Ytterbium doped all-fiber integrated high power laser systems and their applications

Abstract

For the past decades, high-power laser technology has been developing rapidly all over the world. The scientific interest in fiber lasers stems from the rich nonlinear dynamics. Industrial interest is largely due to their practical advantages, such as high power levels, compact size, relatively low cost, excellent beam quality, over established laser technologies. As a result, fiber laser are highly sought after in applications including material processing, especially in high-precision micromachining with ultrafast pulses, medical applications and defence applications, especially for the high power and efficiency levels that fiber laser can offer. The advantage of fiber lasers for high powers is largely due to their geometry, which is a very long cylinder, with an extremely high surface to volume ratio, rendering heat transfer away from the active medium much easier. Fiber lasers diffraction-limited beam quality if operating in the fundamental fiber mode. Average output powers that can be extracted from singlemode fiber lasers can reach up to a few kilowatts without serious thermal problems due to the fiber structure. For many realworld applications, misalignment free operation is important and an all-fiber laser system offers this prospect, but to date, most of the published reports on high-power lasers utilise bulk optics components to couple light in and out of fibers, which detracts from some of the practical advantages of fiber lasers. Ytterbium doped fibers which are preferred as active media for high-power operation, as the technology behind it has led to the development of excellent components and the small quantum defect is extremely useful for high-power applications. Yb-doped continuous wave lasers practically can reach several kilowatt levels, yet the output power of Yb-doped picosecond and sub picosecond pulsed lasers with a small count of bulk optics in the cavity have been limited to several hundred watts. In this thesis, we mainly focus on developing two high-power, robust, fiberintegrated lasers systems. The first system is a laser designed for continuous-wave (cw) operation, reaching up to 200 W level. The second system is a picosecondpulsed system, delivering 100-W, few-ps pulses at 100 MHz repetition rate. The latter is built based on master oscillator power amplifier (MOPA) structure. The multi-stage amplifier of the pulsed system and resonator design for the continuous wave laser system are both based on the all-fiber designs which allow for robust operation and have been optimised through numerical simulations. We expect these systems to find widespread use in material processing applications.