钠信标激光器与钠原子间的耦合效率是其性能评价的核心指标之一, 为对钠信标激光器的激发效率实现精确测量, 在云南丽江1.8 m望远镜上搭建了一套完整的激光钠信标测光系统, 该系统由钠信标激光器、激光中继光路和激光发射望远镜、钠信标接收望远镜、钠原子激光雷达、大气视宁度测量仪等组成。自2011年以来利用该系统对中国科学院理化技术研究所20 W级百微秒脉冲激光器所产生的钠信标进行了相应的测量标定, 成功得到了半高全宽最小为3′(对应到90 km高度处为1.3 m)的钠信标图像, 并测量了在不同的出光功率、偏振状态和中心波长下钠信标的回光结果。实验中分析了滤光片、CCD量子效率曲线等在对钠信标测光时的影响, 对所产生的钠信标回波光子数进行了精确标定, 并提出了一种钠信标V星等的计算方法; 在19 W出光功率, 圆偏光状态下获得了最亮的钠信标, 其在大气层上空的光子数流量为9.55×106 photons·s-1·m-2, 对应7.4 V星等。

Coupling efficiency between the sodium laser and sodium atoms in the mesosphere is one of the most important parameters for the sodium laser guide star system. A photometry system of the sodium beacon had been built on the 1.8-meter telescope at Lijiang Observatory, Yunnan Province, to measure the brightness of the beacon and coupling efficiency of the laser. The system consisted several parts: sodium laser, beam transfer optics and laser launch telescope, receiver telescope, atmospheric coherence length measuring instrument, and ground-based sodium lidar. Measurements of the sodium beacon on different output power, polarization status and central wavelength, excited by the 20 W class quasi-continuous wave sodium laser of Technical Institute of Physics and Chemistry, Chinese Academy of Sciences had been carried out Since 2011. The apparent full width with half maximum of the beacon spot could be as small as 3′, or 1.3 meter at 90 km altitude. Detail of calculating the returned photons had been presented. Taking into account the transmission through astronomical V-band filter and quantum efficiency of the CCD camera, the corrected brightness of the sodium beacon produced by 19 Watts of circular polarized light was equivalent to a 7.4-V-magnitude natural star, or 9.55×106 photons·s-1·m-2 on the top of the atmosphere.