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硅酸盐玻璃的摩尔组成为40SiO2−20 (Na2O+K2O)−20 (CaO+BaO)−20 (ZnO+B2O3),纤芯玻璃掺杂的Tm2O3浓度为8 wt.%。选用分析纯的SiO2、Na2O、K2O、CaO、BaO、ZnO、B2O3,以及纯度为99.999%的Tm2O3制备硅酸盐玻璃。所需的化学原料经称量后均匀混合,放在坩埚中随炉子升温至1600 ℃高温熔融,并在高温阶段保温一段时间,随后让坩埚随炉子冷却到1400 ℃,并将坩埚里的玻璃液浇筑在模具上固化成玻璃块,将玻璃块放到退火炉中冷却至室温后取出。制作出的硅酸盐纤芯玻璃和包层玻璃如图1所示,由于纤芯玻璃中Tm2O3浓度较高,玻璃呈现出淡黄绿色。
纤芯玻璃经过研磨抛光,制得尺寸为15 mm×10 mm×2 mm的玻璃试样进行光谱测试和寿命测试。采用Perkin Elmer公司的Lambda-900 UV-VIS-NIR分光光度计可获得掺铥玻璃在500~2 000 nm范围内的吸收光谱,进而掺铥硅酸盐玻璃的吸收截面σa可由公式(1)得到[8]:
$${\sigma _{\rm{a}}}{\rm{ = }}\frac{{2.303OD(\lambda )}}{{Nd}}$$ (1) 式中:OD(λ)代表光密度,其数值可由分光光度计测量结果直接读出;d代表玻璃样品的厚度;N代表玻璃样品中铥离子的掺杂浓度,其计算公式如下:
$$N{\rm{ = }}\frac{{2w\rho {N_{\rm{A}}}}}{{(1 + w)M}}$$ (2) 式中:w代表玻璃样品中Tm2O3的质量分数;ρ代表玻璃样品的密度;NA为阿伏伽德罗常量;M代表Tm2O3的摩尔质量。采用排水法测试出玻璃的密度为3.688 g/cm3,因此铥离子的掺杂浓度为8.52×1020/cm3,代入公式(1)可求出玻璃的受激吸收截面,如图2所示。在500~2 000 nm波长范围内观测到四个吸收峰,对应波长分别为680、789、1207、1639 nm,分别对应着铥离子的由基态能级3H6向激发态能级3F2,3、3H4、3H5、3F4的跃迁。玻璃样品在500~2 000 nm范围内的主吸收峰在1207 nm处,对应的受激吸收截面大小为6.8×10−21 cm2,第二吸收峰789 nm处的受激吸收截面大小为4.4×10−21 cm2,这与石英玻璃基质下铥离子在两波长处的吸收强弱有较大差异[9],但符合硅酸盐玻璃基质中铥离子的吸收特性[10-11]。玻璃样品在1207 nm的强吸收峰对应着3H6→3H5的泵浦方式,在利用该波长泵浦时,铥离子3H4和3F4能级的能量上转换效应将导致上能级粒子先后跃迁到3F3,2能级和1G4能级,有望产生800 nm红外光和470 nm蓝光。样品1.6 μm的吸收带呈现出双峰的结构,主峰位于1639 nm,次峰位于1720 nm。在常用泵浦波长1570 nm处,样品的受激吸收截面为0.6×10−21 cm2。
图 2 掺铥硅酸盐玻璃的受激吸收截面(插图:测试用的掺铥硅酸盐玻璃试样)
Figure 2. Absorption cross section of Tm-doped silicate glass (Inset: the tested Tm-doped silicate glass)
利用Edinburgh Instruments FLS 980光谱仪测量样品的发射光谱和荧光寿命。选用790 nm激光进行泵浦,测量了样品在1500~2400 nm范围内的发射光谱。根据McCumber理论,在满足稀土离子的能级寿命大于其能级多组态建立热平衡的时间这一前提下,材料的发射截面σe(λ)和吸收截面σa(λ)可以通过公式(3)互相推导[12]:
$${\sigma _{\rm{e}}}{\rm{(}}\lambda {\rm{) = }}{\sigma _{\rm{a}}}{\rm{(}}\lambda {\rm{)}}\dfrac{{{Z_{\rm{l}}}}}{{{Z_{\rm{u}}}}}\exp \left(\dfrac{{{E_0} - h\dfrac{c}{\lambda }}}{{{k_{\rm{B}}}T}}\right)$$ (3) 式中:Zl和Zu分别表示下能级和上能级的配分函数;E0为零线能(即激光下能级的最低能级与激光上能级的最低能级的能量差);h为普朗克常量;c为光速;kB为玻耳兹曼常量;T为开氏温度。上下能级配分函数的定义式为:
$$\left\{ \begin{array}{l} {Z_{\rm{l}}}{\rm{ = }}\displaystyle\sum\limits_j {\exp \left( - \dfrac{{{E_{{\rm{l }}j}}}}{{{k_{\rm{B}}}T}}\right)} \\ {Z_{\rm{u}}}{\rm{ = }}\displaystyle\sum\limits_j {\exp \left( - \dfrac{{{E_{{\rm{u }}j}}}}{{{k_{\rm{B}}}T}}\right)} \end{array} \right.$$ (4) 式中:Elj和Euj表示激光下上能级分裂出的各Stark能级的能量。通过玻璃的归一化吸收和发射光谱可计算出E0和Zl/Zu的数值[12],进而计算出样品中铥离子3F4→3H6能级跃迁的受激发射截面,如图3所示。该受激发射截面覆盖了1550~2 050 nm,在1 862 nm处样品具有最大的发射截面3.7×10−21 cm2,与报道的其他高掺铥玻璃的发射截面大小相当[10-13]。
在波长为790 nm的泵浦光激励下,记录了玻璃样品在1 860 nm发射波长处的荧光衰减曲线,如图4所示。当荧光光强衰减到最大值的1/e时,对应时间235 μs即为样品3F4能级的自发辐射寿命,该数值与石英光纤中铥离子的荧光寿命相当[9],但样品具有比石英光纤更高的铥离子掺杂浓度,因此可以实现更高的激光增益。
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基于8 wt.%铥离子掺杂的硅酸盐玻璃,采用管棒法制备光纤预制棒,并拉制出纤芯直径为7 μm、包层直径为125 μm的掺铥硅酸盐玻璃光纤,光纤端面如图5(a)所示。光纤的纤芯折射率为1.60486,包层折射率为1.59913,NA为0.135。选用Corning公司的SMF-28e+光纤作为掺铥硅酸盐光纤的匹配光纤,两种光纤的参数见表1。利用Vytran GPX-3400光纤熔接平台,采取非对称熔接的方法进行有源光纤与匹配光纤的熔接。图5(b)为利用光学显微镜观察的掺铥硅酸盐玻璃光纤与石英光纤的熔接效果图,图中右侧为掺铥光纤。在泵浦光作用下,熔接点处仅有少量的紫色上转换荧光泄露,说明熔接质量较好,通过进一步测试可知该熔接损耗约为0.6 dB。
图 5 掺铥硅酸盐玻璃光纤的(a)端面和(b)与石英光纤的熔接效果图
Figure 5. (a) End face of Tm-doped silicate glass fiber and (b) the fusion splice of silica fiber and Tm-doped silicate fiber
表 1 硅酸盐光纤和石英匹配光纤的参数
Table 1. Parameters of silicate fiber and matched silica fiber
Parameter Tm-doped silicate fiber SMF-28e+ silica fiber Core diameter/μm 7 8.2 Cladding diameter/μm 125 125 Numerical aperture 0.14 0.135 Effective group index of core refraction @1310 nm 1.4676 1.6049 基于掺铥硅酸盐玻璃光纤与石英光纤的高质量熔接,搭建了掺铥光纤放大器测试新型光纤的增益性能。利用2 cm高掺铥硅酸盐玻璃光纤对功率为1 mW、中心波长为1950 nm的种子源进行放大。记录不同泵浦功率下输出的信号光功率相对于不加泵浦时输出信号光功率的比值,计算出光纤的增益系数为1.7 dB/cm。
Research of novel highly thulium-doped silicate glass fiber and related fiber lasers
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摘要: 具有高稀土离子掺杂浓度的有源光纤一直以来是高性能单频光纤激光器的核心,选用硅酸盐玻璃材料制作高掺杂有源光纤可以有效提升光纤增益。通过高温熔融工艺制备了铥离子掺杂浓度为8 wt.%的高掺杂硅酸盐玻璃,测试了其光谱特性和荧光寿命,并根据McCumber理论计算玻璃的受激发射截面。采用管棒法制备光纤预制棒,拉制出尺寸为7/125 μm的高掺铥硅酸盐玻璃光纤。基于低损耗的异质光纤熔接,测试了该光纤的增益特性,并分别采用2 cm和8 cm的新型掺铥光纤搭建线形腔光纤激光器,获得百毫瓦的1950 nm激光输出。研究表明,在8 wt.%的高浓度掺杂下,铥离子在文中的新型硅酸盐光纤基质中具备良好的发光能力,这种国产高掺杂玻璃光纤在实现高性能单频光纤激光器方面具有明显优势。
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关键词:
- 高掺铥硅酸盐玻璃光纤 /
- 受激发射截面 /
- 荧光寿命 /
- 线形腔光纤激光器
Abstract: Active fiber with high rare earth ion doping concentration has always been the key component of high-performance single-frequency fiber lasers, and silicate glass can be used to fabricate highly doped active fibers with excellent unit gains. Here, highly doped silicate glass with the thulium ion doping concentration of 8 wt.% was prepared by a high-temperature melting process, and its spectral characteristics and fluorescence lifetime were measured. Based on McCumber theory, the emission cross section of the glass was calculated. By the method of rod-in-tube, fiber preforms were prepared, and then the highly thulium-doped silicate glass fiber of 7/125 μm was drawn. Based on low-loss specialty fiber fusion splicing, the fiber unit gain was measured, and linear-cavity fiber lasers based on 2 cm and 8 cm novel thulium-doped fibers were investigated, where 1950 nm laser output of hundred-milliwatt-level power was obtained. This work has shown that under such a high doping concentration of 8 wt.%, which is the highest thulium doping concentration to date, thulium ions still have good luminescence properties in this novel glass host. The homemade highly thulium-doped glass fiber in this paper can be used to fabricate high performance single-frequency fiber lasers. -
表 1 硅酸盐光纤和石英匹配光纤的参数
Table 1. Parameters of silicate fiber and matched silica fiber
Parameter Tm-doped silicate fiber SMF-28e+ silica fiber Core diameter/μm 7 8.2 Cladding diameter/μm 125 125 Numerical aperture 0.14 0.135 Effective group index of core refraction @1310 nm 1.4676 1.6049 -
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