Study On The Composition, Structure And Properties Of PCVD Single Mode Fiber

Quartz glass has high transparency in the visible and near-infrared wavelength region. The chemical and mechanical durabilities of quartz glass are high and its refractive index can be changed by dopants. Therefore, quartz glass is the suitable material for optical fibers. Among the main four core rod fabrication processes, PCVD is considered as the best process for multi-mode fiber fabrication previously because it has high accuracy for RI profile control, high deposition efficiency and flexibility. In this thesis, the current situation of quartz fiber has been reviewed and the development trend of quartz single mode fiber (SMF) has been analyzed. Through analyzing in theory and series of experiments, the material composition, structure and properties of PCVD single mode fiber (SMF) have been researched and summarized in order to improve the properties of PCVD SMF.The PCVD based process for SMF fabrication includes PCVD deposition, deposition tube collapsing, RIT and drawing process in this thesis. The material composition in the core and optical cladding of PCVD SMF is SiO₂-GeO₂-F-(Cl). Through studying the cross sections and RI profiles of the fibers fabricated by different processes, the results show that the advantages of PCVD fiber lie in its excellent material homogenization and high accuracy for RI profile control.To design the material composition and structure in PCVD fiber, the effect of dopants on the refractive index, viscosity and attenuation of quartz glass or fiber has been studied. The viscosity of GeO₂ and F doped quartz glass can be measured by fiber bending and drawing methods. The results show that both GeO₂ and F dopants reduce the viscosity. F has three times greater impact on viscosity than GeO₂ compared to the effects on the refractive index. F dopant can not only reduce the refractive index of doped silica but also reduce the OH^- content in PCVD silica. The test results show that the optimal F dopant level in the core and cladding is around 0.37wt% and 0.67wt% respectively in PCVD low water (LWP) SMF. Besides, the research result has shown that the Cl content in silica reduces with the increase of O/Si in reactants.The gaseous reactants are directly vitrified into dense glass layers on the inside of a fused silica substrate tube in the PCVD-based core rod production process. For the glass with little residual OH^- content (ppb), it is difficult to dehydrate through drying process. Through purifying the raw materials by molecular griddle purifier, online purifying during PCVD by freon, using freon and HF acid to etch off the OH^- contamination layers and the dilution effect with large preform, the OH^- content in PCVD core rod can be reduced to very low level. PCVD process can be used for LWP-SMF production with these arts. PCVD LWP-SMF complies with or exceeds the ITU recommendation G.652 (C and D) and IEC 6093-2-50 type B 1.3 optical fiber specification.To study the effect of defects in optical fiber on the properties, the mechanism of hydrogen sensitivity and test methods have been introduced and studied. Acceleration test for hydrogen sensitivity can be fulfilled by increasing the H₂ partial pressure (concentration) and the reaction temperature. By adopting heat acceleration test, the circle of hydrogen sensitivity can be shortened at the testing conditions of 0.01atm H₂ with 70℃. The experiment results showed that the hydrogen sensitivity test circle is 8-20hours and the induced attenuation at 1240nm is around 0.03-0.06dB/km. From the test results of heat acceleration test, NBOHCs and Si-E' defects were detected in PCVD SMF, but Si-E' defects can automatically heal up.Test results have shown that the hydrogen induced loss increased with the increase of drawing speed. Using D₂ treating the LWP SMF, the hydrogen sensitivity at 1383nm can be reduced and even eliminated. Moreover, the hydrogen sensitivity resistance does not become weak later. The hydrogen sensitivity resistance is stable after D₂ treatment.To improve the properties of PCVD SMF by optimizing the waveguide design, the model of viscosity-matching for SMF has been set up through theoretical analysis and calculation. Making full use of the advantages of PCVD process and combining the waveguide design together with the material composition and structure design effectively are the basis for high-performance PCVD fibers. F and GeO₂ co-deposition and functional graded material (FGM) design are adopted and the viscosity-match is ensured for the material composition and structure design based on the determined waveguide design. The residual internal stress caused in the succedent fiber making process can be released. As a result, not only the residual internal stress but also the broken bonds caused by drawing process have been reduced, which can reduce attenuation, PMD and improve the hydrogen sensitivity resistance as well as microbending resistance properties.By adopting the LWP SMF fabrication techniques, viscosity match and FGM design in the design and fabrication for bending insensitive fiber, the fiber not only has low attenuation, nice dispersion property and hydrogen sensitivity resistance in 1260nm-1625nm but also has excellent bending resistance performance. Such fiber can be compliable with the standard single mode fiber and be acted as transmission media in the FTTH field. It also can be used to minimize the optical components.Besides, adopting UV opaque doped silica as part mechanical cladding for optical fiber preform can prevent the high energy, which may cause defects in optical fiber, going into the core of optical fiber. As a result, the attenuation can be reduced and the hydrogen sensitivity resistance property can be improved.