Fabrication of Fiber Optic Multimode Glass Rods

Multimode fiber optic glass rods are fabricated by depositing ultra-pure silica and doped silica layers onto a preform using processes like MCVD, OVD, or VAD, followed by consolidation and fiber drawing.

Raw Materials

The primary materials for multimode fiber fabrication are silicon tetrachloride (SiCl₄) for the silica base and germanium tetrachloride (GeCl₄) as a dopant to increase the refractive index of the core. Additional dopants such as boron, fluorine, phosphorus, or titania may be used to modify the cladding or core properties. These chemicals are highly purified to minimize optical loss and ensure uniformity in the final fiber.

Preform Fabrication Methods

Modified Chemical Vapor Deposition (MCVD)In MCVD, a high-purity quartz tube is mounted on a rotating lathe. A mixture of SiCl₄, GeCl₄, and oxygen is introduced into the tube. A methane/oxygen torch heats the tube externally, causing the gases to react and form submicron silica and germania particles, known as "soot," which deposit on the inner wall.

The torch traverses the tube repeatedly, and the deposited soot is vitrified into transparent glass layers. By controlling the dopant concentration and deposition parameters, a graded-index or step-index core can be formed for multimode fibers. Outside Vapor Deposition (OVD)OVD involves depositing silica and doped silica soot on the outer surface of a rotating bait rod.

The soot is generated in a flame via hydrolysis reactions of SiCl₄ and GeCl₄. Multiple layers are deposited to form the core and cladding. After deposition, the bait rod is removed, and the porous preform is sintered into a solid glass rod.

OV

D allows high doping levels and precise control of the core-to-cladding ratio. Vapor Axial Deposition (VAD)VAD deposits silica and doped silica axially along a target rod. Gases are injected into a high-temperature furnace, where pyrolysis reactions form soot that accumulates along the rod.

This method is suitable for large-scale production and allows uniform deposition of thick preforms for multimode fibers. Direct Nanoparticle Deposition (DND)DND is a specialized OVD variant where nano-sized particles of silica and dopants are deposited simultaneously. This method achieves high doping levels with minimal clustering, improving optical performance and damage thresholds.

Consolidation and Sintering

After deposition, the preform is heated in a furnace to remove residual porosity and consolidate the glass into a solid, transparent rod. This step ensures mechanical strength and optical clarity. The preform now has the desired core and cladding structure with precise refractive index profiles.

Fiber Drawing

The solid preform is then drawn into thin fibers in a vertical draw tower. The preform tip is softened in a high-purity graphite furnace at temperatures around 1900–2200°C. Gravity and controlled tension pull the molten glass into a continuous fiber.

The fiber is immediately coated with UV-curable acrylate layers to protect against mechanical damage and environmental contamination. Draw speeds typically range from 10 to 20 meters per second, and the core-to-cladding ratio is preserved from the preform.

Quality Control

Throughout the process, mass flow controllers and precise temperature regulation ensure uniform deposition and doping. Optical properties such as attenuation, numerical aperture, and modal dispersion are monitored to meet multimode fiber specifications. In summary, the fabrication of multimode fiber optic glass rods involves chemical vapor deposition of doped silica layers, consolidation into a solid preform, and precise fiber drawing, with careful control of dopants and thermal conditions to achieve the desired optical performance.