Q值引起的圓柱形微腔中WGM激光的輻射特... - 云南

1、Q值引起的圓柱形微腔中WGM激光的輻射特性研究摘 要微米尺寸的光學(xué)圓形微腔，由于其極高品質(zhì)因數(shù)（Q值）的回音壁模式(Whispering Gallery Mode，WGM)和極小的模體積特性，在非線性光學(xué)、腔體量子電動力學(xué)、超低閾值的微腔受激輻射放大過程等研究和應(yīng)用領(lǐng)域受到高度重視。由圓形微腔構(gòu)成的WGM激光器，通常采用側(cè)向光泵浦的消逝波增益耦合方式，由于泵浦光須經(jīng)過包層染料溶液的吸收才能激發(fā)處于微腔WGM的消逝場內(nèi)的染料分子，消逝場外的染料分子徒然損耗了泵浦光能量，增加了產(chǎn)生激光的閾值能量。為了有效地提高激光的泵浦效率，本文采用軸向消逝波激勵增益方式激發(fā)光纖中的WGM激光，研究了這種光纖WG

2、M激光由Q值變化引起的激光輻射特性，具體研究內(nèi)容包括如下三個方面：(1) 將不同直徑的石英光纖分別浸入低折射率的羅丹明6G乙醇和乙二醇混合溶液中，采用沿光纖軸向光泵浦消逝波激勵染料增益的方式，研究回音壁模式光纖激光輻射的閾值能量和混合溶液的折射率的依賴關(guān)系。實驗結(jié)果表明，隨染料混合溶液折射率的增加，對小直徑光纖，閾值能量隨之增加；對大直徑光纖，閾值能量單調(diào)緩慢遞減；對直徑適中的光纖，閾值能量先減后增，存在一個和最小閾值能量對應(yīng)的最佳折射率。用消逝波激勵的回音壁模式激光理論，導(dǎo)出了回音壁模式光纖激光的閾值能量公式。理論計算曲線和不同直徑的光纖閾值能量實驗數(shù)據(jù)完全吻合；(2). 將直徑為93 mm

3、的石英光纖分別浸入一系列低折射率的羅丹明6G乙醇和乙二醇混合溶液中，采用沿光纖軸向光泵浦消逝波激勵染料增益的方式，研究回音壁模式激光輻射的波長及譜線線寬隨增益包層溶液折射率的變化規(guī)律。實驗發(fā)現(xiàn)，隨染料混合溶液折射率的增加，回音壁模式激光輻射的波長向短波方向移動，同時譜線不斷變寬。用回音壁模式染料激光的四能級模型所滿足的激光上能級和所有能級上的分子數(shù)比值g (l)曲線，很好地解釋了波長漂移現(xiàn)象；由電磁波所滿足的特征方程導(dǎo)出了WGM激光的譜線線寬公式，很好地解釋了實驗結(jié)果；(3). 將直徑不同的一系列石英光纖分別浸入包層介質(zhì)折射率為1.396的羅丹明6G乙醇和乙二醇混合溶液中，采用沿光纖軸向光泵浦

4、消逝波激勵染料增益的方式，研究回音壁模式激光輻射的中心波長及譜線線寬的變化規(guī)律。實驗發(fā)現(xiàn)，隨石英光纖直徑的減小，回音壁模式激光輻射的波長向短波方向移動，同時譜線不斷變寬。用回音壁模式染料激光的四能級模型所滿足的激光上能級和所有能級上的分子數(shù)比值g (l)曲線，很好地解釋了波長漂移現(xiàn)象；由WGM激光的譜線線寬公式與實驗寬度進(jìn)行對比分析，二者吻合得很好。實驗和理論分析結(jié)果表明，以上實驗現(xiàn)象都是由Q值變化引起的尚未見文獻(xiàn)報道的微腔光學(xué)新現(xiàn)象。關(guān)鍵詞: 消逝波, WGMs，光纖激光器, 閾值能量 , 波長漂移, 譜線展寬 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