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Mitigating Photon Jitter in Optical PPM Communication

Compensation based partly on photon-arrival statistics would yield gain. A theoretical analysis of photon-arrival jitter in an optical pulse-position- modulation (PPM) communication channel has been performed, and now constitutes the basis of a methodology for designing receivers to compensate so that errors attributable to photon-arrival jitter would be minimized or nearly minimized. Photon-arrival jitter is an uncertainty in the estimated time of arrival of a photon relative to the boundaries of a PPM time slot. Photon-arrival jitter is attributable to two main causes: (1) receiver synchronization error [error in the receiver operation of partitioning time into PPM slots] and (2) random delay between the time of arrival of a photon at a detector and the generation, by the detector circuitry, of a pulse in response to the photon. For channels with sufficiently long time slots, photon-arrival jitter is negligible. However, as durations of PPM time slots are reduced in efforts to increase throughputs of optical PPM communication channels, photon-arrival jitter becomes a significant source of error, leading to significant degradation of performance if not taken into account in design.

Posted in: Briefs, Physical Sciences

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This Month in NASA History

This year, as NASA celebrates its 50th anniversary, we’ll be highlighting technology innovations and important moments in NASA history, leading to our special 50th Anniversary Issue in October.

Posted in: UpFront

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Flexible Silicon Circuits Conform to Complex Shapes

Scientists at the University of Illinois Champaign-Urbana have developed a new form of stretchable silicon integrated circuit that can wrap around complex shapes such as spheres, body parts, and aircraft wings. The circuits can operate during stretching, compressing, folding, and other types of extreme mechanical deformations, without a reduction in electrical performance.

Posted in: UpFront

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Fast Offset Laser Phase-Locking System

Phases can be locked within a microcycle; known phase noise can be added. Figure 1 shows a simplified block diagram of an improved opto- electronic system for locking the phase of one laser to that of another laser with an adjustable offset frequency specified by the user. In comparison with prior systems, this system exhibits higher performance (including higher stability) and is much easier to use. The system is based on a field-programmable gate array (FPGA) and operates almost entirely digitally; hence, it is easily adaptable to many different systems. The system achieves phase stability of less than a microcycle. It was developed to satisfy the phase-stability requirement for a planned spaceborne gravitational-wave-detecting heterodyne laser interferometer (LISA). The system has potential terrestrial utility in communications, lidar, and other applications.

Posted in: Briefs, TSP, Electronics & Computers

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Fiber-Optic Determination of N2, O2, and Fuel Vapor in the Ullage of Liquid-Fuel Tanks

A fiber-optic sensor provides feedback control of onboard inert gas generation systems (OBIGGS) and reduces aircraft operational costs. A fiber-optic sensor system has been developed that can remotely measure the concentration of molecular oxygen (O2), nitrogen (N2), hydro- carbon vapor, and other gases (CO2, CO, H2O, chlorofluorocarbons, etc.) in the ullage of a liquid-fuel tank. The system provides an accurate and quantitative identification of the above gases with an accuracy of better than 1 percent by volume (for O2 or N2) in real-time (5 seconds). In an effort to prevent aircraft fuel tank fires or explosions similar to the tragic TWA Flight 800 explosion in 1996, OBIGGS are currently being developed for large commercial aircraft to prevent dangerous conditions from forming inside fuel tanks by providing an “inerting” gas blanket that is low in oxygen, thus preventing the ignition of the fuel/air mixture in the ullage.

Posted in: Briefs, TSP, Physical Sciences

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Analytical Technique for Healthcare and Industrial Applications

Aria Analytics is commercializing a novel analytical technology to characterize liquids for healthcare and industrial applications. As a drop of liquid evaporates on the vibrating quartz sensor, it undergoes distinctive phase changes representing the unique chemical and physical characteristics of the sample. This unique “fingerprint” represents information that is not readily available from any combination of analytical methods currently in use.

Posted in: Techs for License

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Dr. William (Bill) Farrell, Scientist, Lunar Exploration Program

Goddard Space Flight Center, Greenbelt, MD Dr. William Farrell, a scientist with the Lunar Exploration Program at Goddard Space Flight Center, is an expert on the problem of lunar dust and its effects on astronauts and equipment.

Posted in: Who's Who

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