Defence Research Agency

About: Defence Research Agency is a based out in . It is known for research contribution in the topics: Synthetic aperture radar & Radar. The organization has 1211 authors who have published 1109 publications receiving 31542 citations.

Laser damage issues for mid-IR optical parametric oscillator mirrors

Abstract: A study has been carried out of the design and fabrication of mirrors for Optical Parametric Oscillators (OPOs) operating in the mid-infrared using a variety of coating and substrate materials The major considerations determining the choice of coating materials are refractive index excursion (determines bandwidth of mirror), optical bandgap (multi-photon effects at the pump wavelength), and porosity (adsorbed water causes absorption features in the tuning range of the OPO) The mirror coatings are mostly of quarter wave type design, and have been fabricated from chalcogenides, fluorides and oxides The mirrors have been evaluated at repetition rates and pulse lengths similar to those encountered in operation of the OPO, at both the pump (1064 and 21 micrometers ) and typical output wavelengths Damage data has been gathered for a number of mirrors which form part of the OPO cavity Second Harmonic Generation was observed at 1064 micrometers in Cleartran ZnS substrates

Experimental approach of the EPS passivation condensation phenomena

Abstract: On the request of CNES, ESA/ESTEC has led a study on the disposal of the propellants of the Ariane 5 upper stage (EPS). ESTEC has asked ONERA to propose an experimental assessment of risks of condensation at the exit of dump nozzles of the circuit of N2O4. After an examination of test conditions that would be representative of the reality, a test campaign has been realized at the Propulsion Laboratory of the Center of the Fauga-Mauzac. Tests of particle visualization by laser sheet method have shown that the size of the particles was less than the implemented means limit of resolution (inferior to some microns). Tests based on measurements by Rayleigh scattering have allowed to confirm a submicronic size of condensed particles at the exit of the nozzle. An evaluation of the contamination by the depletion of the N2O4 tanks was realized by ESTEC, by collection of deposited material on a Quartz Crystal Microbalance (QCM). The condensation of N2O4 inside the reservoir during the disposal has been obviously proved during the complete disposal of the stocking sphere at the end of the tests campaign.

Monte Carlo simulation of high-field transport and impact ionization in AlGaAs p+in+ diodes

Abstract: We have used Monte Carlo simulation methods employing both realistic band structure and a simpler analytical approximation to investigate impact ionization in bulk AlxGa1-xAs and also submicron p+in+ diodes for x40%. The calculated impact ionization rates in bulk AlxGa1-xAs compared well with previous experiments and the electron- and hole-initiated current multiplication characteristics of the p+in+ diodes were found to agree very well with our experimental results for both the analytical and the numerical models.

Local gain control and focal accommodation in the self similar stack vision model

Abstract: The aim of this paper is to report an extension to the computationally efficient Self Similar Stack model (Burton et al. Biol., Cybernet. 53, 397-403, 1986) to include the effects of local gain control in the retina. The method employed to do this has been to fit a family of difference-of-Gaussian functions to the human contrast sensitivity function curves of van Nes and Bouman (J. Opt. Soc. Am. 57, 401-406, 1967). The centre frequencies of the DoGs within each family are octave-related, in a simplified manifestation of the DoG channels found by Wilson et al. (Vision Res. 23, 873-882, 1983). The sensitivity of each level, or channel, that formed the original Stack model is modulated individually according to the fitted values, as the local illumination varies within an image. The model was tested against psychometric data obtained by Haig and Burton (Appl. Optics 26, 492-500, 1987) during experiments on visual discrimination. The consistency of the results indicates the validity of the approximations and the robustness of the model, either for machine vision purposes or for predicting human visual performance. A simple algorithm, developed for use with a machine vision application of this model, provides a means by which a TV camera may be focused automatically. The success of this algorithm, using the newly computed channel sensitivities, suggests that human focal accommodation may be regulated by a similar form of mechanism.