Laser modification of thermophoretic deposition

Abstract

The modified chemical vapor deposition process (MCVD) used in the manufacture of optical fiber preforms depends on the thermophoretic deposition of an aerosol of silicon and germanium dioxide particles borne in a background carrier gas. In order to increase the efficiency of this process by increasing the temperature gradient that drives the motion, we consider the absorption of an incident beam of laser radiation by the aerosol particles themselves, and the subsequent conduction of this energy into the background gas. The full set of equations governing the laminar tube flow of an aerosol-gas mixture is formulated. The effect of laser energy deposition is modeled as a source term in the energy equation, and is proportional to the concentration-dependent absorption coefficient. The particle concentration equation contains the effects of axial convection, diffusion, and thermophoretic motion, and is therefore coupled to the energy equation. Scattering and spontaneous emission have been neglected. Analytic results are presented to special cases of the full equations in order to estimate the magnitude of the temperature increase, as well as the change in aerosol concentration, to be expected from laser irradiation. These cases are limited either to circumstances in which the aerosol concentration is decoupled from the temperature (to obtain an upper bound on the temperature increase) or to one-dimensional model problems showing the effect of thermophoretic displacement of the aerosol on modulating the laser heating that can occur. In addition, it is shown that laser heating can cause 100% deposition efficiencies to occur in sufficiently long tubes and some qualitative experiments that indicate the effectiveness of this interaction are described.