共振泵浦2μm掺钬固体激光器

【作者】 陈浩；

【导师】 沈德元；

【作者基本信息】 复旦大学 ， 光学， 2013， 博士

【摘要】 2gm相干光源在军事、工业和医疗等方面日益显现的巨大应用价值,使其成为目前激光技术领域非常重要的研究方向之一。Ho离子掺杂激光增益介质能够直接获得-2.1μm激光,且得益于共振泵浦技术,可以实现极高的光-光转化效率；此外Ho离子掺杂的晶体类材料所具备的储能优势,是获得高能脉冲激光输出的重要可靠保障。本文着重从实验研究方面探索了几种新型的Ho离子掺杂石榴石系激光增益介质激光器的连续波和被动调Q运转特性,包括新型光学功能材料, Ho:LuAG透明陶瓷、混晶Ho:LuYAG和单晶Ho:LuAG。在本论文中,我们通过分析Ho掺杂块体增益介质的激光跃迁能级,确定了以Tm光纤激光器作为Ho激光器共振泵浦源的技术方案,并自建了Tm光纤激光器。该光纤激光器采用体布拉格光栅(VBG)作为波长选择和窄化器件,在792nm波长LD泵浦功率为164W时,获得了44W波长为1907nm的窄线宽激光输出。对于Ho:YAG透明陶瓷激光器,我们首先研究了该增益介质的透射光谱、吸收和发射光谱随掺杂浓度变化的情况。然后采用低损耗的平凹两镜腔结构,通过优化掺杂浓度和耦合输出镜的透射率,在泵浦光外围35W时,获得了21W高效率的激光输出；同时也对比研究了Ho:YAG透明陶瓷掺杂浓度对激光器运行效率的影响,发现掺杂浓度高达4at.%的样品仍然可以发射激光,而未见到同掺杂浓度单晶激光器的相关报道。此外,论文中还使用石墨烯作为可饱和吸收体器件实现了Ho:YAG透明陶瓷的被动调Q激光输出,一方面验证了Ho:YAG透明陶瓷在被动调Q输出方面的可行性,另一方面证明石墨烯材料的宽带可饱和吸收特性(2097nm)。对于Ho:LuAG透明陶瓷,在研究其光谱特性的基础上,实现了连续波激光输出；由于增益介质未镀增透膜,过高的损耗导致激光器的运行效率较低,但在这种高损耗腔中,通过变换耦合输出镜的透射率,观察到了Ho:LuAG透明陶瓷激光器的三种激光跃迁过程(对应三个激光波长)随耦合输出镜透射率变换而发生的竞争现象,并探讨了其成因。对于混晶Ho:LuYAG和单晶Ho:LuAG,重点研究了这两种材料的光谱特性、激光连续波运转和被动调Q运转的差异；由于混晶Ho:LuYAG中,Ho离子既可以取代Lu离子又可以取代Y离子,因此在不同Ho离子位置处的晶体场会有所变化,进而引起光谱的非均匀加宽。在本研究工作中,这两种晶体都获得了目前最高的连续波输出功率；当使用可饱和吸收镜(SESAM)作为被动调Q器件时,两种晶体都获得了较为稳定的脉冲激光序列。总体来说Ho:LuYAG比Ho:LuAG更适合激光运转。在Tm光纤激光器共振泵浦Ho固体激光器的研究中,我们获得了多项重要的成果,主要形成了以下创新点：1、以激光波长为1907nm的可调谐、窄线宽Tm光纤激光器作为共振泵浦源,在透明陶瓷Ho:YAG和单晶Ho:LuAG的激光器中,当泵浦光功率为35W时,分别获得了目前国际最高的21.4W和18W连续波输出功率,相应的激光波长为2097nm和2124.5nm,斜率效率分别达到了63.6%和53.4%。2、在上述相同的泵浦条件下,混晶Ho:LuYAG获得21W高功率、高效率连续波2124nm激光输出,斜率效率为61.5%;这是该材料首次实现激光输出。3、首次实现了透明陶瓷Ho：LuAG(未镀增透膜)的连续波激光输出,并观察到了三种激光跃迁过程(对应3个激光波长)的竞争。4、首次实现了以石墨烯作为被动开关的Ho块体激光器调Q激光输出,在当最大泵浦功率为3.27W时,获得了0.26W的调Q脉冲激光输出,中心波长2097nm,相应获得了最大重复频率64kHz,最小脉宽2.61μs。5、以SESAM作为被动调Q器件,首次实现了混晶Ho:LuYAG和单晶Ho:LuAG的被动调Q激光输出。更多还原

【Abstract】 Recently,2μm coherent light sources have become one of the important research direction in the field of laser technology due to their important applications in the military, industrial and medical. Ho ion doped laser gain medium can be used to directly obtain2.1μn laser with achieving very high optical-optical conversion efficiency which benefits from the in-band pumping technology. In addition, Ho ion doped crystal materials can be used to obtain high energy pulse laser output for their energy storage advantages. In this paper, several new Ho ion doped garnet laser materials are explored to obtain high efficient continuous wave and passively Q-switched operations, including new type of optical functional materials Ho:YAG, Ho:LuAG transparent ceramics, mixed crystal Ho:LuYAG and single crystal Ho:LuAG.In this paper, a technology route of in-band pumped Ho lasers by Tm fiber laser are confirmed after the laser transition level of Ho doped materials are discussed. And a Tm fiber laser was built with a volume Bragg grating (VBG) as wavelength selection and narrowing device. When the LD pumped power is164W (wavelength is792nm), the output power is44W with a very narrow linewidth at1907nm wavelength.For Ho:YAG transparent ceramic laser, we study the transmission spectrum, absorption and emission spectra of the gain medium varying with doping concentration. Using a plane-concave cavity with low loss,21W high efficient laser output power were realized with pumped power of35W, by optimizing the doping concentration and transmittivities of the output mirror. The influence of Ho:YAG transparent ceramics doping concentration on the laser operation were studied comparatively and the sample with doping concentration as high as4at.%could still be launched laser while the Ho:YAG crystal of the same doping concentration was not reported. In addition, Ho:YAG transparent ceramics laser were realized passively Q-switched operation with a graphene as a saturable absorber device. This could verify the Ho:YAG transparent ceramics could be used to obtain pulse laser and the graphene has a broadband saturable absorption properties (at2097nm).For Ho:LuAG transparent ceramics, the spectral characteristics were explorded and continuous wave operation was achieved. Due to the gain medium is not with antireflective coating, the efficiency of the laser is lower. However, three laser transition processes (corresponding to three laser wavelengths) of Ho:LuAG transmission ceramics and their competition phenomenon were observed when transmissivities of output mirrors were changed. Then its causes were discussed.For mixed crystal Ho:LuYAG and single crystal Ho:LuAG, the spectral characteristics, passively Q-switched operation and continuous wavelength operation were studied. The spectra is the line broadening of Ho3+in LuYAG than in LuAG. This inhomogeneous broadening is probably due to the presence of a mixed occupancy of Lu and Y ions on dodecahedral sites, resulting in a variation in the crystal field at different Ho3+sites. In this research work, the highest continuous wave output power have been achieved with both of crystals. When using a saturable absorption mirrors (SESAM) as a passively Q-swithed components, relatively stable laser pulses have been obtained with both kinds of crystals. In general, Ho:LuYAG is more suitable for laser operation than Ho:LuAGSeveral important results have been obtained in this paper on in-band Ho doped solid state laser pumped by Tm fiber laser, including:1. We report on the high-power and high-efficiency operation of a polycrystalline Ho:YAG ceramic laser and a single crystal Ho:LuAG laser in-band pumped by a Tm fiber laser at1907nm. Over21.4W at2097nm and18W at2124.5nm of cw output power have been generated under35W of incident pump power, corresponding to average slope efficiencies with respect to the incident pump power of63.6%and53.4%. To date, these are most high powers corresponding to both of Ho:YAG ceramic and Ho:LuAG single crystal.2. With the same pump condition, the high-power and high-efficiency operation of the mixed crystal Ho:LuYAG was achieved of21W output power at2124nm, corresponding to a sloped efficiency of61.5%. To our knowledge, this was reported for the first time.3. Ho:LuAG ceramic (without antireflective coating) was realized to generate output power for the first time. The competition among three wavelengths was observed and explained.4. Q-switched operation of a polycrystalline Ho:YAG ceramic laser was achieved with a graphene as the saturable absorber for the first time.0.26W at2097nm pulse output power has been generated under a max. incident pump power of3.27W, corresponding to repetition frequency of64kHz at the max. and pulse width of2.6μs at the min.5. For the first time, the mixed crystal Ho:LuYAG and single crystal Ho:LuAG lasers have been used to obtain passively Q-switched pulse with a SESAM as a saturable absorber.更多还原