Cascaded Control for Regulating Soot Geometry in Vapor-phase Axial Deposition

The development of a cascaded feedback control strategy for a vapor-phase axial deposition (VAD) process 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, leading to a more stable process. However, this approach did not address the need to regulate and link the deposition rates of the core and clad torches. To maintain a constant distance between the core and clad deposition surfaces, it is desired to have the core soot and clad soot depositing at the same linear speed 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. Simulations for the process and control scheme demonstrate a potential improvement in the uniformity of the core and clad soot geometry over the soot product length. INTRODUCTION This paper investigates a process control improvement for a vapor-phase axial deposition (VAD) process, a commonly used 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 the core and clad geometry to create a uniform product, necessary for high bandwidth optical transmission and costeffective production. 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. This leads to varying diameters of core and clad soot regions which affect the usable length of the final glass. VAD was invented at NTT Laboratories in Japan and is the dominant process for Japanese manufacturers of optical fiber. VAD is an improvement on the Corning OVD (outside vapor deposition) process [1,2]. Much has been written and documented about the VAD process (Refi [3], Choi [4], MacChesney [5]). However, developments in modeling and control of the process are still actively pursued in industry [6]. VAD is a multi-step process for creation of high purity glass soot for optical fiber. This work focuses on the creation of the soot preform step. 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, silicon-dioxide, oxygen, and fuel (typically hydrogen). A pure silicon-dioxide soot layer is also 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 available 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 tip position. As the soot core tip grows, it blocks the light signal and causes the servo stage to move upward. This upward movement is commonly referred to as pull speed. The pull speed is a result 1 Copyright © 2006 by ASME of position control on the core tip to keep it in the same location as the soot preform grows. Thus, the pull speed is the core soot deposition linear 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 to form a solid glass preform, 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 Figure 1. VAD process: core and clad torches PROBLEM DESCRIPTION Common practice in VAD processing is for the core and clad soot deposition rates, as well as the related surface temperature, to run essentially open-loop. Each deposition torch (core and clad) has regulated flow rates of chemicals and gases (determined a priori by trial and error approaches). While the goal is to perfectly match the depositions of the clad and core, it is rarely accomplished. The core region may grow faster than the clad, or vice versa. In addition to causing the soot preform tip length to grow or to shrink, relative to the cladding, this situation causes the diameter ratios of the clad to core (D/d) to vary. This (D/d) variation results in a less uniform product requiring more processing or waste. These diameters and variations can be visualized in Fig. 2. Final soot performs show open-loop process variation as indicated by the non-uniform or tapering outside diameters or varying length core tips.