Fraunhofer Absorption Lines

9008-075

In certain circumstances, the wavelengths corresponding to the Fraunhofer absorption lines in the stellar continuum may be preferred for communications because they may slightly increase the obtainable signal-to-noise ratio.

Table 9006-025 and the summary charts of 9008-073 & 9008-074 indicate some of the more significant Fraunhofer lines. It will be recalled that for analysis purposes, the H line at 656 nm was chosen for the modelled visible laser frequency. The typical absorption profile of this line from a quiet Sun is such that the minimum intensity in the dark line is about 20% of the intensity of the general continuum. The so-called dark line is really not that dark, and in the case of the H line, amounts to a contrast ratio of only 5:1. Thus if a ETI transmitter operates at this wavelength, the Signal-To-Planck Ratio (SPR) within the dark line is increased by about 7 dB.

Typical Solar-Type Spectra

Except in the case of incoherent communication links, and coherent communication links operating within the Solar System where the Planckian noise is very intense, this SPR improvement has no effect since the noise-floor is set by signal quantum noise.