Term Paper: Communications Broadband in Space: Nanowire Applications

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Communications

Broadband in Space: Nanowire Applications in Interplanetary Communications

New applications for developments in nanotechnology seem to multiply faster than we can keep up with them. Nonetheless, it is good to note that some new examples of nanotechnology can be turned to existing problems that plague the communications industry. Nanowires, for example, show incredible promise for revolutionizing certain aspects of telecommunications. Nanowires are, quite simply, wires that are constructed on the scale of nanometer -- 10-9 meters. In general, nanowires have structures that are limited to widths that can be measured in a handful of nanometers, though they may be as long as is required for the desired application. At this incredibly small scale, nanowires are affected by quantum effects, which increase the unpredictability of the material but also opens up new uses and applications. At that scale, nanowires that are properly designed and deployed are capable of detecting individual photons, a fact that makes the technology aptly suited for the development of new communications technology.

It may not be immediately obvious how nanowires can be of direct benefit as a communications technology. Everyday information is transmitted around the globe at a blinding pace. The potential uses for nanowires isn't necessarily apparent, save perhaps in obscure situations, or if we are referring to applications in computer chip technology. But that sidesteps the direct relevance of nanowires to the communications industry. In fact, in this essay, I will examine a very particular manifestation of the nanowire, developed at MIT, which has important potential benefits as a communications technology. These nanowires, organized into structures known as superconducting nanowire single-photon detectors (SNSPDs), are incredibly useful for one type of telecommunication: interplanetary communication. Interplanetary communication, because of the incredible distances involved requires equipment that is exceptionally sensitive and able to detect relatively weak signals over long distances. What's more, as in all aspects of communications technology, there is a constant push towards developing technology that is able to transmit more data than before at faster rates than previously experienced.

This communications issue isn't one that generally faces the general public. Interplanetary communication is a relatively narrow branch of specialization. As such, general knowledge about the problems in communication facing interplanetary communication aren't as obvious as for other communication issues: like improving the technology that extends the reach of our cellular networks. Interplanetary communication mundanely affects orbital spacecraft like the Space Shuttle, as well as all of the satellites currently in orbit around the planet. More exotically, interplanetary communication difficulties affect long-distance probes sent to other planets in the solar system -- as well as the possibility of future manned missions to those destinations. Especially in the case of manned interplanetary missions, the importance of fast, information-rich communications technology could be crucial to the lives of the individuals participating in the mission. Over such incredible distances, even minute communications problems are magnified significantly, increasingly undermining signal integrity and data transmission rates. When lives are at stake, these losses should be deemed unacceptable.

Currently, interplanetary communication is handled primarily by radio frequency (RF) transmissions. This is an obvious technology for any communication application, but one that is fraught with difficulties in the context of interplanetary communication. One of the most pressing issues is the fact that RF antennae are relatively large and heavy. Sending this type of technology into orbit, or on missions throughout the solar system, means that the cost of launching these items into orbit is significantly higher than for smaller technologies (Berggren and Kerman). What's more, RF transmissions don't boast very high data transmission rates. True, RF signals travel at the speed of light -- the universal speed limit, so to speak -- but that does not mean that RF signals are capable of densely transmitting large amounts of information. The speed at which an RF signal travels is irrelevant to the amount of information that said signal can carry during its travel. Information density is entirely a product of the wavelength of the transmission medium.

Consider this example for technical context. The current Mars Odyssey orbiter uses RF transmission technology to send information about the mission back to controllers on Earth. The transmission rate for the signal is a mere 128 kilobits/second. By contrast, the optical laser that was designed for the Mars Telecommunications Orbiter -- a mission that was not launched -- would have sent information back to Earth on an infrared (IR) laser that could have transmitted between 1 megabits/second and 30 megabits/second, depending on the distance between the two planets at the time of transmission (Groshong). The difference in the density of the data transmission had nothing at all to do with the speed at which the signal could cross the distance, but instead how much information could be packed into the signal. The IR laser that the Mars Telecommunications Orbiter would have used transmitted at a wavelength of 1.06 microns; this is a wavelength thousands of times shorter than an RF signal.

In other words, the shorter wavelength means better data compression and more information being transmitted in the same amount of time. The slower data rates of an RF signal, combined with the cost of putting the technology in orbit, makes it a less than ideal prospect for interplanetary communications (Groshong; Berggren and Kerman). True, the technology has been in use for some time, but so have coal-fired power plants. This fact does not stop us from trying to develop alternative energy sources that are able to more effectively, efficiently, and with less environmental damage produce electricity. The same is true for communications technology; RF interplanetary communications devices continue to operate effectively, but the improved efficiency of optical transmission devices is so great that researchers have turned their focus toward the development of photo-detectors capable of receiving information-rich data transmissions via optical laser over interplanetary distances.

Obviously, then, optical communications technology seems to be the ideal method for improving the state of interplanetary communications. Optical technology promises smaller transmitters not to mention higher data transmission rates. If researchers can produce very sensitive photo-detectors, then the benefits will be two-fold. On the one hand, sensitive photo-detectors will mean that the optical lasers used for transmission can be smaller and require less energy to operate. This would mean that the transmission technology could be smaller and consequently less costly to launch into space. In addition, these photo-detectors would be able to receive optical signals that are capable of transmitting data in much denser streams than available for RF transmissions. Existing technology suggests that data transfer rates in the range of 100 megabits/second are achievable. At this rate of transmission, unmanned or manned interplanetary missions would be able to stream video back to Earth live (Berggren and Kerman; Gawel 25).

However, researchers are discovering that the incorporation of nanowires into the detectors used to pick up the optical signal can significantly improve the performance of those detectors and theoretically push data transfer rates even higher and simultaneously make them more reliable. To make this communications technology a reality one needs two basic components: a laser to transmit the data signal, and a photo-detector to receive that signal. Because optical signals transmitted by laser are highly affected by interference -- such as, say, clouds -- the signal that reaches the detector might be highly diffused and much weaker than it was when it left its point of origin. Sensitivity is very important in an effective photo-detector; in addition to helping to minimize the effects of environmental interference, higher degrees of sensitivity can help reduce the size of the transmitting laser required to send a coherent signal (Gawel 25; Groshong).

The problem comes into focus when nanowires are integrated with the photo-detectors in very specific ways. The incorporation of a thin nanowire detector is the key to the overall system's high demonstrated efficiency and sensitivity (Groshong). As should be apparent, there are a number of key factors that will affect the success of any photo-detector, factors that are largely handled by nanowire integration. The most important factors include sensitivity to photonic input, the speed at which the detector can receive information, the amount of optical loss inherent in the design, and the overall simplicity of the packaging required to maintain the system itself. It may never be possible to achieve all of these ends simultaneously. Nonetheless, the SNSPD developed at MIT produces a high degree of success for all of these considerations, with the possible exception of the simplicity of the packaging (Berggren and Kerman). Since it is somewhat unreasonable to expect a developing technology to be ideal in all respects, the fact that the MIT SNSPD is as effective as it is should be taken as an important first step in the development of optical interplanetary communication technology.

The key to MIT's ultra sensitive photo-detector is combination of a superconducting nanowire with an optical photon trap that increases the likelihood that incoming photons will be absorbed and detected by the nanowire (Gawel 25; Groshong; Rosfjord et al. 528). The primary issue… [END OF PREVIEW]

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