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Introduction to Optical SETI

 

The electromagnetic spectrum (SETI Institute). (5243 bytes)

 

 

The relative position of Sol in our galaxy (SETI Institute). (14349 bytes) In the summer of 1990, the author commenced revisiting the optical approach to the Search for Extraterrestrial Intelligence (Optical SETI), an idea almost as old as modern-day Microwave SETI.  Star Trek has an impressive history of stretching the viewer's imagination with regard to the possibilities that exist out in deep space.  The author has long believed in the existence of Extraterrestrial Intelligence (ETI), an idea that Star Trek's ingenuity has made seem almost obvious, and hopes that one day it will even become possible for mankind to travel between the stars - that we will not be locked forever within the confines of our star system.  However, if it is not found to be possible to travel at warp speed, discover worm holes that might be used as short cuts through the back of the universe or develop so-called "sub-space communications", then electromagnetic communications will probably be the only means we have to establish contact with other intelligent civilizations.  Nevertheless, compared with actual space travel, the development of laser technology by ETIs is trivial and should be non-taxing to their technical prowess.   Free-space laser communication for them would be no more difficult or expensive than its microwave counterpart.  At any rate, in an infinite universe, with infinite possibilities, it might well be that while some civilizations invent "warp-speed" technology or discover "worm-holes", others stay home and just communicate.

 

Antenna Gain Versus Diameter (12520 bytes)

 

Wide beamwidth microwave beacon signal. (3635 bytes) The age of Microwave SETI can be said to have started in 1959, while the still-born Optical SETI first saw the light of day in 1961.   For various reasons, by the early '70's, SETI rationales became based on the microwave approach, and the viability of the visible or infrared laser approach had largely been discounted.  A review of the SETI literature of the past 34 years appears to indicate that the report on "Project Cyclops" had much to do with the subsequent lack of interest in the optical approach to SETI.  The comparisons in that report between the various forms of electromagnetic SETI severely limited the potential antenna gain advantage afforded by optical telescopes.  In those days, the word "optical" was still synonymous with the word "visible".  However, over the past two decades, as the fiber-optics industry has matured, "optical" has been redefined to cover the entire electromagnetic regime from the far-infrared to the ultra-violet.  The technology of "Photonics" or "Optoelectronics" is the major force that is shrinking our planet today, aka the "Global Village".   It is in this redefined sense that the word "optical" is used here - a superset of both the visible and infrared regimes.  Perhaps within the Milky Way, the Encyclopedia Galactica is being exchanged between civilizations on an optically-based Internet, but we have yet to find or establish a node!  For sure, the future of terrestrial communications is one that is largely photonics based.  Some time in the next 30 years, men will for the first time walk upon the surface of Mars.  The historic High Definition (HDTV) pictures of that event will be transmitted back to Earth via a laser link, probably from the Martian surface to a Martian orbiter via laser, from the orbiter to Earth via laser and around the Earth via laser-linked geostationary satellites and into peoples' home via fiber-optics.  Truly, the future is photonic.   Photonic for our terrestrially-based future and so too for our space-based future.   To argue otherwise is to fly in the face of reason.

 

1 kW beacon signals at 100 light years (16262 bytes)

 

An optical beacon! (14979 bytes) Initially, the author compared the effectiveness of free-space interstellar communications using visible and infrared lasers operating in a very monochromatic continuous wave (cw) mode and with the use of heterodyne detection techniques, with that of radio-frequency systems.  By January 1993, after communication with Monte Ross, and SPIE's first international conference on Optical SETI, the author decided to concentrate on the assumption that most ETIs employed pulsed lasers and that our receiving systems should employ incoherent photon-counting techniques.   It can easily be shown that pulsed lasers are very effective for both beacon signals and wideband channels.  The main advantage is that the high peak to mean power levels, allow for the received pulses to easily overcome the effects of photon noise caused by the sky and stellar backgrounds, without the need to employ narrow-band optical filters.  This means that there is no need to postulate a "magic frequency" or "magic wavelength" - only a "magic wavelength regime".  It also obviates the need to employ complex and expensive coherent optical heterodyne techniques with local oscillator lasers.

 

Detectability of pulsed laser beacons (20068 bytes)

 

palomar.jpg (2003 bytes) Sometimes it is overlooked that most discussion of SETI involves the reception of "beacon signals", not the more wideband channel that would convey rich data.  That there should be some connection between the "beacon" channel and the "data" channel is often ignored. It is the latter that will probably govern the placement of both signals in the electromagnetic spectrum.  Indeed, without full consideration of how a wideband signal can be transmitted over interstellar distances, we are likely to draw erroneous conclusions about so-called "magic wavelengths or frequencies".

 

Acquisition Signal (11078 bytes)

 

The above is based on the introduction from Stuart Kingsley's paper in the OSETI II Proceedings:

  

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