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Optical fibers are typically heated and drawn from silica preforms, which usually consist of two concentric cylinders called the core and the cladding, in a high-temperature furnace. For optical communication purpose, the core always has a higher refractive index than the cladding. In order to investigate the effect of core-cladding structure on th...
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... flows of the double-layer glass preform and aiding purge gas in a cylindrical furnace are considered. A schematic diagram of the double-layer optical fiber drawing process is shown in Fig. 1. The governing equations for the glass layers and the gas are given ...
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... initial values and constants are defined as 26: n d 0 =0, n p 0 =7 10 22 g −1 , E p = 6.4087 10 −19 J, E d = 0.3204 10 −19 J, v =8 10 −3 s −1 , and K = 1.380658 10 −23 J / K. As expected, the concentration of E defects is found to be larger in the doublelayer preform due to higher preform temperature, as shown in Fig. 10. It is indicated that the fiber quality is degraded with an increase in refractive index difference between the core and the cladding in terms of E defects. Neglecting the shear force exerted by the gas, the draw tension can be obtained from the following equation ...
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... calculated forces including draw tension F T , and forces due to viscous stress F , surface tension F , inertia F I , and gravity F g , are shown in Fig. 11. It is seen that a smaller draw tension is obtained for the double-layer preform drawing. It is because the force due to viscous stress is much smaller since viscosity of silica decreases drastically with increasing temperature, although the magnitude of the force due to gravity decreases as ...
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... refractive index is taken as uniform in the double-layer preform. Figure 12 indicates the temperature distributions along the centerline for both the double-layer and the single-layer preforms. It is interesting to note that when the absorption coefficient in the cladding gets larger, the centerline temperature drops due to the lower transmissivity of the cladding. ...
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... along the centerline for both the double-layer and the single-layer preforms. It is interesting to note that when the absorption coefficient in the cladding gets larger, the centerline temperature drops due to the lower transmissivity of the cladding. The effect of change in the absorption coefficient on the temperature lag is shown in Fig. 13. It is seen that the difference between the surface temperature and centerline temperature becomes larger near the entrance. This is because the cladding is heated up faster near the entrance with larger absorption coefficient and the centerline temperature becomes lower as shown in Fig. 12. Since the temperature of preform decreases ...
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... coefficient on the temperature lag is shown in Fig. 13. It is seen that the difference between the surface temperature and centerline temperature becomes larger near the entrance. This is because the cladding is heated up faster near the entrance with larger absorption coefficient and the centerline temperature becomes lower as shown in Fig. 12. Since the temperature of preform decreases with an increase in absorption coefficient of the cladding, the neck-down region of double-layer preform starts farther from the entrance than that for the single-layer preform, as shown in Fig. 14. Figure 15 shows that the concentration of E defects along the free surface in the double-layer ...
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... the entrance with larger absorption coefficient and the centerline temperature becomes lower as shown in Fig. 12. Since the temperature of preform decreases with an increase in absorption coefficient of the cladding, the neck-down region of double-layer preform starts farther from the entrance than that for the single-layer preform, as shown in Fig. 14. Figure 15 shows that the concentration of E defects along the free surface in the double-layer preform is lower than that in the single-layer preform, as expected. The draw tension for double-layer preform drawing is also obtained, as shown in Fig. 16. It is found that a larger absorption coefficient in the cladding has a stronger ...
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... is because the cladding is heated up faster near the entrance with larger absorption coefficient and the centerline temperature becomes lower as shown in Fig. 12. Since the temperature of preform decreases with an increase in absorption coefficient of the cladding, the neck-down region of double-layer preform starts farther from the entrance than that for the single-layer preform, as shown in Fig. 14. Figure 15 shows that the concentration of E defects along the free surface in the double-layer preform is lower than that in the single-layer preform, as expected. The draw tension for double-layer preform drawing is also obtained, as shown in Fig. 16. ...
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... preform starts farther from the entrance than that for the single-layer preform, as shown in Fig. 14. Figure 15 shows that the concentration of E defects along the free surface in the double-layer preform is lower than that in the single-layer preform, as expected. The draw tension for double-layer preform drawing is also obtained, as shown in Fig. 16. It is found that a larger absorption coefficient in the cladding has a stronger effect on forces due to viscous stress F than forces due to gravity F g . This results in larger draw tension with larger absorption coefficient in the ...
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... Then, each separate layer in the glass is assumed to have a uniform refractive index, bounded by diffuse surfaces. The zonal method is applied to calculate the radiation transfer inside the three enclosures (Chen and Jaluria 2007). The two-band model presented by Myers (1989) and given below may be used for the absorption coefficient a of pure silica. ...
This chapter discusses the fabrication of optical fibers, focusing on the drawing, cooling, and coating of fibers. The basic transport mechanisms that arise are discussed, along with results from analytical, numerical, and experimental studies. Starting with the fabrication of the preform, this chapter discusses the thermal transport in the draw furnace and the flow in silica glass. Of particular interest are the neck-down profile, defects generated during the process, feasibility of the process, and possible optimization of the process. The consideration is also extended to hollow, microstructured, and doped fibers that are of interest in different applications. The cooling and the liquid polymer coating processes are similarly considered, to discuss the underlying phenomena and present characteristics results. In all these considerations, a particular aspect that is brought up is the quality of the fiber, as well as of the coating, in terms of defects, imperfections, entrapment of bubbles in the coating, dopant distribution, viscous rupture, and so on. Also considered is the production rate, as indicated by the drawing speed and the ranges over which operating conditions may be varied to obtain a high-quality coated fiber with low transmission loss. Even though the presentation is directed at the fabrication of optical fibers, many of the basic aspects, methods for analysis, concerns, and trends are similar to those applicable to other manufacturing processes, such as wire drawing. Some of these similarities are discussed in other chapters as well in order to link the basic aspects of different manufacturing processes and thus impact on new and existing processes and systems.
... With recent developments in optical physics and nanotechnology, fiber drawing techniques have been adapted to the creation of new types of optical fibers [14,24] and nanoparticles [25]. Although considerable numerical modeling has been conducted on solid fiber drawing, few investigations have been carried out on structured fiber drawing [26] with most analyses being based on a leading order (i.e., one-dimensional) description of the velocity profile along with a two-dimensional description of the temperature field [27][28][29][30][31][32][33][34]. ...
Considerable recent research has focused on the ability of microstructured fibers to exhibit diverse optical functionalities. However, accurately preserving the structure imposed at the preform stage after drawing it down to fiber, while avoiding Rayleigh-Plateau style instabilities, has proven to be a major fabrication challenge. This modeling/analytical study was carried out in support of an experimental program into possible fabrication options for various microstructured optical fibers and considers the generic case of the nonisothermal drawing of a capillary preform to fiber. Model development was carried out in two stages. Initially, a fully conjugate multiphase model, which includes all heat transfer modes within an operational fiber drawing furnace, was validated against available experimental data. To evaluate the external radiative heat flux using the net-radiation method, a Monte Carlo ray-tracing (MC-RT) method was coupled to the commercial polyflow package to obtain all view factors between the various furnace walls and the deforming preform/fiber. A simplified model was also developed (to shorten simulation run times) by explicitly calculating the convective heat transfer between the air within the furnace and the preform/fiber surface using a heat transfer coefficient determined by matching predicted results with those obtained from the multiphase model.
... Knowledge of the spectral absorption coefficient of fused silica optical fibers is important in modeling heat transfer in the processes and applications in which these fibers are used. These include optical fiber thermometers [1][2][3][4][5][6], the splicing of optical fibers using lasers [7], the fabrication of fiber couplers and tapers [8], and the manufacture of optical fibers by drawing [9][10][11][12][13]. Myers [14] introduced a two-band model of the spectral absorption coefficient of fused silica that has been widely used to model heat transfer in optical fiber drawing processes [9][10][11][12]. ...
... These include optical fiber thermometers [1][2][3][4][5][6], the splicing of optical fibers using lasers [7], the fabrication of fiber couplers and tapers [8], and the manufacture of optical fibers by drawing [9][10][11][12][13]. Myers [14] introduced a two-band model of the spectral absorption coefficient of fused silica that has been widely used to model heat transfer in optical fiber drawing processes [9][10][11][12]. This model neglects absorption at wavelengths less than 3 μm. ...
... The refractive index of silica is relatively constant over this spectral band [29]. Therefore, in Eq. (11), it was assumed that the critical angle does not vary with wavelength. Figure 3 also shows spectral absorption coefficient measurements of low OH fused silica made by Izawa and co-workers [15,16], which compare favorably with those made in this work. ...
Knowledge of the spectral absorption coefficient of fused silica optical fibers is important in modeling heat transfer in the processes and applications in which these fibers are used. An experimental method used to measure the spectral absorption coefficient of optical fibers is presented. Radiative energy from a blackbody radiator set at different temperatures is directed through the optical fibers and into an FTIR spectrometer. Spectral instrument response functions are calculated for different fiber lengths. The ratios of the slopes of the instrument response functions for the different lengths of fibers are used to solve for the spectral absorption coefficient of the fibers. The spectral absorption coefficient of low OH pure fused silica optical fibers is measured between the wavelengths 1.5 and 2.5 μm.
... The governing equations derived in earlier papers [11,12] are used in this paper. The flows in the glass and the gas are assumed to be laminar and axisymmetric, due to the low mass flow rates. ...
... The ratio of diameters of the core and the preform is set at a typical value of 1:2. Radiation transport at the end of the finite-sized furnace is considered in terms of the iris, opening and moving fiber [12]. The diameter of the furnace is taken as 7 cm and the temperature distribution for the furnace is taken as [13], ...
... 2 =1.24 F/GeO 2 =2.12 ...
Optical fibers are typically drawn from silica preforms, which usually consist of two concentric cylinders called the core and the cladding, heated in a high-temperature furnace. For optical communication purposes, the core generally has a higher average refractive index than the cladding to obtain total internal reflection. This paper investigates the effects of adding dopants to the core or to the cladding, to change the refractive index values, on the optical fiber drawing process. Employing an analytical/numerical model developed earlier to simulate the core-cladding structure of a typical optical fiber, the paper considers different dopants and the effects resulting from the consequent changes in properties, particularly the radiation absorption properties, on the temperature distributions, flow, neck-down profile, thermally induced defects and draw tension. The zonal method is applied to model the radiation transfer in the glass perform and the purge gas is taken as non-participating. The numerical model has been validated by comparing with results available in the literature, wherever possible. It is found that the effects are significant because of changes in refractive index and absorption of radiation, which give rise to significant changes in temperature and tension. These can, in turn, substantially affect fiber quality and characteristics. Therefore, for an accurate and realistic modeling of the process, the effects of property changes due to dopants on the draw process must be included.
... Radiation heat transfer is the dominant mode of transport in the heating process and the viscosity of the glass, which is a subcooled liquid. is directly related to the draw tension, may affect the transmission properties and also result in strength degradation [3]. Another important class of optical fibers, hollow or microstructure fibers, has grown in interest in recent years since these have many advantages over the conventional solid-core optical fibers. ...
... Pressurization of the gas in the core is neglected. The starting neck-down profiles are taken as the results for the solid-core double-layer fiber obtained by Chen and Jaluria [3]. The profile correction scheme is applied to get the final converged neck-down profiles. ...
In many practical circumstances, the flow in microchannels is driven by moving surfaces that impart shear to the fluid. The shear may generate a pressure differential or the pressure may be imposed to affect the resulting flow and heat transfer. Processes where such flows arise include coating, cooling and thermal treatment of moving wires, fibers, and microscale devices. One particularly important circumstance is the optical fiber coating process, where both the entrance of the moving surface into a reservoir of fluid, as well as the exit, are of interest, since the thermal transport in the relevant microchannels influences the resulting coating very substantially. Similarly, the drawing of microscale fibers and the cooling of fibers after drawing are important in the overall manufacturing process and involve microchannel transport. This paper discusses the basic considerations in such processes, particularly the flow that arises in drawing and coating and the menisci that are observed at the inlet and outlet regions of the two microchannels. Experimental and analytical/numerical work has been carried out on these flows and the results obtained are presented. An important aspect is the pressure rise in the channel for narrowing flow domains, such as those employed in dies, and a comparison with imposed pressures. It is found that, in many practical problems, the shear generates much higher pressures than the typically imposed pressures and, thus, the flow is largely dominated by the shear effects due to the moving surfaces. It is also found that the flow develops very rapidly in forced convective cooling by gases, such as nitrogen and helium, resulting in thermal transport in a largely developed flow region. Comparisons between experimental and numerical results show fairly good agreement, indicating the validity of the model for such complex microchannel flows.
This paper reviews the microscale transport processes that arise in the fabrication of advanced materials. In many cases, the dimensions of the device being fabricated are in the micrometer length scale and, in others, underlying transformations that determine product quality and characteristics are at micro- or nanoscale levels. The basic considerations in these transport phenomena are outlined. A few important materials processing circumstances are considered in detail. These include the fabrication of multilayer and hollow optical fibers, as well as those where micro- and nanoscale dopants are added to achieve desired optical characteristics, thin film fabrication by chemical vapor deposition, and microscale coating of fibers and devices. It is shown that major challenges are posed by the simulation and experimentation, as compared with those for engineering or macroscale dimensions. These include accurate simulation to capture large gradients and variations over relatively small dimensions, simulating high pressures and viscous dissipation effects in microchannels, modeling effects such as surface tension that become dominant at microscale dimensions, and coupling micro- and nanoscale mechanisms with boundary conditions imposed at the macroscale. Similarly, measurements over microscale dimensions are much more involved than those over macro- or industrial scales because of difficult access to the regions of interest, relatively small effects such as tension, buoyancy effects, viscous rupture, bubble entrapment, and other mechanisms that are difficult to measure and that can make the process infeasible. It thus becomes difficult to achieve desired accuracy for validating the mathematical and numerical models. This paper reviews some of the approaches that have been adopted to overcome these difficulties. Comparisons between experimental and numerical results are included to show fairly good agreement, indicating the validity of the modeling of transport.
This paper discusses microscale transport processes that arise in the fabrication of advanced materials. In many cases, the dimensions of the device being fabricated is in the microscale range and, in others, underlying transformations that determine product quality and characteristics are at micro or nanoscale levels. The basic considerations in these transport phenomena are outlined. Three important materials processing circumstances are considered in detail. These include the fabrication of multilayer and hollow optical fibers, as well as those where microscale dopants are added to achieve desired optical characteristics, thin film fabrication by chemical vapor deposition and microscale coating of fibers and devices. It is shown that major challenges are posed by the simulation as well as experimentation over microscale dimensions. These include accurate simulation to capture large gradients and variations over relatively small dimensions, simulating high pressures and viscious dissipation effects in microchannels, modeling effects such as surface tension that become dominant at microscale dimensions, and coupling micro and nanoscale mechanisms with boundary conditions imposed at the macroscale. Similarly, measurements over microscale dimensions are much more involved that those over macro or industrial scales because of access to the regions of interest, small tension effects and other mechanisms that are difficult to measure and that can make the process infeasible, and difficulty in achieving desired accuracy for validating the mathematical and numerical models. The paper presents some of the approaches that may be adopted to overcome these difficulties. Comparisons between experimental and numerical results are included to show fairly good agreement, indicating the validity of the modeling of transport.
This paper presents a mathematical model to simulate the silica hollow optical fiber drawing process. Two neck-down profiles, which represent the inner and outer surfaces of the hollow fiber, are generated by using an iterative numerical scheme. The zonal method is applied to calculate the radiative transport within the glass. The effects of variable properties for air and buoyancy are investigated and results indicate that these can be neglected for simulating the draw process. The validation of the model is carried out by comparing the results with those obtained by using the optical thick method as well as those for a solid-core fiber. The effect of drawing parameters like the temperature of the furnace, feeding speed and drawing speed on the temperature and velocity distributions and on the draw tension are studied. It is found that the geometry and qualities of the final hollow optical fiber are highly dependent on the drawing parameters, especially the drawing temperature and the feeding speed.
Microscale transport mechanisms play a critical role in the thermal processing of materials because changes in the structure and characteristics of the material largely occur at these or smaller length scales. The heat transfer and fluid flow considerations determine the properties of the final product, such as in a crystal drawn from silicon melt or a gel from the chemical conversion of a biopolymer. Also, a wide variety of material fabrication processes, such as the manufacture of optical glass fiber for telecommunications, fabrication of thin films by chemical vapor deposition, and surface coating, involve microscale length scales due to the requirements on the devices and applications for which they are intended. For example, hollow fibers, which are used for sensors and power delivery, typically need fairly precise microscale wall thicknesses and hole diameters for satisfactory operation. The basic transport mechanisms underlying these processes are discussed in this review paper. The importance of material characterization in accurate modeling and experimentation is brought out, along with the coupling between the process and the resulting properties such as uniformity, concentricity, and diameter. Of particular interest are thermally induced defects and other imperfections that may arise due to the transport phenomena involved at these microscale levels. Additional aspects such as surface tension, stability, and free surface characteristics that affect the material processing at microscale dimensions are also discussed. Some of the important methods to treat these problems and challenges are presented. Characteristic numerical and experimental results are discussed for a few important areas. The implications of such results in improving practical systems and processes, including enhanced process feasibility and product quality, are also discussed.
