[论文解读] Mesosphere Sodium Column Density and the Sodium Laser Guide Star Brightness

Jian Ge, J. R. P. Angel+8 more|arXiv (Cornell University)|Aug 29, 1997

Adaptive optics and wavefront sensing被引用 3

一句话总结

本研究首次在北纬32°实现了对大气层中间层钠柱密度与钠激光导星（LGS）亮度的同步测量，结果表明，与线性偏振相比，圆偏振激光光可使荧光回波强度提高约30%。在钠柱密度为3.7×10⁹ cm⁻²时，1 W的圆偏振激光可产生视星等R = 10.3等的LGS（每平方米每秒8.4×10⁵个光子），从而可根据季节性和昼夜柱密度变化，精确预测实现目标导星亮度所需的激光功率。

ABSTRACT

The first time simultaneous measurements of sodium column density and the absolute flux from a sodium laser guide star, created by a monochromatic 3 W cw laser, tuned to the peak of the sodium D2 hyperfine structure, were conducted at the MMT and CFA 60 inch telescope in 1997. The results show that linearly and circularly polarized laser returns are proportional to the simultaneous sodium column density. Moreover, circularly polarized laser provides about 30% increase in fluorescent return over linearly polarized laser. A laser guide star with R = 10.3 mag. or absolute flux of 8.4x10^5 photons/s/m^2, could be formed from a 1 watt projected circularly polarized sodium laser beam when sodium layer abundance N(Na) = 3.7x10^9 /cm^2. Together with the distributed column density measurements (e.g. seasonal and diurnal variations), we can project laser power requirements for any specified guide star brightness. The mesosphere sodium column density variation was measured above Tucson sky throughout the year, through sodium absorption line measurements in stellar and solar spectra. Previous measurements, e.g. Papen et al, 1996, have not been made at this latitude (32 degrees). Further, our absorption method is more direct and may be more accurate than the lidar methods normally used. The seasonal variation amplitude is smaller than that at higher latitudes. While the annual mean sodium column density tends to be lower than at higher latitudes. Diurnal sodium column density tends to vary by as much as a factor of two within an hour.

研究动机与目标

• 利用恒星和太阳吸收光谱法测量北纬32°处中间层钠柱密度。
• 将钠柱密度与3 W连续波激光产生的钠激光导星的绝对通量相关联。
• 确定激光偏振态（线性与圆偏振）对激光导星亮度的影响。
• 提供一种直接、准确的方法，基于柱密度变化预测实现特定导星视星等所需的激光功率。
• 表征中纬度站点钠柱密度的季节性和昼夜变化特征。

提出的方法

• 通过从MMT望远镜和CFA 60英寸望远镜观测的恒星和太阳光谱，利用吸收线光谱法测量钠柱密度。
• 使用波长锁定在钠D2超精细结构峰值的3 W连续波激光，同步开展激光导星实验。
• 在相同柱密度条件下，比较线性偏振与圆偏振激光束的荧光回波强度。
• 利用测得的柱密度，推算实现目标导星亮度所需的激光功率。
• 采用直接吸收法，避免依赖激光雷达技术，以提高柱密度估算的准确性。
• 对全年数据进行分析，评估钠丰度的季节性和昼夜变化。

实验结果

研究问题

• RQ1 北纬32°处钠柱密度的季节性和昼夜变化如何？
• RQ2 钠柱密度与钠激光导星的绝对通量之间存在何种关系？
• RQ3 与线性偏振光相比，圆偏振激光光是否显著提高激光导星的亮度？
• RQ4 测得的柱密度能否准确预测实现目标导星视星等所需的激光功率？
• RQ5 与传统激光雷达技术相比，吸收法在测量中间层钠柱密度时的准确性如何？

主要发现

• 北纬32°处钠柱密度在单小时内可出现高达两倍的昼夜变化。
• 北纬32°处的季节性变化幅度小于高纬度地区，且年均柱密度低于高纬度地区。
• 对于线性和圆偏振光，荧光回波通量均与同步测量的钠柱密度呈线性正比关系。
• 在相同条件下，圆偏振激光光比线性偏振激光光的荧光回波强度高出约30%。
• 在钠柱密度为3.7×10⁹ cm⁻²时，1 W的圆偏振激光束可产生视星等R = 10.3等的激光导星，其绝对通量为8.4×10⁵ photons/s/m²。
• 与传统激光雷达技术相比，直接吸收法能更准确、更简便地测量柱密度。