Today's communication networks are characterized by "fibre optics." With the trend of copper, FTTX, and all-IP networks, the entire metropolitan area network is gradually constructed of fiber optic cables from the core network to the access network. Today, Class A multimode fiber and Type B single mode fiber for communications are made from quartz glass. In the production process of quartz glass fiber, there is inevitably slight unevenness in the glass substrate, high-temperature melt quenching drawing causes uneven stress distribution on the surface, and environmental micro-cracks, mechanical damage, etc., cause some micro-cracks on the surface of the optical fiber. In the high-speed wire drawing, these micro-crackes are subjected to a large drawing tension, and further expansion occurs, resulting in a decrease in the optical fiber strength. Preform defect is the main cause of fiber breakage The occurrence of low-intensity breaks in the optical fiber during production is mainly caused by defects in the optical fiber. These defects can be roughly classified into internal defects and surface defects. The main cause of internal defects is inclusion of air bubbles and impurities in the preform. The main form of surface defects is micro-cracking and micro-dust contamination. Bubbles and impurities are inevitably present in the preform production process. For a bubble of a certain diameter inside the preform, cracking may occur during drawing, or it may be reduced to a very fine gas line with a serious effect on the strength of the optical fiber. For the defects caused by internal impurities, not only can it not be allowed to heal and shrink during the drawing process, but on the contrary, at the time of high temperature melting, the impurities and the glass body interface can also be cracked due to the cracks. Cracks continue to grow during wire drawing, and the crack size is much larger than the size of the impurities themselves. The surface defects are mainly micro-cracks and surface contamination. The micro-cracks on the preform surface will inevitably turn into smaller micro-cracks on the surface of the fiber during the drawing process. When the external stress of the optical fiber is larger than the critical stress of these small microcracks, the small microcracks gradually increase and eventually cause the optical fiber to break. The contamination of the surface will reduce the binding density of the bare fiber surface and the inner coating. At present, there are two main treatments for surface defects: one is flame polishing and the other is HF acid treatment. The flame polishing can effectively cure the micro-cracks on the surface of the preform. The HF acid can wash off the impurities adhering to the surface of the preform. Therefore, in the actual production, the preform is subjected to HF pickling and flame polishing secondary treatment to increase the optical fiber strength. Coating and curing determine mechanical protection strength The role of the optical fiber coating is to protect the surface of the optical fiber from mechanical damage and moisture and maintain its original strength. If the coating is too thin or eccentric, it will lose its mechanical protection. Concentricity of the coating is easily changed during the drawing process, so care must be taken during drawing. Figure 1 shows the influence of different coating concentricity on the fiber strength based on actual drawing. According to actual optical fiber production methods, photopolymerization technology methods are widely used at present. The use of UV radiation causes the photoinitiator to excite into an active (radical or cation). The active body reacts with the C=C double bond in the prepolymer and monomer to form a growing chain. This growing chain reacts further to form longer polymer chains. If a multi-energy polymer or monomer is present, a cross-linked structure will result, and finally the coupling and disproportionation of the active body will terminate the reaction. With the improvement of technology, the drawing speed in the current production has been increased to 20m/s to 30m/s, and the residence time of the optical fiber in the curing oven is only 0.1s to 0.2s. To ensure the curing effect of the coated optical fiber, the curing oven is required. Sufficient UV light energy can be provided to meet the energy required for photoinitiator activation into the active body. At the same time, a certain proportion of inert gas is introduced into the curing furnace to prevent oxygen from inhibiting the growth of the polymer chain and improve the curing effect. Fig. 2 and Fig. 3 are statistics of the strength of optical fibers of different degree of cure and optical fibers that are surface-tacky due to an excessively high oxygen content. As can be seen from the figure, when the degree of solidification of the optical fiber is higher than 80%, the strength of the optical fiber does not increase as the degree of solidification of the optical fiber rises, but it is a random distribution. In the figure, the surface-tacked optical fibers caused by the high oxygen content in the curing oven have a higher number of breaks per 1000 KM, from 12.1 to 12.8, compared with normal optical fibers. Furnace temperature affects fiber mechanical strength The occurrence of point defects during high-temperature wire drawing will lead to deterioration of the mechanical strength of the optical fiber. One of the most important point defects discovered is the breakage of Si-O chains, and the breakage and re-linking of Si-O chains are dynamically changed. E The concentration of defects depends on the balance of the Si-O chain breaking and re-linking. The concentration of E-defects increases with the length of the heating zone in the drawing furnace and decreases with increasing drawing speed. The length of the heating zone causes the preform to lengthen at the high temperature zone, resulting in a higher frequency of Si-O chain fractures. Studies have shown that when the heating furnace temperature increases from 2200K to 3000K, the defect concentration of the bare fiber emerging from the heating furnace will increase by two orders of magnitude. At the same time, due to the high temperature, the graphite in the furnace volatilizes to produce a chemical reaction, resulting in a SiC particle with higher hardness. If the bare fiber is touched by the SiC particles in the heating furnace, defects and cracks may occur on the surface of the optical fiber. The higher the temperature in the heating furnace, the greater the number of SiC particles generated by the reaction, so the probability of bumping the surface of the bare fiber is higher, and the more defects on the fiber surface, the lower the fiber strength.
Another factor that affects the strength of the fiber during the coating process is the air bubbles in the coating. The generation of bubbles is mainly due to the shift of the position of the optical fiber in the mold during drawing, which causes the tilt of the half-moon liquid surface formed by the coating, the pressure on the smaller side of the coating to increase, and the gas easily brought into the coating by the optical fiber; or the coating temperature. Changes, coating pressure fluctuations and other factors will produce bubbles in the coating. The bubbles in the coating reduce the adhesion between the coating and the coating and between the coating and the bare fiber. And the presence of bubbles increases the possibility of cracks in the coating under tension, resulting in a decrease in fiber strength.