Efficient excitation of a mesospheric sodium laser guide star by intermediate-duration pulses
Efficient excitation of a mesospheric sodium laser guide star by intermediate-duration pulses
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DOI：
10.1364/josaa.11.000832
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发表时间：
1994-02
影响因子：
1.9
通讯作者：
J. Morris
中科院分区：
文献类型：
--
作者：
J. Morris
本文计算了激光导引星星中中间层钠原子在脉宽为30 ns ~ 0.9 μs的脉冲激发下的后向散射辐射。在饱和辐照度值下有效使用这种脉冲需要~3 GHz的光谱展宽，以获得D2线的全部吸收光谱。通过频率调制提供加宽。使用一组密度矩阵来解释所有24个超精细态和非均匀多普勒展宽。在宽带（3GHz）饱和辐照度为4 W/cm ~ 2时，线偏振和圆偏振激光束的发射率均超过了0.9 μs脉冲最大速率方程速率的60%。正如预期的那样，圆偏振产生更多的反向散射比线偏振，但增强从来没有超过1/3的计算报告。对地球磁场中状态进动的简要估计表明，即使实现这种增强也需要将光泵浦的时间尺度保持在1 μs以下，这将需要使用大于0.7 W/cm 2的辐照度和整个3 GHz超精细加多普勒吸收轮廓的光谱覆盖，至少直到大多数布居被泵出F = 1基态。
Calculations of backscatter emission of mesospheric sodium atoms in a laser guide star that is excited by pulses ranging from 30-ns to 0.9-μs duration are described. The efficient use of such pulses at saturating irradiance values is shown to require ~3 GHz of spectral broadening to provide access to the full absorption spectrum of the D2 line. The broadening is provided by frequency modulation. A set of density matrices was used to account for all 24 hyperfine states and inhomogeneous Doppler broadening. At the broadband (3-GHz) saturation irradiance of 4 W/cm2, both linearly and circularly polarized laser beams are shown to produce emission rates exceeding 60% of the maximum possible rate-equation rate for the 0.9-μs pulses. As expected, circular polarization produced more backscatter than did linear polarization, but the enhancement never exceeded 1/3 in the calculations that are reported. A brief estimate of state precession in the Earth’s magnetic field suggests that achieving even this enhancement will require that the time scale for optical pumping be held to less than 1 μs, which will require the use of irradiances greater than 0.7 W/cm2 and spectral coverage of the full 3-GHz hyperfine-plus-Doppler absorption profile, at least until most of the population is pumped out of the F = 1 ground states.