Study And Fabrication Of High Concentration Erbium Doped Optical Fiber

Erbium doped fibers and related devices have great applications in the optical communications, sensing, material process and military fields. In this dissertation, we concentrate on the fabrication of the highly erbium-doped fibers (EDF) with small erbium-cluster ratios, analysis of the erbium-cluster ratio in highly erbium-doped fibers, and the design of double-clad highly erbium-doped fibers. And the work is supported by the National High Technology Research and Development Programs of China, "Rare-earth doped fibers" and "Novel special optical fibers". The main achievements of this dissertation are listed as follows:The key issues about the fabrication of highly erbium-doped fibers with small erbium-cluster ratios are researched. The erbium-doped optical fiber preforms are fabricated by the combination of the MCVD process and solution doping technique. Firstly, the structure of deposited soots is observed by EPMA. The influnce of the deposition temperature on the soot is analyzed and it is pointed out that the microstructure of soot is an important factor which affects the erbium's concentration and clustering. The cross sections of different preforms that are doped with different dopants are also observed by EPMA, and the influnces of soots with different co-dopants are analyzed. By experimental comparision, we find that increasing the concentration of Bi³⁺ or Al³⁺ in the solution will increase the erbium concentration in the fiber. It's contributed to the chemical adsorption effects which happen during the solution doping process. It's proved that co-doped Al³⁺ can reduce the erbium clustering. Finally, several kinds of highly erbium-doped fibers are manufactured, including highly erbium-doped photosensitive fibers, highly erbium-doped fibers with only Al³⁺ co-doped, highly erbium-doped fibers by multi-component soot deposition, and highly erbium-doped fibers codoped with multi-metal elements. In these erbium-doped fibers, erbium concentrations exceed 0.1mol%, with the highest valueA simple method is proposed to measure the erbium-cluster ratio in erbium-doped fibers. We measure the erbium-cluster ratios of self-made erbium-doped fibers, and prove this method correct by comparision the numerical simulation and experimental results. The measurement results show that cluster ratios of all self-made fibers are below 0.1 except highly erbium-doped photosensitive fibers which are co-doped with high concentration GeO₂. By analysing the measured results, we find that Al³⁺ codopants can effectively reduce the erbium clustering, and high concentration GeO₂ will induce big erbium-cluster ratio which is contributed to the bigger pore sizes and wider pore size distribution of soots codoped with GeO₂. It's showed that the key to eliminate erbium clustering are codopants selection and the soot depostion.Based on the erbium cluster model, we analyze the effects of erbium-clusters on the amplification performance of EDFA. It is found that erbium clusters will damage the amplifier's gain performance, deteriorate the noise figure, and reduce the quantum conversion efficiency. The effects of fiber core parameters on the performance of EDFA are also analyzed. Increasing the refractive index and reducing the core diameter are favorable to improve the gain performance. Reducing the erbium-doped region in fiber core can make the EDFA's performance insensitive to the core's refractive index and diameter. Meanwhile, we measured the ASE spectrum and gain spectrum of self-made erbium-doped fibers. The results show that only l²m long erbium doped fibers can reach 30dB small signal gain. Thus the erbium doped fiber length can be reduced. According to the amplification performance, it's proved that codoped Al³⁺ affects the energy structure Stark split, and thus broadens the amplification band. The numerical model of double clad highly erbium-doped fiber considering the erbium clustering is set up. We analyze the effects of erbium clusters on the performance of double clad fiber amplifier. Compared with the core-pumped EDFA, cladding-pumped EDFA is less infiunced by erbium clusters. That' to say, double clad waveguide structure can weaken the influnce of erbium clusters. We simulation the pump absorption in double clad fibers with different shapes of inner clad. It's showed that those double clad which seriously destroyed circular symmetry can maintain the pump adsorption coefficient. Reducing the area of inner clad can improve the gain performance of double clad EDFA, but this must be accompanied with higher NA of inner clad, otherwise the pump coupling efficiency will be reduced. The effects of parameters of fiber core are also analysed. Increasing the core diameter and reducing the refractive index can improve the amplification performance, which is contrary to the core pumped EDFA. Finally, we simulate the micro-structured fibers with air holes introduced into inner or outer clad.