Dependence of sodium laser guide star photon return on the geomagnetic field
Dependence of sodium laser guide star photon return on the geomagnetic field
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DOI：
10.1051/0004-6361/200811411
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发表时间：
2009-07
影响因子：
6.5
通讯作者：
N. Moussaoui;R. Holzlöhner;W. Hackenberg;D. B. Calia
中科院分区：
文献类型：
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作者：
N. Moussaoui;R. Holzlöhner;W. Hackenberg;D. B. Calia
目标。在连续波激光导星（LGSs）中，增加中间层钠原子背散射发射的光泵浦效率会显著降低，在最坏的情况下，地磁场的作用会消除光泵浦的效率。我们的目标是对几个望远镜点的这种效应进行估计。方法。钠原子在绕磁量子数循环的磁力线周围进动，降低了光泵浦的有效性。我们的方法是基于计算地磁场存在下的钠磁亚能级种群和在Starfire光学距离（SOR）上进行的钠LGS辐射返回的实验测量。结果。我们提出了一个相对简单的半经验公式，用于估计地磁场对圆偏振连续波单频激光束光抽运增强LGSs光子返回的影响。从地磁场效应的计算结果与实验测量结果吻合较好出发，为了更真实地估计钠离子LGSs的光子返回，我们引入了到中间层钠离子层的距离和大气衰减的影响。这三个因素的综合影响，计算了几个望远镜站点。结论。在计算LGSs的返回通量时，通常只假设最佳返回条件，依赖于圆偏振激光器的强光泵浦。然而，人们只能在天空中沿着一个特定的激光方向获得这种最佳回报，在这个方向上，地磁场线与激光束平行。对于大多数望远镜，在超出观测极限的望远镜方位处可以获得最优。对于靠近地磁极的望远镜来说，光泵浦的好处比靠近地磁赤道的望远镜要重要得多。
Aims. The efficiency of optical pumping that increases the backscatter emission of mesospheric sodium atoms in continuous wave (cw) laser guide stars (LGSs) can be significantly reduced and, in the worst case, eliminated by the action of the geomagnetic field. Our goal is to present an estimation of this effect for several telescope sites. Methods. Sodium atoms precess around magnetic field lines that cycle the magnetic quantum number, reducing the effectiveness of optical pumping. Our method is based on calculating the sodium magnetic sublevel populations in the presence of the geomagnetic field and on experimental measurements of radiance return from sodium LGS conducted at the Starfire optical range (SOR). Results. We propose a relatively simple semi-empirical formula for estimating the effect of the geomagnetic field on enhancing the LGSs photon return due to optical pumping with a circularly polarized cw single-frequency laser beam. Starting from the good agreement between our calculations and the experimental measurements for the geomagnetic field effect, and in order to more realistically estimate the sodium LGSs photon return, we introduce the effect of the distance to the mesospheric sodium layer and the atmospheric attenuation. The combined effect of these three factors is calculated for several telescope sites. Conclusions. In calculating the return flux of LGSs, only the best return conditions are often assumed, relying on strong optical pumping with circularly polarized lasers. However, one can only obtain this optimal return along one specific laser orientation on the sky, where the geomagnetic field lines are parallel to the laser beam. For most of the telescopes, the optimum can be obtained at telescope orientations beyond the observation limit. For the telescopes located close to the geomagnetic pole, the benefit of the optical pumping is much more important than for telescopes located close to the geomagnetic equator.