Browsing by Subject "Fiber optics."

Now showing 1 - 14 of 14

Open Access
33 Femtosecond Yb-doped optical frequency comb for frequency metrology applications
(2013) Şenel, Çağrı
Optical frequency combs have enabled many applications (high precision spectroscopy, table-top optical frequency metrology, optical atomic clocks, etc.), received considerable attention and a Nobel Prize. In this thesis, the development of a stabilized Yb-doped femtosecond optical frequency comb is presented. As a starting point in the development of the frequency comb, a new type of fiber laser has been designed using numerical simulations and realized experimentally. The developed laser is able to produce pulses that can be compressed to 33 fs without higher-order dispersion compensation. After realization of the laser, a new type of fiber amplifier has been developed to be used for supercontinuum generation. The amplifier produces 6.8 nJ pulses that can be compressed to 36 fs without higher-order dispersion compensation. The dynamics of supercontinuum generation have been studied by developing a separate simulation program which solves the generalized nonlinear Schr¨odinger equation. Using the simulation results, appropriate photonic crystal fiber was chosen and octave-spanning supercontinuum was generated. Carrier-envelope-offset frequency of the laser has been obtained by building an f-2f interferometer. Repetition rate and carrier-envelope offset frequency of the laser have been locked to Cs atomic clock using electronic feedback circuits, resulting in a fully stabilized optical frequency comb. The noise performance and stability of the system have been characterized. Absolute frequency measurement of an Nd:YAG laser, which was stabilized using iodine gas, has been performed using the developed optical frequency comb.
Open Access
Biophotonic applications of ultrafast fiber lasers: from biomaterial surface modification to sub-cellular nanosurgery
(2014) Erdoğan, Mutlu
Just a year after the invention of the LASER in 1960, it was demonstrated that lasers could be used for the treatment of certain skin abnormalities. At present, lasers are extensively used in a broad range of medical treatments. After the development of femtosecond pulse lasers in the 1980s, even more exciting possibilities in a diverse range of fields have been realized. Accordingly, ultrashort pulse lasers are widely used in biological applications in recent years. In parallel to these, fiber laser systems have increasingly been utilized in a wide range of scientific and biomedical applications, since they are highly compatible systems for being employed for industrial and biomedical applications. Consequently, the aim of this Ph.D. thesis proposal is to develop compact, simpler to operate, and cost-efficient ultrafast fiber lasers with different repetition rates and pulse energies. By using such systems, we demonstrate the biophotonic applications of these lasers on two different biological research fields. As a part of this thesis study, we develop ultrafast fiber lasers and apply them in biomaterial surface modification. We demonstrate that different surfaces with micro- and nano-scale topographies can be generated at high speed, precision and repeatability. The outcomes of biomaterial surface modification with different laser parameters are compared in terms of topographical uniformity and repeatability. Additionally, a variety of topographical modifications are assessed with respect to the efficiency on cell attachment and proliferation on metal implants.As the second part of this thesis, we develop a custom-built ultrafast fiber laser-integrated microscope system for nanosurgery and tissue ablation experiments. Subsequently, we employ this system in order to make high-precision cuts onto different biological specimens ranging from the tissue level to subcellular level, such as a part of an axon or a single organelle. Finally, we improve this integrated system in a way that it becomes capable of generating optical pulses in any desired sequence possible. This is achieved by using acousto-optic modulators (AOM) and custom-developed field-programmable gate arrays (FPGA).
Open Access
Engineering the nonlinear dynamics of photonic systems : demonstration of the soliton-similariton fiber laser and nonlinear laser lithography
(2013) Öktem, Bülent
Nonlinear effects easily and unavoidably arise in ultrafast optics, often acting as sources of limitation to performance. However, many fascinating phenomena, from generation to utilization of ultrashort laser pulses rely on the very same nonlinear effects. Deep understanding of the governing dynamics, coupled with mechanisms through which they can be controlled or manipulated holds potential for observing new phenomena, as well as achieving new functionalities, which can be difficult or even impossible to achieve otherwise. This thesis presents a series of work, starting from a novel mode-locked oscillator for generating ultrashort pulses, followed by amplification of ultrashort pulses to microjoule-level energies, finally a novel nanostructuring mechanism relying on the nonlinear interaction of such pulses with the surface of a metal. The novel mode-locked laser developed in this thesis is one in which pulses propagate self-similarly in the presence of amplification, as similaritons, in one part of the cavity and as soliton-like pulses in the rest of the cavity. The coexistence of the seemingly incompatible similariton and soliton-like waves subject to the boundary conditions of a laser oscillator requires in the presence of a narrow bandpass filter and result in spectral breathing of the waves by unprecedented one order of magnitude, constituting the observation of the strongest nonlinear effects in any mode-locked laser to date. The laser reduces to the dispersion-managed laser in limit of large filter bandwidth and to the all-normal-dispersion laser in the limit of vanishing anomalous-dispersion fiber. Thus, all the four basic modelocking regimes are covered. As such, we believe the unraveling of this regime can be instrumental in deeper understanding of all the mode-locking regimes. Importantly, by showing that two attractor solutions can co-exist in a single laser cavity opens the door to new future designs. From an applications point of view, the laser is easy to mode-lock and exhibits excellent short-term and longterm stability, indicating high potential for high precision materials processing applications. We also illustrate, to our knowledge, the first high-energy, all-fiber implementation of the nonlinear chirp pulse amplification technique, which allows us to achieve in-fiber peak powers of 57 kW. We demonstrate a fiber amplifier with no free space beam pump or signal beam propagation, producing 70-ps chirped pulses with 3 μJ and 4 μJ pulse energies, which are compressible to 140 fs and 170 fs, respectively, via a grating compressor. The amplified output can be used directly, as a picosecond source, or compressed externally in a grating compressor. This approach results in a completely robust, misalignment-free system, with peak powers approaching 10 MW. This was, at the time of publication, the highest peak power obtained from an integrated fiber amplifier. Finally, we apply the laser systems we developed, together with the lessons learned from our implicit control of the nonlinear dynamics to demonstrate a method that utilizes positive nonlocal feedback to initiate, and negative local feedback to stabilize growth of self-organized metal oxide nanostructures, initiated and controlled by ultrafast pulses. We achieve unprecedented uniformity at high speed, low cost, and on non-flat or flexible surfaces. By exploiting the nonlocal nature of the feedback to stitch the nanostructures seamlessly, we are able to cover indefinitely large areas with sub-nm uniformity in periodicity. We demonstrate our approach through fabrication of TiO2 and tungsten oxide nanostructures, which can be extended in principle to a large variety of materials.
Open Access
The Examination of the effect of polarization on the radiation losses of bent optical fibers
(1990) Tanyer, Süleyman Gökhun
It has long been recognized that the bending losses in weakly guiding optical fibers, is independent of the polarization for large bend radius. We showed this fact using the volume equivalent current method. The procedure is then applied to a helically bent fiber, and it is shown that the radiation from the helical fiber is also independent of the polarization as long as the fiber is weakly guiding.
Open Access
Fabrication of novel core-shell nanostructures for photonics applications
(2013) Khudiyev, Tural
Developments in nanoscale fabrication and characterization techniques have led to fundamental changes in the scientific understanding of many fields, and novel nanostructures have been utilized to investigate the conceptual underpinnings behind a diverse array of natural phenomena. However, nanofabrication methods are frequently hindered by issues such as misalignment, small batch sizes, high production costs and constraints in material choice or nanostructure diversity, which decrease their potential utility and prevent their widespread application in nanoscale optics and photonics. In this work, a new top-down nanofabrication method is described, which is called Iterative Size Reduction (ISR), where step-by-step reduction is utilized to decrease structure dimensions from macro- to nanosizes and produce indefinitely long one-dimensional core-shell nanostructures with properties highly suitable for use in optical applications. Plateau-Rayleigh instabilities are then utilized to thermally degrade ISR-produced nanowire arrays into complex core-shell schemes, which are produced successively in a hitherto-undescribed transitory region between core-shell nanowires and core-shell nanospheres. A diverse array of optical phenomena have been observed on fabricated novel core-shell nano-platforms, which are utilized in the design of novel nanostructures for emerging photonics applications. Briefly, (a) the resonant Mie scattering behavior is characterized on glass-polymer nanostructures and these nanostructures are designed for large-area structural coloration, (b) a novel non-resonant Mie scattering regime responsible for the scattering characteristics exhibited by all-polymer core-shell nanowires is described, (c) a nanoscale analogue to the thin film interference phenomenon is demonstrated that occurs on the core-shell boundary of ISR-produced micro- and nanostructures, (d) an unusual photonic crystal structure observed in the neck feathers of mallard drakes is investigated and imitated, (e) a series of all-polymer core-shell nanowires to function as novel light-trapping platforms and sensors are engineered and (f) the generation of supercontinuum light in well-ordered arrays of As2Se3 nanowires is reported.
Open Access
Fiber optical network design problems : case for Turkey
(2013) Yazar, Başak
The problems within scope of this thesis are based on an application arising from one of the largest Internet service providers operating in Turkey. There are mainly two different problems: the green field design and copper field re-design. In the green field design problem, the aim is to design a least cost fiber optical network from scratch that will provide high bandwidth Internet access from a given central station to a set of aggregated demand nodes. Such an access can be provided either directly by installing fibers or indirectly by utilizing passive splitters. Insertion loss, bandwidth level and distance limitations should simultaneously be considered in order to provide a least cost design to enable the required service level. On the other hand, in the re-design of the copper field application, the aim is to improve the current service level by augmenting the network through fiber optical wires. Copper rings in the existing infrastructure are augmented with cabinets and direct fiber links from cabinets to demand nodes provide the required coverage to distant nodes. Mathematical models are constructed for both problem specifications. Extensive computational results based on real data from Kartal (45 points) and Bakırköy (74 points) districts in Istanbul show that the proposed models are viable exact solution methodologies for moderate dimensions.
Open Access
High power all-fiber laser-amplifier systems for materials processing
(2011) Özgören, Kıvanç
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.
Open Access
Hollow core photonic bandgap fibers for medical applications
(2009) Vural, Mert
The design, fabrication and characterization of photonic band gap (PBG) based optical polymer fibers is discussed. Unlike conventional total internal reflection (TIR) fibers, used primarily in telecommunications, PBG fibers can be made hollow core and can be used to guide infrared radiation of any wavelength, a property known as wavelength scalability. Since the electromagnetic radiation is transmitted in the hollow core of the fiber, the intrinsic absorption of the fiber core as well as the insertion Fresnel losses at front and end faces are avoided, giving rise to extraordinarily high power densities to be delivered. The fiber production line includes material characterization, and the design of nanoscale quarter wavestacks using common thermoplastic polymers (poly ether sulphone and poly ether imide) and chalcogenide glasses (As2S3, As2Se3, Ge15As25Se15Te45). The fiber preform is fabricated using rolling mechanism of thermally evaporated chalcogenide glasses on large area polymers. Subsequently, the fiber preforms are thermally drawn to obtain nano-structured PBG fibers.Two different fibers are designed and produced, signifying wavelength scalability of the overall process, for the widely used holmium (Ho:YAG) and carbon dioxide (CO2) medical lasers. The transmission characteristics of the fibers proved that they can be used to safely deliver 15Wlaser power, along a 3 meter fiber with external diameter of 1.5 mm and hollow core diameter of 0.5 mm, corresponding to a laser power density of 1kW/cm2 with a loss of -10dB/m. The PBG fibers are expected to be widely used in high precision surgical laser for incision, photoablation and coagulation where infrared radiation is the radiation of choice for its superior laser-tissue interaction properties.
Open Access
Light scattering from core-shell nano-structures : structural coloration
(2013) Dolaş, Muhammet Halit
In this work, we produced kilometer-long semiconducting cylindrical nanostructures by using a top-to-bottom nano-fabrication technique which was recently developed in our research group. Comparison of commonly used methods of producing nano-structures such as electrospinning and nano-imprint lithography versus iterative thermal size reduction (ISR) is done in terms of uniformity, geometry control, multi-material compatibility, yield and device integrability. While the others cannot fulfil all requirements, ISR shows impressive results in all aspects. From very beginning to end, all steps of production and characterization of nano-wires produced by ISR, the design, chalcogenide glass production, preform preparation, fiber drawing, iterative size reduction, chemical etching and imaging are explained in details. In addition, production and characterization of nanospheres by in-fiber fluid instability which is based on Plateau-Rayleigh instability is also demonstrated. Theoretical study on scattering from small particles, Mie scattering, which is one of the mechanisms for structural coloration together with thin film interference, multilayer interference, diffraction grating and photonic crystals is done. Structural coloration due to scattering from small particles is simulated using Finite Domain Time Difference (FDTD) method and compared with theoretical results estimated for nano-wire and nano-sphere cases. Results are confirmed with observation of structural coloration by taking dark field optical microscopy images of the final products of ISR and in-fiber fluid instability processes
Open Access
Polymer / glass hollow-core photonic band gap fibers for infrared laser beam delivery
(2011) Köylü, Özlem
Photonic band gap fibers are proposed for the medical applications of laser light transmission into body. Conventional optical fibers guide light via total internal reflection. Due to light guiding mechanisms and materials they have limited frequency range, fiber flexibility and laser power. On the other hand, it is possible to scale operating wavelengths of PBG fibers just by changing a few parameters during fabrication process. Besides, hollow core of PBG fibers eliminates material absorptions and non-linearities during light guiding. PBG fiber production starts from material characterization; and selection; and continues with fiber design, thin film coating, preform preparation and fiber drawing. Studies on theoretical calculations and material properties have shown that best candidate materials for CO2 laser delivery are As2Se3 and poly-ethersulfone (PES). For this purpose, As2Se3 coated PES films are rolled to form a preform and consolidated before thermal drawing. Characterization of drawn fibers indicated that CO2 laser can be transmitted with loss levels of > 1 dB/m and 32 W output power is observed from a 1.2 m long fiber. After fabrication and characterization of PBG fibers, a prototype infrared laser system is built and tested on various applications. In our group laser tissue interactions are examined to see effectiveness of CO2 laser on tumor tissue. Experiments showed that tumor tissue is affected in a very distinctive way compared to healthy tissue. Absorption of cancerous lung tissue at CO2 laser wavelength (10.6 µm) is higher than absorption of healthy tissue at the same wavelength. This study proposes a wide use of PBG fiber for not just CO2 lasers, but also other laser systems used in different medical operations, such as Ho:YAG lasers. PBG fibers for high power laser delivery are novel structures for fast, painless and bloodless surgeries.
Open Access
Prediction and characterisation of intensity noise of ultrafast fiber amplifiers and low noise vibrometer for biological applications
(2013) Gürel, Kutan
We report on the experimental characterisation and theoretical prediction of intensity fluctuations for ultrafast fibre amplifiers. We formulate a theoretical model with which the intensity noise of a Yb-doped fiber amplifier can be predicted with high accuracy, taking into account seed and pump noise, as well as generation of amplified spontaneous emission. Transfer of pump and seed signal modulations to the amplified output during fibre amplification is investigated thoroughly. Our model enables design and optimisation of fiber amplifiers with regards to their intensity noise performance. As a route to passively decreasing the noise imparted by multi-mode diodes in cladding-pumped amplifiers, we evaluate the impact of using multiple, low-power pump diodes versus a single, high-power diode in terms of the noise performance. We use this gathered intuition on intensity noise to build a low noise fibre interferometer that is able to detect sub-5 nm vibrations for biological experiments.
Open Access
Repetition-rate stabilization of femtosecond stretched-spectrum fiber laser
(2008) Ülgüdür, Coşkun
Passively modelocked lasers produce trains of femtosecond pulses, with the temporal separation between the pulses being determined by the length of the laser cavity. The repetition rate of the laser is inverse of this temporal separation. For a free-running laser, the repetition rate is very stable over short time scales (less than 1 ms), but drifts due to environmental effects on a longer time scale. For applications demanding a precise repetition rate to be maintained, such as optical frequency metrology, the laser needs to be locked to an RF or microwave reference source with a feedback loop acting on an actuator within the laser cavity. In this work, repetition-rate stabilization of a “stretched-spectrum” fiber laser is reported, which corresponds to a new modelocking regime. As the name implies, the laser produces pulses undergoing periodic breathing of the spectra during a complete round trip through the cavity. To the best of our knowledge, this breathing is the strongest modification observed in a laser to date. It is noteworthy that even under such strong nonlinearity the laser is more robust than the regular stretched-pulse laser. Encouraged with its robustness, it is proposed that the stretched-spectrum fiber laser is a promising alternate to laser oscillators for frequency metrology applications and laser master oscillators in use with accelerator based next-generation light sources. After photodetection of the laser output, one of the upper harmonics of the laser is locked to a highly stable dielectric resonator oscillator (DRO) at 1.3 GHz. In order to reduce the environmental effects on the laser, a handmade encasing was developed and temperature control of the fibers in the cavity was implemented. Remarkably, the custom encasing of the laser dramatically improved the laser’s stability, outperforming the DRO up to a 5 kHz bandwidth. Since the heating-loop is not sensitive enough, latter upgrade does not decrease the phase noise of the laser, but ensures the temperature stability stays within limits in unclimatized environment. With the present setup, we observe a maximum locking range of a few kHz. The system has the potential to stay in-lock indefinitely, as long as the excessive perturbations on the system are prevented.
Open Access
Ultra-low noise fiber laser systems and their applications
(2014) Budunoğlu, İbrahim Levent
Fiber laser systems are intensely studied for and already utilized in a wide range of scientific, biomedical and industrial applications. Scientifically, fiber lasers are widely used for spectroscopy, laser-matter interactions, nonlinear and quantum optics experiments, among others. The industrial applications range from the well-established, such as laser-material processing, laser marking, and various forms of optical sensing to niche or upcoming applications such as highspeed circuit testing, inspection of packaged foods, additive manufacturing. In all applications outside the research laboratory, long-term stability of the lasers operation is of paramount importance. Fiber lasers are clearly advantageous in this respect, as the optical fibers provide isolated paths for light propagation, minimizing the impact of environmental effects, and generally render the laser system nearly or completely free from mechanical misalignment. In addition to long-term stability of the laser operation, short-term (typically less than 1 second) stability, or fluctuations of the laser output is of crucial importance as in many situations, it effectively determines the signal-to-noise ratio, sets the resolution or otherwise limits the quality of the measurement. Fluctuations or noise impact both the intensity and phase of the laser output. As part of this thesis, first, the intensity noise of mode-locked fiber lasers is characterized systematically for the major mode-locking regimes over a wide range of parameters. It is found that equally low-noise performance can be obtained in all regimes. Losses in the cavity influence noise strongly without a clear trace in the pulse characteristics. Noise level is found to be virtually independent of pulse energy below a threshold for the onset of nonlinearly induced instabilities. Instabilities that occur at high pulse energies are characterized. It is found that continuous-wave peak formation and multiple pulsing influence noise performance moderately. However, at high pulse energies, an abrupt increase of the intensity noise is encountered, corresponding to up to 2 orders of magnitude increase in noise. These results effectively constitute guidelines for minimization of the laser noise in mode-locked fiber lasers. For the high-power laser systems that utilize external amplification in fiber amplifiers, the added noise due to amplification is usually predominantly determined by the pump source, assuming that the amplifier design is correctly made and amplified spontaneous emission (ASE) is minimized. Many high-power amplifiers utilized multi-mode pump diodes, which have much higher noise levels. A high-power fiber laser system where the amplifiers are seeded by low intensity noise pulses is analyzed in detail. When operating at its maximum power level (10 W), the amplified output exhibits an integrated (from 3 Hz to 250 kHz) intensity noise of 0.2%, whereas the seed signals intensity noise is less than 0.03%. The origins of the added noise is analyzed systematically using modulation transfer functions to ascertain contributions of the pump source. The transfer of the noise in the seed signal is also analyzed, as well as contributions of ASE, which can be significant. Prediction of intensity noise by modulation transfer functions supplies a lower limit for the intensity noise of fiber lasers and amplifiers. The second part of the thesis applies the know-how on low-noise fiber lasers that was developed in the first part to a scientific problem. As part of a collaboration with researchers from Ruhr-University at Bochum, Germany, we have developed a custom, low-noise laser system for spectroscopy of micro-plasma discharges. Absorption spectroscopy is a commonly used technique to determine the presence of a particular substance or to quantify the amount of substance present in the plasma discharge. However, the absorbance is usually small, at the level of one part in a thousand or less. Therefore, low-noise laser signals are required to detect such low differences. We developed a low-noise fiber laser system for the absorption spectroscopy studies of reactive species in a micro-plasma discharge. The laser setup also produces high-energy picosecond pulses, which are powerful enough to trigger the plasma ignition and transition into other transient states of plasma. Since both pulses are generated from the same mode-locked oscillator, they have excellent mutual synchronization. We demonstrate the possibility for pump-probe experiments by initiating breakdown on a picosecond time scale (pump) with a high-power beam and measuring the broadband absorption with the simultaneously provided supercontinuum (probe). The third part of this thesis the laser-noise know-how to address a technological problem, namely the development custom, low-noise fiber lasers for LADAR applications. Two different fiber laser systems are constructed as transmitter sources of direct detection and coherent detection LADAR systems and tested for realistic scenarios. Both LADAR systems succeeded to detect 1 cm-diameter wire from a distance of 1 km in a measurement time shorter than 100 s, which is comparable to the best performing commercial LADAR systems.
Open Access
Ytterbium doped all-fiber integrated high power laser systems and their applications
(2013) Yılmaz, Saniye Sinem
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.