High power all-fiber laser-amplifier systems for materials processing
Author(s)
Advisor
İlday, F. ÖmerDate
2011Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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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.
Keywords
Fiber laseroscillator
amplifier
all-fiber structure
high power lasers
materials processing