基于双星激光外差干涉测距的超高精度激光指向控制研究

The satellite-satellite heterodyne laser interferometer has the characteristics of millions of kilometers working distance and pico-meter range accuracy, which would become the preferred measurement methodology of the space gravitational waves detection. In space, the satellites would be disturbed by the non-conservative forces such as the cosmic rays, atmosphere and solar wind and so on, resulting in the attitude of which being changed. As a result, the laser beams would jitter, and a ranging error of the laser interferometer would be generated. To eliminate the error, it needs to implant a laser pointing control system to make the two interfering laser beams keep a high degree of parallel. Therefore, the study on the laser pointing control based on the satellite-satellite laser heterodyne interferometer ranging becomes a primary key technique for the space gravitational waves detection. The theory predicts that only the laser pointing noise lower than 1nrad/√Hz can make it possible to meet the ranging accuracy requirement of the space gravitational waves detection. Currently, there is no technology system to meet such technical requirement. Therefore, this project will propose a new methodology combining laser pointing control and laser pointing elimination. By establishing a satellite-satellite laser pointing control system demonstration on-ground, and using a laser pointing control methodology based on a conversion technology of angle-phase, to make the accuracy of the laser pointing control better than 10nrad/√Hz. Then, using the laser pointing elimination method to make the laser pointing control improved by one or two orders of magnitude, to achieve the building of laser pointing control system, the pointing noise of which will be lower than 1nrad/√Hz.

双星激光外差干涉具有百万公里工作距、皮米测距精度等特点，在空间引力波探测方案中成为首选的测量方法学。空间中，卫星受宇宙射线、大气和太阳风等非保守力的扰动，导致卫星姿态发生改变，引起激光光束产生抖动，造成激光外差干涉产生测距误差。为消除此误差，需引入激光指向控制系统，以确保两干涉激光束的超高平行度。因此，双星激光外差干涉测距的激光指向控制研究成为空间引力波探测的关键环节。理论预测，激光指向抖动噪声需低于1nrad/√Hz才能满足空间引力波探测的测距精度要求。目前，尚没有能满足此指标要求的技术系统。为此，本项目提出激光指向控制结合激光指向消除的新方法，通过构建双星激光指向控制技术系统，利用基于角度、相位转换技术的激光指向控制法，使得激光指向抖动控制精度优于10nrad/√Hz，然后采用激光指向消除法，使得抖动控制水平再提高1～2个量级，从而实现激光指向抖动噪声低于1nrad/√Hz的技术系统。

双星激光外差干涉具有百万公里工作距、皮米测距精度等特点，在空间引力波探测方案中成为首选的测量方法学。空间中，卫星受宇宙射线、大气和太阳风等非保守力的扰动，导致卫星姿态发生改变，引起激光光束产生抖动；同时，航天器的接收望远镜因受镜片平整度与打磨精度等各种因素的影响，造成接收激光产生波前畸变，波前畸变激光会导致干涉信号的相位发生变化，从而引起激光干涉测距系统产生测相误差，降低激光干涉测距系统的测距精度。为消除此测相误差，需引入激光指向控制系统，以确保两干涉激光束的超高平行度。为此，本项目设计、构建了面向未来空间引力波探测需求的双星激光指向控制实验技术系统，针对此实验技术系统进行了相关光路及控制系统的模拟仿真，研究发现双星的激光指向控制不存在耦合性问题；本项目利用基于差分波前检测技术的激光指向控制法，发展了噪声等效功率密度仅为4.5×10-12W/Hz1/2的低噪弱光四象限探测器，研制出测相精度高达2π×10-6rad/√Hz的四通道数字相位计，同时发展了读出噪声仅为2nrad/√Hz的指向敏感器；利用模糊自适应和PID相结合的控制策略，有效地将运动频率为10mHz，幅值为5μrad/√Hz的激光指向抖动抑制到1nrad/√Hz（in-loop）以下，实现了超高精度的激光指向控制，达到了本项目的研究目标。为未来高精度、高稳定性的双星激光指向控制研究提供了方案解决途径，并为未来空间引力波探测提供了技术支撑。