High power all-fiber laser-amplifier systems for materials processing

Abstract

When the fiber lasers first appeared in 1970s, their average powers and pulse energies were so low that they remained as a laboratory curiosity for a long time. The scientific interest in fiber lasers continued due to their inherited practical advantages over the established solid state lasers. First of all, in single-mode operation, fiber lasers deliver diffraction-limited beam quality since light is always guided in the fiber by total internal reflection. Beam qualities of other type of lasers deteriorate with increasing power due to thermal effects like thermal lensing. Second, their structures are well suited to power-scaling due to their enormous surface area to volume ratio. In theory, output power level of a fiber laser should be able to go up to the 1-10 kW range without serious thermal problems. Third, the small signal gain and optical efficiency are very high compared to other types of lasers because of the intense interaction with the active ions over long lengths. Efficiency of an ytterbium fiber laser can reach 80%, depending on the design parameters. Therefore, single-pass amplification is practical, whereas most other gain media do not have enough gain for single-pass amplification. Consequently, the vast majority of high-power fiber lasers are based on master-oscillator power-amplifier (MOPA) structure, where the signal is first created in an oscillator and then amplified in an (single or multi stage) amplifier. Fourth, beam propagation through all the optical elements comprising a fiber laser can be guided propagation and, in theory, this enables misalignment-free operation. Fiber lasers are increasingly used outside the basic laser research laboratory in material (particularly metal) processing, medical, metrology, defense applications, as well as scientific research. For many of these applications, flexibility and misalignment-free operation is important. However, there are still many systems in use, including many reported in the academic literature, where the pump light is coupled into the fiber through free space optics, and components such as isolators, grating stretchers are frequently employed in bulk optics form. In this thesis, we mainly focus on all-fiber designs, with the specific aim of developing high-power, robust, fiber-integrated systems delivering high technical performance without compromising on the practical aspects. The laser systems developed in this thesis are also applied to material processing. This allows us to gain first-hand experience in the actual utility of the lasers that we develop in real-world applications, generate valuable feedback for our laser development efforts and produce laser systems, which are ready for industrial implementation. The thesis begins with introductory chapters on the basic physics and technology of highpower fiber lasers, including a brief discussion of the material processing applications. In Chapter 1, we focus on optical fiber itself, where the manufacturing and structure are explained briefly, followed by some theoretical information on guidance of light, dispersion and nonlinear effects in fibers. In Chapter 2, we focus on the theory of fiber lasers. Firstly, propagation of ultrashort pulses in fibers is explained and nonlinear Schrödinger equation (NLSE) is introduced. Then gain in rare-earth doped fibers, mode- locking mechanism, and different mode-locking regimes are described. Following a survey on current situation of fiber lasers in world market, we introduce the current fiber architectures, discuss the main limitations encountered in high power fiber laser design, nonlinear effects, fiber damage and excessive thermal loads. Then, the possible application areas of these lasers in materials processing are described. Chapter 3 reports on the development of a high-power and high-energy all-fiber-integrated amplifier. In Chapter 4, we introduce a new and low-cost technique that allows the construction of all-fiberintegrated lasers operating in the all-normal dispersion regime. In Chapter 5, an all-fiberintegrated laser system delivering 1-ns-long pulses with an average power of 83 W at a repetition rate of 3 MHz is introduced that combines the positive aspects of micromachining with ultrashort pulses in terms of precision and long nanosecond pulses in terms of ablation speed. In Chapter 6, we report on the development of an all-fiber continuous-wave fiber laser producing more than 110 W of average power. Chapter 7 is on the use of these laser systems in systematic material processing experiments, where we compare the influence of three different laser systems, producing approximately 100 ps, 1 ns and 100 ns pulses. The final chapter provides the concluding remarks.