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performance of such a waveguide for long-distance-communication application.3 Mater ial aspectsThe losses of the dielectric-fibre waveguide are governedby the bulk losses of the materials which constitute thefibre and the surrounding medium. The relative contribu-tion to the total loss is determined by the proportion ofenergy within and outside the fibre and the relative lossesof the two media. In general, it is desirable to have lowbulk losses for both media, in order to achieve a satisfac-tory fibre waveguide with low attenuation.3.1 Material-loss characteristicsThe bulk loss in dielectrics is caused by absorption andscattering phenomena. The particular mechanism involveddiffers for each material and depends on the operatingwavelength. The material-loss property is to be examinedbetween wavelengths of 100 and 0.1 /an, where the physi-cal size and the information capabilities of the dielectric-fibre waveguide are convenient.3.1.1 Scattering: Scattering arises as a result of(a) lack of order of the structure of the material

(b) structural defects(c) particle inclusion(d) random fluctuation.

For crystalline materials, the first two mechanisms are pre-dominant. Polycrystalline materials and materials whichare partly amorphous and partly crystalline show lack oforder of the structure; this results in high scattering loss.Single-crystal materials are ordered but may have structur-al defects; if these are few and of small size compared withthe wavelength, the scatter loss may not be very high.However, such materials are usually difficult to obtain inlong lengths.For amorphous materials, such as organic polymersand inorganic glasses, mechanisms (c) and (d) are moreimportant. Organic polymers often contain all-chemicaldust particles much larger than 1 fim in diameter, causedby the uncontrolled environment in which they are manu-factured. This undesirable feature is likely to be eliminatedby the use of a dustfree environment and freshly redistilledmonomers and catalysts during manufacture. For inorga-nic glasses, the temperatures involved are high enough tocause chemical decomposition of most particle inclusion,resulting in such particles appearing as impurity centres.

The glassy state is a result of the supercooling of aliquid; thus the glassy-state solid retains some of the fun-damental behaviour of the liquid state. Therefore localisedmaterial-density fluctuation can take place. The scatteringdue to this can be described by the following expression[2] :

in I)236 x 103 - - i 1 - kTpc decibels per metreAFor inorganic glass with a fictive temperature of 1000C,the scattering loss is of the order of 1 dB/km. Fictive tem-perature is the temperature at which glass viscosity hasincreased to a value where the glass is regarded as a solid.

Crystallite formation is a structural defect for glassy-state materials. The sizes of the crystallites in a glassymaterial can be controlled by the rate of cooling. For afibre, the rate of cooling is high; this results in fewer andsmaller crystallites. The scattering due to crystallites inrapidly cooled glasses obeys the Rayleigh scattering law;

i.e. loss is proportional to X 4. It is estimated that the lossis of the order of a few decibels per kilometre at 1 /anwavelength.3.1.2 Absorption: Absorption bands in solids are usuallybroad, owing to the close packing of the molecules. Theyarise from the natural-vibration frequencies of the molecu-lar and electronic systems. Near such frequencies, theenergy of the external electromagnetic field couples energyinto the vibration of the molecules and electrons. In thewavelength region between 100 and 1 /an, many longitudi-nal and rotational resonances of molecules are present inalmost all substances, especially the long-chain polymers.Strong absorption takes place throughout most of theregion. In the 0.3-0.1 /mi region, electronic-resonanceabsorption bands are present. In the intermediate region(i.e. 1-0.3 /an), resonance-absorption phenomena are rela-tively absent. This represents a region for the material tohave low loss.

In inorganic glasses, it is known that absorption canoccur owing to the presence of impurity ions. It is knownthat, in high-quality optical glasses, the main contributionto absorption loss in the 1-3 /an region is due to the Fe+ +and Fe+ + + ions. The ferrous ion has an absorption bandcentred at about 1 /mi, while the ferric ion has one atabout 0.4 /an. At band centre, the absorption due to 1 partper million of Fe+ 2 in certain glass systems3 is estimatedto result in an absorption coefficient of less than20 dB/km.3.1.3 Present state of low-loss material: The presentknown low-loss materials in the frequency range of interestare mainly in the visible part of the spectrum. This isbecause transparent materials in this frequency range havebeen in high demand. The best transparent materialsknown in the visible spectrum are high-quality opticalglasses, fused quartz, polymethyl methachrylate and poly-styrene. The best absorption coefficient for glass isreported as 0.05% per cm, which is equivalent totan (5 = 1 x 10~8 at 1 /mi, giving a bulk loss of about200 dB/km. The published data on polymethyl meth-achrylate give 0.2% per cm, equivalent to a bulk loss ofabout 600 dB/km at 0.7 /an wavelength. This is for acommercial-grade material which is known to suffer fromhigh particle-scattering losses.

Typical absorption/wavelength curves can be seen inFigs. 1, 2 and 3, showing the measurements made on glass,

Fig. 1 Attenuation in SW6 glassquartz and polymethyl methachrylate samples, respec-tively. Work is in progress towards obtaining lower-absorption glasses. Currently this is being given anadditional boost, owing to laser-glass requirements. It isforeseeable that glasses with a bulk loss of about20 dB/km at around 0.6 /an will be obtained, as the iron-impurity concentration may be reduced to 1 part permillion.

192 IEE PROCEEDINGS, Vol. 133, Pt. J, No. 3, JUNE 1986

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4 Electromag netic aspectsThe choice of the mode of propaga t ion for the fibre wave-guide used for communication purposes is governed by the

0-6 " 0 7 0-8 0-9wavelength, >jmFi g . 2 Attenuation infused quartz

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Fi g . 3 Attenuation in polymethyl methacrylateconsideration of loss characteristics and informationcapacity.4.1 Dielectric lossFor a fibre-dielectric waveguide with free space as itslossfree outside medium, it is an advantage to choose theradius, the dielectric constants and the mode of propaga-tion so tha t the ratio of the energy in free space to theenergy in the dielectric fibre is large. Examining the char-acteristic eqn. 1 of this system, it can be shown that th eradial-decay coefficient in the outside medium decreaseswhen a particular mode is near cutoff, corresponding tothe proportion of energy in the outside region increasing.The characteristics of the E o , H o and H E U modes areshown in Figs. 4 an d 5; the effective losses in decibels areshown in Figs. 6 and 7.

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2 3 4 5 KOOFi g . 4 Ho and Eo mode characteristicsIEE PROCEEDING S, Vol. 133, Pt. J, No. 3, JUNE 1986

This improvement in loss characteristics has alreadybeen explored at microwave frequencies [4], the E o mode

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