Chebyshev approximation of the null function by an affine combination of complex exponential functions

We describe the theoretical solution of an approximation problem that uses a finite weighted sum of complex exponential functions. The problem arises in an optimization model for the design of a telescope array occurring within optical interferometry for direct imaging in astronomy. The problem is to find the optimal weights and the optimal positions of a regularly spaced array of aligned telescopes, so that the resulting interference function approximates the zero function on a given interval. The solution is given by means of Chebyshev polynomials.     

Three-dimensional optical data storage using third-harmonic generation in silver zinc phosphate glass

We demonstrate the possibility of three-dimensional optical data storage inside a specific zinc phosphate glass containing silver by using third-harmonic generation (THG) imaging. Information is stored inside the glass with femtosecond laser irradiation below the refractive index modification threshold. We use the same laser for THG readout. The capability of storage with this technique is discussed.     

High-power and long-lifetime inGaN blue- violet laser diodes grown by molecular beam epitaxy

InGaN 405 nm multiple quantum well laser diodes grown by molecular beam epitaxy (MBE) with a continuous-wave (CW) lifetime of up to 42 h are reported. The CW threshold current density of the ridge waveguide laser diodes is 3.6 kA/cm² and the slope efficiency for uncoated facets is 0.42 W/A per facet with a maximum CW output power of 45 mW per facet. Statistical variation of CW lifetime with dissipated power is presented for MBE-grown laser diodes from five different wafers.     

Satellite antenna depointing originating from attitude control and station keeping specifications

The paper establishes practical formulae in order to determine the depointing angle for a geostationary satellite antenna. Both satellite attitude and station-keeping errors are considered. The attitude control and station-keeping specifications give the amplitude of the angular motion of the satellite about its centre of mass and the amplitude of the displacement of the satellite with respect to its nominal position, respectively. The effects of these angular motions and displacements on the aiming of the satellite antenna are analysed. A set of formulae is derived which permits the calculation of the overall satellite antenna depointing angle. These formulae can be used for the estimation of the satellite antenna gain loss or interference contribution at a given location on the Earth.     

Quelques applications des boucles à verrouillage de phase aux problèmes de synchronisation dans les liaisons spatiales

This paper deals with a presentation of carrier and bit synchronizers used for low signal-to-noise ratio space digital communications. Such synchronizers are closed loop phase estimators and carry out a mechanization of the maximum likelihood criterium. The paper presents well known carrier synchronizers such as the decision feedback synchronizer, the Costas loop and the x² non-linearity synchronizer. Bit synchronizers such as the early late gate, the data transition tracking loop and others synchronizers associated with a non-linearity are also introduced.     

Photocrosslinking of elastomer systems. I. UV light-induced reactions in a blend of EPDM-benzoin derivatives

Photocrosslinking experiments were performed on elastomeric polymer films (EPDM) that contained benzoin derivatives as photosensitive initiators. Infrared (IR) and ultraviolet (UV) spectroscopy investigations were used in conjunction with physical property measurements. The results all together make it possible to discuss the crosslinking mechanism and pose interesting questions about the reactivity of benzoin derivatives in their excited states in relation to their efficiency in the crosslinking process.     

Performance of regenerative/conventional satellite systems

A great many papers have been devoted in the last few years to the advantages of regenerative communications satellite systems when compared to conventional ones. This paper attempts to summarize the most significant results which are scattered in the literature, and to provide a tutorial overview of this comparison. Linear and non-linear satellite channel performances, with and without interference, are discussed. The major impacts on system design provided by on-board regeneration are also presented.     

Structured assemblies of ferromagnetic particles through covalent immobilization on functionalized polymer surfaces obtained by surface segregation

We report a strategy to immobilize magnetic particles on polymer surfaces in an organized manner. Surface segregation of binary polymer blends provided surfaces with the desired chemical functions (carboxylic functions). These functional groups were demonstrated to be accessible and were thus able to react with magnetic particles functionalized with amine functions. The presence of a magnetic field during the covalent attachment step in direct surface patterning produced particle chains oriented parallel to the field.