Optical fiber waveguides and their transmission characteristics have been considered in some details in chapter 2 and 3. However we have yet to discuss the practical considerations and problems associated with the production, application and installation of optical fibers within a line transmission system.

In this chapter, we therefore consider the first three of the above practical elements associated with optical fiber communications.

4.2    Liquid-phase (melting) techniques

The first stage in this process is the preparation of ultra pure material powders which are usually oxides or carbonates of the required constituents. These include oxides such as SiO₂,GeO₂,B₂O₂, and A₂O₃ and carbonates such as Na₂CO₃, K₂CO₃,CaCO₃ and BaCO₃ which will decompose into oxides during the glass melting. Very high initial purity is essential and purification accounts for a large proportion of the material cost ; nevertheless these compound are commercially available with total transitions metal contents below 20 parts in 109 and below 1 part in 109 for some specific impurities.

Silica crucibles can give dissolution into the melt which may introduce inhomogeneities into the glass, especially at high melting temperatures. A techniques for avoiding this involves melting the glass directly into a radiofrequency (RF approximately 5 MHz) induction furnace while cooling the silica by gas or water flow, as shown in Figure 4.2 [Refs. 6 to 81]. The materials are preheated to arc the 1000 ° C where they exhibit sufficient ionic conductivity to enable coupling between of the melt and the RF field. The melt is also protected from any impurities in the crucible by a thin layer of solidified pure glass which forms due to the temperature difference between the melt and the cooled silica crucible.

In both techniques the glass is homogenized and dried by bubbling pure through the melt, whilst protecting against any airborne dust particles originating in the melt furnace or present as atmospheric contamination. After the melt has been suitably processed, it is cooled and formed into long rods (cane) multicomponent glass.

The first stage in this process is the preparation of ultra pure material powders which are usually oxides or carbonates of the required constituents. These include oxides such as SiO2,GeO2,B2O2, and A2O3 and carbonates such as Na2CO3, K2CO3,CaCO3 and BaCO3 which will decompose into oxides during the glass melting. Very high initial purity is essential and purification accounts for a large proportion of the material cost ; nevertheless these compound are commercially available with total transitions metal contents below 20 parts in 109 and below 1 part in 109 for some specific impurities.

Silica crucibles can give dissolution into the melt which may introduce inhomogeneities into the glass, especially at high melting temperatures. A techniques for avoiding this involves melting the glass directly into a radiofrequency (RF approximately 5 MHz) induction furnace while cooling the silica by gas or water flow, as shown in Figure 4.2 [Refs. 6 to 81]. The materials are preheated to arc the 1000 ° C where they exhibit sufficient ionic conductivity to enable coupling between of the melt and the RF field. The melt is also protected from any impurities in the crucible by a thin layer of solidified pure glass which forms due to the temperature difference between the melt and the cooled silica crucible.

In both techniques the glass is homogenized and dried by bubbling pure through the melt, whilst protecting against any airborne dust particles originating in the melt furnace or present as atmospheric contamination. After the melt has been suitably processed, it is cooled and formed into long rods (cane) multicomponent glass.

The traditional technique for producing the fine optical fiber waveguides is to make a preform using the rod in tube process. A rod of core glass is inserted into the tube of cladding glass and the preform is drawn in a vertical muffle furnaces illustrated in Figure 4.3. This technique is very useful for the production of step index fiber with large core and cladding diameters where the achievement of low attenuation is not critical is there is a danger of including bubbles and particulate matter at the core-cladding interface. Subsequent development in drawing of optical fibers (especially graded index) produced by liquid-phase has concentrated on the double crucible method. In this method the core and cladding glass in the form of separate rods is fed into two concentric platinum crucibles, as illustrated in Figure 4.5. The assembly is usually located in muffle furnace capable of heating the crucible contents to a temperature of between 800 and 1200 C. The crucibles have nozzles in their bases from which the clad fiber is drawn directly form the melt, as shown in Figure 4.5 . Index grading may be achieved through the diffusion of mobile ions across the core-cladding interface within the molten glass. It is possible to achieve a reasonable refractive index profile via this diffusion process, although due to lack of precise control it is not possible to obtain the optimum near parabolic profile which yields the minimum pulse dispersion (see Section 3.10.2). Hence graded index fibers produced by this technique are subsequently less dispersive than step index fibers, but do not have the bandwidth-length products of optimum profile fibers. Pulse dispersion of I to 6 ns km^-1 [Refs. 12, 13] is quite typical, depending on the material system used.

Some of the material systems used in the fabrication of multicomponent glass step index and graded index fibers are given in Table 4. 1. Using very high purity melting techniques and the double crucible drawing method, step index and graded index fibers with attenuations as low as 3.4 dB km [Ref. 14] and 1.1 dBkrn-1 [Ref. 21, respectively, have been Produced. However, Such low losses cannot be consistently obtained using liquid-phase techniques and typical losses for multicomponent glass fibers prepared continuously by these method are between 5 and 10 dB km^-1. Therefore, liquid-phase techniques the inherent disadvantage of obtaining and maintaining extremely pure glass which litnits their ability to produce low loss fibers. The advantage of these techniques is in the possibility of continuous production (both inelting and dra@kirw) of optical fibers.

4.5 Vapour-phase deposition techniques

Vapour-phase deposition techniques are used to produce silica-rich glasses of the highest transparency and with the optimal optical properties. The starting material are volatile compounds such as SiCl₄, GeC1₄, SiF₄, BCl₃, O₂, BBr₃ and POCl₃ which may be distilled to reduce tile concentration of most transition metal impurities to below one part in 10⁹, giving negligible absorption losses front these elements. Refractive index modification is achieved through tile formation of dopants from the nonsilica starting materials. These vapour -phase dopants include TiO₂, GeO₂, P₂O₅, AI₂O₃, B₂O₃, and F, tile effects of which on the refractive index of silica are shown in Figure 4.6 [Ref. 2]. Gaseous mixtures of the silica containing compound, the doping material and oxygen are combined in a vapour-phase oxidation reaction where the deposition of oxides occurs. File deposition is usually on to a substrate or within a hollow tube and is built tip as a stack of successive layers. Hence the dopant concentration may be varied gradually to Produce a graded index profile or maintained to give a step index profile. In the case of the substrate this directly results in a solid rod or preform whereas the hollow tube must be collapsed to give a solid preform from which the fiber may be drawn.

There are a number of variations of vapour-phase deposition which have been successfully utilized to produce low loss fibers. The major techniques are illustrated in Figure 4.7, which also indicates the plane (horizontal or vertical) in which, the deposition takes place as well as the formation of the preform. These vapour-phase deposition techniques fall into two broad categories: flame hydrolysis and chemical vapour deposition (CVD) methods. The individual techniques are considered in the following sections.

4.6 Outside vapour-phase oxidation (OVPO) process

This process which uses flame hydrolysis stems from work on ‘soot’ processes originally developed by Hyde [Ref. 161 which were used to produce the first fiber with losses of less than 20 dB km^-1 [Ref. 17]. The best known technique of this type is often referred to as the outside vapour-phase oxidation process. In this process the required glass composition is deposited laterally from a 'soot' generated by hydrolyzing the halide vapours in an oxygen-hydrogen flame. Oxygen is passed through the appropriate silicon compound (i.e. SiC1₄) which is vaporized, removing any impurities. Dopants such as GeC1₄ or TiCl₄, are added and the mixture is blown through the oxygen-hydrogen flame giving the following reactions:

The silica is generated as a fine soot which is deposited oil a cool rotating mandrel, as illustrated in Figure 4.8(a) [Ref. 181]. The flame of' the burner is reversed back, and forth over the length of the mandrel until a sufficient number of' layers of' silica (approximately 200) are deposited oil it. When this process is completed tile mandrel is removed and the porous mass of' silica soot is sintered (to form a glass body), as illustrated in Figure 4.8(b). The preform may contain both core and cladding glasses by properly varying the dopant concentrations during the deposition process. Several kilometres (around 10 km of 120 Am core diameter fiber have been produced [Ref. 21]) can be drawn from the preform by collapsing and closing the central hole, as shown in Figure 4.8(c). Fine control of the index gradient for graded index fibers may be achieved using this process as the gas flows can be adjusted at the completion of each traverse of the burner. Hence fibers with bandwidth-length products as high as 3 GHz km have been achieved [Ref. 19) through accurate index grading with this process.

The purity of the glass fiber depends on the purity of the feeding materials and also upon the amount of OH impurity from the exposure of the silica to water vapour in the flame following the reactions given in Eqs. (4. 1) to (4.4). Typically, the OH content is between 50 and 200 parts per million and this contributes to the fiber attenuation. It is possible to reduce the OH impurity content by employing gaseous chlorine as a drying agent during sintering. This has given losses as low as I dBkm^-1 and 1.8 dBkm^-1 at wavelengths of 1.2 and 1.55 m m respectively [Ref. 20] in fibers prepared using the OVPO process.

Other problems stem from the use of the mandrel which can create some difficulties in the formation of the fiber preform.

This process was developed by lzawa et al. [Ref. 22i in the search for a continuous (rather than batch) technique for the production of low loss optical fibers. The VAD technique uses an end-ort deposition oil to I rotating fused silica target, as illustrated in Figure 4.9 [Ref. 23). The vaporized constituents are injected from burners and react to form silica root by flame hydrolysis. This is deposited on the of the end of the starting target in the axial direction forming a solid porous glass preform in the shape of a boule. The preform which is growing in the axial direction is pulled upwards at a rate which corresponds to the growth rate. It is initially dehydrated by heating with SOC1₂ using the reaction:

and is then sintered into a solid preform in a graphite resistance furnace at all elevated temperature of around 1500 OC. Therefore, in principle this process mav be adapted to draw fiber continuously, although at present it tends to be operated as a batch process partly because the resultant preforms can yield more than 100 km of fiber [Ref. 21].

A spatial refractive index profile may be achieved using the deposition properties of SiO₂-GeO₂ particles within the oxygen-hydrogen flame. The concentration of these constituents deposited on the porous preform is controlled by the substrate temperature distribution which can be altered by changing the gas flow conditions. Fibers produced by the VAD process still suffer from some OH impurity content due to the flame hydrolysis and hence very low loss fibers have not been achieved using this method. Nevertheless, fibers with attenuation in the range 0.7 to 2.0 dB km^-1 at a wavelength of 1. 181 m m have been reported [Ref. 24] .

Chemical vapour deposition techniques are commonly used at very low deposition rates in the semiconductor industry to produce protective SiO2 films on silicon semiconductor devices. Usually an easily oxidized reagent such as SiH₄ diluted by inert gases and mixed with oxygen is brought into contact with a heated silicon surface where it forms a glassy transparent silica film. This heterogeneous reaction (i.e requires a surface to take place) was pioneered for the fabrication of optical fibers using the inside surface of a fused quartz tube [Ref. 25]. However, these processes gave low deposition rates and were prone to OH contamination due to the use of hydride reactants. This led to the development of the modified chemical vapour deposition (MCVD) process by Bell Telephone Laboratories [Ref. 261 and Southampton University, UK [Ref. 27], which overcomes these problems and has found widespread application throughout the world.

The MCVD process is also an inside vapour-phase oxidation (IVPO) technique taking place inside a silica tube, as shown in Figure 4. 10. However, the vapour-phase reactants (halide and oxygen) pass through a hot zone so that a substantial part of the reaction is homogeneous (i.e. involves only one phase; in this case the vapour phase). Glass particles formed during this reaction travel with the gas flow and are deposited on the walls of the silica tube. The tube may form the cladding material but usually it is merely a supporting structure which is heated on the outside by an oxygen-hydrogen flame to temperatures between 1400 'C and 1600 'C. Thus a hot zone is created which encourages high temperature oxidation reactions such as those given in Eqs. (4.2) and (4.3) or (4.4)

(not Eq. (4. 1)). These reactions reduce the OH impurity concentration to levels below those found in fibers prepared by hydride oxidation or flame hydrolysis.

The hot zone is moved back and forth along the tube allowing the particles tube deposited on a layer by layer basis giving a sintered transparent silica him on the walls of the tube. The film may be up to 10 m in thickness and uniformity is maintained by rotating the tube. A graded refractive index profile can be created by changing the composition of the layers as the glass is deposited. Usually, when sufficient thickness has been formed by successive traverses of the burner for the cladding, vaporized chlorides of germanium (GeC1₄) or phosphorus (POC1₃) are added to the gas flow. The core glass is then formed by the deposition of successive layers of germanosilicate or phosphosilicate glass. The cladding layer is important as it acts as a barrier which suppresses OH absorption losses due to the diffusion of OH ions from the silica tube into the core glass as it is deposited. After the deposition is completed the temperature is increased to between 1700 and 1900° C.

The tube is then collapsed to give a solid preform which may then be drawn into fiber at temperatures of' 2000 to 2200 'C as illustrated in Figure 4. 10.

This technique is the most widely used at present as it allows the fabrication of fiber with the lowest losses. Apart from the reduced OH impurity contamination tile MCVD process has the advantage that deposition occurs within an enclosed reactor which ensures a very clean environment. Hence, gaseous and particulate impurities may be avoided during both the layer deposition and the preform collapse phases. The process also allows the use of a variety of materials and glass compositions. It has produced GeO₂ doped silica single-mode fiber with minimum losses of only 0.2 dB km^-1 at a wavelength of 1.55 m m [Ref. 28]. More generally, the GeO₂-B₂0₃-SiO₂ system (B₂0₃ is added to reduce the viscosity and assist fining) has shown minimum losses of 0.34 dB km^-1 with multimode fiber at a wavelength of 1.55 m m [Ref. 29]. Also, graded index germanium phosphosilicate fibers have exhibited losses near the intrinsic level for their composition of 2.8, 0.45 and 0.35 dB km-' at wavelengths of 0.92, 1.3 and 1.5 m m respectively [Ref. 301.

The MCVD process has also demonstrated the capability of producing fibers with very high bandwidths, although still well below the theoretical values which may be achieved. Multimode graded index fibers with measured bandwidth-length products of 4.3 GHz km and 4.7 GHz km at wavelengths of 1.25 and 1.29 m m have been reported [Ref. 31]. Large-scale batch production (30 000 km) of 50 m m core graded index fiber has maintained bandwidth-length products of 825 MHz km and 735 MHz/km at wavelengths of 0.825 and 1.3 turn respectively [Ref. 301. The median attenuation obtained with this fiber was 3.4dBkm-l at 0.825,um and 1.20 dB km at 1.3 m m. Hence, although it is not a continuous process, the MCVD technique has proved suitable for tile mass production of high performance optical fiber and is the predominant technique used to prepare polarization maintaining fibers (see Section 3.13.21). Moreover it can be scaled-up to produce preforms which provide 100 to 200 km of fiber .