Large Mode Area Microstructured Fiber For High Power Laser Application

Along with the enhancement of laser output power, nonlinear effects become the main obstacle to improving laser beam quality and output power. Meanwhile, the lack of effective transmission waveguide also limits the development of high power laser system. Due to the flexible structure and excellent properties, microstructured fiber provides an effective solution to develop large mode area fibers. Supported by National Nature Science Foundation Project, Innovation Foundation for Excellent Doctorial Candidates of Beijing Jiaotong University and the EU Framework Program, oriented to the requirement of compact, integrated high power fiber lasers for large mode area microstructured fibers, as well as the lack of transmission waveguide for high power laser, from the point of view of structure innovation and fabrication feasibility, this dissertation carries out the investigation on large mode area microstructured fibers and hollow core photonic bandgap fibers. The main achievements of the dissertation are listed as below:1. A fiber geometry reconstruction model is put forward. Combined with the finite element analysis model, an analysis method for modeling the optical properties of actual microstructured fiber is established. To effectively eliminate the noise during the acquisition process, a total variation-wavelet denoising model is introduced. And the point spread function estimation model based on kalman filtering is also introduced to recover the blurred image accurately. With the proposed analysis model, the optical characteristics of two kinds of commercial microstructured fiber are investigated. Analysis results indicates that the proposed model can evaluate the optical properties of the actual microstructured fiber accurately and rapidly. The proposed analysis model is a fast, low cost method for evaluating the optical properties of the actual microstructured fiber.2. A large mode area microstructured fiber with air holes arranged in rectangle-lattice is proposed and successfully fabricated for the first time. Theoretical results indicates that at the wavelength of 2 μm, the fabricated fiber achieves a mode field area as large as 2471 μm2 at straight state. When it is bent at the radius of 30 cm, the fabricated fiber is robust single mode operation and the bend loss for the fundamental mode is as low as 0.0015 dB/m. The mode field area at bent state is 2349 μm2, the decrement of mode field area induced by bend is as low as 3.7%, indicating the fabricated fiber has a strong capacity of resisting bend distortion, which is superior to current reports. Moreover, experimental results show that in wavelength of 2 μm, at the bend radius of 30 cm, the actual bend loss of the fabricated fiber is only 0.062 dB/m.3. A triangular-core large mode area microstructured fiber with leakage channels is proposed. Compared with current reports, there are only two sizes of air holes in the design, which greatly reduces the fabrication difficulty. In the wavelength of 1.064 μm, the fiber conforms to single mode operation conditions at the bend radius of 30 cm, and the bend loss and mode field area of the fundamental mode is 0.0025 dB/m and 815 μm2, respectively. Meanwhile, the bend orientation angle in which the fiber can be single mode operation is optimized to ±55°, and that is superior to the bend orientation angle of the reported asymmetric structure(±7°).4. In order to further reduce the difficulty of fabricating two kinds of air hole and improve mode field area, a modified structure is proposed with the introduction of doped fluoride rod to replace the inner ring of air holes. Through the analysis of the correlation between structure parameters and bend properties, the optimized fiber structure parameters is obtained. In the wavelength of 1.064 μm, the modified structure is single mode operation with a mode field area of 1154 μm2 at the bend radius of 30 cm. Compared to the original triangle-core structure, the modified structure realizes an increment of 340 um2 in mode field area. Moreover, the bend orientation angle in which the fiber can be single mode operation is optimized to ±180°, which makes a breakthrough in reducing the bend orientation sensitivity.5. A bend resistant large mode area fiber with multi-trench core is proposed. The importance of the multi-trench core in modulating mode field distribution, suppressing bend distortion, ensuring single mode operation is proved. In the wavelength of 1.064 μm, robust single mode operation with a mode field area of 1100 μm2 is achieved under a tight bend radius of 15 cm, while according to the current reports, the mode field area of the multi-trench fibers is limited to 890 μm2 when the bend radius is smaller than 20 cm. Moreover, a modified structure is proposed to further improve the mode field area to 1310 μm2 at the bend radius of 15 cm.6. A novel phase extraction method for the broadband dispersion measurement of hollow core bandgap fiber is proposed. Based on the proposed phase extraction method, the measurement system based on Mach-Zehnder interferometer is set up for measuring the dispersion of hollow core bandgap fiber. Combining with the determination of the peak and center of symmetry points, the phase information is obtained directly. The fundamental mode dispersion curve of a 19 cell hollow core photonic bandgap fiber from the wavelength of 1400 nm to 1630 nm is obtained. Moreover, four high order modes dispersion curves is obtained for the first time. The measurement results are in accordance with the simulation results.