Transverse mode instabilities in high-power fiber-laser systems
Seiten
2016
|
1. Aufl.
Mensch & Buch (Verlag)
978-3-86387-680-7 (ISBN)
Mensch & Buch (Verlag)
978-3-86387-680-7 (ISBN)
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The demonstration of the ruby laser by Maiman in 1960 represented the starting point of the prolonged development of lasers as a general tool in science and industry. Until now, a large variety of different laser architectures has been successfully demonstrated and with it the field of applications has been dramatically expanded. Naturally, with an increasing number of possible applications the demands on the laser itself become more and more stringent. For instance, higher average powers are beneficial for faster processing speeds, enable to work thicker materials or accelerate imaging applications by allowing the use of shorter integration times. Nowadays, demands on the footprint and wall-plug efficiency of the entire laser system also play an important role. In order to address these increased demands, the laser architecture has evolved to overcome current limitations. In the particular case of solid-state lasers, the initial rod-type design
has been improved leading to the development of the slab-, thin-disk- and fiber-laser. Each one of these concepts is the subject of intense research to explore its particular opportunities and limitations.
This thesis is focused on the concept of fiber lasers. The trademark characteristic of fiber lasers is their unique combination of an excellent average-power scalability with a nearly diffraction-limited beam-quality. These attributes are a consequence of the fiber-geometry itself, which offers excellent cooling conditions due to a large surface-to-active-volume ratio. Simultaneously, the waveguide structure can be chosen in such a way that only the Gaussian-shaped fundamental mode is guided, which ensures a nearly-diffraction limited beam-quality. Within the last two decades the rapid development of fiber lasers has led to the establishment of several average output-power records in both continuous-wave and ultra-short pulse operation. However, this evolution suddenly stopped around the year 2010/11.
Since that time, the power scaling of fiber-lasers has come at the cost of reduced beam quality and stability. The reason for this is the onset of a new phenomenon: transverse mode instabilities (TMI). This effect causes an initially stable and nearly diffraction-limited beam to fluctuate rapidly with time which, additionally, significantly reduces its beam quality. This effect is caused by the excitation of several transverse modes in the fiber, which interact with each other in a complex manner. To start these unwanted dynamics a certain average output power, the so-called TMI threshold, has to be reached. Depending on the particular fiber-laser system, the TMI threshold ranges from hundreds of watts up to several kilowatt of average power.
TMI are a serious threat for the reputation of fiber lasers and, therefore, it is exceptionally important to overcome this limitation. However, the characterization and the understanding of TMI were in a very early stage in 2010/11 and both aspects being fundamental requirements to develop mitigation strategies. This situation marks the starting point of this thesis and, simultaneously, determines its goals: a detailed characterization of TMI that allows developing a theoretical understanding of the effect as well as effective ways to overcome the current limitation imposed by it.
This work is structured as follows: in chapter 2 the necessary fundamental background of optical fibers and fiber amplifiers is given. Furthermore, photodarkening and the modification of the refractive index during light amplification are also discussed, since both effects significantly influence TMI. Besides that, a brief introduction to fiber gratings is given, because the theoretical description of TMI is based on the creation of self-induced fiber gratings. In the subsequent chapter 3 a comprehensive experimental investigation of TMI is presented and a robust definition of the TMI threshold is given. Furthermore, the impact of several parameters of a fiber-laser system on the TMI threshold is demonstrated. Chapter 4 explains these findings in the context of the current theoretical understanding of TMI. The explanations are supported by simulations of TMI done within the time-frame of this work. Most importantly, a simplified approach to predict the TMI threshold has been developed, which is also introduced in this work. In chapter 5 the current understanding of TMI is used to deduce particular mitigation strategies. Moreover, first experimental realizations of the suggested mitigation approaches are also shown. Finally, the work is closed by a conclusion and an outlook.
has been improved leading to the development of the slab-, thin-disk- and fiber-laser. Each one of these concepts is the subject of intense research to explore its particular opportunities and limitations.
This thesis is focused on the concept of fiber lasers. The trademark characteristic of fiber lasers is their unique combination of an excellent average-power scalability with a nearly diffraction-limited beam-quality. These attributes are a consequence of the fiber-geometry itself, which offers excellent cooling conditions due to a large surface-to-active-volume ratio. Simultaneously, the waveguide structure can be chosen in such a way that only the Gaussian-shaped fundamental mode is guided, which ensures a nearly-diffraction limited beam-quality. Within the last two decades the rapid development of fiber lasers has led to the establishment of several average output-power records in both continuous-wave and ultra-short pulse operation. However, this evolution suddenly stopped around the year 2010/11.
Since that time, the power scaling of fiber-lasers has come at the cost of reduced beam quality and stability. The reason for this is the onset of a new phenomenon: transverse mode instabilities (TMI). This effect causes an initially stable and nearly diffraction-limited beam to fluctuate rapidly with time which, additionally, significantly reduces its beam quality. This effect is caused by the excitation of several transverse modes in the fiber, which interact with each other in a complex manner. To start these unwanted dynamics a certain average output power, the so-called TMI threshold, has to be reached. Depending on the particular fiber-laser system, the TMI threshold ranges from hundreds of watts up to several kilowatt of average power.
TMI are a serious threat for the reputation of fiber lasers and, therefore, it is exceptionally important to overcome this limitation. However, the characterization and the understanding of TMI were in a very early stage in 2010/11 and both aspects being fundamental requirements to develop mitigation strategies. This situation marks the starting point of this thesis and, simultaneously, determines its goals: a detailed characterization of TMI that allows developing a theoretical understanding of the effect as well as effective ways to overcome the current limitation imposed by it.
This work is structured as follows: in chapter 2 the necessary fundamental background of optical fibers and fiber amplifiers is given. Furthermore, photodarkening and the modification of the refractive index during light amplification are also discussed, since both effects significantly influence TMI. Besides that, a brief introduction to fiber gratings is given, because the theoretical description of TMI is based on the creation of self-induced fiber gratings. In the subsequent chapter 3 a comprehensive experimental investigation of TMI is presented and a robust definition of the TMI threshold is given. Furthermore, the impact of several parameters of a fiber-laser system on the TMI threshold is demonstrated. Chapter 4 explains these findings in the context of the current theoretical understanding of TMI. The explanations are supported by simulations of TMI done within the time-frame of this work. Most importantly, a simplified approach to predict the TMI threshold has been developed, which is also introduced in this work. In chapter 5 the current understanding of TMI is used to deduce particular mitigation strategies. Moreover, first experimental realizations of the suggested mitigation approaches are also shown. Finally, the work is closed by a conclusion and an outlook.
| Erscheinungsdatum | 04.11.2016 |
|---|---|
| Verlagsort | Berlin |
| Sprache | englisch |
| Einbandart | gebunden |
| Themenwelt | Naturwissenschaften ► Physik / Astronomie ► Optik |
| Schlagworte | fiber amplifiers • fiber-laser • TMI |
| ISBN-10 | 3-86387-680-6 / 3863876806 |
| ISBN-13 | 978-3-86387-680-7 / 9783863876807 |
| Zustand | Neuware |
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