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THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE (SETI) IN THE OPTICAL SPECTRUM
by Dr. Stuart A. Kingsley
Fiberdyne Optoelectronics, Columbus, Ohio
From the author of the January 1992 six-part EJASA (THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC) article (Vol. 3, No. 6A-6F) on Optical SETI (OSETI).
The author would like to acknowledge that this Electronic Journal has been instrumental in the organization of this conference, for without last January's publication, this author would not have been invited by SPIE to put this conference together.
You are encouraged to remail this material to anyone you know with interests in SETI or to print it out and pin it up on your astronomical society, company, faculty, or school notice board. Some of the following material was featured in the October 1992 issue of EJASA (Vol. 4, No. 3) and the December 1992 issue of SPIE's OE REPORTS:
Almost three years ago I began "lobbying" the scientific community to reconsider the optical approach to the electromagnetic search for extraterrestrial intelligence (SETI), first described by Nobel laureate Charles Townes (1964 - masers/lasers) in 1961. Unfortunately, many of the strongest proponents for electromagnetic SETI have become dogmatic and will not countenance open discussion of alternatives to microwave SETI, believing that the issue of the relative efficiencies of microwave and optical SETI was settled years ago in favor of microwaves. Optical SETI has received very poor press ever since the skewed ETI laser transmitter assumptions in the Project Cyclops report, two decades ago. This NASA design study report described a microwave array consisting of up to nine hundred 100-meter diameter dishes which if fully built would have occupied an area 6.4 kilometers in diameter and cost, in 1970s currency, some ten billion dollars. This grand project was never fun- ded but the report itself has had a profound effect on SETI thinking.
In the comparison table that appeared on page 50 of that report, the optical system modeled described an interstellar laser communication system that employed a 1.06 micron Nd:YAG laser, and a 22.5 cm diameter transmitting telescope. There were various reasons for limiting the aperture of the ETI transmitter. One reason arose out of unnecessarily constraining both parts of the system to operate on a planetary surface, within an atmosphere, and thus be limited by the atmospheric coherence cell size. Another reason was to avoid the production of beams that were smaller than the zones of life around nearby targeted stars. However, the net effect was to cripple the potential very high Effective Isotropic Radiated Power (EIRP) of the optical transmitters.
It would be far better to build larger transmitting telescopes and defocus them when targeting nearby stars, if, in the unlikely event, the ETI civilization did not possess the technical prowess to aim narrow beams into nearby stars. In this way, the long-range EIRP would not be unnecessarily degraded. The modeled "toy" ETI uplink telescopes put the onus on the young and technically immature receiving civilization (humanity) to build very large and expensive telescopes to receive weak signals from mature ETI civilizations, instead of the converse. It was this part of an otherwise excellent report that so distorted its conclusions concerning the efficacy and relative cost (to us) of the optical approach to SETI.
If we allow for transmitting and receiving apertures to be 10 meters in diameter or larger, it can be shown for transmitter powers comparable to those for microwave systems that relatively small diffraction- limited laser systems are capable of supporting far higher signal-to- noise ratios and data rates than the much larger microwave systems. The extremely high gains of optical antennas more than make up for the additional quantum noise and stellar background radiation noise. The lack of (currently) easily identifiable "magic optical frequencies", equivalent to the microwave waterhole between 1.420 and 1.662 GHz, save for the major CO2 transition at 10.6 microns, is not a reason to conclude that ETIs would not use lasers to signal Earth. Indeed, the effectiveness of pulsed laser signals is so high that there is less need to be concerned about the exact laser frequency.
Indeed, from the viewpoint of communications with extraterrestrial intelligences (CETI), which is not presently being proposed, terrene SDI lasers of the late Twentieth Century are certainly capable of "reaching out and touching ETIs" across one thousand or more light years. The problem today is that we do not know where to point our lasers and we lack the means to provide precise forward predictive targeting of extrasolar planets. Even more basic to this problem is that direct visual observations of other planets around nearby star systems await the technological developments of the next century. In the meantime, we must take the passive and perhaps safer approach of listening for ETI signals.
Let it be noted here that the word "optical" is used in a manner familiar to optoelectronics (photonics) engineers and scientists, as an umbrella term. It is a superset of both "visible" and "infrared." The word "optical" is not to be taken as being synonymous with the word "visible", since the former (for communication engineers) covers all electromagnetic frequencies from the far-infrared to the ultra-violet.
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