星际通讯。X.光学SETI的颜色
Interstellar communication. X. The colors of optical SETI
摘要：
最近有人从激光工程学的角度提出，光学SETI只有几种神奇的颜色。这些主要是1064 nm的Nd：YAG谱线及其二次谐波532.1 nm。次佳选择将是Nd：YAG和Nd：YLF激光线的和频和/或二次谐波产生，393.8 nm（接近Fraunhofer CaK）、656.5 nm（H $α$）和589.1 nm（NaD 2）。在本文中，我们研究了星际消光，大气透明度和闪烁，以及这些激光线的噪声条件。对于强信号，我们发现，光学波长是最佳的距离$d\lesssim\,$ kpc。在大多数条件下，$λ=1{,}064\,$ nm处的Nd：YAG也是一个类似的好选择，在2倍之内，并且超出$d\lesssim3\,$ kpc。对于较弱的发射器，其中的信噪比相对于混合的主机星是相关的，最佳波长取决于背景源，如恒星类型。夫琅和费谱线虽然提供较低的恒星背景噪声，但在大多数使用情况下是不相关的，因为它们被其他因素压倒。激光推动的太空飞行概念，如“突破性的Starshot”，将产生比OSETI以往任何时候都更明亮和更紧密的光束。这样的激光飞船在kpc的距离外看起来就像是肉眼可见的恒星。如果激光物理学已经成熟，并集中在最有效的技术，激光线的选择，为给定的情况下（例如，Nd：YAG（用于强信号）可以通过窄滤光片进行观察，以显著降低背景噪声，从而在快速勘测中实现大视场观察。
英文摘要：
It has recently been argued from a laser engineering point of view that there are only a few magic colors for optical SETI. These are primarily the Nd:YAG line at 1064 nm and its second harmonic 532.1 nm. Next best choices would be the sum frequency and/or second harmonic generation of Nd:YAG and Nd:YLF laser lines, 393.8 nm (near Fraunhofer CaK), 656.5 nm (H$α$) and 589.1 nm (NaD2). In this paper, we examine the interstellar extinction, atmospheric transparency and scintillation, as well as noise conditions for these laser lines. For strong signals, we find that optical wavelengths are optimal for distances $d\lesssim\,$kpc. Nd:YAG at $λ=1{,}064\,$nm is a similarly good choice, within a factor of two, under most conditions and out to $d\lesssim3\,$kpc. For weaker transmitters, where the signal-to-noise ratio with respect to the blended host star is relevant, the optimal wavelength depends on the background source, such as the stellar type. Fraunhofer spectral lines, while providing lower stellar background noise, are irrelevant in most use cases, as they are overpowered by other factors. Laser-pushed spaceflight concepts, such as "Breakthrough Starshot", would produce brighter and tighter beams than ever assumed for OSETI. Such beamers would appear as naked eye stars out to kpc distances. If laser physics has already matured and converged on the most efficient technology, the laser line of choice for a given scenario (e.g., Nd:YAG for strong signals) can be observed with a narrow filter to dramatically reduce background noise, allowing for large field-of-view observations in fast surveys.
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