4,448 research outputs found
Shuttle/GPSPAC experimentation study
The utilization is discussed of the GPSPAC, which is presently being developed to be used on the low altitude host vehicle (LAHV), for possible use in the shuttle avionics system to evaluate shuttle/GPS navigation performance. Analysis and tradeoffs of the shuttle/GPS link, shuttle signal interface requirements, oscillator tradeoffs and GPSPAC mechanical modifications for shuttle are included. Only the on-orbit utilization of GPSPAC for the shuttle is discussed. Other phases are briefly touched upon. Recommendations are provided for using the present GPSPAC and the changes required to perform shuttle on-orbit navigation
Advanced flight control system study
The architecture, requirements, and system elements of an ultrareliable, advanced flight control system are described. The basic criteria are functional reliability of 10 to the minus 10 power/hour of flight and only 6 month scheduled maintenance. A distributed system architecture is described, including a multiplexed communication system, reliable bus controller, the use of skewed sensor arrays, and actuator interfaces. Test bed and flight evaluation program are proposed
The correspondence between small-scale coronal structures and the evolving solar magnetic field
Solar coronal bright points, first identified in soft X-rays as X-ray Bright Points (XBPs), are compact, short lived and associated with small bipolar magnetic flux. Contradictory studies have suggested that XBPs are either a primary signature of the emerging flux spectrum of the quiet Sun, or that they are representative of the disappearance of pre-existing flux. Results are presented using coordinated data obtained during recent X-ray sounding rocket flights on 15 August and 11 December 1987 to determine the correspondence of XBPs with time-series, ground based observations of evolving bipolar magnetic structures, He-I dark points, and the network. The results are consistent with the view that coronal bright points are more likely to be associated with the annihilation of pre-existing flux than with emerging flux
Nonlocal explanation of stationary and nonstationary regimes in cascaded soliton pulse compression
We study soliton pulse compression in materials with cascaded quadratic
nonlinearities, and show that the group-velocity mismatch creates two different
temporally nonlocal regimes. They correspond to what is known as the stationary
and nonstationary regimes. The theory accurately predicts the transition to the
stationary regime, where highly efficient pulse compression is possible.Comment: 3 pages, 2 figures, published verison in Optics Letters. Contains
revised equations, including an updated mode
Scaling laws for soliton pulse compression by cascaded quadratic nonlinearities
We present a detailed study of soliton compression of ultra-short pulses
based on phase-mismatched second-harmonic generation (\textit{i.e.}, the
cascaded quadratic nonlinearity) in bulk quadratic nonlinear media. The
single-cycle propagation equations in the temporal domain including
higher-order nonlinear terms are presented. The balance between the quadratic
(SHG) and the cubic (Kerr) nonlinearity plays a crucial role: we define an
effective soliton number -- related to the difference between the SHG and the
Kerr soliton numbers -- and show that it has to be larger than unity for
successful pulse compression to take place. This requires that the phase
mismatch be below a critical level, which is high in a material where the
quadratic nonlinearity dominates over the cubic Kerr nonlinearity. Through
extensive numerical simulations we find dimensionless scaling laws, expressed
through the effective soliton number, which control the behaviour of the
compressed pulses. These laws hold in the stationary regime, in which
group-velocity mismatch effects are small, and they are similar to the ones
observed for fiber soliton compressors. The numerical simulations indicate that
clean compressed pulses below two optical cycles can be achieved in a
-barium borate crystal at appropriate wavelengths, even for picosecond
input pulses.Comment: 11 pages, 8 figures, resubmitted version, to appear in October issue
of J. Opt. Soc. Am. B. Substantially revised, updated mode
Limits to compression with cascaded quadratic soliton compressors
We study cascaded quadratic soliton compressors and address the physical
mechanisms that limit the compression. A nonlocal model is derived, and the
nonlocal response is shown to have an additional oscillatory component in the
nonstationary regime when the group-velocity mismatch (GVM) is strong. This
inhibits efficient compression. Raman-like perturbations from the cascaded
nonlinearity, competing cubic nonlinearities, higher-order dispersion, and
soliton energy may also limit compression, and through realistic numerical
simulations we point out when each factor becomes important. We find that it is
theoretically possible to reach the single-cycle regime by compressing
high-energy fs pulses for wavelengths in a
-barium-borate crystal, and it requires that the system is in the
stationary regime, where the phase mismatch is large enough to overcome the
detrimental GVM effects. However, the simulations show that reaching
single-cycle duration is ultimately inhibited by competing cubic nonlinearities
as well as dispersive waves, that only show up when taking higher-order
dispersion into account.Comment: 16 pages, 5 figures, submitted to Optics Expres
Correspondence between solar fine-scale structures in the corona, transition region, and lower atmosphere from collaborative observations
The Soft X-Ray Imaging Payload and the High Resolution Telescope and Spectrograph (HRTS) instrument were launched from White Sands on 11 December 1987 in coordinated sounding rocket flights to investigate the correspondence of coronal and transition region structures, especially the relationship between X-ray bright points (XBPs) and transition region small spatial scale energetic events. The coaligned data from X-ray images are presented along with maps of sites of transition region energetic events observed in C IV (100,000 K), HRTS 1600 A spectroheliograms of the T sub min region and ground based magnetogram and He I 10830 A images
Measurement of Gamma Radiation in an Automobile Mechanic Village in Abuja, North Central, Nigeria
Environmental radiation measurement was carried out in an automobile mechanic village, Apo, Abuja, Nigeria. An in-situ measurement approach was adopted using RDS-200 Universal Survey Meter and a handheld Global Positioning System (Garmin GPS 76S) equipment. It was observed that the dose equivalent varied from 0.04 μSv/h to 0.22 μSv/h with a mean of 0.10± 0.03 μSv/h which is below the standard background radiation of 0.133 μSv/h The study also revealed that the average annual effective dose rate is approximately 0.20±0.06 mSv/yr which is lower than the value of 1.0 mSv/yr averaged over five consecutive years according to the dose limit placed by the Basic Safety Standards (BSS) SCHEDULE II and the International Commission on Radiological Protection (ICRP) REPORT 60. This indicates that the automobile technicians, craftsmen and the people living and working within the area are safe and are not exposed to high doses of radiation as a result of activities in the Apo Automobile Mechanic Village.Keywords: Radiation Measurement; Apo Automobile Mechanic Village; Background Radiation; DoseEquivalent; Annual Effective Dose; Automobile Technician
Demonstration of a cavity-enhanced optical parametric chirped-pulse amplification system
Cataloged from PDF version of article.The use of a low finesse enhancement cavity resonant with a low average power (< 1W) and a high repetition rate (78MHz) pump source is shown to achieve 55% conversion efficiency into signal and idler from the coupled pump in an optical parametric process, whereas an equivalent amount of pump power in a single-pass configuration leads to negligible conversion. Careful comparison of the intracavity conversion process to the single-pass case is performed to assess the underlying impedance matching that yields the high conversion results. (C) 2011 Optical Society of Americ
Optical Cherenkov radiation by cascaded nonlinear interaction: an efficient source of few-cycle energetic near- to mid-IR pulses
When ultrafast noncritical cascaded second-harmonic generation of energetic
femtosecond pulses occur in a bulk lithium niobate crystal optical Cherenkov
waves are formed in the near- to mid-IR. Numerical simulations show that the
few-cycle solitons radiate Cherenkov (dispersive) waves in the
\lambda=2.2-4.5\mic range when pumping at \lambda_1=1.2-1.8\mic. The exact
phase-matching point depends on the soliton wavelength, and we show that a
simple longpass filter can separate the Cherenkov waves from the solitons. The
Cherenkov waves are born few-cycle with an excellent Gaussian pulse shape, and
the conversion efficiency is up to 25%. Thus, optical Cherenkov waves formed
with cascaded nonlinearities could become an efficient source of energetic
near- to mid-IR few-cycle pulses.Comment: Extended version of Nonlinear Optics 2011 contribution
http://www.opticsinfobase.org/abstract.cfm?URI=NLO-2011-NTuA7. Submitted for
Optics Express special issue for NLO conferenc
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