用于高精度视向速度测量的高稳定光纤传输技术研究

The astronomical laser frequency comb was installed to calibrate the HRS of 2.16m telescope at Xinglong by Nanjing Institute of Astronomical Optic & Technology. The calibration repeatability could be 0.23m/s when the speckle noise of calibration fiber was suppressed. However, in order to reach the radial velocity (RV) precision of 1m/s, we must reduce other errors due to the variations of spectral line spread function, which deteriorate the Doppler shift measurement. The temperature of HRS has been well controlled, the stability of temperature will be improved and the pressure controlling system will be added soon. Meanwhile, the HRS is planned to upgrade to synchronous calibration mode. However, the errors caused by the variation of seeing, guiding error of telescope and instability of observation fiber have not been considered yet, that could introduce several meters per second RV error. So, the high stability technology of optical fiber transmission was proposed to reduce these errors. For the HRS of 2.16m telescope, the scrambling gain of observation fiber should be over 1000 to satisfy the RV precision of 1m/s, and over 10000 to 10cm/s.

南京天光所采用天文激光频率梳为兴隆2.16米望远镜HRS光谱仪进行光谱定标，在消除了定标光纤散斑噪声的情况下，重复定标精度可以达到0.23m/s。然而为了实现1m/s的视向速度精度，我们必须降低其他由于谱线扩散函数改变引起的误差，这些误差会干扰多普勒频移的测量。HRS光谱仪已经增加了温度控制系统，之后还将提高温度稳定性，增加气压控制系统。同时，双通道同步定标的改造工作也在计划当中。由于视宁度变化、望远镜指向误差和天体光纤不稳定造成的误差可以达到几米每秒，然而这类误差还未进行有效抑制。为此，我们提出采用高稳定光纤传输技术来解决此问题。针对2.16米望远镜HRS光谱仪，天体光纤的扰模增益要超过1000才能满足1m/s的视向速度精度，超过10000才能满足10cm/s的视向速度精度。

南京天文光学技术研究所为兴隆2.16米望远镜高分辨率光纤光谱仪（HRS）安装了天文激光频率梳。定标精度不再是限制视向速度精度的因素。由于视宁度变化和望远镜跟踪误差引起的照明不稳定性能导致几米每秒的误差。这类误差无法通过定标过程来追踪，幸运的是可以通过光纤扰模技术进行抑制。本项目专注于光纤和光纤扰模器的扰模特性研究。光线追迹模拟结果显示：八边形光纤加双光纤扰模器的输出近场和远场分布与入射条件无关。实验研究结果也显示：八边形光纤加双光纤扰模器的扰模增益能超过20,000，足以满足10cm/s视向速度测量。鉴于此，八边形光纤加双光纤扰模器的技术方案是高精度视向速度光谱仪的最佳选择。2.16米望远镜的HRS已经进行了一系列升级，包括恒温、恒压系统，定标系统，当然也包括光纤连接系统。相关的天文观测和数据处理正在进行中，我们相信视向速度精度肯定能有明显提高。