Characteristics of Pulsated Oscillation of Supersonic-flow Chemical Oxygen-Iodine Laser

超声速流化学氧碘激光器（S-COIL）作为一种高功率激光源具有突出的潜力。利用调q操作，可以从S-COIL中提取出极强的脉冲。为了开发和优化该脉冲激光系统，必须准确地评估激光器的光束质量、功率、时间依赖性等特性。因此，本研究发展了数值模拟激光光学的方法。计算代码由两部分组成，一部分是气体流动和化学反应的代码，另一部分是光学的代码。由于这两部分现象的特征时间不同，后一部分计算每300个时间步与前一部分计算1个时间步相耦合。在前一部分中，采用全隐式有限差分法求解了三维可压缩Navier-Stokes方程和包含21种化学反应和12种化学物质的化学动力学模型。假设腔内的流动来自喉道混合系统的下游，这在之前的论文[1]中有显示。在第二部分中，引入了一个近轴波动方程来描述光束的衍射和干涉。在麦克斯韦方程的基础上，简化了激光束的单频和传播方向几乎为轴向的两个假设，导出了近轴波方程。另外，采用几何光学代替波动光学进行计算，考察了衍射和干涉的影响。结果表明，波光学和几何光学光束功率的时间依赖性在性质上是相似的。振荡开始后90ns，峰值功率达到最大值。波光学的峰值比几何光学的峰值小9%。计算得到的光束扩展角减小，在25000 ns处的扩展角比在90ns处的扩展角减小20%。这意味着与连续提取相比，最大功率下的激光质量略差。少

A supersonic-flow chemical oxygen-iodine laser (S-COIL) is expected to have outstanding potential as a high-power laser source. Extremely strong pulse would be extracted from the S-COIL by utilizing Q-switch operation. In order to develop and optimize this pulse laser system, characteristics of the laser such as beam quality, power, time-dependence, so on, should be accurately evaluated. Thus, in this study, numerical method is developed for simulating the laser optics.The calculation code consists of two parts one is a code for the gas flow and chemical reactions, and the other is that for the optics. Because of the difference of characteristic times between the phenomena in these two parts, every 300-time-steps of the latter calculation is coupled with one-time-step of the former. In the former part, three-dimensional, compressible Navier-Stokes equations and a chemical kinetic model encompassing 21 chemical reactions and 12 chemical species are solved by means of full-implicit finit … More e difference method. The flow in the cavity is assumed to be of the downstream of the throat mixing system, which is shown in the previous paper [1]. In the latter part, a paraxial wave equation is introduced so as to describe the diffraction and interference of the beam. The paraxial wave equation is derived from the Maxwell equation by simplifying with two assumptions that the laser beam is of single frequency, and that the propagating direction of it is almost axial. Additionally, another calculation adopting geometric optics instead of wave optics is conducted in order to examine the effects of diffraction and interference.The results indicate that the time dependence of the beam power is qualitatively similar between the wave and geometric optics. The peak power reaches to the maximum value at 90 ns after the initiation of oscillation. The peak value for the wave optics is 9 % less than that for the geometric optics. The calculated spreading angle of the beam is shown to decrease: its value at 25,000 ns is 20 % less than that at 90 ns. This means the laser quality at the maximum power is slightly worse compared to that of continuous extraction. Less