In selecting materials for optical fibers, a number of requirements must be satisfied. For example:
（1）It must be possible to make long, thin flexible fibers from the material.
（2）The material must be transparent at a particular optical wavelength in order for the fiber
to guide light efficiently.
（3）Physically compatible materials that have slightly different refractive indices for the core
and cladding must be available.
Materials that satisfy these requirements are glasses and plastics.
The majority of fibers are made of glass consisting of either silica (SiO2) or a silicate. The variety of available glass fibers ranges from high-loss glass fibers with large cores used for short-transmission distances to very transparent (low-loss) fibers employed in long-haul applications. Plastic fibers are less widely used because of their substantially higher attenuation than glass fibers. The main use of plastic fibers is in short-distance applications (several hundred meters) and in abusive environments, where the greater mechanical strength of plastic fibers offers an advantage over the use of glass fibers.
1.Glass Fibers
Glass is made by fusing mixtures of metal oxides, sulfides, or solenoids. The resulting material is a randomly connected molecular network rather than a well-defined ordered is structure as found in crystalline materials. A consequence of this random order is that glasses do not have well-defined melting points. When glass is heated up from room temperature, it remains a hard solid up to several hundred degrees centigrade. As the temperature increases further, the glass gradually begins to soften until at very high temperatures it becomes a viscous liquid. The expression“ melting temperature” is commonly used in glass manufacture. This term refers only to an extended temperature range in which the glass becomes fluid enough to free itself fairly quickly of gas bubbles.
The largest category of optically transparent glasses from which optical fibers are made consists of the oxide glasses. Of these, the most common is silica (SiO2), which has a refractive index of 1.458 at 850 nm. To produce two similar materials that have slightly different indices of refraction for the core and cladding, either fluorine or various oxides (referred to as departs), such as B2O3, GeO2, or P2O5, are added to the silica. As shown in the addition of GeO2 or P2O5 increases the refractive index, whereas doping the silica with fluorine or B2O3 decreases it. Since the cladding must have a lower index than the core, examples of fiber compositions are
1. GeO2-SiO2 core; SiO2 cladding
2. P2O5-SiO2 core; SiO2 cladding
3. Sio2 core; B2O3-SiO2 cladding
4. GeO2-B2O3-SiO2 core; B2O3-SiO2 cladding
Here, the notation GeO2-SiO2, for example, denotes a GeO2-doped silica glass. Referred to as either silica glass, fused silica, or vitreous silica. Some of its desirable properties are a resistance to deformation at temperatures as high as 1000,a high resistance to breakage from thermal shock because of its low thermal expansion, good chemical durability, and high transparency in both the visible and infrared regions of interest to fiber optic communication systems. Its high melting temperature is a disadvantage if the glass is prepared from a molten state. However, this problem is partially avoided when using vapor deposition techniques.
2.Halide Glass Fibers
In 1975 researchers at the Universities de Rennes discovered fluoride glasses that have extremely low transmission losses at mid-infrared wavelengths (0.2-8um,with the lowest loss being around 2.55um).Fluoride glasses belong to a general family of halide glasses in which the anions are from elements in group Ⅶ of the periodic table, namely fluorine, chlorine, chlorine, bromine, and iodine.
The material that researchers have concentrated on is a heavy metal fluoride glass, which uses ZrF4 as the major component and glass network former. Several other constituents need to be added to make a glass that has moderate resistance to crystallization. Table 2-3 lists the constituents and their molecular percentages of a particular fluoride glass referred to as ZBLAN(after its elements ZrF4,BaF2, LaF3, AlF3 and NaF).This material forms the core of a glass fiber. To make a lower-refractive-index glass, one partially replaces ZrF4 by HaF4 to get a ZHBLAN cladding.
Although these glasses potentially offer intrinsic minimum losses of 0.01-0.001 dB/km, fabricating long lengths of these fibers is difficult. First, ultra pure materials must be used to reach this low loss level. Second, fluoride glass is prone to diversification. Fiber-making techniques have to take this into account to avoid the formation of microcry stallites, which have a drastic effect on scattering losses.
3.Active Glass Fibers
Incorporating rare-earth elements into a normally passive glass the resulting material new optical and magnetic properties. These new properties allow the material to perform amplification, attenuation, and phase retardation on the light passing through it. Doping can be carried out for both silica and halide glasses.
Two commonly used materials for fiber lasers are erbium and neodymium. The ionic concentrations of the rare-earth elements are low to avoid clustering effects. By examining the absorption and fluorescence spectra of these materials, one can use an optical source which emits at an absorption wavelength to excite electrons to higher energy levels in the rare-earth do pants. When these excited electrons drop to lower energy levels, they emit light in a narrow optical spectrum at the fluorescence wavelength. Chapter 11 discusses the applications of erbium-doped fibers to optical amplifiers.
4.Chalgenid Glass Fibers
In addition to allowing the creation of optical amplifiers, the nonlinear properties of glass fibers can be exploited for other applications, such as all-optical switches and fiber lasers. Chalgenide glass is one candidate for these uses because of its high optical nonlinearity and its long interaction length. These glasses contain at least one chalcogen element (S, Se, or Te) and typically one other element such as P, I, Cl, Br, Cd, Ba, Si, or Ti for tailoring the thermal, mechanical, and optical properties of the glass. Among the various chalgenide glasses, As2S3is one of the most well known materials. Single-mode fibers have been made using As40S58Se2 and As2S3 for the core and cladding materials, respectively. Losses in these glasses typically range around 1dB/m.
5.ic Optical Fibers
The growing demand for delivering high-speed services directly to the workstation has led fiber developers to create high-bandwidth graded-index polymer optical fibers (POF) for use in a customer premises. The core of these fibers is either polymethylmethacrylate or a per fluorinated polymer. These fibers are hence referred to as PMMA POF and PFP POF, respectively. Although they exhibit considerably greater optical signal attenuations than glass fibers, they are tough and durable. For example, since the modulus of these polymers is nearly two orders of magnitude lower than that of silica, even a 1-m-diameter graded-index POF is sufficiently flexible to be installed in conventional fiber cable routes. Compared with silica fibers, the core diameters of plastic fibers are 10-20 times larger, which allows a relaxation of connector tolerances without sacrificing optical coupling efficiencies. Thus, inexpensive plastic injection-molding technologies can be used to fabricate connectors, splices, and transceivers.
发表于 2005-4-23 15:42:00
