Multimode optical fiber, as a crucial transmission medium in modern communications networks, is widely used for short-distance, high-speed data transmission due to its ability to support multiple optical modes. Its production process is complex and sophisticated, involving multiple disciplines, including materials science, optical engineering, and automated control. This article systematically explains the complete manufacturing process for multimode optical fiber, from raw materials to finished product, focusing on key technical aspects and quality control measures in key process steps.
Raw Material Preparation and Pretreatment
The core raw material for multimode optical fiber is high-purity quartz glass (SiO₂), typically purified by vapor deposition using chemical raw materials such as silicon tetrachloride (SiCl₄). The preform is the initial form of optical fiber production, and its preparation quality directly affects the performance of the final fiber. During the production process, the concentration and distribution of dopants (such as germanium and phosphorus) must be strictly controlled to regulate the optical fiber's refractive index profile-a key structural feature that enables multimode transmission.
Quartz tubing undergoes ultrasonic cleaning and drying before entering the deposition process to ensure that the surface is free of particle contamination. Some manufacturers use oxyhydrogen flame polishing to further improve the smoothness of the tube wall and reduce the probability of defects during subsequent deposition.
Fiber Preform Manufacturing
Chemical Vapor Deposition (CVD) Process
The mainstream manufacturing methods for multimode fiber preforms are modified chemical vapor deposition (MCVD) or axial vapor deposition (VAD). In MCVD, for example, a high-purity carrier gas (such as helium) carries reactant gases such as SiCl₄ and GeCl₄ into a rotating quartz tube. Under the high temperature of the oxyhydrogen flame (approximately 1800°C), a hydrolysis reaction occurs, producing silica and doped oxide particles that are deposited on the tube's inner wall. By precisely controlling the thickness and doping ratio of each layer, a graded-index (GI) or step-index (SI) refractive index profile can be created. The GI is more suitable for multimode fiber to optimize mode coupling efficiency.
Rod-in-Tube Auxiliary Process
Some companies use the rod-in-tube method to combine a high-refractive-index core rod with a low-refractive-index cladding quartz tube, adjusting the core-to-cladding ratio through high-temperature stretching. This method allows for flexible adjustment of refractive index profile parameters, but requires strict matching of the thermal expansion coefficients of the two to avoid stress defects.
After deposition, the preform undergoes a collapse treatment at temperatures exceeding 2000°C, transforming the hollow quartz tube into a solid rod while eliminating internal bubbles and impurities.
Fiber Drawing and Coating
High-Temperature Drawing Process
After cutting and end-face grinding, the preform is vertically fixed to the top of a drawing tower. In a graphite furnace heated at 2100-2300°C, the preform tip softens to form a molten glass droplet. This droplet is then continuously drawn into a filament (bare optical fiber) with a diameter of approximately 125 microns by gravity and traction. The drawing speed is typically controlled between 1000 and 3000 m/min. A computer system monitors diameter deviation in real time and adjusts heating power and pulling tension based on feedback.
Double-Layer Coating Protection
To enhance the mechanical strength and environmental resistance of the optical fiber, the bare fiber is immediately coated with a buffer layer using a UV-curable coating system. The inner layer is a low-modulus acrylate coating (approximately 10-30 microns thick) to provide stress buffering, while the outer layer is a high-modulus coating (approximately 20-50 microns thick) to provide resistance to lateral pressure. The coating cures in less than 1 second, and the concentricity error with the fiber must be maintained within 1 micron.
Quality Inspection and Finished Product Processing
Optical Performance Testing
Key parameters of the drawn optical fiber are measured using in-line testing equipment:
Attenuation Coefficient: Loss in the 1310nm/850nm band is measured using the shear method or an OTDR (Optical Transmitter and Detector) with a typical value of ≤3.5 dB/km (850nm) for multimode fiber.
Bandwidth Characteristics: Modal dispersion performance is assessed using the injection current method or far-field scanning method to ensure compliance with standards such as OM3/OM4.
Numerical Aperture (NA): Reflects optical coupling capability. The NA for multimode fiber is typically 0.275 ± 0.015.
Mechanical and Environmental Testing
Samples undergo reliability tests such as tensile strength (≥600 kpsi), bend radius (≤30 mm), and temperature cycling (-60°C to +85°C).
Qualified optical fibers are coated with ink (e.g., type and batch number), wound into reels (typically 2–5 km in length), and then packaged and stored. High-end products may include loose tubes or armor layers to accommodate specialized installation environments.
Process Optimization Trends
Currently, multimode optical fiber production is moving toward intelligent and green processes:
Intelligent Control: Introducing machine learning algorithms to predict deposition layer uniformity and dynamically adjust gas flow parameters;
Low-Loss Technology: Using fluorine-doped cladding to reduce Rayleigh scattering loss;
Environmentally Friendly Processes: Recycling chlorosilane byproducts to reduce the use of hazardous chemicals such as hydrofluoric acid.
Through this refined management and control, the production efficiency and quality of modern multimode optical fibers have reached globally unified standards (such as IEC 60793-2-10), providing reliable high-speed transmission solutions for data centers, industrial Ethernet and other fields.
