Material Aspects of Standard Transmission Optical Fibers
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Material Aspects of Standard Transmission Optical Fibers P. Guenot Abstract This article reviews loss components of standard transmission silica optical fibers and their connections to glass structure and manufacturing parameters. Loss-reduction possibilities are described. In particular, the Rayleigh scattering component, which is the main contributor to fiber losses within the transmission window, can be significantly reduced by lowering the fictive temperature (the temperature at which the liquid structure is “frozen” during cooling) and tuning the glass composition. The issue of a compromise between manufacturing productivity and minimization of excess loss is also addressed. Keywords: absorption, attenuation, fictive temperature, optical communications, optical fibers, photonic materials, preforms, Rayleigh scattering, silica glass.
Introduction Although the principle of dielectric waveguides (i.e., optical fibers) proposed by Hondros and Debye in 1910 was theoretically very interesting, it was not considered for many years because no material with a low optical loss could be found. Material attenuation is actually the severest limiting factor in optical fibers because it reduces signal power propagating through the waveguide and thus limits transmission length. In the early 1960s, the invention of the laser spurred research activities in the area of optical communications. In 1968, Kao and Davies1 proposed the use of fusedsilica glass fiber as a medium for guided optical transmissions. In 1970, Kapron et al.2 demonstrated an attenuation coefficient of 20 dB/km at a 0.85 m wavelength for a multimode silica glass fiber. In other words, the transmission efficiency of injected signal power is 1% after 1 km of fiber. During the following decade, major developments were made: the transmission window shifted from 0.85 m to 1.31 m and finally to 1.55 m—where the loss spectrum of silica glass exhibits its minimum value—and attenuation values were drastically reduced. Single-mode fibers
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(SMFs) exhibiting an attenuation as low as 0.20 dB/km at 1.55 m were demonstrated in 1979.3 The current absolute attenuation record is 0.152 dB/km at 1.55 m.4 In other words, the transmission efficiency of an injected signal power is 1% after 130 km of fiber. This approaches the minimum theoretical limit of transparency. Several factors contribute to fiber loss within transmission fibers. Lowering fiber attenuation is still an important issue, as it permits an increase in the span length between two amplifiers and thus reduces the cost of the optical link (based on the hypothesis that dispersion compensation and nonlinear effects5 are still properly mastered). In the late 1980s, the emergence of wavelength-division multiplexing (WDM) transmissions allowed a drastic increase of fiber bandwidth by the use of multiple transmission channels (each of which corresponds to a distinct wavelength). Hence, the optical-fiber loss spectrum must be optimized for a broad wavelength range and not only at 1.55 m. The conventional transm
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