Novel approaches to ultrafast fiber laser design for ablation-cooled material removal
The past few decades in particular have witnessed the tremendous beneficial impact of innovations in laser technology ranging from biomedical to industrial applications in response to enhancing community's quality of life. From the beginning, laser technology, especially ultrafast lasers have provided a very convenient platform for producing need-based laser signals, which are addressed to a wide variety of scientific and technological problems. However, there is a scarcity of utilization of ultrafast lasers as well-developed tools for various applications, mostly in industry and research laboratories due to their complexity, low reliability and high cost as a result of the dominance of solid-state lasers. Fiber lasers, on the other hand, are inherently inexpensive, compact in size and robust in their operation under harsh conditions. Applications of ultrafast laser material processing have become extremely diverse, yet ultrafast material processing is still extremely complex, costly and quite slow in terms of material removal, which is particularly taxing for biological tissue removal, rendering ultrafast lasers uncompetitive compared to mechanical techniques. This thesis represents a series of work about developing fiber laser systems which address this technological problem. The motivation of this thesis is to develop fiber laser systems for applying the ablation cooled laser material removal idea which has recently proposed by our group  for tissue and material processing. Ablation cooling becomes significant above a certain repetition rate, which depends on the thermal diffusivity of the target material. Besides, the speed with which the laser beam can be repositioned over a target is limited. As a remedy, burst-mode operation, also proposed by our group  has been implemented, where the laser produces groups of high repetition rate pulses, which are, in turn, repeated with a lower frequency. Consequently, the burst-mode fiber laser system operating at 1 µm was demonstrated with an all-fiber architecture and we scaled it to 100 MHz intra-burst repetition rate and 1 MHz burst repetition rate with the average power of 150 W for high power applications. Additionally, a detailed investigation on the limits of continuously-pumped all-fiber burst mode laser system was reported. Besides all the practical advantages of the ablation cooling idea compared to other laser-material interactions, laser ablation depends on laser operating wavelength because materials have wavelength dependent absorption and scattering values. In terms of underlying laser technology, ultrafast tissue ablation experiments require a laser system operating around 2 µm where laser tissue interaction is much stronger due to the local peak of water absorption for achieving a high ablation efficiency. Therefore, this thesis also focuses on transferring know-how on burst-mode operation to the Tm/Ho doped fiber system, operating around 2µm, which addresses requirements for an effcient tissue ablation process without any collateral damage. The physics of the laser-material interaction assisted by ablation cooling idea is also valid for tissue ablation, so the repetition rates of several GHz are necessary for fully exploiting this effect. Toward this goal, we developed core technologies, which were constituted by three different stages: (i) starting from a novel mode-locked oscillator with a repetition rate in the GHz range, (ii) followed by the construction of a Tm-doped pump source based on the WDM cascade architecture and (iii) finally the amplification of the Ho-doped fiber with a dual wavelength pumping concept.