Claude Brehm

Segregation of impurities and striation in single crystals of double niobates BaxSr1−xNb2O6 and Ba2NaNb5O15☆

Abstract Heterogeneities of refractive index, striae, in single crystals of double niobate such as Ba x Sr 1−x Nb 2 O 6 and Ba 2 NaNb 5 O 15 , are a major reason for limiting the optical quality of these materials, so that large volume applications are not possible. The nature and origin of these defects are discussed. Observations of fracture surfaces show that the heterogeneity of mechanical properties can be correlated to striae. A secondary ion microscope analyzer allows one to see that a periodical repartition of some alkaline and alkaline-earth impurities could be the reason for the striation phenomenon. This segregation is caused by variations of the growth speed which can be attributed to thermal instabilities near the crystal-melt interface. By avoiding thermal fluctuations and using raw materials of higher purity, it is possible to make striation-free crystals of good optical quality.

Crystal growth of strontium barium niobate BaxSr1−xNb2O6

Abstract Study of solid state reactions which occur during the synthesis of barium strontium niobate revealed the presence of a previously unrecorded monoclinic phase belonging to the P21/a space group in coexistence with the desired tetragonal phase. Under certain preparation conditions, formation of this phase can produce growth defects in the single crystal. The production of single crystals without defect requires use of raw materials which do not contain this monoclinic phase. Single crystals pulled using the Czochralski method contain striations; these periodic variations of refractive index were shown to be related to a heterogeneous distribution of small quantities of impurities such as calcium and potassium. The growth of single crystals free of striations can be obtained using very pure raw materials by carrying out the pull without rotation and by thermally insulating the upper part of the crucible. These single crystals were tested by the determination of the half wave voltage which was shown to be 85 V, and by the extinction ratio, which is on the order of 30. Experiments of light modulation were also carried out.

New scattering method for the measurement of optical fiber outer dimension

A new contactless method is presented to measure the variations in the outer diameter of an optical fiber. The fiber is illuminated by a cw He-Ne laser beam that is perpendicular to its axis. The technique is based on an analysis of the light scattered by the fiber in a direction nearly perpendicular to the incident beam. This part of the pattern is very sensitive to a variation in the fiber outer diameter. A method to monitor the outer diameter during the drawing of the fiber has been deduced from the computation of the scattering pattern.

Accroissement d’atténuation prévisible sur des fibres unimodales placées en atmosphère d’hydrogène

AnalyseCet article décrit le comportement à diverses températures et en atmosphère d’hydrogène de fibres unimodales à cæur dopé au GeO2 et à gaine compensée, optimisées soit pour 1,3 μm soit pour 1,55 μm. Dans le câble sousmarin mis en æuvre, l’accroissement d’atténuation prévisible après 25 ans à 3° C ne devrait pas dépasser 10−4 dB/km aux longueurs d’onde d’utilisation.AbstractBehavior of GeO2core doped, matched cladding singlemode fiber structures fitted for 1.3 μm and for 1.55 μmare studied at high temperature in hydrogen atmosphere. Loss increase prediction for submarine cables after 25 years at 3° C is less than 10−4dB/km at operating wavelengths.