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1、Pure and it was not819820JAMES W. MITCHELLpossible to identify the impurities present. As a result, analytical research to develop extremely sensitive, specific methods was stimulated and has con- tinued to be a primary factor essential in technological research and devel- opment.As is already evide
2、nt instrumental and chemical methods for trace analysis have been developed and applied to a wide variety of materials character- ization problems. Many materials in controlled states of purity are required for fabricating various components of telecommunications systems. In fact the production of m
3、aterials with important properties has been one of the keys to technological advancement in the communications industry. A few examples of materials and important properties are listed in Table 1.TABLE 1. Materials required in controlled states of purityMaterialSiliconGermaniumFused silicaApplicatio
4、nDevice productionRadiation detec- torsContainers, fiber optic claddingControlled ImpuritiesDefects, Na, B, P, Cl, As, 0, AlTransition elements(Tr ,Ms)MCVD pre formsOptical fibersFe, Cu, Ni, Cr, Mn, Co, OHGaP, GaAsQuartzLEDs ,LasersPiezoelectric resonators0, C, N, Cl, P,other Tr.MsFe, Al, OHPlatinum
5、GoldContainersPlatingFe, IrFe, Sn, Pb, CCu-Ni-SnGraphiteSilicon carbideSpring alloysVarister manu- factureP, Pb, Ti, Si, YAl, FeThe properties of some materials change dramatically with impurity content and with variation in composition. Low ppb levels of certain impurities in semiconductors and gla
6、ss will render the material completely useless for the intended application while certain properties are only obtained by precisely doping materials with known amounts of important trace elements.The most recent critical need in the telecommunications industry for pure materials and trace element an
7、alyses occurred with the initiation of research to develop optical waveguide systems. Although todays telephone systemsdepend largely on transmission of electrical signals via copper conductor cables, systems of the future will be based on combinations of electrical and optical transmission componen
8、ts. A schematic diagram of a representative optical waveguide cable is shown in Fig. 1.Fig. 1. Optical waveguide cable systemSTEEL VPOLYETHYLENEOPTICALFIBERS IITrace analysis for optical waveguide technology821An optical wavegulde is essentially a light pipe into whIch light of appropnate characteri
9、stics is launched from a suitable LED or laser source and then Is propagated through the fiber via total internal reflection from the corecladding interface. Because the transmission bandwidth capacity of the fiber is extremely large and its physical dimensions exceedingly small in comparison to oth
10、er systems in use today, the worldwide telecommunications industry is very active in applied research, development and deployment of optical waveguide systems.Glass has emerged as the most suitable material for fabricating optical wave guides. To be useful for this purpose the material must be devoi
11、d of impurities that would attenuate the propagating light via absorption losses. Projected tolerance limits for certain transition elements in glasses of various composition are given in Table 2.TABLE 2. Impurity levels (PPB) introducing 1 dB/Km absorption losses at 800 nm in various glassesImpurit
12、yNaCa-SiNa-B-SiCo0.240100Cr2.1220Cu5.1100l(II)*, l0(I)*Fe2.120l(II)*, l000(III)*Mn10.150100Ni2.14010V10.1125*(I),(II), (III) are oxidation states of ImpuritiesSCOPE AND OBJECTIVES OF ANALYTICAL PROGRAMThe analytical information required for supporting the waveguide research pro- gram included, rapid
13、 screening of available glasses and glassmaking rawmaterials to determine their suitability for waveguide production, identify- ing impurities present in these materials, detecting sources of contamination during glassmaking and fiber drawing, and developing procedures for preparingmaterials of suff
14、icient purity in the event commercially available products were too impure. To obtain this information extremely sensitive and reliable quantitative methods were required since small changes in the levels of var- ious traces would need to be distinguished. The appropriate support method could be cho
15、sen from the listing of analytical techniques in Table 3. Neutron activation analysis (NAA) was selected as the primary support method because of its characteristic features of 1 sufficient sensitivity for direct subppm and subppb detection of a large number of trace elements, 2J application for rap
16、id, semiquantitative survey determinations of a large number of elements or highly accurate, selective, single element determinat- ions using chemical separations, and 3 freedom from blank problems that could occur during postirradiation chemical processing of the sample. X-ray fluorescence (XRF), atomic absorption spectroscopy (AAS), and laser intra cavity absorption spectrophotometry have also been applied.TABLE 3. Techniques for trace and ultratrace analysisAtomic absorpt
