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Incoming photons were either reflected off of the initial detector surface or they passed directly through the device without being absorbed by the nanowire filament. Optical transmission losses such as these represented a major stumbling block for researchers intent on improving the efficiency of these systems, which had only been able to reach efficiencies of around 20% (Gawel 25; Rosfjord et al. 528). Efficiencies as low as this do not represent sufficient sensitivity for these photo-detectors to be effectively implemented in interplanetary communications.
The design of the MIT photo-detector is relatively simple from an engineering standpoint. This is especially true considering the high degree of efficiency it is capable of producing. Of course, the simplicity of the design belies the significant calibration the device requires: the nanowire must be cooled to almost absolute zero, the glass gap of the photon trap must be a very specific function of the wavelength of incoming photons, and the use of an anti-reflective coating on the surface of the device is critical. The design consists of a photon trap with a nanowire detector followed by a gap of glass, and then a mirrored surface. The nanowire is wrapped in a tight coil in order to maximize its absorption of incoming photons, and the nanowire is cooled to close to absolute zero, three degree Kelvin to be precise, which transforms the nanowire into a very small superconductor. As a superconductor, the nanowire responds in specific ways to photons that impact the nanowire allowing for detection. The photon trap and the mirrored surface help to contain any photons that aren't initially absorbed, until they eventually contact with the nanowire. The trap itself is a cavity between glass and a gold mirror that reflects those photons that would ordinarily pass straight through the detector or else be reflected away from it. As the mirror reflects the photons back through the cavity and trap, the photons eventually come into contact with the nanowire filament and are absorbed and detected (Gawel 25; Groshong).
The engineering genius of combining a superconductive nanowire with an optical cavity, mirrored surface, and anti-reflective coating on the incoming surface helps the photo-detector significantly minimize the optical losses that have plagued other photo-detectors, even ones that incorporated nanowires (Rosfjord et al. 528). When we look back at our original criteria for an effective photo-detector, we find that the MIT device meets nearly all of our ideal criteria. To refresh, the ideal photo-detector would be sensitive, able to operate at high speeds, minimize optical loss, and would come in an efficient packaging system. The MIT SNSPD is very sensitive, almost three times as efficient as the best photo-detector currently being produced. The detection efficiency for the device is 57% at 1550 nanometers, the wavelength at which broadband data transmissions are sent. At 1064 nanometers, the device is even more efficient: 67% absorption (Rosfjord et al. 528; Berggren and Kerman; Gawel 25). Contrasted with the best efficiency rates of other devices, roughly 20%, this is a remarkable enhancement.
Secondly, an ideal photo-detector should be able to operate at high speeds. In fact, at 1550 nanometers, this SNSPD boasted only a 3-nanosecond reset time between photon detection. The optical loss of the device has also been significantly reduced thanks to the presence of the photon trap, the anti-reflective coating, and other design considerations. Total detection jitter was quite minimal. The only issue that this device has is in its packaging. Because the nanowire needs to take on superconducting properties in other to efficiently register the incoming photons, the nanowire must be cooled to three degree Kelvin, very close to absolute zero. To do so requires an elaborate cryocooler that can reduce the temperature around the nanowire filament to this degree and then maintain that low temperature during operation of the photo-detector (Berggren and Kerman). While this is certainly not as ideal as a pocket-sized photo-detector that requires no intensive cooling, the impressive gains made in receiver efficiency more than make up for the less-than-ideal packaging requirements.
Currently, this is the best IR photo-detector in production in the world. Tests performed on the device at MIT produced incredible results: error-free photon counting communication at a data transfer rate of 781 megabits/second (Bergren and Kerman). Contrasted with RF data transmission rates of only 128 kilobits/second this is a phenomenal increase in efficiency and effectiveness as a communications medium. Even contrasted with existing photo-detector technology, this device represents a significant quantum leap forward. Previous data transmission rates for similar devices were only expected to be as high as 100 megabits/second. A nearly 800% increase in data transmission rates is incredible considering the relative simplicity of the design. The MIT SNSPD, capable of detecting individual photons, makes an ideal photo-detector that could effectively be employed as an interplanetary communications device sometime in the near future.
It is the use of a superconducting nanowire -- of course in conjunction with a well-designed photon trap -- that makes this photo-detector and its high efficiencies possible. The tightly coiled nanowire is the heart of the device and the component that makes individual photon detection possible. Without that kind of at least theoretical sensitivity, engineering a photo-detector capable of receiving optical signals over interplanetary distances would have been most likely a challenge left for a future generation of researchers. The incorporation of the nanowire, cooled to three degrees Kelvin, with superconducting properties allows the detector to pick up very minute incoming signals and interpret the incoming data at a very rapid pace. The addition of a photon trap and other engineering tricks to minimize optical losses, such as anti-reflective coatings, further enhances the efficiency of the device by reducing drastically the number of incoming photons that would have passed directly through the device or else been reflected off of its surface. Once trapped within the device, the superconducting nanowire absorbs and detects the photons as they bounce around within the photon trap.
Advances in communications technology require researchers who can effectively pursue the twin goals of all communications technology: reliability and speed. This maxim is no less true in the context of interplanetary communication than it is in terms of more mundane technologies such as broadband Internet or cellular telephony. While RF transmission have been able to shore up communications on an interplanetary scale, the reliability of this type of transmission has of late been unable to make up for its other deficiencies: namely poor data transmission densities and the necessity of heavy equipment. Optical technology promises improved interplanetary communications that is remarkably faster -- because of information density -- than RF signals could ever hope to achieve. But to make optical technology a reality over the extreme distances of the interplanetary scale, researchers have had to go to another extreme of scale and combine superconducting nanowire filaments with existing photo-detector technology. The results have been incredibly impressive thus far and point the way towards more reliable, efficient, and generally speedier forms of interplanetary communication.
Berggren, Karl K. And Kerman, Andrew J. "Emerging Detector Devices: Nanowires Detect Individual Infrared Photons." Laser Focus World (1 Sept. 2006). 8 Dec. 2007 http://www.laserfocusworld.com/articles/272171.
Gawell, Richard. "Photon Detector Speeds Up Interplanetary Communications." Electronic Design (27 Apr. 2006): 25.
Groshong, Kimm. "Photon Detector Is Precursor to Broadband in Space." New Scientist Space (21 Mar. 2006). 8 Dec. 2007 http://space.newscientist.com/article/dn8877-photon-detector-is-precursor-to-broadband-in-space.html.
Rosfjord, Kristine M., Yang, Joel J.W., Dauler, Eric a., Kerman, Andrew J., Anant, Vikas, Voronov, Boris M., Gol'tsman, Gregor N., and…[continue]
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