【作者】 王可嘉；

【导师】 刘劲松；

【作者基本信息】 华中科技大学 ， 物理电子学， 2007， 博士

【摘要】 随机激光器具有许多类似于传统激光器的特性,例如阈值特性和辐射光谱线很窄。在传统激光器中,光波被局限于具有增益介质的光腔中从而获得受激辐射放大。但是在随机激光器中,则没有传统意义上的光腔,在此系统中为光波受激辐射放大提供反馈的是介质的无序特性。对于随机激光器的理论研究是基于局域和激光物理理论。本文中,我们研究了随机激光的辐射光特性,结果如下:(1)利用随机激光器的准态模理论分析了一维四能级系统随机介质中光波局域态的动态演化过程。结果表明,泵浦数率越高,局域态形成的时间越短。(2)基于随机激光器的半经典理论数值地研究了一维随机激光器中准态模式的饱和特性。结果显示,准态模式的某些特征参数,例如局域程度,将会影响系统的饱和特性。利用这一结果,可以通过单频抽运的方法选择具有低阈值、高饱和强度的准态模式作为整个激光系统的稳态输出模式。(3)基于随机激光准态模理论研究了随机激光的形成过程、准态模的放大特性以及二者之间的关系。结果显示非增益介质中的准态模相当于传统激光腔的腔模。当增益大于阈值时,一些准态模在增益介质中被放大,形成随机激光。(4)利用随机激光半经典理论,分析了二维相干反馈随机激光模式的阈值与饱和特性。结果表明随机激光模式具有与传统激光模式相类似的阈值与饱和特性。进一步的研究显示随机激光模式的阈值与许多因素有关,其中模式的寿命为主要因素,计算得出两者的关系为:寿命越长的模式,阈值越低。(5)基于随机激光中的半经典理论,数值研究了二维随机激光器中横磁模式的输出光谱线宽度,获得了谱线宽度与泵浦强度的关系曲线,由此曲线可以计算横磁模式的很多特征参数。从谱线宽度的角度来看,横磁模式的输出特性结果与已有的横电模式结果相比,横磁模式具有较好的输出特性。(6)研究了一系列相同随机构型,不同外部形状无源二维随机介质中功率谱与介质形态的关系。结果表明,二维随机激光器的辐射光谱是形态相关的。实践中可以利用形态特性来控制随机激光器的辐射光谱,建立一种基于形态的模式选择技术。(7)利用具有增益项扩散方程模型考查了双光子泵浦随机激光器辐射光的时间演化过程和饱和效应。数值求解了具有增益项的扩散方程。结果显示了随机激光器中辐射光能量从自发辐射开始,经历小信号增益状态得到放大,再到由于增益饱和形成稳态输出的整个演化过程;对饱和特性的讨论表明,一方面可以通过调整增益介质的参数来获得更高的稳态输出值;另一方面可以通过改变随机构型来获得不同的稳态输出行为。更多还原

【Abstract】 Random lasers, which have many features, e.g. threshold for lasing action and frequency narrowing, in common with more conventional lasers based on an optical gain medium enclosed in a cavity with two mirrors to enhance stimulated emission, are strongly scattering media with optical gain. Eventually, the random lasers have many different optical properties, the most important one of which is in absence of well-defined cavity mode, from the conventional lasers. Randomness, which is the actual lasing feedback mechanism for the random lasers, is the essential element in such system. The theoretical investigation on the random lasers is based on the localization and laser physics.In this dissertation we study the emission properties in random lasers as follows:(1) We study the dynamical evolution of the localized states of the emission by use of the quasi-state modes theory of random lasers in one-dimension random media with four-level atomic system. Results show that the larger the pump rate is, the shorter the formation time of the localized states is.(2) The saturation effects of quasi-state modes in one-dimension random lasers are numerically investigated based on the time-dependent theory. The results show that many properties of quasi-state modes, such as the localization length, will influence the saturation effects of the random laser system. By investigating these effects, one can choose the best quasi-state mode, which has a low threshold and high saturation intensity, as a steady output in such system with one single frequency pump.(3) We study the relationship between the emission of random lasers and quasistate modes by use of the finite difference time domain (FDTD) method. The spatial profile and the spectra of qusaistate modes are obtained for the passive and active random media. The results show that random lasing modes directly originate from the localized modes inside random media. In the presence of gain, any one of the localized modes can be amplified and serves as random lasing modes.(4) The pumping rate dependence of the peak intensity of the individual lasing mode in two-dimensional active media is investigated. The results show that each of these modes has a typical threshold gain behavior and different threshold pumping rate. The relationship between the mode’s threshold and lifetime, which is that the longer the lifetime is, the lower the threshold is, is obtained.(5) The spectral width of transverse magnetic (TM) lasing modes in two-dimensional random media is theoretically investigated by using the time-dependent theory. The evolution curve, in which one can calculate some characteristic parameters of the emission, e.g. the threshold, of the spectral width of TM-polarization modes versus the pump intensity is obtained. The results show that the emission, from the view of spectral width, of the TM-polarization lasing modes have better out-put property than that, which was obtained previously, of TE-polarization.(6) The power spectra from a set of two-dimensional passive random media with the same randomness and different shapes have been investigated by numerically solving the Maxwell equations. The results show the power spectra of such set of passive random system are morphological-dependent. This character is promising for proposing a new mode-selecting technique for random lasers.(7) Based the diffusion model, we study the temporal evolution and the saturation of the emission from two-photon pumping random lasers. Numerical results show how the intensity of emitted light varies from the spontaneous radiation, which undergoes amplification by small-signal gain, to the steady output due to the saturation effect. From the analysis of normalized emitted intensity and saturation intensity, one can obtain a method to control steady output-intensity of two-photon pumping random lasing by adjusting some parameters of the random media.更多还原