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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: Photonics, Briefs

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PX Series Photoelectric Sensors

The KEYENCE PX Series rugged photoelectric sensors from Keyence Corp. of America, Woodcliff Lake, NJ, feature an IP-69K environmental rating for highpressure (1,400 psi) applications at temperatures to 176°F. They feature stainless steel casings, sensor heads backfilled with epoxy under vacuum conditions, and scratch-resistant lenses. A dual-output feature enables users to see when the sensor activates and when the low-light alarm turns on. A Dynamic Stability Control function continuously and automatically adjusts the set point according to the environmental conditions. Seven water- and oil-resistant sensor heads are available. Ultra-high-intensity LEDs provide power by combining infrared or four-element red LEDs with optical- quality glass lenses. Stainless steel guarded units are available to protect the sensor heads. A Zero-Shift function enables users to adjust the displayed value. For Free Info

Posted in: Products

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MACOS Version 3.31

Version 3.31 of Modeling and Analysis for Controlled Optical Systems (MACOS) has been released. MACOS is an easy-to-use computer program for modeling and analyzing the behaviors of a variety of optical systems, including systems that have large, segmented apertures and are aligned with the technology of wavefront sensing and control. Two previous versions were described in “Improved Software for Modeling Controlled Optical Systems” (NPO-19841) NASA Tech Briefs, Vol. 21, No. 12 (December 1997), page 42 and “Optics Program Modified for Multithreaded Parallel Computing” (NPO-40572) NASA Tech Briefs, Vol. 30, No. 1 (January 2006) page 13a. The present version incorporates the following enhancements over prior versions:

Posted in: Photonics, Briefs

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Simulated Models Test Design of Space Shuttles and Rocket Engines

Finite element modeling and analysis Dynamic Concepts Huntsville, AL 256-922-9888 www.dynamic-concepts.com NASA tasked Dynamic Concepts (DCI) with assessing the structural dynamics of the rollout process, whereby the space shuttle orbiter, external tank, and solid rocket booster assembly is moved via a crawler transporter from the Vertical Assembly Building to the launch pad. DCI used Femap finiteelement modeling software from Siemens PLM Software (Plano, TX) to create an integrated model of all the shuttle components, and used Siemens’ NX Nastran to analyze the simulated vibration environment. The analysis helped NASA resolve issues with support structures and determine target rollout speeds that minimized potentially damaging vibration.

Posted in: Application Briefs

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Advanced Position Sensors to Aid NASA in Future Spaceflight

Silicon carbide-based position sensors INPROX Technology Corp. Boston, MA 617-573-5158 www.inproxtechnology.com INPROX Technology Corp. (ITC) has entered into a Space Act Agreement (SAA) with NASA’s John H. Glenn Research Center in Ohio to develop advanced silicon carbide (SiC)- based position sensors aimed at potential uses in future spaceflight, turbine engine controls, and automotive engine applications. Under this SAA, high-temperature SiC electronics from NASA will be prototyped into ITC’s proprietary linear position sensor technology platform.

Posted in: Application Briefs

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Performance of 1mm2 Silicon Photomultipliers

A silicon photomultiplier (SPM) is a new type of semiconductor detector that has the potential to replace the photo- multiplier tube (PMT) detector in many applications. In common with a PMT detector, the output of an SPM is an easily detectable current pulse for each detected photon and can be used in both photon counting mode and as an analogue (photocurrent) detector. However, the SPM also has a distinct advantage over PMT detectors. The photon-induced current pulse from a PMT varies greatly from photon to photon, due to the statistics of the PMT multiplication process (excess noise). In contrast, the current pulse from an SPM is identical from photon to photon. This gives the SPM a distinct advantage in photon counting applications as it allows the associated electronics to be greatly simplified. Identical pulses also mean that the SPM can resolve the number of photons in weak optical pulses, so-called photon number resolution. This is critical in a number of applications including linear-optics quantum computing.

Posted in: Articles

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Attitude and Translation Control of a Solar Sail Vehicle

A report discusses the ability to control the attitude and translation degrees-of-freedom of a solar sail vehicle by changing its center of gravity. A movement of the spacecraft’s center of mass causes solar- pressure force to apply a torque to the vehicle. At the compact core of the solar-sail vehicle lies the spacecraft bus which is a large fraction of the total vehicle mass. In this concept, the bus is attached to the spacecraft by two single degree-of-freedom linear tracks. This allows relative movement of the bus in the sail plane. At the null position, the resulting solar pressure applies no torque to the vehicle. But any deviation of the bus from the null creates an offset between the spacecraft center of mass and center of solar radiation pressure, resulting in a solar-pressure torque on the vehicle which changes the vehicle attitude. Two of the three vehicle degrees of freedom can be actively controlled in this manner. The third, the roll about the sun-line, requires a low-authority vane/propulsive subsystem.

Posted in: Briefs, TSP

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