【作者】 冯先旺；

【导师】 龙兴武；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2012， 博士

【摘要】 氦氖气体激光具有优质的光学质量，其单色性好、线宽窄、发散角小、频率稳定性高、功率稳定度好。调制后的氦氖气体激光，广泛地应用在光学测量、光学传感以及光学研究等领域。外电光调制是氦氖激光普遍采用的一种调制方式，但由于调制装置的核心器件——电光晶体的半波电压很高，因此外电光调制的调制深度不大，调制效率不高。半导体激光调制方便，但相比氦氖气体激光而言，它的光束质量较差，稳定性不够好，从而限制了其在很多场合的应用。如果能将两者的优点结合起来，研制出调制效率高、调制深度大、调制方便并且光束质量好的氦氖气体激光器，可以对调制激光器的应用产生一定的促进作用。一个有代表性的例子是调制氦氖气体激光器用于腔衰荡测量时，内调制方式的调制电压远远小于外调制方式。本文从结构方案、器件选择、调制理论、配置优化等方面对内电光调制氦氖气体激光器进行了较为系统的研究，并且成功地构建了一套基于内调制氦氖气体激光器的腔衰荡损耗测量系统。本文的主要内容和结论如下：1.设计了一种由BBO(β-BaB2BO4)电光晶体、布儒斯特窗和微晶玻璃激光器构成的内电光调制氦氖气体激光器。通过对综合性能的比较，结合实际应用的需求，选择了合适的内调制激光器器件和设计方案。对激光的内电光调制原理进行了分析，理论上证实了内调制方法调制效率高、调制深度大。2.系统研究了影响内调制氦氖气体激光器调制效率的各种因素，包括布儒斯特窗个数、调制元件在腔内的位置分布、晶体安装方位角等等。通过理论和实验研究，发现晶体光轴相对激光器腔轴的倾斜会对激光的调制效率造成非常大的影响。以激光的关断电压为例，仅仅几分的倾斜角，就可能导致激光的关断电压由几百伏变成数千伏。理论上给出了激光调制达到预定指标所允许的晶体安装误差裕量。通过分析，最终得到了内调制激光器的最佳结构配置方案：布儒斯特窗数目为1个；晶体光轴与激光器腔轴平行；激光在布儒斯特窗上的入射面平行于晶体的x轴或y轴。对于同一种电光晶体，在最佳配置状态下，内调制激光器的关断电压仅仅约为外调制方式的1/25。3.通过氦氖气体激光器的速率方程，仿真了激光开启和关断的瞬态变化过程，发现内调制氦氖气体激光器的关断时间比电光效应的弛豫时间长，一般为10ns以上。发现并在实验上印证了激光关断时，内调制激光器对加载在晶体上的电压振荡具有抑制作用，即关断电压的振荡对激光关断光强波形影响不大。当内调制激光器用于腔衰荡损耗测量时，抑制效应大大降低了对晶体施加电压的设计要求。4.根据理论和实验分析得出的结论，优化了调制激光器系统的设计参数，结合实际的工程技术经验，设计了一种便于晶体安装调整、激光输出稳定的一体化内电光调制氦氖气体激光器。5.对激光外调制腔衰荡损耗测量系统进行了初步的研究。激光外调制腔衰荡测量的一个技术瓶颈为电光晶体上施加电压信号的设计。根据电压信号的要求，找到了一种电压幅度为数千伏、上升沿或下降沿达纳秒量级的快沿高压波形发生器设计方案，并利用Pspice进行了仿真分析。对容易造成电路工作失效的原因进行了剖析，并提出了解决方案。6.采用内电光调制氦氖气体激光器设计了一套腔衰荡损耗测量系统。对系统的各个组成部分进行了详细的介绍说明。分析了光电探测器带宽对测量结果造成的影响。设计了测量系统的外围工作电路，并对其性能进行了测试。构建了直腔和折叠腔两种无源腔结构，对腔的损耗进行了测量。测量结果与半导体激光器的测量结果做了对比，两者的测量结果很接近，且氦氖激光的测量结果方差更小。测量得到了其中一个高反膜损耗为77.423ppm，对应的反射率为99.99225%。更多还原

【Abstract】 He-Ne gas laser has an excellent optical quality for its high monochromaticity,narrow linewidth, small diverging angle, good frequency stability and high intensitystability. After being modulated, it is used extensively in the field of opticalmeasurement, optical sensing and optical research etc.. External electro-optical (EO)modulation is generally employed for He-Ne gas laser. Unfortunately, due to the highhalf-wave voltage of EO crystal which is the critical element in a modulation system,external EO modulation has the disadvantage of limited modulation depth and lowmodulation efficiency. Semiconductor laser can be modulated easily, but compared withHe-Ne gas laser, its optical quality and stability is relatively bad, which limits itsapplication. Combining the merits of He-Ne gas laser and semiconductor laser, aninternal modulated He-Ne gas laser with deep modulation depth, high modulationefficiency, easy modulation method and excellent optical quality, would account for theapplication of modulation laser. One typical of such applications is the cavity ring downmeasurement, in which the modulation voltage of internal EO modulation technique ismuch lower than that of the external modulation method. In this paper, an internal EOmodulation He-Ne laser is proposed and investigated systematically, including thestructural scheme, the elements choice, the modulation theory, the optimal configurationand so on. And a cavity ring down loss measurement system is established based onsuch an internal modulation He-Ne gas laser. The main contents and results of thispaper are listed as follows:1. An internal EO modulation He-Ne gas laser is designed with BBO (β-BaB2BO4)EO crystal, Brewster window and laser constructed with Zerodur crystallite glass.Suitable modulation elements and design scheme are selected according to theirperformance and the application requirement. The internal EO modulation theory ofHe-Ne laser is deduced, which confirms the high modulation efficiency and deepmodulation depth of the internal modulation technique.2. All factors that influence the modulation efficiency of internal modulation laserare analyzed systematically, including the number of Brewster window, distribution ofmodulation elements and orientation of EO crystal. Through experimental andtheoretical investigation, we found that the tilt of EO crystal with respect to cavity axisof laser has a significant influence on modulation efficiency. Take switching-off voltageof laser beam as an example, only several minutes of tilt angle may cause theswitching-off voltage change from hundreds of volts to thousands of volts. Thepermitted mounting error of the EO crystal for certain modulation requirement could beevaluated by the theoretical analysis. In the end, an optimal configuration of the internalmodulation laser is obtained, namely, the number of Brewster window is one, the optical axis of the EO crystal is in parallel to the cavity axis of laser resonance, and theincidence plane of Brewster window is in parallel or perpendicular to the x axis of theEO crystal. For a certain EO crystal, it is validated that the switching-off voltage of theinternal modulation method is only~1/25of the external modulation method.3. By employing the rate equation of He-Ne gas laser, the transition of turn-on andturn-off of the laser light is simulated. It is found that the switching-off time of theHe-Ne laser, which is generally longer than10ns, is much longer than the relaxationtime of EO effect. Besides, it is discovered and validated by experiments that internalmodulation laser could reduce the influence of ringing effect of voltage applied on EOcrystal on switching-off light signal. When the internal modulation laser is used incavity ring down loss measurement, this effect could greatly reduce the requirement ofthe voltage signal applied on EO crystal.4. Based on the conclusion of theoretical and experimental analysis, the designparameters of internal modulation He-Ne gas laser are optimized. And according to theengineering experience, an integration internal modulation He-Ne gas laser is designedwith convenient mounting structure for EO crystal and stable output laser beam.5. The cavity ring down loss measurement system with external modulation laser isstudied preliminarily. It is known that a bottleneck of this technique is the requirementof the voltage applied on EO crystal. A scheme of high voltage generator is found toaccommodate the requirement with amplitude of thousands of volts and rise or fall timeof nanosecond. In addition, it has been studied by PSpice simulation. The reason for theeasy-fault of such a circuit is analyzed, and a solution has been put forward.6. A cavity ring down loss measurement system with internal modulation He-Negas laser is built. The constituent part of the system is introduced in detail. Especially,the influence of the bandwidth of the photodetector on loss measurement result isanalyzed. The working circuit of the system is designed and tested to ensure the properoperation of the system. Two passive resonant cavities, namely, a linear cavity and afold cavity are constructed, and their cavity losses are measured respectively. Comparedwith the results that measured by a semiconductor laser, it is found that the two resultsare close to each other. In addition, the variances of the results measured with He-Nelaser are smaller. The loss of one film is measured as77.423ppm, which corresponds toa reflectivity of99.99225%.更多还原