Numerical study on the dopant concentration and refractive index profile evolution in an optical fiber manufacturing process

The index of refraction is an important property of optical fibers, since it directly affects the bandwidth and optical loss during information transmission. The refractive index is governed by the dopant concentration distribution across the fiber cross section, which is strongly influenced by the processing conditions. An understanding of the effects of process parameters on the dopant concentration profile evolution is important to design the drawing process for tailored refractive index and optical transmission characteristics. Although the heat and momentum transport in optical fiber drawing have been studied extensively, little has been reported in the open literature on dopant concentration and index of refraction profile development during processing. This paper presents a two-dimensional numerical analysis on the flow, heat and mass transfer phenomena involved in the drawing and cooling process of glass optical fibers using a finite difference approach based on primitive variables. The effects of several important parameters are investigated in terms of nondimensional groups, including: fiber draw speed, inert gas velocity, furnace dimensions, gas properties, and dopant properties on the flow, temperature and dopant concentration distribution.     

Feasibility and optimization of the hollow optical fiber drawing process

The drawing process for the fabrication of a hollow optical fiber involves the flow of glass, which is largely heated by thermal radiation, in an inert gas environment. It is critical to maintain the central core, which can collapse if the thermal conditions are not properly imposed and controlled. This paper presents the analysis and simulation of this complicated process. A numerical model is developed, validated, and applied to simulate the hollow optical fiber drawing process under a wide range of boundary conditions, particularly draw speed, tension, and temperature. A feasible domain of the drawing process is identified to give the range of the drawing parameters for which the geometry of the fiber is maintained and collapse of the core and viscous rupture of the fiber are avoided. The effects of drawing temperature and feeding speed, which are crucial factors in determining the geometry and quality of the fiber, are investigated in detail. A multi-variable unconstrained optimal design problem is posed and considered in terms of the feasible domain. An appropriate objective function, comprised of the maximum velocity lag, thermally induced defect concentration and draw tension, is proposed to quantify the quality of the hollow fiber. The univariate search method is then applied to obtain the optimal drawing temperature and feeding speed. This study provides a basis for the optimization of hollow fiber drawing process and indicates that a substantial improvement in fiber quality can be achieved.     

TEMPERATURE DISTRIBUTION OF AN OPTICAL FIBER TRAVERSING THROUGH A CHEMICAL VAPOR DEPOSITION REACTOR

A fundamental understanding of how reactor parameters influence the fiber surface temperature is essential to manufacturing high-quality optical fiber coatings by chemical vapor deposition (CVD). In an attempt to understand this process better, a finite-volume model has been developed to study the gas flow and heat transfer of an optical fiber as it travels through a CVD reactor. This study showed that draw speed significantly affects fiber temperature inside the reactor, with temperature changes over 50% observed under the conditions studied. Other parameters affecting fiber temperature include fiber radius, fiber coating emissivity, and gas flow velocity at inlet. Multiple heat transfer modes contribute to these phenomena, with convection and radiation heat transfer dominating the process. The numerical model is validated against analytical cases.     

High sensitivity hydrogen sensor based on tilted fiber Bragg grating coated with PDMS/WO3 film

A hydrogen sensor based on tilted fiber Bragg grating (TFBG) coated with PDMS/WO₃ composite film was proposed. The WO₃ powder can be adhere well to the surface of TFBG by the flexible PDMS film. The exothermic reaction between WO₃ and hydrogen made the effective refractive index of the cladding of the TFBG change, and the intensities of cladding modes coupled from the core mode changed accordingly. The proposed fiber hydrogen sensor showed the sensitivity of 0.596 dB/% under the hydrogen concentrations from 0% to 1.53%. The average response time and recovery time of the sensor were 93 s and 107 s, respectively. Moreover, the TFBG based sensor can be made inherently temperature self-compensation by referencing all cladding-mode wavelengths to the wavelength of the core mode resonance of the grating, which is isolated from the effective refractive index changes of the fiber surroundings.     

Effect of Furnace Thermal Configuration on Optical Fiber Heating and Drawing

ABSTRACT

The thermal configuration of the draw furnace, which involves the wall temperature profile, the temperature level, and the length of the heated zone, is an important aspect in high-speed optical fiber manufacture. This article presents a computational study on the effect of the furnace thermal configuration on the draw process and on physical quantities such as velocity and temperature difference across the preform/fiber, tension, and neck-down profile. Considering a cylindrical graphite furnace, the study solves a conjugate problem, which involves both the moving silica glass rod or fiber and the inert gases in the furnace. The flow and heat transfer in the two regions is linked due to the boundary conditions at the surface of the glass. Force balance conditions are used to determine the neck-down profile. A fairly versatile finite-difference numerical scheme is employed to consider different temperature distributions along the furnace wall, as well as a range of heating-region lengths. Besides the flow and thermal transport, the tension in the fiber and thermally induced defects that affect fiber quality are also calculated. A range of fiber draw speeds is also considered, and the effect on the variables that determine the fiber characteristics is studied.     

A Finite-Element Model for the Thermal Radiative Properties of Graded Index Fiber Coated with Thin Absorbing Film

A finite-element model in combination with the wave optical approach is developed based on the radiative transfer equation for graded index medium in cylindrical coordinate system to predict the total hemispherical thermal radiative properties of semitransparent graded index fiber coated with thin absorbing film. The film is made of a strong absorbing medium with thickness less than or on the order of the wavelength of peak magnitude of thermal radiation. Radiative absorptance of the fiber-film system is directly obtained by solving the radiation deposited in the system. Radiative transfer in the fiber is solved by a least squares finite-element method, while radiative transfer in the thin film is treated through wave optics, and the film is formulated as a special kind of semitransparent boundary condition for the fiber medium. The results obtained by the finite-element model for uniform index fiber are in good agreement with the results in the literature obtained through the ray tracing model. The effects of fiber refractive index distribution on predicted thermal radiative properties are investigated. For the fiber with or without film, the variation of refractive index distribution has a substantial influence on the effective emittance.     

Fixed Cartesian grid based numerical model for solidification process of semi-transparent materials II: Reflection and refraction or transmission of the thermal radiation at the solid–liquid interface

Mathematical and numerical models of solidification process in materials which were semi-transparent both in the solid and liquid phases were developed in this paper. These models took into account different optical and thermophysical properties in phases, herein for the first time different refractive index in the solid and the liquid phase. Also optical phenomena like reflection and refraction or transmission of the thermal radiation at transparent as well as either specular, partially specular and partially diffusive or diffusive solid–liquid interface were also considered. Conditions for the radiation intensity at the transparent solid–liquid interface were formulated according to the specular reflection and Snell’s laws. The numerical model was based on the Fixed Grid Front Tracking Method combined with the Immersed Boundary Technique for phase change process and on the Pixelation Technique for optical phenomena at the solid–liquid interface. Subsequently, comparisons of obtained results with results presented in the literature for one-dimensional two layers slab proved the correctness and accuracy of the proposed approach. Also the effect of different refractive index in the solid and the liquid phase on solidification process in an idealized square cavity were studied.     

Light leakage in optical fibers: experimental results, modeling and the consequences for solar concentrators

Optical fibers used to transport sunlight exhibit considerable light leakage within their nominal numerical aperture. Of particular interest in the design and diagnosis of solar fiber-optic concentrators is the dependence of this leakage on: (a) incidence angle, (b) the optical properties of the core and the cladding, and (c) fiber length. We present measurements of fiber angular transmission, along with a theoretical model. The implications for solar fiber-optic concentrators are also assessed.     

Temperature dependence of the optical band gap and refractive index of poly(ethylene terepthalate) oligomer–DDQ complex thin film

The temperature dependence of the optical band gap and refractive index dispersion of thin film of poly(ethylene terepthalate) oligomer–DDQ charge transfer complex has been investigated. The absorption edge shifts to the lower energy as consequence of the thermal annealing on film and the fundamental absorption edge corresponds to a direct energy gap. The temperature coefficient of the optical band gap for the film was found as dE_g/dT = − 3.15 × 10⁻³ eV/K. The temperature dependence of the refractive index has also been investigated and it is observed that the refractive index changes by annealing temperatures.     

Investigations on thermal and optical performances of a glazing roof with PCM layer

The thermal and optical performances of a roof in a building containing phase change material (PCM) were investigated in this paper. The glazing roof model consists of two layers of glass and one layer of PCM. The purpose of filling the roof structure with PCM is to utilize the solar energy efficiently. The effectiveness of thermal and optical performances of the roof PCM system was determined by analyzing the heat flux and temperature at the indoor surface with different absorption coefficients and refractive index of PCM in solid and liquid states. The results show that the absorption coefficients and refractive index of solid and liquid PCMs have both effects on thermal performance in the roof PCM system. Of all the thermal performances, the effect on internal temperature, temperature lag, and total transmitted energy is smaller and the effect on solar transmittance and transmitted solar energy is bigger. The absorption coefficients have the opposite effect with the refractive index on interior temperature lag. Considering the indoor daylight, increasing the refractive index and absorption coefficient of liquid PCM is a better method to better the thermal performance of a roof PCM system. Copyright © 2017 John Wiley & Sons, Ltd.     

EFFECT OF DRAW FURNACE GEOMETRY ON HIGH-SPEED OPTICAL FIBER MANUFACTURING

Optimal design of the draw furnace is particularly desirable to meet the need of high-volume production in the optical fiber industry. This article investigates the thermal transport and flow in optical fiber drawing at high draw speeds in a cylindrincal graphite furnace. A conjugate problem involving the glass and the purge gases is solved. The transport in the two regions is coupled through the boundary conditions at the free glass surface. The neck-down profile of the preform at steady state is determined by a force balance, using an iterative numerical scheme. To emphasize the effects of draw furnace geometry, the diameters of the preform and the fiber are kept fixed. Only the length and the diameter of the furnace are changed. For the purposes of comparison, a wide domain of draw speeds, ranging from 5 m/s to 20 m/s, is considered, and the form of the temperature distribution at the furnace surface remains unchanged. The dependence of the preform/fiber characteristics on the furnace geometry are demonstrated quantitively. Based on these numerical results, an optimal design of the draw furnace can be developed.     

Heat treatment effects on the optical properties of Sol–gel Ta2O5 thin films

In this work optical properties of Ta₂O₅ thin films with respect to heat treatment temperature were investigated. Ta₂O₅ thin films were prepared by sol–gel process using dip-coated method with a constant speed of 107 mm/min. Optical properties have been calculated from optical transmission measurements as a function of heat treatment temperature. The refractive indices and absorption coefficients were affected by heat treatment. The refractive index at λ=550 nm increased from 1.84 to 2.04 and absorption coefficient increased from 241 to 5668 cm⁻¹ when heat treatment temperature increased from 100°C to 500°C. The thickness of the film decreased from 272 to 190 nm and their optical band gap decreased from 3.68±0.09 eV to 3.51±0.08 eV for the film heated from 100°C to 500°C.     

Numerical Investigation of Opto-Energy Phenomena in Thin Gold Films Irradiated by Femtosecond Pulse Laser Considering Quantum Effects

Nonequilibrium energy transport and optical characteristics in thin gold film structures irradiated by a femtosecond pulse laser are examined numerically. With the use of a two-temperature model, the quantum effect is considered to determine various thermo-optical properties such as electron heat capacity, electron thermal conductivity, collision frequencies, reflectivity, and absorption rates. As a result, estimation on the electron temperature considering the quantum effect is in better agreement with measurements than those without considering quantum effect. During a femtoseond laser pulse, it is found that the electron and lattice are in nonequilibrium, the electron thermal conductivity changes rapidly with time, and reflectivity decreases substantially because of the changes in dielectric function and refractive index of gold films.     

Concentrating characteristics of the ellipsoidal concentrator–radiation source system in an optical medium

The features of radiation concentration by an ellipsoidal concentrator are considered for the case in which the concentrator, the radiation source, and the receiver are in the optical medium. Investigations are conducted based on a photometric model that we developed that takes into account the features of the source radiation in the optical medium and change in the radiation indicatrix medium. It is found that, in contrast to direct radiation, in the case of radiation concentration in an optical medium, the irradiance will always increase, since, at the second ellipsoid focus, it increases in proportion to the square of the refractive index of the medium. It is shown that these effects do not depend on the refractive index of the material of the radiation source bulb. Thus, since the radiation source flux is constant both in air and in medium, increasing irradiance leads to a reduction of the source image spot in the medium.     

Analytical Monte Carlo Ray Tracing simulation of radiative heat transfer through bimodal fibrous insulations with translucent fibers

In this study, a Monte Carlo Ray Tracing (MCRT) simulation technique is developed to study steady-state radiative heat transfer through fibrous insulation materials. The simulations are conducted in 3-D disordered virtual fibrous media with unimodal and/or bimodal fiber diameter distributions consisting of fibers whose surfaces are specularly reflective, and are translucent to Infrared (IR) radiation. Scattering within the realm of geometric optics is incorporated into our MCRT simulations using Snell’s Law for ray refraction. Fibers’ optical properties are obtained from Fresnel’s law and Beer’s law based on the refractive index of the material. Two different treatments of “high” and “low” conductivities are considered for the fibers and their effects are discussed. Our results indicate that heat flux through a fibrous medium with translucent fibers decreases with increasing packing fraction of the fibers. It was observed that IR transmittance through the media increases with increasing through-plane orientation of the fibers, but is independent of their in-plane orientations. It was also found that fiber orientation has generally a negligible effect on the temperature profile across the media’s thickness. However, for the case of high-conductivity fibers, increasing fibers’ through-plane orientation tends to flatten the temperature profile. The results obtained from simulating bimodal fibrous structures indicate that increasing the fiber-diameter dissimilarity, or the mass fraction of the coarse fibers, slightly increases the radiation transmittance through the media, and accordingly reduces the temperature gradient across the thickness. Our simulation results are compared with those from the two-flux model and good agreement is observed.     

OPPC预绞式耐张线夹处光纤温度研究

针对光纤复合架空相线(OPPC)预绞式耐张线夹处光纤温度对OPPC使用寿命影响的问题,文章提出在耐张线夹处加装一根截面相等的分流线的方案,并采用光纤测温法对3种试验规格的OPPC(630/45、400/50、185/30)进行“电流-温升”试验。试验结果表明,未加装分流线时,当非耐张线夹处OPPC表面温度为设计要求的70℃时,此处光纤温度小于80℃,而3种规格的OPPC耐张线夹处内部光纤温度均大于95℃,超过了公认的85℃的光纤最高安全运行温度;在加装分流线后,耐张线夹处光纤温度降至85℃以下,避免了长期的高温环境加速光纤涂层的老化而失去对光纤的保护,保证了OPPC的30年设计寿命。     

Coupled radiation and solidification heat transfer inside a graded index medium by finite element method

Effects of thermal radiation on solidification heat transfer must be considered inside semitransparent media. This paper investigates coupled heat transfer of solidification and radiation within a two-dimensional rectangular semitransparent medium having gradient index. Solidification process is supposed to happen at some temperature range, and accordingly three zones including liquid-, solid- and mushy-zones exist in phase-change media. In different phase field, parameters of thermophysical property are assumed different and those of radiative property are assumed same. Governing equation includes conduction, radiation and phase-change terms, and radiation and phase-change are treated as source terms in the equation, respectively. A Galerkin finite element method is used to solve energy equation of coupled radiation and phase-change heat transfer. This paper analyzes effect of thermal radiation on phase-change heat transfer and those of refractive index distributions on temperature fields and liquid fraction distributions during radiation–solidification coupled heat transfer. From the results, we can find that refractive index gradient has a major influence on phase-change process and compared with the case of smaller index gradient, bigger gradient can speed up phase-change heat transfer in semitransparent media.     

Combined non-gray radiative and conductive heat transfer in solar collector glass cover

A rigorous approach for the radiative heat transfer analysis in solar collector glazing is developed. The model allows a more accurate prediction of thermal performance of a solar collector system. The glass material is analysed as a non-gray plane-parallel medium subjected to solar and thermal irradiations in the one-dimensional case using the Radiation Element Method by Ray Emission Model (REM by REM).This method is used to analyse the combined non-gray convective, conductive and radiative heat transfer in glass medium. The boundary surfaces of the glass are specular. The spectral dependence of the relevant radiation properties of glass (i.e. specular reflectivity, refraction angle and absorption coefficient) are taken into consideration. Both collimated and diffuse incident irradiation are applied at the boundary surfaces using the spectral solar model proposed by Bird and Riordan. The optical constants of a commercial ordinary clear glass material have been used. These optical constants (100 values) of real and imaginary parts of the complex refractive index of the glass material cover the range of interest for calculating the solar and thermal radiative heat transfer through the solar collector glass cover. The model allows the calculation of the steady-state heat flux and temperature distribution within the glass layer. The effect of both conduction and radiation in the heat transfer process is examined. It has been shown that the real and imaginary parts of the complex refractive index have a substantial effect on the layer temperature distribution. The computational time for predicting the combined heat transfer in such a system is very long for the non-gray case with 100 values of n and k. Therefore, a simplified non-gray model with 10 values of n and k and two semi-gray models have been proposed for rapid computations. A comparison of the proposed models with the reference non-gray case is presented. The result shows that 10 bandwidths could be used for rapid computation with a very high level of accuracy.     

Computational Study of Convective Transport in a Coating Applicator for a Non-Newtonian Fluid

Convective transport in an optical fiber coating applicator and die system has been simulated for a non-Newtonian fluid. Low-density polyethylene (LDPE) is employed for the numerical analysis, though ultraviolet (UV) curable acrylates are more commonly used, because of a lack of property information for acrylates and similar behavior of these two materials. The equations governing fluid flow and heat transfer are transformed to obtain flow in a cylindrical domain. A numerical scheme similar to the SIMPLE algorithm is developed and employed with a nonuniform grid. Variable fluid properties are employed because of the strong dependence of these on the temperature. In contrast to the isothermal case, streamlines for the non-Newtonian fluid are found to be quite different for various fiber speeds. The temperature level in the applicator is much higher for the Newtonian case, due to the larger fluid viscosity and associated viscous dissipation. The shear near the fiber is found to be lower for the Newtonian fluid. As expected, the effects become larger with increasing fiber speed. A fairly high temperature rise is observed in the die, regardless of fiber speed. This study focuses on the computational modeling of non-Newtonian effects during the coating process, and several interesting and important features, as compared to the Newtonian case, are observed.     

Radiation effects on mixed turbulent natural and forced convection in a horizontal channel using direct numerical simulation

The purpose of this study is to clarify the radiation effects on mixed turbulent convection in a horizontal channel. The present study provides turbulence statistics using direct numerical simulation (DNS) in an optically thin medium. When the radiation effect is considered, the flow structure and the temperature distribution in the channel change with an increase in the optical thickness of the fluid. The radiation effect changes the distributions of the temperature fluctuation intensity and the turbulent heat flux. These radiation effects on mixed convection can be clearly explained by the turbulence statistics obtained from the DNS results.