It is possible that aliens have been beaming high-power relatively wide-band signals towards Earth in the visible or near visible region of the spectrum for some considerable period of time, only that we have been looking at the wrong wavelengths, totally oblivious to what is out there in the optical region of the electromagnetic spectrum. Present microwave SETI systems assume sub-hertz to a few kilohertz type bandwidths. However, the potential of optical SETI is that in order to facilitate the maximum efficiency in communicating data, the bandwidths employed may be quite large.
Indeed, it is possible that slow-scan or real-time TV type signals occupying bandwidths of about 0.1 to several MHz are being sent, as these signals would be the easiest to interpret. Such high data rates would be possible if the laser transmitting power is increased from 1 kW to 100 kW, or even 1 MW. This power level would not be unreasonable for a society only slightly more technically advanced than our own, and most alien technical civilizations (if they exist) will be far more advanced. Thus, most Microwave SETI approaches may be misguided in looking for the odd "bits/second" - rather we should be looking for powerful and fast data streams!
After all, if you wished to upload or download, depending on one's perspective, vast amounts of information concerning one's civilization, culture and history, would you only "speak" at a bit/second if you had the means to speak faster! Note that in high power Optical SETI systems, high bandwidths tend to be preferred because of laser linewidth and Doppler shift considerations. Since linewidth is inversely proportional to laser oscillator power, high power transmitting lasers will tend to have very narrow linewidths. The receiver's local oscillator, if its the same type of laser, will inherently have a greater linewidth.
Continuous Wave (CW) powers in the gigawatt levels may be possible for an advanced civilization, which would produce an EIRP > 2 x 1024 W, and allow data rates approaching a GHz to be achieved over 10 light years. At this power level, which is about two orders of magnitude less energy than leaves our Sun through every square meter of its surface every second, the effective Apparent Stellar Magnitude of the signal at 10 L.Y. is about +8, still too low to be seen by the naked eye. We assume of course, that the alien civilization has the means to construct laser transmitters of such power without incurring mirror damage due to the very high flux densities.
Note that other, more sophisticated modulation schemes, e.g., frequency, pulse or digital, could be employed to increase the signal-to-noise ratio levels by exchanging bandwidth for SNR. Since the SNR is inversely proportional to the square of the distance, directed communications up to 1000 light years distance should be possible if one has the patience for a reply! This range encompasses over one million solar type stars.
Alternatively, instead of using the extra laser power to increase the data rate in any particular direction in space, the laser could be time and space multiplexed. A powerful alien laser transmitter with a scanning mirror system (or other means) might be able to address many likely star systems (or spaceships/probes) in a time sequential manner, thereby increasing the likelihood that its signal will be received by at least one technical civilization. The multiplexing aspect may be another reason why optical communications would be preferred.
It is even possible that we might intercept communications not meant for us (eavesdropping mode), i.e., communications between other advanced civilizations or between such a civilization and an exploration space-ship in our region of the galaxy. In such a situation, the format of the transmission may be such that we might not be able to decode it or even realize that it was there. Of course, in the centuries to come, other modes of communication may be discovered and developed that are more efficient than the electromagnetic spectrum and even perhaps faster - the "sub-light" mode of communications which is the boon to so many science fiction writers!
The 10.6 µm wavelength of a carbon dioxide laser seems a reasonable wavelength to search for Extra-Terrestrial Intelligence (ETI) if only because of the relatively plentiful supply of CO2 in the planetary atmospheres supporting life, and the high efficiency and narrow linewidth of the laser. Unless dynamic receiving mirrors are used, wavefront distortion due to atmospheric turbulence requires that even the receiving telescope must be operated outside a planetary atmosphere. In this case, the argument that a visible laser wavelength must be employed because of the atmospheric window would not be tenable.
However, not withstanding these comments, the potentially higher gain of a visible laser system, for a given telescope size at both ends of the link, might be sufficiently attractive as to dictate operation in the visible region of the spectrum.
In the light of these findings, the author is even more perplexed that Optical SETI has been ignored and believes that the scientific community may be suffering from "group-think". After all, lasers were invented 30 years ago, yet there doesn't appear to have been a systematic investigation using the technology to see if there are any signals out there.
The Columbus Optical SETI Observatory