Manufacture of optical waveguide preforms by modified chemical vapor deposition

Abstract

Manufacture of optical waveguide preforms by modified chemical vapor deposition (MCVD) is theoretically investigated. For the first time, a model accounting for the concurrent heat transfer, chemical kinetics, and silica aerosol dynamics during lightguide preform fabrication by MCVD is presented. Silica particles are formed by high-temperature oxidation of Si CI,, grow by coagulation, and deposit to the preform walls by thermophoresis and Brownian diffusion. Assuming first-order SiCI, oxidation and approximating the aerosol size distribution by a log norma1 function throughout the process, five partial differential equations describe this process. The emphasis of this study is on the achievement of high process yield (deposition efficiency of MCVD).Process conditions for operation in the particle transport-limited and reaction-limited regimes are quantitatively identified. Operation in the former regime results in complete oxidation of the inlet SiCI, but only about half of the product SiO, particles deposit to the tube wall. Operation in the latter regime results in limited oxidation of inlet SiCI, (about 40%) but almost all product SiO, particles deposit to the tube wall. This study shows that high process yields and deposition rate can be achieved in lightguide preform manufacture by MCVD by combining operation in the reaction-limited regime with recycling of the exit gases from the preform tube.

CFD simulation of laser enhanced modified chemical vapor deposition process

“…(12) particle diffusivity of silica in both radial and axial directions is neglected in comparison to thermophoretic flux. The thermophoretic coefficient, K is constant and equal to 0.55 for the particles of SiO 2 less than 0.2 m in diameter at the operating temperature of the MCVD process (Kim and Pratsinis, 1988). The radial thermophoretic velocity is defined as v T = K ((∂ ln T)/∂r), where is the kinematic viscosity of the gaseous mixture.…”

“…Choi et al (1987) carried out detailed analysis of the effects of tube rotation and variable properties on the three-dimensional temperature fields, particle trajectories and deposition for a fast moving torch. Kim and Pratsinis (1988) were first to present a model accounting for the concurrent heat transfer, chemical kinetics, and silica aerosol dynamics during waveguide preform fabrication by MCVD. Simultaneous diffusion, thermophoresis and coagulation of submicron aerosol in laminar tube flow were studied by Pratsinis and Kim (1989).…”

Simulation of aerosol dynamics and transport in chemically reacting particulate matter laden flows. Part I: Algorithm development and validation

“…(1) viz, moment formulation, sectional formulation and discrete-sectional formulation. In the moment formulation, it is assumed that the aerosol size distribution will always be log-normal in shape and thus the ÿrst three moments of the distribution are su cient to describe the aerosol dynamics (Kim and Pratsinis, 1988). The governing equations for the sectional and the discrete-sectional formulations are described in the next section.…”

“…Using this technique, a closed set of equations is obtained by assuming a speciÿc mathematical form of the size distribution function. The commonly assumed form is the log-normal function (Pratsinis, 1988;Kim and Pratsinis, 1988;Bai and Biswas, 1990). The moment method although is computationally very e cient, it is plagued by the assumption of a ÿxed form of the size distribution function that is necessary for closure.…”

Jet Assisted Aerosol Chemical Vapor Deposition for Optical Fiber Synthesis

“…Therefore the enhancement of particle deposition ef ciency (and rate) and the uniformity needs the proper control of temperature elds in a rotating silica tube as well as ow control that governs particle trajectories. Aerosol dynamics in the MCVD was investigated using the moment method (Kim and Pratsinis 1988) and the sectional method (Park et al 1999). Heat and mass transfer aspects relevant to MCVD have been investigated by several researchers (Kim and Pratsinis 1990;Lin et al 1992;Park and Choi 1998).…”