,信号光输出功率与耦合效率的大小成正比,而同相位超模在总输出功率中所占比例几乎与耦合效率差成正比,从而证明采用耦合效率差衡量同相位超模选模机制的科学性。优化后均可以得到同相位超模的单模输出,从而为实现高功率高亮度的全光纤多芯光纤激光器提供了理论依据。5.首次实验获得LD侧泵Nd:GGG晶体激光器在1110nm处的连续运转。首次实现了LD侧泵Nd:GGG晶体激光器1110nm和1105nm双波长连续运转,并采用插入高透平面镜从而改变腔内损耗的方法对两种波长之间的竞争进行控制,最终获得1110 nm处的单波长运转。更多还原

【Abstract】 The research in this dissertation includes two parts. One is the theoretical research on multicore fiber laser, the other is the experimental research on Nd:GGG crystal laser operating at novel wavelengths.As the application of the high power fiber laser extends from optical communication to mechanical processing, material processing, laser medicine, thermonuclear fusion, aviation, military and other fields, high power fiber laser with hundreds or even thousands of watt output power has become one the hottest topics in the present optoelectronic technology area.At the same time, the output power of traditional single core fiber laser is limited by lots of factors, such as nonlinear effect, amplified spontaneous emission, and thermal damage threshold, and so on. Therefore scientists have applied methods like double-cladding pump technique, photonics crystal fiber, coherent beam combination of fiber lasers to scale the output power of fiber laser. And muticore fiber, which can guarantee high power laser output, stable phase-locking and good beam beam quality simultaneously, is lately raised for coherent beam comibination and is the best method to accomplish high power fiber laser.It is very important to develop new wavelengths to meet various application requirements. Compared to other Nd-doped crystal, Nd:GGG crystal has good lasing properties, as well as physical and chemical properties. It has been widely used for high power heat capacity lasers and LD pumped lasers. So far the reported Nd:GGG crystal lasers generally operate at 1062 run,1331 nm and 938 nm.Up to now 1110 nm laser has been obtained from Ytterbium-doped fiber lasers, and applied for communication, mechanical working and other fields. As a light source for second harmonic generation,1110 nm can be frequency-doubled to generate yellow-green laser at 555 nm, which is most sensitive for human eyes and has important application for medicine, biological test and so on.There are two general research topics in this thesis:one is theoretical research on supermode propagation properties of multicore fiber, supermode selection and muticore fiber laser; the other is research on lasing performance of Nd:GGG crystal, experimentally verifying and studying its operation at around 1110 nm.The content studied in this paper mainly includes the following several respects: 1. The coupled-mode equation in multicore fiber is derived, and the supermode characteristics in three-core, four-core, six-core and seven-core fibers are calculated and analyzed, to obtain the near-field and far-field intensity distributions of each supermode. The M2 factor and power-in-bucket(PIB) are calulated to weight their beam quality during diffraction and propagation. The in-phase supermode of any mutlicore fiber always has the maximum propagation constant and best beam quality, and it has a Gaussian-like far-field distribution. The power-in-bucket(PIB) has better application for non-continuous mode fields. The analytical coupled-mode theory can be used for any multicore fiber with weakly-coupling, and is more intuitive than numerical methods.2. Take three-core, four-core, six-core and seven-core fibers as examples to discuss the coupling efficiency between supermodes of ring-type and concentric-type multicore fiber and fundamental mode of single-mode (SM) fiber. The coupling efficiencys are analyzed and optimized with core radius of SM fiber and the gap distance between the two fibers as variables. The theoretical results show their couplings with fundamental mode of SM fiber are different, which can be attributed to their different supermode profiles. The study provides good theoretical foundation for the coupling and connection between the two types of fibers.3. A new mode selection method by using single-mode fiber is proposed based on the analysis of coupling efficiencys of three-core, four-core, six-core and seven-core fibers. A parameter of coupling efficiency difference is put forward to measure the in-phase supermode selection efficiency in different mode selection schemes. The coupling efficiency difference is optimized and then compared to that in traditional Talbot cavity. The simulation results show the SM fiber mode selection scheme has superiority and flexiblility for in-phase supermode selection for both ring-type and concentric-type multicore fibers, and the scheme has great potential for pratical applications.4. Take ring-type six-core fiber and concentric-type seven-core fiber as examples, rate equations are established for multicore fiber laser with SM fiber mode selection scheme, to obtain the signal power properties. The SM fiber mode selection provides boundary condition for solving rate equations. So the signal power is affected by the coupling efficiency. Simulation results reveal the output power is propotional to the coupling efficiency, and the ratio of in-phase supermode output to total output is almost propotional to the coupling efficiency difference. It is proved to be reasonable using coupling efficiency difference to weight in-phase supermode selection. Simulation results show that single in-phase supermode output with high optical conversion efficiency can be achieved after optimization in six-core and seven-core fiber lasers based on SM fiber mode slection scheme. This study provides us theoretical foundation for high power, high brightness multicore fiber laser.5. Investigate the lasing behavior of Nd:GGG crystal and obtain 1110nm laser. In experiment, dual wavelength operation at 1110nm and 1105nm is obtained by diode-pumped Nd:GGG crystal laser, with competition existing between them. 1110 nm can be selected successfully by inserting a third plane mirror, which has high transmission at 1110nm,1105nm and 808nm. Adjusting its orientation slightly will change the intracavity losses of different wavelengths. Consequently 1105nm is suppressed, and single wavelength operation at 1110nm is achieved. The innovations in the thesis include:1. For the first time, systematic analysis and comparison has been done for three-core, six-core ring-type and four-core, seven-core concentric-type fibers based on coupled-mode theory and Kirchhoff diffraction theory. The Beam quality of supermodes is calculated with M2 factor and power-in-bucket (PIB). In-phase supermode is proved to have much better beam quality than other supermodes. The theoretical model established is suitable for any multicore fiber if weakly-coupling is satisfied.2. For the first time, the coupling efficiency between SM fiber and multicore fiber is systematically analyzed and optimized. The coupling efficiency and coupling efficiency difference are optimized by varing SM fiber core radius and the gap distance between two fibers. Different coupling characteristics of two types of multicore fiber with SM fiber have been demonstrated. Simulation results provide detailed and reliable basis for coupling between SM fibers and multicore fibers in practical operation.3. The mode selection scheme based on SM fiber is proposed for the first time. And the proposed scheme for two different types of multicore fiber is optimized and compared. In ring-type multicore fiber, the SM fiber can suppress all the high-order supermodes. In concentric-type multicore fiber, the SM fiber can suppress all the high-order supermodes except in-phase supermode and anti-phase supermode. The coupling efficiency difference is put forward to weight in-phase supermode selection. Compared to Talbot cavity, SM fiber can improve in-phase mode selection more than 100%, proving its effectiveness and superiority.4. For the first time, single mode output all-fiber multicore fiber laser based on SM mode selection is obtained theoretically. Based on rate equations, simulations on six-core ring-type and seven-core concentric-type fiber lasers have verified that, output power is propotional to coupling efficiency, and the ratio of in-phase supermode output to total output is almost propotional to coupling efficiency difference. Therefore coupling efficiency difference is proved to be quite reasonable to weight in-phase supermode selection. Single in-phase supermode output can be achieved after optimization. This study provides us theoretical foundation for high power, high brightness all-fiber multicore fiber laser.5. For the first time, continous wave operation at 1110 nm is obtained in diode-pumped Nd:GGG crystal laser in experiment. And continous wave dual wavelength operation at 1110 nm and 1105 nm are obtained for the first time. By inserting a third high transmission mirror to change the intracavity loss, single wavelength operation at 1110 nm is finally achieved.更多还原