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The Search for Extraterrestrial Intelligence (SETI) in the

Optical Spectrum


Picture Showing OSETI I Workshop Panel

From left to right:  Dr. Barney Oliver, Dr. Mike Klein, Dr. Kent Cullers, Professor Frederick Johnson, Professor Charles Townes (Moderator), Dr. Stuart Kingsley (Chairman), Professor Neil Tennant, Monte Ross (Co-Chairman) and Dr. James Lesh.

Hear this entire workshop in RealAudio realaudo.gif (1207 bytes)


Charles Townes: Come up around the table. Barney, you too, come and join us. I'm not sure what we are supposed to do here [laughter], except that it is an opportunity to round out the conference and see how people react and what they feel that the conclusions are, and to bring up additional points that you feel need to be clarified or added. I think all the people here at the table should keep that in mind, say anything you feel would be useful at this point. I suggest that three or four people at the table say what they like and then hesitate for the audience to pursue these a little further, then we will go on down the line. Let's start at the end of the table.

Jim Lesh: I don't know how many people in the room are communicators. I know that a lot of people are interested in detecting signals and searching the skies. I think that a lot of people come from astronomy backgrounds. A lot of the discussion I heard did not have the same language that communicators tend to use all the time. I thought perhaps I might be able to add something from a communication flavor, just to try to give you a different perspective of things.

JPL has for years sent spacecraft out and tried very hard to communicate data back. Now, this is an intentional communication, where they were trying to send data back from deep space, and of course that is a tough job to do. What we find is that as time goes on, the frequencies have continued to increase, and the aperture sizes have continued to increase. In other words, the directivity for sending that information back has been on the rise all along since we started.

To show where this can go, this is one of the slides that I use in an optical communications presentation, and it just shows the benefit of going to optical from a real practical standpoint. What I am showing here is the Voyager spacecraft with its RF X-band antenna for transmitting back from Saturn. It produces about a thousand earth diameter beamwidth by the time that it gets back to the earth over 10 AU. That's with a 3.7 meter antenna. Instead, if you take a telescope about "that" big at optical and transmit it back, you get about one earth diameter. That's why we are developing the technology for such systems.

I heard talk about 10 meter systems, sometimes they were referred to as Keck telescopes. We have in the plan right now the construction of a large 10 meter photon-bucket system. This would have a much worse aperture surface quality than the Keck telescope, and the whole purpose is to drive costs down. You don't need to preserve phase across the aperture if what you are trying to do is to detect very weak photon pulses coming back.

We also have some work in narrow-band filters to get rid of a lot of the background noise you have to contend with, whether that background comes from our Sun or somebody else's Sun. This, for example, is a Faraday rotation filter that has a bandwidth that can be selected out that's about 1 GHz wide. It has quite high throughput, projected to be as high as 80% total throughput efficiency of that particular filter. So, the technologies are coming along for that.

I'd also like to say that in some of the modelling that's being done, there are some concerns that I have and those concerns stem around two things. One, is the fact that if you take a look at - I hear a lot of people talking about - quantum noise, etc. There are some misconceptions that I'd like to have you at least think about. If we take an optical signal, and turn it on, a beam, a laser, a CW laser, let's say, and if we have a fairly high intensity, then because of the quantum nature of that signal, you will actually see a fluctuation. We call that "quantum noise".

Typically, what happens is that people say "that's roughly the square root of the intensity, so we can calculate a signal-to-noise ratio (SNR)". In other words, there is a mean signal and there is a fluctuation that we call noise. A lot of people use that SNR as one of their characteristic parameters of interest.

In fact, there are other scenarios. Another scenario is that we are sending back in a fairly background- or noise-free environment a series of pulses, and those pulses are intentionally sent back. By the time they get back to the target they are fairly weak. The number of photons that are in there are fairly small. So, I am showing here an average intensity of let's say two photons per pulse. If you choose to look at this signal as being a mean level with a quantum fluctuation around it, that's your choice. But I can tell you that from the standpoint of a communications system, that's not the right way of looking at it. In fact, the primary way that this system in a communications system makes errors is because occasionally when you send a pulse that may have an average intensity of two photons, actually, quantum fluctuations result in zero photons getting to the receiver. That's called an erasure, and the probability of that event occurring is e-lambda, which is the intensity of the signal.

The point that I am trying to make here is that when you compute communication performance, or if you are talking about signal detection performance in the communication sense, that SNRs can be extremely misleading. That the detection statistics depend on the intensity of the signal and on the intensity of the noise or the background that you deal with; not just the ratio of those two. So, it is important to be aware of this.


Copyright , 1993, SPIE

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