The process development history of optical fiber preforms

Since the large-scale production of optical fibers began in the late 1970s, the research and improvement of optical fiber preform manufacturing technology has never been interrupted. After AT&T (Lucent) of the United States invented the modified chemical vapor deposition (MCVD) process, Corning Company of the United States subsequently developed the outside vapor deposition (OVD) method suitable for mass production of optical fibers. ) process, and then the OVD process has been continuously improved. Currently, the seventh-generation process has been released, which has greatly reduced production efficiency and production costs; while Japan's NT&T improved on the basis of OVD and launched the vapor phase axial deposition method ( VAD, Vapor Axial Deposition) process; Alcatel of France has developed an advanced plasma vapor deposition (APVD, Advance Plasma Vapor Deposition) preform production process using high-frequency plasma technology; Philips of the Netherlands has developed plasma chemistry (PCVD ,Plasma Chemical Vapor Deposition) process forced supply has been successfully applied in production.

The early optical fiber preform manufacturing technology adopted a one-step method. In the early 1980s, the casing method began to be used to prepare optical fiber preforms, thus realizing the transformation of the optical fiber preform manufacturing process from a one-step method to a two-step method, that is, first The preform core rod is manufactured, and then different technologies are used to manufacture the outer cladding outside the core rod to increase the drawable kilometers of a single preform rod to improve production efficiency. It is generally believed that the manufacturing of the core rod determines the transmission performance of the optical fiber, while the outer cladding determines the manufacturing cost of the optical fiber. In the manufacturing technology of the core rod, MCVD and PCVD are called in-tube deposition processes, and OVD and VAD are external deposition processes; in the outer cladding process, the external deposition technology refers to OVD and VAD, and the external spraying technology mainly refers to plasma spraying quartz sand. Craftsmanship. Today's optical fiber outer cladding manufacturing technologies include the casing method, the plasma spray method invented by Alcatel (Alcatel), the flame hydrolysis method (SOOT), and the sol-gel method (Sol-gel method invented by Lucent Technologies of the United States). ), where the SOOT method generally refers to flame hydrolysis external deposition processes such as OVD and VAD.

The MCVD method now uses external deposition technology to replace the casing method to produce large preforms, forming a hybrid process that combines the MCVD external deposition process, thus changing the shortcomings of the traditional MCVD process of low deposition speed and poor geometric dimensional accuracy. , reducing production costs and improving the quality of preforms. Since then, some companies have developed low-cost, large-size casing processes. The casing preparation processes are Sol-gel and OVD methods.

Preform rod preparation process OVD method has developed from single blowtorch deposition to multiple blowtorch simultaneous deposition in the past twenty years. The deposition rate has doubled, and one device can deposit multiple rods at the same time, and from sequential deposition The one-step method of making preforms from core cladding has evolved into a two-step method, that is, first preparing a large-diameter core rod, and then drawing it into a small-diameter core rod or not attenuating, and then using outer cladding technology to prepare an optical fiber preform. Improved production efficiency and reduced production costs. Moreover, the MCVD method, especially the PCVD method, OVD and VAD methods, are easier to accurately control the radial refractive index distribution of the core rod, and are therefore more effective in preparing multi-mode optical fiber MMF and non-zero dispersion optical fiber DZDF core preforms.

In the past 20 years, there have been many developments in optical fiber preform outer cladding technology. The American company CORNING first adopted SOOT outsourcing technology instead of the casing method for industrial production. In 1990, Alcatel plasma spraying technology and Sol-gel outsourcing technology developed by Lucent Corporation of the United States replaced the casing technology. Therefore, manufacturers that use the casing method to prepare optical fiber prefabricated VAD to manufacture optical fiber core rods all use SOOT outsourcing technology. ●The initial MCVD was to sequentially perform cladding deposition, core deposition, and melting into preforms on a lathe. This is a typical "one-step method." At present, Alcatel has separated deposition and melting. After deposition, another special lathe is used to melt and shrink into rods, and a graphite induction furnace is used instead of the hydrogen-oxygen flame as the heat source to melt and shrink into rods.

●Use large-diameter synthetic quartz tubes instead of small-diameter quartz tubes made from natural crystal powder as substrate tubes. The outer diameter of synthetic quartz substrate tubes currently used in production is about 40mm. Length 1.2~1.5m.

●The most important thing is that various external deposition technologies have been used to replace the casing method to make large prerods. For example, flame hydrolysis cladding and plasma cladding technology are used to make the cladding on the core rod, forming MCVD Hybrid process combined with external deposition process. This new technology makes up for the shortcomings of low deposition rate and poor geometric dimensional accuracy of the traditional MCVD process, reducing costs, improving quality, and enhancing competitiveness.

●Develop low-cost, high-quality, large-size casing manufacturing methods (such as sol-gel method, OVD method) for casing use.

●The VAD process of the 1970s, in which the core and cladding are deposited and sintered at the same time, is known as a prefabricated continuous manufacturing process.

●The VAD process in the 1980s was to first make a large-diameter mandrel, then draw the large-diameter mandrel into multiple small mandrels, and then use the casing method to make preforms. From "one step" method" developed into the "two-step method".

●In the 1990s, SOOT outsourcing was used instead of the casing method to make optical fiber preforms.

●Since the 1990s, the number of manufacturers using VAD has increased. In addition to Japan's Furukawa and Tokura, Shin-Etsu, Hitachi, Mitsubishi, Showa and other companies have obtained licenses from Japan's NTT to use the VAD process to produce optical fibers. , and implemented redevelopment to realize the commercial VAD process. Lucent also purchased a license to use the VAD process from Sumitomo Corporation. In addition, it established a joint venture optical fiber factory with Sumitomo in the United States based on the VAD method, giving it the opportunity to observe VAD optical fiber production for many years. ,Since then, Lucent has introduced the VAD process to its Atlanta fiber optic plant. The American company SpecTran also acquired the VAD process while purchasing the assets of ENSIGN-BICKFORD. By the way, SpecTran was acquired by Lucent in the United States at the end of 1999. ●From single blowtorch deposition to multi-jet simultaneous deposition, the deposition rate is doubled.

●From one device depositing one rod at a time to one device depositing multiple rods simultaneously.

●From the "one-step method" of sequentially depositing cores and cladding to continuously make preform rods, to the "two-step method"; that is, first using a ceramic rod or graphite rod as the target rod, and only depositing the core material ( (containing a small amount of cladding) to make a large-diameter core rod. After dehydration and sintering, the large-diameter core rod is drawn into multiple small-diameter core rods, and then these small-diameter core rods are used as target rods to deposit cladding. into optical fiber preforms, greatly improving productivity and reducing costs. ●Like MCVD, the current PCVD process also uses large-diameter synthetic quartz tubes instead of quartz tubes made of natural crystal as the substrate tube.

●The Dutch POF company has developed the fourth generation PVCD process. The inner diameter of the substrate tube has increased from the initial 16mm to 60mm, the deposition rate has increased to 2~3g/min, and the deposition length has been increased to 1.2~1.5m. .

●At present, the casing method is still used to make large preforms, but a casing weighs several kilograms.

●In principle, like MCVD, a hybrid process that combines PCVD and external deposition processes can also be formed, but there has been no report so far.

Currently, manufacturers are adopting various technical routes. The optical fibers they produce can meet international standards and have certain competitiveness in the market. With the diversity of market demand for optical fiber products, manufacturers are required to produce economically competitive optical fiber products with different performances to meet this diverse demand.

Ordinary G.652 single-mode optical fiber is widely used in the market today. For long-distance trunk lines, G.655 optical fiber is used, and local area networks use data optical fiber. However, not all processes can be optimized for production. All fiber optic varieties. As far as the production of G.652 optical fiber is concerned, the external deposition technology (DVD, VAD) of the core is superior to the internal deposition technology (MCVD, PCVD). The main advantages of external deposition technology are: it does not use expensive synthetic quartz tubes, and the deposition rate, The number of deposited layers is not limited by the diameter of the lining tube, which is particularly beneficial for manufacturing large preforms at high deposition rates. In addition, external deposition technology can produce G.652 (C) low water peak fiber. As far as the production of G.655 optical fiber is concerned, the core-in-tube deposition technology (PCVD process or MCVD process) has considerable advantages. The biggest advantage compared with DVD and VAD is: it can accurately control the radial refractive index distribution (RIP). This advantage is particularly beneficial to the manufacture of the latest generation of communication optical fibers, such as large effective area optical fibers, large effective area optical fibers with flat local dispersion, straight wave optical fibers with reduced dispersion slope, etc. These optical fibers are usually responsible for multi-cladding. RIP structure, data optical fiber has become a new generation of multi-mode optical fiber production, and the PCVD process is more competitive.