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Popular Science: Overview of Mainstream Fiber Optic Prefabrication Rod Technology


There are four globally recognized and mature technologies for fiber optic preforms, collectively known as the "vapor deposition method", which can be divided into "in tube method" and "out of tube method" according to the firing method, as shown in the following figure:


1. OVD: Outside Vapor Deposition (OVD) is a simple process developed by Kapron of Corning Inc. in 1970. The chemical reaction mechanism of the OVD process is flame hydrolysis, which means that the required core glass composition is obtained by gradually depositing "powder" layer by layer through the gaseous halides (SiCl4, etc.) carried in the hydrogen oxygen flame or methane flame. The OVD process involves two specific steps: deposition and sintering. Firstly, the porous glass preform core is deposited according to the designed fiber refractive distribution requirements (the growth direction of the preform is radially from the inside out), and then the deposited preform core is sintered to remove residual water, in order to obtain a transparent and water free fiber preform core. The latest development of the OVD process has gone from single spray lamp deposition to simultaneous deposition of multiple spray lamps, By depositing one rod at a time from one device to multiple rods at a time, productivity is greatly improved and costs are reduced.



2. VAD: Vapor Axial Deposition (VAD) is a continuous process invented by Nippon Nippon Nippon Nippon et al. in 1977 to avoid disputes with Corning Inc.'s OVD patent. The chemical reaction mechanism of VAD process is the same as that of OVD process, which is also flame hydrolysis. Unlike the OVD process, the growth direction of the preform obtained by VAD deposition is vertical and axial from bottom to top. Sintering and deposition are completed simultaneously in different spaces within the same equipment, i.e. continuous manufacturing of prefabricated rods. The latest development of VAD technology has gone from the simultaneous deposition and sintering of cores and packages in the 1970s, to the two-step method of depositing core rods and then sleeves in the 1980s, and then to the use of dust cladding to replace sleeves in the 1990s to produce fiber optic preforms.



3. MCVD. Modified Chemical Vapor Deposition (MCVD) is a classic process developed by Machessney et al. of Bell Labs of AT&T in 1974. The MCVD process is a method used by companies such as Lucent. The MCVD process is a vapor deposition process that occurs inside a high-purity quartz glass tube using a hydrogen oxygen flame heat source. The chemical reaction mechanism of MCVD process is high-temperature oxidation. The MCVD process consists of two steps: deposition and rodding. Deposition is the process of obtaining the refractive index distribution of the fiber core required by the design, while rodding is the process of melting and shrinking the deposited hollow high-purity quartz glass tube into a solid fiber preform core rod. The current MCVD process uses large diameter synthetic quartz glass tubes and outsourcing technology, such as using flame hydrolysis outsourcing and plasma outsourcing technology to produce large prefabricated rods. These outsourcing technologies make up for the shortcomings of traditional MCVD processes such as low deposition rate and poor geometric dimension accuracy, improve quality, reduce costs, and enhance the competitiveness of MCVD processes.



4. PCVD. Plasma Chemical Vapor Deposition (PCVD) is a microwave process proposed by Koenings of Philips in the Netherlands in 1975. The process similarity between PCVD and MCVD is that they both undergo vapor deposition and high-temperature oxidation reactions in high-purity quartz glass tubes. The difference lies in the heat source and reaction mechanism. The heat source used in the PCVD process is microwave, which activates the gas to generate plasma and ionizes the reaction gas. The ionized reaction gas is charged with ions. The heat energy released during the recombination of charged ions melts the gaseous reactants, forming a transparent quartz glass deposition layer. The PCVD process for preparing core rods involves two specific steps, namely deposition and rodding. Deposition is the direct deposition of gaseous halides and oxygen into a high-purity quartz glass deposition tube using low-pressure plasma at a high temperature of approximately 1000 ℃ to form the fiber core glass required by the design. Rod forming involves moving the deposited quartz glass tube onto a glass lathe used for rod forming, and using the high-temperature effect of a hydrogen oxygen flame to melt the tube into a solid optical fiber preform core rod. The latest development of PCVD technology is the use of large diameter synthetic quartz glass tubes as deposition substrate tubes, with a deposition rate increased to 2-3g/min and a deposition length of 1.2-1.5m.



The following is a comparison of several processes. Of course, in actual production, combination processes are commonly used, such as OVD for outer layer production and MCVD for core rod production, or VAD+MCVD.