Production of Optical Fiber Preform

The Royle Pilot Plant
The production of optical fiber starts with the making of a silica based preform. The optical fiber preform is the basic component from which optical fiber is drawn and subsequently cabled. The preform is a solid rod made of an oxygenated compound of silicon, present in a great number of minerals, such as quartz, chalcedony and opal. The finished preform is thereafter heated and drawn as optical fiber.

The core of the fiber is made up of ultra pure germanium doped silica. A silica of less refractive index surrounds the core, forming the optical sheath. A difference of index between the core and the sheath is made by incorporating doping agents, such as germanium tetrachloride, which increases the refractive index in the core. Boron and fluorine compounds decrease refractive index of the sheath. Generally, a single-mode fiber preform of one meter long and eight centimeters in diameter can be stretched to approximately 400 kilometers of single-mode fiber.

Different Methods of Production
There are four production methods of optical fiber commonly used around the world: (1) Modified Chemical Vapor Deposition (MCVD, also known as the inside process); (2) Outside Vapor Deposition (OVD, also known as the outside process); (3) Vapor-phase Axial Deposition (VAD); and (4) Plasma-activated Chemical Vapor Deposition (PCVD). All four processes have been successful in producing low attenuation, high transmission property and durable optical fiber (preform) for industrial production.

The deposition rate determines the efficiency of the fiber manufacturing process. The higher the deposition rate, the more efficient the fiber manufacturing process. The deposition rates of the respective processes are listed below:

Process Depositing rate (g/minute)
OVD 15 to 20
VAD 15 to 20
MCVD 1.0 to 2.0
PCVD 0.5 to 1.0

The MCVD and PCVD processes have substantially lower deposition rates, which require less depositing time, whereby 75% of the substrate tube and jacket tube of fiber will result in non-transmission properties. Since MCVD and PCVD processes have this slower depositing rate, the efficiency rate is not as competitive for producing single-mode fiber in large diameters or quantities. The capital investment for MCVD and PCVD is relatively lower than the other processes, and the production process is easier to operate and control. It is the preferred process for making complicated refractive index profile and developing new products. For production of larger quantity, the production cost of MCVD is very high since the production is dependent on the supply of synthetic silica tubes, for which there are only limited global manufacturers.

The OVD and VAD processes have higher depositing rates, which require lower material costs. The OVD process ensures high uniformity in the quality and refractive index profile of fibers and produces the purest glass, resulting in stronger and more consistent fiber. The OVD process was exclusively used by Corning since the 1970s, and the patent of such technology has been expired since July 2000.

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Hybrid Process

Royle has successfully developed an optical fiber preform production process called the Hybrid Process. The Hybrid Process places significant emphasis on higher efficiency and depositing rate of fiber and a lower cost of production.

The Hybrid Process is an integrated process of both the inside process (MCVD or PCVD) and the outside process (OVD) into producing a single-mode fiber preform. The MCVD process produces a core rod, and overcladding is then deposited on the core rod using the OVD process. The Hybrid Process is capable of producing high quality single-mode fiber with a very low production cost and can produce multimode fiber efficiently and economically with a low capital requirement. Unlike the MCVD process, the Hybrid Process uses basic material and components that are readily available around the world and substantially reduces the risks of shortage of raw materials and high production costs.

In 1996, Optex commenced its research program to develop a production process that is cost effective and reliable for the manufacture of optical waveguide fiber preforms. In 1998, Optex and Royle completed the initial hybrid technology research and development. They studied the optical fiber market and determined that 95% of the world's optical fiber usage was for telecommunications, which utilizes a fiber structure called single-mode. In a single-mode fiber, the core, or light transmission median, comprises only about 1% of the entire fiber area. The remaining 99% is basically pure SiO2 and is called the cladding. Optex has initially concentrated its Hybrid Process on single-mode fiber, as demand and profit margin for single-mode fiber has caused an accumulated annual growth rate of 30% for the past 5 years, and this growth is expected to continue over the next decade. A pilot plant was constructed and is located in the factory of Royle Systems Group in Pompton Lakes, New Jersey, United States to demonstrate and optimize the Hybrid Process.

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MCVD to Produce the Primary Preform for the Hybrid Process

Nearly 50% of the world's optical fiber is produced by a production process called MCVD, which deposits core glass that acts as the waveguide on the inside of a purchased thin-wall tube. This tube is generally made from synthetic silica due to the low water content. Once the desired thickness of core glass is deposited on the inside surface of the tube, it is collapsed into a rod, which becomes the primary preform. Most manufacturers then insert this primary preform into a purchased thick-wall tube that becomes the outer cladding. The larger tube is then collapsed onto the primary preform to produce a large diameter preform that provides much longer lengths of fiber. This process is practiced under the U.S. patent #4,596,589 assigned to Mr. Gregory Perry and has become known in the industry as the rod-in-tube process.

In the world today, there are only three manufacturers of these synthetic silica tubes. The high cost of manufacture, as well as the demand and limited supply has caused the price of these tubes to range between US $250 and US $450 per kilogram of glass. The market conditions of high price and short supply of synthetic silica tubes caused Mr. Perry to develop an alternative process to the one in his patent #4,596,589. Instead of synthetic tubes, It was decided to use the basic source material, silicon tetrachloride, as he knew that it was available at about US $10 per kilogram.

Optex research determined that the primary preform as developed for the rod-in-tube process could be used as the primary preform for the new Hybrid process. Optex specified a new MCVD system to its sister company, Royle Systems Group, who designed a complete MCVD system for the manufacture of the “primary preform”. The system is completely computer controlled, utilizes a water cooled burner, is manufactured using all stainless steel gas flow lines which incorporate orbital welds, and is enclosed in a self contained laminar air flow housing. MCVD is a widely used and well-published technology. Most MCVD systems use nearly the same methodology of operation, but Optex and RSG developed their system to be user friendly and easily maintained. It uses components that are readily available anywhere in the world. Its software is an open architecture that is configurable and easy to use. The system design is modular, which means more up time, due to ease of maintenance and interchangeable parts.

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OVD to Produce the Overcladding for the Hybrid Process

Optex commenced its research program in 1996 to develop a hybrid process for the manufacture of single-mode preforms by overcladding MCVD primary preforms using soot (silicon tetrachloride.) The goals of this program were to take advantage of the economics of the soot process for the generation of synthetic silica to form the 99% cladding glass of the waveguide preform and to utilize the well known and well published MCVD process for manufacturing the “primary preform” or core rod.
Optex identified the parameters felt to be essential for the successful manufacture of soot. In that the by-products of the process are mainly hazardous materials, an efficient neutralization system was necessary for safety of operators and to comply with environmental laws. This system was designed to remove the soot particles from the exhaust by the use of cyclone technology combined with hydrostatic wash down methods. The hydrostatic wash is water and sodium hydroxide, which fully neutralizes the HCl and Cl2. The system design is capable of removing soot and neutralizing by-products at a rate in excess of 600 cfm at 1 meter of static pressure.

The research determined that much of the critical deposition technology involved the burner. Optex concentrated on burner design, so as to maximize efficient performance of the soot deposition system. Optex research determined that the metallic burners used by others limit the process efficiency. Optex chose materials for the construction of the burner that are compatible with the gases and the hazardous atmosphere in which the burner must operate. The burner development has passed through three generations of evolution, so that Optex is able to express strong confidence in its performance. A less critical but very important consideration to maximize rate of deposition is the fume extraction process. It was determined that the rate of airflow across the soot boule creates a dramatic effect on both the rate of deposition and the density of the soot. Optex research has optimized this parameter, which has dramatically improved deposition rates and efficiency of usage of materials.
Optex specified a new OVD system to its sister company, Royle Systems Group, who designed a complete OVD system for the manufacture of large diameter preforms by overcladding the MCVD “primary preform” to make a large diameter preform. The system consists of a lathe that traverses and is completely computer controlled. It is enclosed in a self-contained laminar airflow housing. OVD is not widely used outside of Corning, so it is not a well published technology. RSG developed the OVD system to be user friendly and easily maintained. RSG uses components that are readily available anywhere in the world. RSG's software is an open architecture that is configurable and easy to use. The system design is modular, which means more up time, due to ease of maintenance and interchangeable parts.

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Sintering and Dehydration of the Large Diameter Preform for the Hybrid Process

Optex specified the important parameters of the sintering furnace to Royle Systems Group, who designed the system. It has the capability to reach the high temperatures required for the sintering process. The furnace shell is a split design for ease of maintenance. The sintering and dehydration require the use of helium and chlorine, and the furnace has quartz tubing at top and bottom of the shell, which are sealed to prevent leakage of gases. There are ports for entry and exhaust of gases. The sintering system is fully tested to assure no leakage of hazardous gases and is supplied with all necessary gas safety alarm systems, thereby guaranteeing operator safety and compliance to environmental standards.

The sintering system consists of a tower frame, on which is mounted the furnace, a preform chuck and a downfeed mechanism. The chuck drive provides for rotational movement of the preform in either direction. The downfeed drive provides for up or down movement of the preform. All controls (rotational speed and direction as well as up/down speed) utilize a programmable controller with specific holding sequences. The tower is designed to support four sintering systems to conserve factory floor space.

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Raw Materials
The raw materials used for the production of optical fiber preform include substrate tube, silicon tetrachloride (SiC14), germanium tetrachloride (GeC14), phosphorous oxychloride (POC13), chlorine (C12), silicon tetraflouride (SiF4), coating DSM Acrylate, power, gas and water. The quality of raw materials is specified critically and will only be purchased through qualified suppliers worldwide such as Japan, Germany and China.

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Environmental Protection
The production process of optical fiber preforms contains certain hazardous chemical waste harmful to human health and environment. Optex designed a system capable of removing soot and neutralizing hazardous by-products using the cyclone technology combined with hydrostatic wash down method. The neutralizing system will be installed to process the waste gases and polluted water produced and will turn hazardous contents into harmless solid, water and oxygen. The neutralizing system will exceed the standards required by the Environmental Protection Agency.

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