用于开环液晶自适应光学系统的模式预测技术研究

摘要

In order to reduce the time delay of the liquid-crystal （LC） adaptive optics system （AOS） which reduces the image resolution of the observed objects, we present a new technique for the first time which is called recursive least square （RLS） modal prediction of turbulent wavefront. First, we introduce the structure of the open-loop LC AOS with RLS predictor. Second, we present the RLS modal prediction algorithm in detail. Third, an actual open-loop LC AOS is designed and built, and the RLS prediction is carried out on it. It is shown that under a pure time delay system and the turbulent condition with Greenwood frequency of 35 Hz and Fried parameter of 6 cm, after prediction the residual wavefront error reducs to 0.15 wave （wave -- 785 nm） from 0.26 wave that is obtained through the direct open loop correction. The prediction gain reaches 42%. Finally, the images obtained by the open-loop AOS with and without prediction are demonstrated. With direct correction without prediction, the image resolution reache 25.4 cycles/ram. After the correction with RLS prediction, the image resolution reaches 32.0 cycles/mm which is equal to 0.9 of the diffraction limit resolution of the system. Therefore, with respect to correction without prediction, a relative gain of 26% in image resolution is achieved with RLS prediction. In conclusion, the RLS modal prediction can improve the image resolution of the open-loop LC AOS effectively.%时间延迟误差是液晶自适应光学系统的一个最主要的误差源．本文提出了一种利用智能模式预测一迭代最小二乘（RLS）模式预测算法来克服其对成像分辨率的影响．首先，介绍了具有RLS模式预测能力的开环液晶自适应光学系统的结构和工作原理．其次，详细讨论了RLS模式预测算法的实现过程．再次，设计和搭建了一套带有液晶湍流模拟器的开环液晶自适应光学系统，对RLS模式预测算法的预测效果进行了分析，并和直接开环校正做了比较．分析结果表明：当系统处于中等强度湍流条件（大气相干长度r0=6cm，Greenwood频率fG=35Hz）和只有时间延迟误差情况下，经过RLS预测后，残差波面的RMS值由直接校正的0．26波长（1波长=785nm）降低到了0．15波长，校正效果提高了42％．最后，对预测前后自适应光学系统的成像效果进行了对比试验．实验结果显示，经过预测以后，系统的成像分辨率由直接开环校正的25．4cycles／mm提高到了32．0cycles／mm，成像分辨率提高了26％，达到了0．9倍的衍射极限分辨率．因此，RLS模式预测技术可以有效的提高开环液晶自适应系统的成像分辨率．