Q值引起的圆柱形微腔中WGM激光的辐射特... - 云南

1、Q值引起的圆柱形微腔中WGM激光的辐射特性研究摘 要微米尺寸的光学圆形微腔，由于其极高品质因数（Q值）的回音壁模式(Whispering Gallery Mode，WGM)和极小的模体积特性，在非线性光学、腔体量子电动力学、超低阈值的微腔受激辐射放大过程等研究和应用领域受到高度重视。由圆形微腔构成的WGM激光器，通常采用侧向光泵浦的消逝波增益耦合方式，由于泵浦光须经过包层染料溶液的吸收才能激发处于微腔WGM的消逝场内的染料分子，消逝场外的染料分子徒然损耗了泵浦光能量，增加了产生激光的阈值能量。为了有效地提高激光的泵浦效率，本文采用轴向消逝波激励增益方式激发光纤中的WGM激光，研究了这种光纤WG

2、M激光由Q值变化引起的激光辐射特性，具体研究内容包括如下三个方面：(1) 将不同直径的石英光纤分别浸入低折射率的罗丹明6G乙醇和乙二醇混合溶液中，采用沿光纤轴向光泵浦消逝波激励染料增益的方式，研究回音壁模式光纤激光辐射的阈值能量和混合溶液的折射率的依赖关系。实验结果表明，随染料混合溶液折射率的增加，对小直径光纤，阈值能量随之增加；对大直径光纤，阈值能量单调缓慢递减；对直径适中的光纤，阈值能量先减后增，存在一个和最小阈值能量对应的最佳折射率。用消逝波激励的回音壁模式激光理论，导出了回音壁模式光纤激光的阈值能量公式。理论计算曲线和不同直径的光纤阈值能量实验数据完全吻合；(2). 将直径为93 mm

3、的石英光纤分别浸入一系列低折射率的罗丹明6G乙醇和乙二醇混合溶液中，采用沿光纤轴向光泵浦消逝波激励染料增益的方式，研究回音壁模式激光辐射的波长及谱线线宽随增益包层溶液折射率的变化规律。实验发现，随染料混合溶液折射率的增加，回音壁模式激光辐射的波长向短波方向移动，同时谱线不断变宽。用回音壁模式染料激光的四能级模型所满足的激光上能级和所有能级上的分子数比值g (l)曲线，很好地解释了波长漂移现象；由电磁波所满足的特征方程导出了WGM激光的谱线线宽公式，很好地解释了实验结果；(3). 将直径不同的一系列石英光纤分别浸入包层介质折射率为1.396的罗丹明6G乙醇和乙二醇混合溶液中，采用沿光纤轴向光泵浦

4、消逝波激励染料增益的方式，研究回音壁模式激光辐射的中心波长及谱线线宽的变化规律。实验发现，随石英光纤直径的减小，回音壁模式激光辐射的波长向短波方向移动，同时谱线不断变宽。用回音壁模式染料激光的四能级模型所满足的激光上能级和所有能级上的分子数比值g (l)曲线，很好地解释了波长漂移现象；由WGM激光的谱线线宽公式与实验宽度进行对比分析，二者吻合得很好。实验和理论分析结果表明，以上实验现象都是由Q值变化引起的尚未见文献报道的微腔光学新现象。关键词: 消逝波, WGMs，光纤激光器, 阈值能量 , 波长漂移, 谱线展宽 Cavity-Q-driven phenomena in an evanesce

5、nt-wave pumped andgain coupled whispering-gallery-mode fiber laserAbstractA micro sized optical cavity with whispering gallery modes (WGMs) can have high factor and small mode volume, which has attracted great interest in nonlinear optics, cavity quantum electrodynamics and ultra-low threshold of mi

6、cro-cavity laser research recent years. To excite the gain molecules optically around a circular cavity, laser beam is usually pumped from the outside of gain medium. This pumping scheme suffers low pump efficiency because the pump energy is absorbed by the whole excited molecules, only the molecule

7、s residing in the evanescent field of Whispering Gallery Modes (WGMs) of a circular cavity contribute to the lasing gain. To solve this problem, evanescent-wave pumping scheme has been used in capillary, microfiber knot, and fiber lasers in recent years, owing to a good spatial overlap between the e

8、vanescent-wave of WGMs and the surrounded gain medium, the pump efficiency is considerably improved. In this thesis, the evanescent field pumped and gain coupled WGM fiber laser has been studied, the main topic focuses on three aspects: Firstly, quartz fibers of different diameters were immersed int

9、o ethanol and ethylene glycol mixed solution doped by rhodamine 6G dye molecules, the refractive index of the solution was variable and lower than that of quartz fiber. We find that the dependence of lasing threshold on the refractive index of the mixed solution differs from different diameter of qu

10、artz fiber. With the increase of refractive index of the mixed solution from 1.362 to 1.432, the threshold energy decreases slowly and monotonously for fibers of diameter larger than 296 mm, but increases for fibers of diameter smaller than 93 mm; for a fiber of diameter 197 mm, the threshold energy

11、 decreases slowly at first, and then increases sharply, there is an optimum refractive index of the mixed solution which matches a minimum threshold energy. Based on the theory of evanescent-wave pumped WGM fiber laser, we deduce a formula of lasing threshold, the calculated curves of lasing thresho

12、ld are consistent with the experimental data well.Secondly, Quartz fiber with a diameter of 93 mm was immersed into a mixed solution of ethanol and ethylene glycol doped by rhodamine 6G dye molecules, the refractive index of the solution was variable and lower than that of quartz fiber. Pumped by ev

13、anescent-wave along the fiber axis, the lasing wavelength of WGM has been found to blue shift and the spectra line-width has widened with the increase of refractive index of dye solution. The experimental observations of lasing wavelength blue shift are well explained by the g (l) curve, which is a

14、ratio of molecular density for the upper and total lasing energy levels and deduced from the four-energy level model of dye lasing from a WGM fibre laser; The line-width widen are also well explained by the approximate formula of WGM laser line-width, which we derived from the characteristic equatio

15、n of electromagnetic wave.Thirdly, quartz fibers of different diameters were immersed into a mixed solution of ethanol and ethylene glycol doped by rhodamine 6G dye molecules, the refractive index of the solution was 1.396. Pumped by evanescent-wave along the fiber axis, the lasing wavelength of WGM

16、 has been found to blue shift and the spectra line-width has widened with the decrease of fiber diameter. The experimental observations of lasing wavelength blue shift and line-width widen are also well explained by the g (l) curve and approximate formula of WGM laser line-width.Experimental results an