All-optical pulse reshaping and retiming systems incorporating pulse shaping fiber Bragg grating

This paper demonstrates two optical pulse retiming and reshaping systems incorporating superstructured fiber Bragg gratings (SSFBGs) as pulse shaping elements. A rectangular switching window is implemented to avoid conversion of the timing jitter on the original data pulses into pulse amplitude noise at the output of a nonlinear optical switch. In a first configuration, the rectangular pulse generator is used at the (low power) data input to a nonlinear optical loop mirror (NOLM) to perform retiming of an incident noisy data signal using a clean local clock signal to control the switch. In a second configuration, the authors further amplify the data signal and use it to switch a (low power) clean local clock signal. The S-shaped nonlinear characteristic of the NOLM results in this instance in a reduction of both timing and amplitude jitter on the data signal. The underlying technologies required for the implementation of this technique are such that an upgrade of the scheme for the regeneration of ultrahigh bit rate signals at data rates in excess of 320 Gb/s should be achievable

Novel Fiber Design for Wideband Conversion and Amplification in Multimode Fibers

We propose an operating principle to achieve broadband and highly tunable mode conversion and amplification exploiting inter-modal four wave mixing in a multimode fiber. A bandwidth of 30 nanometers is demonstrated by properly designing a simple step-index silica fiber.Comment: Ecoc conference 201

The effects of heat: their treatment and prevention with special reference to military practice

Effects of Heat, even under modern conditions and in peace time, constitute a considerable menace to the health of the soldier in hot climates, and in this Thesis measures are described by which the incidence of these conditions may be lessened and their affects mitigated.The Regulation of body temperature is discussed, and it is shown that in response to the need for heat loss, considerable amounts of water and of sodium chloride may be lost since the most effective means of cooling the body in high environmental temperatures, is the evaporation of sweat from its surface. The results of failure of the heat regulation are described under four headings:- Circulatory Insufficiency, Dehydration, Salt Loss and Hyperpyrexia.The most important factors leading to such failure are atmospheric conditions unfavourable to the evaporation of sweat, especially a high relative humidity in association with a high environmental temperature, or if the humidity is low the presence of a scorching wind. Whilst Heatstroke can occur without any exposure to the sun, it is thought to be impossible in the light of our present knowledge to dismiss the possibility that sunlight may play a part in the causation of these conditions.The influence of subsidiary factors, such as unsatisfactory environmental conditions, failure to take the precautions, and to make the adjustments in the manner of living called for by residence in hot climates, and illhealth, is discussed and the general effects of hot climates on white men are briefly described.Clinical conditions comprised under the heading of Effects of Heat are described in terms of the four main forms of failure in the adaptation of the body to heat which are mentioned above.The primary indications for Treatment differ somewhat in these four conditions, but in all of them the water lost in sweating must be replaced. The same generalisation cannot be made in the case of the replacement of salt, since although such replacement is specifically indicated in cases of salt loss, and greatly benefits many cases of the other three types, there are certain contraindications to its administration in severe dehydration and in Heat Hyperpyrexia (see pp.156,157: 195,196).Whatever its place in Treatment the provision of a generous supply of salt is invaluable in the prevention of all these conditions, and for the !maintenance of health in hot climates since it is probable that minor degrees of hypochloraemia resulting from continued loss of sodium chloride in sweat without adequate replacement, is responsible for many of the adverse effects of hot climates.The replacement of the large quantities of water lost in sweating is also of great importance, since without such replacement the body cannot maintain the degree of sweating required for cooling purposes,, and the excretory functions of the kidney, except at the expense of the bodily hydration.The practice of limiting the intake of water by soldiers on the march is therefore condemned and it is pointed out that any adverse effects from drinking water can be prevented by the addition of salt. Other prophylactic measures include the education of the soldier in adapting himself to hot climates, the provision of suitable housing and clothing, and the regulation of his meals, his sleep, his work, and his play, in such a way that no avoidable strain is imposed upon his temperature - regulating mechanism

Cross and magnetic helicity in the outer heliosphere from Voyager 2 observations

Plasma velocity and magnetic field measurements from the Voyager 2 mission are used to study solar wind turbulence in the slow solar wind at two different heliocentric distances, 5 and 29 astronomical units, sufficiently far apart to provide information on the radial evolution of this turbulence. The magnetic helicity and the cross-helicity, which express the correlation between the plasma velocity and the magnetic field, are used to characterize the turbulence. Wave number spectra are computed by means of the Taylor hypothesis applied to time resolved single point Voyager 2 measurements. The overall picture we get is complex and difficult to interpret. A substantial decrease of the cross-helicity at smaller scales (over 1-3 hours of observation) with increasing heliocentric distance is observed. At 5 AU the only peak in the probability density of the normalized residual energy is negative, near -0.5. At 29 AU the probability density becomes doubly peaked, with a negative peak at -0.5 and a smaller peak at a positive values of about 0.7. A decrease of the cross-helicity for increasing heliocentric distance is observed, together with a reduction of the unbalance toward the magnetic energy of the energy of the fluctuations. For the smaller scales, we found that at 29 AU the normalized polarization is small and positive on average (about 0.1), it is instead zero at 5 AU. For the larger scales, the polarization is low and positive at 5 AU (average around 0.1) while it is negative (around - 0.15) at 29 AU.Comment: 14 pages 5 figures. Accepted for publication on European Journal of Mechanics B/Fluids (5/8/2015

Solar array subsystems study

The effects on life cycle costs of a number of technology areas are examined for a LEO, 500 kW solar array. A baseline system conceptual design is developed and the life cycle costs estimated in detail. The baseline system requirements and design technologies are then varied and their relationships to life cycle costs quantified. For example, the thermal characteristics of the baseline design are determined by the array materials and masses. The thermal characteristics in turn determine configuration, performance and hence life cycle cost

Turbulence in the solar wind: spectra from Voyager 2 data at 5 AU

Fluctuations in the flow velocity and magnetic fields are ubiquitous in the Solar System. These fluctuations are turbulent, in the sense that they are disordered and span a broad range of scales in both space and time. The study of solar wind turbulence is motivated by a number of factors all keys to the understanding of the Solar Wind origin and thermodynamics. The solar wind spectral properties are far from uniformity and evolve with the increasing distance from the sun. Most of the available spectra of solar wind turbulence were computed at 1 astronomical unit, while accurate spectra on wide frequency ranges at larger distances are still few. In this paper we consider solar wind spectra derived from the data recorded by the Voyager 2 mission during 1979 at about 5 AU from the sun. Voyager 2 data are an incomplete time series with a voids/signal ratio that typically increases as the spacecraft moves away from the sun (45% missing data in 1979), making the analysis challenging. In order to estimate the uncertainty of the spectral slopes, different methods are tested on synthetic turbulence signals with the same gap distribution as V2 data. Spectra of all variables show a power law scaling with exponents between -2.1 and -1.1, depending on frequency subranges. Probability density functions (PDFs) and correlations indicate that the flow has a significant intermittency.Comment: 14 pages, 7 figures. Discussion improved since the previous versio

Coboson formalism for Cooper pairs used to derive Richardson's equations

We propose a many-body formalism for Cooper pairs which has similarities to the one we recently developed for composite boson excitons (coboson in short). Its Shiva diagram representation evidences that $N$ Cooper pairs differ from $N$ single pairs through electron exchange only: no direct coupling exists due to the very peculiar form of the BCS potential. As a first application, we here use this formalism to derive Richardson's equations for the exact eigenstates of $N$ Cooper pairs. This gives hints on why the $N(N-1)$ dependence of the $N$-pair ground state energy we recently obtained by solving Richardson's equations analytically in the low density limit, stays valid up to the dense regime, no higher order dependence exists even under large overlap, a surprising result hard to accept at first. We also briefly question the BCS wave function ansatz compared to Richardson's exact form, in the light of our understanding of coboson many-body effects

A method for spatial deconvolution of spectra

A method for spatial deconvolution of spectra is presented. It follows the same fundamental principles as the ``MCS image deconvolution algorithm'' (Magain, Courbin, Sohy, 1998) and uses information contained in the spectrum of a reference Point Spread Function (PSF) to spatially deconvolve spectra of very blended sources. An improved resolution rather than an infinite one is aimed at, overcoming the well known problem of ``deconvolution artefacts''. As in the MCS algorithm, the data are decomposed into a sum of analytical point sources and a numerically deconvolved background, so that the spectrum of extended sources in the immediate vicinity of bright point sources may be accurately extracted and sharpened. The algorithm has been tested on simulated data including seeing variation as a function of wavelength and atmospheric refraction. It is shown that the spectra of severely blended point sources can be resolved while fully preserving the spectrophotometric properties of the data. Extended objects ``hidden'' by bright point sources (up to 4-5 magnitudes brighter) can be accurately recovered as well, provided the data have a sufficiently high total signal-to-noise ratio (200-300 per spectral resolution element). Such spectra are relatively easy to obtain, even down to faint magnitudes, within a few hours of integration time with 10m class telescopes.Comment: 18 pages, 6 postscript figures, in press in Ap

The case for a wet, warm climate on early Mars

Arguments are presented in support of the idea that Mars possessed a dense CO2 atmosphere and a wet, warm climate early in its history. The plausibility of a CO2 greenhouse is tested by formulating a simple model of the CO2 geochemical cycle on early Mars. By scaling the rate of silicate weathering on Earth, researchers estimated a weathering time constant of the order of several times 10 to the 7th power years for early Mars. Thus, a dense atmosphere could have existed for a geologically significant time period (approx. 10 to the 9th power years) only if atmospheric CO2 was being continuously resupplied. The most likely mechanism by which this could have been accomplished is the thermal decomposition of carbonate rocks induced directly or indirectly by intense, global scale volcanism