【作者】 李文萃；

【导师】 宣丽；

【作者基本信息】 中国科学院研究生院（长春光学精密机械与物理研究所） ， 光学， 2011， 博士

【摘要】 HPDLC光栅是一种由液晶层和聚合物层周期交替排列的光栅结构,这种光栅制备方法简单可控、一级衍射效率高,而且可以利用电场进行光强调谐。基于HPDLC光栅的DFB激光器是分布式反馈激光器,其光栅主体取代了在传统激光器中腔的作用,并可使光波发生多重光反射,产生特有的窄线宽、低阈值、且亮度可调谐的激光,拓展了反馈激光器的应用领域。另外这种小型化的激光发射器件在光纤通信、航空航天领域、天文学以及激光地面定位系统等都有极其广泛的应用前景。基于HPDLC光栅结构的DFB激光器,其关键的制备步骤是微型激光谐振腔即HPDLC光栅的形成效果。但是由于聚合物与液晶的相分离是很复杂的物理化学过程,完美的控制很难实现,存在着体系内残余未反应单体较多、光栅结构轮廓不光滑、光栅周期很难做到波长量级,光栅散射损失严重、衍射效率低等问题,从而导致DFB激光器的激射谱线过宽、泵浦能量阈值高,严重制约了该器件的实用化。为了解决上述问题,本论文进行了如下研究。首先从影响聚合物与液晶相分离的微观物理机理出发,分析了残余未反应单体较多、衍射效率低的原因,主要是反应速率与扩散速率不匹配,因此从反应单体的光敏度即平均官能度、曝光强度、制备温度等方面进行优化调整实验,得出当平均官能度为2.4、曝光强度在3.9~4.3mW/cm~2时,在室温下就可制备出波长级周期的HPDLC光栅,其衍射效率提高到75%,散射损失由22%减小到12%;为了进一步减小散射,进行了液晶的取向处理,实验表明取向处理对散射具有非常明显的抑制作用,散射损失降低到0.3%,得出非取向液晶畴在HPDLC中的重要散射机理。经过以上光栅制备条件的改进,获得了衍射效率高达98%、波长级周期的HPDLC光栅。基于HPDLC光栅的制备技术,制备了周期更小的HPDLC光栅,尝试了DFB激射泵浦实验,获得波长为610nm,宽度为6nm的发光谱线,判断这只是窄化的荧光光谱。在高倍率显微镜下观测,光栅的结构轮廓不够清晰,说明周期更小的HPDLC光栅的反应度不高。为此增添了交联剂NVP,不仅增加了双键转化率即反应度,使其高达84%,而且提高了光敏单体的聚合速率,光栅的衍射效率恢复到98%。再次进行泵浦实验,获得了中心波长为620.2nm的激光,实验结果与理论值相符,其泵浦阈值能量很低,只有5.91μJ,能量转化效率约为2‰,达到了国际领先水平。论文进一步分析了液晶分子排列取向对DFB激光器的影响。由于对液晶分子进行了取向处理,几乎消除了光栅散射损,液晶层和聚合物层的折射率差值增加,大幅度提高了光栅的衍射效率,制备出的DFB激光器泵浦阈值能量降低到3.44μJ,能量转化效率提高到3.5‰,线宽降低到0.4nm,超过了目前文献报道的最好水平。与此同时,由于染料分子和液晶分子都具有相似的棒状结构,使得激光器对泵浦光具有偏振依赖性,只有当泵浦激光的偏振矢量与液晶取向平行时出射光为激光,线宽为0.4nm。最后进行了更短波长的DFB激光器的制备以及DFB激光器的调谐特性研究。当HPDLC光栅周期为373nm时,获得了DFB激光器的最短出射波长585.7nm,能量转换效率为3‰,线宽为0.3nm,泵浦阈值为4.3μJ,这种波长的激光器目前还没有商业化的产品。另外通过外加电场改变液晶层的折射率,可以实现幅度高达90%的输出激光强度调制;通过改变温度,则可以实现出射波长在10nm范围的连续调谐,中心波长由626.5nm变为615.6nm,而且在调制波长的同时输出激光强度变化较小。本论文的探索性研究,展示了一种更有希望的DFB激光器。论文中所得出的学术结论为极有应用价值的光聚合反应研究提供了理论和实验依据。更多还原

【Abstract】 Holographic polymer dispersed liquid crystal (HPDLC) grating is an alternate structure of liquid crystal lamella and polymer lamella making use of photo-initiated polymerization-induced phase separation. Due to its compactibility, high first order diffraction efficiency and electrically tunable characteristics, this grating exhibits a bright prospect in the fields of optical communication, integrated optics, data storage and laser techniques. In a distributed feedback (DFB) laser based on HPDLC transmission grating, laser cavity is substituted by the grating structure. Photons corresponding to some specific frequencies selected by the periodic structure will undergo multi-reflections which results in effective light feedback. And then, lasing action with narrow linewidth, low threshold and switchable luminance can be obtained. This kind of compact laser has been explored a wide range of applications in the fields of fiber-optic communication, aerospace and astronomy.Based on HPDLC transmission grating, the output lasing properties are inherently dependent on the nature of the HPDLC transmisstion grating. Limited by photochemical properties of monomers, chemical structure of molecules, and some complicated molecular dynamic problems, it is very difficult to manipulate the PIPS process, leading to low monomer conversion, incomplete phase separation and low diffraction efficiency. So that the FWHM of the DFB laser is broad and the lasing threshold is high, which restrict its applications. In order to resolve these problems from the root, this paper has done the following research.To fabricate HPDLC grating with high diffraction efficiency and low scattering loss, the effects of reaction system average functionality, exposure energy, fabricating temperature and the surface alignment treatment on HPDLC grating are discussed. It is found that when the average functionality is 2.4, exposure energy is 3.9~4.3mW/cm~2 and at room temperature, the diffraction efficiency increases from 47.8% to75.6% and the scattering reduced to 12.2%. In order to decrease the scattering loss further, the LC domains are aligned uniformly and the scattering was disappeared thoroughly. Through theoretical analysis, the key problem of scattering loss is the non-uniform alignment of LC domains in the grating. Experimental results indicate that the diffraction efficiency of the grating increases to 98.1% which is almost the same with theoretical result, and the scattering loss is decreased from 12.2% to 0.3%.Optically pumped lasing with the 6nm linewidth at 610nm was achieved from dye-doped laser HPDLC grating. It actually is the amplified spontaneous emission (ASE) due to the imperfections within the grating. In order to get narrow linewidth lasing action, mono-vinyl NVP is added to the reaction system. NVP significantly increases the rate of polymerization in HPDLC photopolymerization, and also it facilitates additional conversion of pendant double bonds otherwise trapped in the polymer network. As NVP concentration is 15%, the reaction system conversion increases from 55 % to 84%. And consequently, the emitted lasing with the 0.4nm linewidth at 620.2nm was obtained.The effect of liquid crystal alignment on DFB laser is also demonstrated. When the liquid crystals are parallel oriented, the pumping threshold is very low (3.44μJ) due to the low scattering loss of the grating. It is also found that the lasing phenomenon is dependent on the pumping light polarization. This can be explained by the anisotropic alignment of the dye molecules inside the HPDLC. Experimental results indicate that S-polarized pump laser will generate much more lasing emission.At the end, the wavelength and intensity switchable properties of DFB laser are studied. The experimental results demonstrated that the emitted laser can be obtained at different wavelength varied from 585nm~680nm, corresponding to the different grating spacings. And the lasing emission can also be electrically and thermally switchable due to the change of the refractive index modulation. The maximum switchable extent can reach 90.2%.The exploratory studies in this dissertation, exhibits a kind of more promising DFB laser. The academical conclusion can provide an important theoretical and experimental basis for the photo-polymerization reaction with high application value.更多还原