It is possible that powerful alien optical transmitters may be hidden behind the light of their stars. We should not assume that if optical signals are being sent that they would be obviously visible, i.e., like heliographs, and at very low data rates similar to morse-code or semaphore. The aliens may have sound logical and philosophical reasons for not making their presence known to other worlds unless these worlds made a determined effort to find such signals. Today we have the technology to "see" what has previously been invisible. In order to narrow down our search of the optical cosmic haystack we will need to make educated guesses as to the most likely laser wavelengths that will be used. This analysis has shown that on a watt for watt transmitter comparison basis, the system performance of Optical SETI should be somewhat better than Microwave SETI, even if we allow for some, say 10 dB, inefficiency in our optical heterodyne receivers.
The Optical Cosmic Haystack illustrated in 9008-020 indicates the magnitude of the task facing us when we extend our "search" into the optical spectrum.
We have also shown that the choice facing the alien civilization in selecting a particular optical wavelength need not be governed by the availability of Fraunhofer lines in its star's spectrum, though it would be sensible to avoid selecting a wavelength coinciding with a solar emission line. The fact that Planck (black body) radiation is substantially less than quantum noise in moderate bandwidths, also implies that it should be possible to use some form of optical parallel signal processing on a star-field to reduce the time required for a search.
Note that with large telescopes outside the Earth's atmosphere, it would be possible to spatially separate (resolve) the image of a nearby star from its planetary system, thereby eliminating the Planck radiation consideration almost entirely.
The analysis indicates that for Optical ETI transmitters at light year distances, the noise floor is set by receiver quantum noise which itself is determined by the local oscillator level. For all intents and purposes, it is unaffected by the level of Planck starlight that passes through the optical pre-filter. Thus, the CNR is independent of optical pre-filter bandwidth. This system behavior is unique to coherent optical receivers, and is one of its major advantages.
This investigation has confirmed that we need to reconsider the present scientific dogma concerning the Search for Extraterrestrial Intelligence. Coherent optical receivers should be designed and installed in some of the world's best telescopes (including the ill-fated Hubble) to further the Optical Search for Extra-Terrestrial Intelligence. Certainly, the 30 year old assumption that the microwave water hole or the microwave region in general, is the natural place to gather, does not withstand the light of this analysis. It appears that the scientific community suffers from myopic vision, as much as the Hubble Space Telescope! There is no doubt that mankind will be forever changed, if as a result of these SETI investigations, be it microwave or optical, we discover that there is a Galactic Communications Network.
The technology for Optical Heterodyne SETI would also be of considerable benefit to the light sciences. Even though a device like the Hubble Space Telescope has sufficient aperture to resolve the planetary system of a nearby star (once its present mirror problem is corrected), scattering imperfections in the telescope, its side lobe suppression factor, and interstellar scattering, limits the ability to separate the planet from the star. The light from a star is far more intense than the light reflected from one of its planets.
An optical heterodyne receiver may allow us to analyze the weak spectra found in the Fraunhofer absorption bands. By looking for Doppler shifted "bright" lines produced by the planet within the "dark" Fraunhofer bands of its star, it may then be possible to filter out and discern the very weak light from the planet, and determine its orbital parameters. If these lines are further modified by a planetary atmosphere, we could then determine the constituency of that atmosphere and also obtain information about the planet's magnetic field.
One advantage of Optical SETI has not been mentioned, and that is that for the moment, there is almost no chance that if a signal is received it will be of terrestrial origin.
When Dan Rather got mugged in New York a few years ago the mugger asked him a question, which until this day has remained a mystery - "What's the frequency?". We have yet to find out!
When we at last receive our subscription to the "Galactic Central Office" we will be able to reach out and touch an alien!
The Columbus Optical SETI Observatory