Simulation Study of Deposition Parameters for Elliptical-Shaped Preforms by VAD for Polarization-Maintaining Fibers

A novel fabrication method of elliptical-shaped preforms for polarization-maintaining fibers, developed using the VAD technology and based on the variation of the perform rotation velocity from zero to a maximum velocity in repeated cycles of 180°, allows modifying and controlling the preform geometry by choosing the process parameters, such as the maximum rotation velocity and the delay time on zero velocity positions. The effect of each process variable was studied through a mathematical modeling and simulation of the methodology, comprising a simplified model of the soot deposition process. Results demonstrated that preforms with high elllipticity can be obtained by increasing the delay time or decreasing the maximum rotation velocity. It was also observed that the effect of the delay time is more noticeable on the preform geometry than the contribution of maximum velocity. These facts were confirmed when compared to real deposition results, using a VAD chamber with gases fluxes set to 2.33×10-6 m3/s (SiCl4) and 0.8×10-6 m3/s (GeCl4), and setting the burner-target angle to a high deposition rate condition. It is expected that the improvement of the mathematical model can be very useful in order to determine the process conditions necessary to achieve any desired preform geometry, even before the deposition stage.