Study On Novel Erbium-Doped Fibers And Related Fiber Lasers

Novel erbium-doped fibers including the photosensitive erbium-doped fiber, the high-concentration erbium-doped fiber and the double-clad erbium-doped fiber have great potential applications in optical fiber communication systems, fiber sensing systems, optical access systems, medical lasers, industrial processing systems and space optical communication systems. Consequently, novel erbium-doped fibers have attracted a worldwide attention. Under the supports of the projects of National High Technology Research and Development Program of China with Grant No. 2001AA312230 and 2004AA31G200, this dissertation is focused upon the theoretical and experimental studies on these types of novel erbium-doped fibers and related fiber lasers. The main research results have been achieved as follows.(1) Some crucial stages, including soot deposition process, drying process and collapsing process, which can affect the final concentration of GeO₂ in the photosensitive erbium-doped fiber core, are investigated. By confirming the optimal fabrication conditions, a photosensitive erbium-doped fiber with a perfect performance is manufactured. Its peak absorption coefficient at 1530 nm is 27 dB/m, and a 5-m-long photosensitive erbium-doped fiber pumped with the power of 20 dBm can amplify a small signal by about 30 dB. Fiber Bragg gratings with their length of 25 mm and their reflectivity of 98.7% are inscripted into the photosensitive erbium-doped fiber by exploiting the phase mask method. A polarization-maintaining photosensitive erbium-doped fiber with the birefringence of 3.13×10^-4 is further fabricated by using the lateral grooving method.(2) A stable and switchable dual-wavelength erbium-doped fiber laser with two fiber Bragg gratings written in the photosensitive and polarization-maintaining erbium-doped fiber directly is presented and experimentally demonstrated. Simultaneous dual-wavelength oscillation at room temperature and two stable single-wavelength lasing lines are achieved. The power fluctuation and the wavelength shift of single-wavelength oscillations are measured to be less than 0.24 dB and 0.013 nm over 2 hours.(3) A compact single-polarization erbium-doped fiber laser by adopting vernier effect is experimentally demonstrated. The linear laser cavity consists of a piece of photosensitive and polarization-maintaining erbium-doped fiber with two high-birefringence fiber Bragg gratings written at its both ends respectively. The single-polarization regime of our proposed laser is achieved by applying a radial stress upon one of two gratings and stretching the other grating axially to adjust the reflection peak match. Two single-polarization oscillations are realized respectively with the maximal power fluctuation of 0.2 dB and the wavelength shift of 0.01 nm over an hour.(4) The fact that the doping level of erbium ion in the fiber core can be increased rapidly by codoping bismuth ion is analyzed and comprehended in details. A highly erbium-doped fiber with its peak absoption coefficient of 97.7 dB/m at 1530 nm is manufactured by codoping bismuth ion. The erbium ion concentration reaches 7650 ppm which is, to the best of auther's knowledge, the highest doping level of erbium ion in silica host fiber by using solution doping technology.(5) A quasi-three dimensional ray tracing model for analyzing the absorption properties of double-clad fibers is proposed based on geometry optics. The influences of injection area of pump lights, the size and the position of the fiber core, the shape of the inner cladding on absorption efficiency are investigated. On the basis of the numerical model, double-clad high-concentration erbium-doped fiber and double-clad erbium/ytterbium-codoped fiber are designed and optimized, respectively, to realize an approximately uniform absorption along the fibers.