Is Anyone Out There

While preparing last monthcolumn we began to think about the various unknown properties of transmission in and beyond the THz region. You will remember that at the beginning of radio (as we know it) the frequencies above a MHz or so were considered useless and relegated to the amateurs. It took our ranks (as well as many avid experimenters) to achieve transatlantic communications on the frequencies well below 200 meters (which we then lost, but that is another story). As a result, I would like to pass on the results of a couple of experiments I performed that might perhaps goad you into experimenting in this region. As regular readers of this column are aware, my company is involved with the design and manufacture of fiber-optic transmission equipment. These transmitters and receivers use optical wavelengths between 850 nm and 1550 nm, well within the THz region but lower in frequency than visible light. Our various modulation schemes, however, involve turning a laser (or LED) on and off for digital modulation (basically CW, but at rates that could go as high as a GHz) or varying its intensity (essentially AM) for non-sampling analog signals. There is no finesse or true FM yet (which in the optical world would amount to modulation”). I wonder if the technologically more advanced civilizations that are most assuredly there”use something along those lines and unique propagation modes that allow extreme distances to be covered. It certainly seems like a reasonable assumption, so as a result letlook at how we might proceed to search in this region.

LEDFig. 1 is our first experiment. Here we have coupled a laser to an optical fiber (which is really just a waveguide for light) and the output of the fiber to a lens assembly which focuses the resulting beam onto a photodiode. In a similar manner, we have coupled a second laser to another lens assembly and directed its output beam onto the same photodiode. The output from the photodiode therefore is the sum and difference of the frequencies (wavelengths) of the two lasers (as well as their individual frequencies). The sum and the individual frequencies are obviously well beyond the range of the photodiode, but since the lasers are basically at the same frequency, the difference can be in the MHz or GHz region, one we (and the photodiode) can readily deal with. If we then modulate one of the lasers with audio, the photodiode should reproduce the audio.

This, by the way, is without any specific AM detection circuitry, but simply because of the fact that the result will be the difference between laser frequency 1 and laser frequency 2. The problem is that the lasers are not at all stable in the kHz range. As a result, one can only get slight occasional bursts of audio with this arrangement, since the lasers drift rather rapidly with respect to each other. To reiterate, this sporadic output is, of course, the difference frequency. But what about the sum? It still exists, but it is well into the THz region. The problem now is to build a practical detector for it. None of the common electronic components we currently possess will work beyond a few hundred GHz, so it seems that we are stuck (for the moment). To double the frequency of a laser (like any electromagnetic radiation generator) one usually needs a non-linear device. In the case of low-frequency RF this is a job that can often be accomplished with a diode. In the optical region certain nonlinear crystalline materials such as lithium niobate (LiNb03) exist and can be (and in fact are) used for this purpose. LiNb03 is rather expensive and somewhat rare. However, a couple of companies occasionally do have small samples available. If you search the web you should be able to find them. Because this material is non-linear it can be thought of as an diode.”

LEDThis leads us to experiment 2, as shown in fig. 2. Here we used a laser as a sort of optical local oscillator coupled to a small piece of LiNb03 through a short length of optical fiber and a lens. Coupled to the same piece of LiNb03 is another optical fiber and lens. The far end of this fiber is at the focus of a large, long focal-length lens assembly. This entire arrangement is in essence an optical super-heterodyne receiver. The long focal-length lens assembly is really a telescope, and in fact, for our experiments we actually used a 6-inch Newtonian telescope which we had built many years ago when the astronomy bug bit. Also coupled to the LiNb03 crystal is our high-speed photodiode. The basic theory of this arrangement was that if a signal was received (from there”), it would mix with the output of the laser and produce an intermediate (IF) frequency that could be detected by the photodiode and then amplified by a high-gain, wide-band amplifier). Again, the frequency stability of the laser was marginal, but we had to at least try!

Believe it or not, we actually scanned the sky with this arrangement for a while and for the most part received nothing more than static or occasional bursts of noise. Last April 1st (by coincidence), however, momentarily we did actually get something (which was clearly a signal of sorts), but it was not intelligible (to us) and unfortunately we could not repeat the results even though we tried again for several days. I donknow what we actually stumbled upon or whether the timing was right,”but perhaps you will be luckier than we were. I also donknow if LiNb03 is necessarily the best crystal to use, and I encourage you to experiment with others such as quartz, various precious and semi-precious stones, or whatever you might have or imagine. Who would have thought that lead sulfide (galena) could detect radio waves in 1800? There is definitely something or someone out there, of that I am convinced. The first person or group to intercept extraterrestrial communications could easily be someone within our ranks who is not afraid to experiment, tinker, and dream! It is now exactly one year since my result”and another April 1st has come. Donyou think ityour turn? Look for yourself information about indie games? But all the same? How do I know what you mean, until recently, he was the same. But here's a great solution forum is a place where you can chat with other people about it.

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