QQ客服 官方微博 反馈留言

题名	提高液晶/聚合物光栅激光器转化效率的研究
作者	刁志辉
学位类别	博士
答辩日期	2015-05
授予单位	中国科学院大学
导师	宣丽
关键词	液晶/聚合物光栅 分布反馈式激光器 转化效率 波导结构
其他题名	Studies on the conversion efficiency improvement of the laser based on holographic polymer dispersed liquid crystal grating
学位专业	光学
中文摘要	液晶/聚合物光栅是由液晶层和聚合物层交替排列组成的周期结构。该光栅制备简易、成型快速，可作为分布反馈式激光器的谐振腔。通过向其中加入激光染料，即可获得窄线宽、低阈值的激光输出。由于液晶材料的引入，激光的出射强度或波长可主动调谐，因此液晶/聚合物光栅激光器在光谱分析、集成光学等领域有着十分重要的应用价值。 然而，在已查到的国际报道中，该类激光器的能量转化效率很低，不足0.5%，严重阻碍其实用化进程。为了解决这一问题，本论文开展了提高液晶/聚合物光栅激光器转化效率的研究。 首先探索了转化效率低的内在原因，是由于光栅的折射率调制量Δn很低，激光在光栅中振荡时无法获得足够的反馈作用。针对这一问题，定量化研究了光栅的内部结构特点，并以此为基础，确定了Δn对转化效率的影响：Δn的增加有助于提高激光器的转化效率。基于这一结论，采用高折射率单体制备光栅，通过提高聚合物层折射率来获取更大的Δn，相比于原有体系，激光器的转化效率由0.3%提高到1.2%，出射阈值由3.5 μJ降低为0.8 μJ。进一步地，提出减小光栅周期的办法，利用小周期光栅的几何效应改变液晶取向，使光通过液晶层时的折射率增大，从而达到提高光栅Δn的目的，激光器的转化效率提高到2.4%，出射阈值降低为0.5 μJ。 针对激光器中存在的多模激光现象进行了研究，该现象会严重降低某一特定波长的转化效率。研究发现，光栅与其两侧的玻璃基板会形成平板波导结构，在波导结构和光栅的双重作用下产生了多模激光。想要抑制多模激光的形成，需要减小衬底层与光栅层的折射率差值。为此，实验中利用聚合物混合方法设计出一种折射率可调的衬底层代替玻璃基板，进行激光器的制备，获得了稳定的单模激光输出，使激光器的出射能量可集中在单一波长之上。进一步测定激光波长与转化效率间的关系，选取最高发光效率的出射波长，使激光器的转化效率提高到2.8%。 然而，受限于浓度猝灭效应，激光染料的可使用浓度低于1 wt%，对泵浦光的利用率仅为15%，使激光器的转化效率停滞在2.8%。为了提高泵浦光的利用率，改用对泵浦光有高吸收特性的MEH-PPV作为增益介质。鉴于MEH-PPV无法和光栅体系互溶，提出MEH-PPV增益层位于光栅之下的激光器结构，并从理论和实验角度证实了该结构能够有效地形成激光。结果表明，出射激光具有单一模式，由于MEH-PPV对泵浦光的利用率高达67%，激光器的转化效率提高到7.1%，出射阈值降低为0.09 μJ。进一步地，尝试将MEH-PPV和激光染料结合在同一激光器结构中，制备出了波长可独立控制的双波长激光器。在两种增益介质的共同作用下，泵浦光的利用率提升到85%，激光器的转化效率提高到9.0%。 本论文从提高光栅Δn、单模光栅激光器的制备以及提高泵浦光利用率三个方面对液晶/聚合物光栅激光器的转化效率进行了提升，所得转化效率为已知文献报道中的最好水平。本文中给出的研究结论为该类激光器的实用化进程奠定了坚实的基础。
英文摘要	Holographic polymer dispersed liquid crystal (HPDLC) grating forms a periodic structure composed of alternating liquid crystal and polymer layers. The HPDLC grating has the advantages of ease of processing and rapid prototyping, which can be used as a resonator cavity in distributed feedback (DFB) laser. By doping laser dyes in the grating, the laser emission with narrow linewidth and low threshold can be achieved. For the introduction of liquid crystal, the laser intensity or wavelength can be tuned actively, so the DFB laser based on HPDLC grating possesses important application value in the research fields of spectrum analysis and integrated optics. Nevertheless, the energy conversion efficiency of the DFB laser based on HPDLC grating is lower than 0.5% among the known international reports, which severely hinders its practical process. To overcome that problem, this dissertation develops studies on the conversion efficiency improvement of the DFB laser based on HPDLC grating. Firstly, the internal reason for the low conversion efficiency is explored and attributed to the low refractive index modulation Δn of the grating, so that the laser can not obtain enough feedback when propagating in the grating. To solve this problem, the characteristics of the internal grating structure are investigated quantitatively, and on that basis, the effect of Δn on the conversion efficiency is determined: a larger Δn will lead to a higher conversion efficiency. Based on the above conclusion, a monomer with high refractive index is applied to fabricate the HPDLC grating to achieve larger Δn by increasing the refractive index of polymer layer. Compared to the original system, the conversion efficiency is enhanced from 0.3% to 1.2% and the threshold is decreased from 3.5 μJ to 0.8 μJ. Furthermore, a method of decreasing grating period is proposed to change the liquid crystal orientation due to the geometrical effect of small-period grating, so Δn can be increased since the refractive index of liquid crystal layer is increased for the propagating light. In this way, the conversion efficiency is enhanced to 2.4% and the threshold is reduced to 0.5 μJ. A research is carried out on the multi-mode laser phenomenon, which will severely decrease the conversion efficiency of the specific wavelength. The results show that the grating and glass substrates on both sides define a typical planar waveguide, and the multi-mode laser is generated by the double effects of waveguide and grating structures. To inhibit the multi-mode laser, the refractive index difference between the grating and substrate layers needs to be decreased. Therefore, a substrate layer with controllable refractive index is designed by blending two different polymers in the experiment. By substituting the new substrate for the glass, a stable single-mode laser emission is achieved, so the emission energy can be concentrated on a single wavelength. Furtherly, the relationship between laser wavelength and conversion efficiency is determined. By choosing the right wavelength with highest efficiency, the laser conversion efficiency is increased to 2.8%. However, the workable concentration of laser dye should be lower than 1 wt% because of its concentration quenching effect, which induces a low pump light utilization rate of 15% and limits the laser conversion efficiency to 2.8%. To improve the utilization rate, the gain material MEH-PPV with higher absorption to pump light is used. In view of the insolubility of MEH-PPV and HPDLC grating system, a new laser structure with MEH-PPV under HPDLC grating is proposed, and it is proved that this laser structure can effectively generate laser from the aspects of theory and experiment. The results show that the laser emission has single-mode, the conversion efficiency is increased to 7.1% and the threshold is decreased to 0.09 μJ due to the higher pump light utilization rate of 67%. Moreover, the MEH-PPV and laser dye are applied in the same laser structure and dual-wavelength laser emission is obtained. Since the two gain materials can work simultaneously, the utilization rate of pump light is increased to 85% and the laser conversion efficiency is enhanced to 9.0%. In this dissertation, the conversion efficiency of the DFB laser based on HPDLC grating is improved from three aspects: increase of grating Δn, single-mode laser fabrication and enhancement of pump light utilization. The obtained conversion efficiency is the highest among the known reports. The research conclusions presented in this dissertation lay a solid foundation for the practical process of the DFB laser based on HPDLC grating.
公开日期	2015-12-24
内容类型	学位论文
源URL	[http://ir.ciomp.ac.cn/handle/181722/48834]
专题	长春光学精密机械与物理研究所_中科院长春光机所知识产出
推荐引用方式 GB/T 7714	刁志辉. 提高液晶/聚合物光栅激光器转化效率的研究[D]. 中国科学院大学. 2015.

个性服务

查看访问统计

相关权益政策

暂无数据

收藏/分享

除非特别说明，本系统中所有内容都受版权保护，并保留所有权利。