Isfa 2008 U _ 108 Axial Deposition Control in Vapor - Phase Axial Deposition

An advanced feedback control strategy for a vapor-phase axial deposition (VAD) is investigated in this paper. VAD is a widely used process in the creation of high purity glass for optical fiber. In previous work a soot tip surface temperature controller was developed for the VAD process to reduce the effects of core soot temperature variation on deposition geometry, leading to a more stable process. However, it is desired to regulate both the core soot and clad soot deposition such that they deposit at the same axial rate to provide a more uniform product. This paper presents the design and development of a cascaded controller strategy and process model to couple and regulate the surface temperature and deposition rates of core and clad soot. Simulation studies demonstrate a potential improvement in the uniformity of the core and clad soot geometry over the soot product length. NOMENCLATURE A Area, effective heat flow normal area D Clad diameter of preform d Core diameter of preform GCORE Core axial deposition function GLENGTH Core tip length function GP Plant transfer function ∆H2 to TCORE GPI PI controller transfer function GTEMP Core temperature function kTH Thermal conductivity Ki Integral gain Kp Proportional gain LCORE Length of core soot tip L0 Initial length of core substrate soot tip PI Proportional, integral control Pull speed Rate of preform deposition axial growth qCLAD,SS Heat flow from clad substrate QCLAD Volumetric deposition rate of clad torch TCORE,0 Core soot substrate temperature initial TCORE Core soot substrate temperature TCLAD Clad soot substrate temperature VAD Vapor-phase Axial Deposition CORE X& Core soot axial growth rate, or pull speed CLAD X& Clad soot axial growth rate β Heat flux (constant) ∆H2(s) Hydrogen flow rate change to core torch ∆L Change in length of core soot section ∆TCORE_LENGTH Core substrate temperature change from core tip length change INTRODUCTION This paper proposes a process control improvement for a vapor-phase axial deposition (VAD) process, a commonly used multi-step process for the manufacture of high quality glass for optical fiber. The process deposits a glass soot mixture of silicon-dioxide and germanium-dioxide to create the light guide core and cladding around the core. It is desirable to maintain consistent core and clad geometry throughout the manufacturing process to create a high performance optical fiber product for high bandwidth data transmission. Common practice in the VAD process is for the core and clad soot deposition rates, as well as the related surface temperature, to run essentially open-loop while regulating constant flow rates of gases and chemicals to the deposition torches. This situation yields varying diameters of core and clad soot regions reducing the usable length of the final glass and lowering product yield. VAD was invented at NTT Laboratories in Japan and is the dominant process for Japanese manufacturers of optical fiber. VAD is an improvement of the Corning OVD (outside vapor deposition) process [1,2]. The VAD process has been the 1 Copyright © 2008 by ASME subject of previous studies (Refi [3], Choi [4], MacChesney [5]). However, developments in modeling and control of the process are still actively pursued in industry [6]. This work focuses on the glass soot creation step in the VAD process. Soot making and deposition are typically accomplished via two torches in a vertical process chamber with a rotating chuck (Fig. 1). A core torch creates circular inner core soot from a mixture of germanium-dioxide, silicondioxide, oxygen, and fuel (typically hydrogen). A pure silicondioxide soot layer is concurrently deposited from a second (clad) torch, as part of the final cladding around the core. The germanium-dioxide component of the core region increases the refractive index of the light guide core over the index of the surrounding cladding glass in the resulting optical fiber. (Basic glass chemistry and flame hydrolysis reactions for the glass process in VAD are reviewed in several references [3, 4, 7]). The rotating chuck moves upward as glass soot is deposited to form a ‘preform’. The preform moves upward by a control loop using laser light to indicate the core tip position. As the soot core tip grows from deposited soot, it blocks the light signal causing the servo stage to move upward. This upward movement is commonly referred to as pull speed. The pull speed is a result of position control to keep the bottom of the core tip in the same location as the soot preform grows. Thus, the pull speed is the core soot deposition axial growth rate. In contrast, the cladding growth is not controlled. After the soot preform has reached the design length (1m or larger), a sequential sintering operation is used to consolidate the glass soot into a solid glass perform. It is then, nearly ready to draw into optical fiber. Clad Torch (SiO2) Core Torch (SiO2 +GeO2) Deposition Growth Rate or Pull Speed Soot Preform Core Tip Chuck with Starting Rod Core End Position Sensor for Pull Speed Fig. 1: VAD process depicting core and clad torches depositing glass soot onto the rotating preform. The core tip position is held constant causing the preform and chuck to move upward.