Browsing by Subject "High power lasers."

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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
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
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.