Browsing by Subject "Lasers."
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Item Open Access 33 Femtosecond Yb-doped optical frequency comb for frequency metrology applications(Bilkent University, 2013) Şenel, ÇağrıOptical 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.Item Open Access Engineering the nonlinear dynamics of photonic systems : demonstration of the soliton-similariton fiber laser and nonlinear laser lithography(Bilkent University, 2013) Öktem, BülentNonlinear 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.Item Open Access High power all-fiber laser-amplifier systems for materials processing(Bilkent University, 2011) Özgören, KıvançWhen 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.Item Open Access A novel compression algorithm based on sparse sampling of 3-D laser range scans(Bilkent University, 2010) Dobrucalı, Oğuzcan3-D models of environments can be very useful and are commonly employed in areas such as robotics, art and architecture, environmental planning and documentation. A 3-D model is typically comprised of a large number of measurements. When 3-D models of environments need to be transmitted or stored, they should be compressed efficiently to use the capacity of the communication channel or the storage medium effectively. In this thesis, we propose a novel compression technique based on compressive sampling, applied to sparse representations of 3-D laser range measurements. The main issue here is finding highly sparse representations of the range measurements, since they do not have such representations in common domains, such as the frequency domain. To solve this problem, we develop a new algorithm to generate sparse innovations between consecutive range measurements acquired while the sensor moves. We compare the sparsity of our innovations with others generated by estimation and filtering. Furthermore, we compare the compression performance of our lossy compression method with widely used lossless and lossy compression techniques. The proposed method offers small compression ratio and provides a reasonable compromise between reconstruction error and processing time.Item Open Access Prediction and characterisation of intensity noise of ultrafast fiber amplifiers and low noise vibrometer for biological applications(Bilkent University, 2013) Gürel, KutanWe 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.Item Open Access Repetition-rate stabilization of femtosecond stretched-spectrum fiber laser(Bilkent University, 2008) Ülgüdür, CoşkunPassively 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.Item Open Access Ultrashort and short pulsed fiber laser development for transparent material processing, imaging and spectroscopy applications(Bilkent University, 2015) Yavaş, SeydiSince the invention of the laser in the 60s, the main advances in laser technology were done in two directions; shorter pulses and higher powers. In order to achieve this purpose, many laser types are developed and always replaced with simpler, smaller, cheaper alternatives that can deliver the same or better parameters. In the past 20 years, ber lasers have become an important alternative that can match and even enhance the performance of currently used lasers while reducing the complexity, costs and instability. Optical bers, which are the main components of ber lasers, were rst de- veloped just as a substitute for conventional cables since they offer much less attenuation in carrying signals over long ranges. So, most of the studies were focused on making the bers better for communication channels. After realizing that ber lasers offer better beam qualities, which is also a vital parameter for many laser applications, researches started nding ways to use bers for lasers and they achieved this in 80s by the rst ever utilization of low-attenuation ac- tive bers. After the invention of double-clad bers, utilization of diode lasers for pumping and development of efficient rare-earth doped bers, ber lasers became more than just a research topic in the laboratory and began to nd use in many applications. The utilization of ber lasers for short (nanoseconds) and ultrashort (picosec- onds, femtoseconds) pulse generation was a difficult task for researchers. The biggest challenge to overcome was nonlinear effects caused by the con nement of the beam into small volumes. By using smart designs like chirped pulse ampli - cation and highly doped lasers, pulse energies and peak powers close to solid-state ultrafast lasers can be achieved. These nonlinear effects were not just problems in the power scalability of ber lasers, on the other hand, they were an opportunity for new possible applications. For example, using these nonlinear effects inside bers, supercontinuum generation was demonstrated and found usage in many areas like spectroscopy, imaging and metrology. Today, more than 50 worldwide companies sell short-pulse ber lasers for ap- plications as diverse as ophthalmology, micromachining, medical imaging and precision metrology. Especially, ber-laser-based micromachining is routinely im- plemented in the fabrication processes for widely used consumer products. New applications of ber lasers are being continuously developed. Consequently, in this Ph.D. thesis study, new application areas of ber lasers are investigated. Ultrashort and short pulsed ber lasers are developed and uti- lized for biological and transparent material processing, spectroscopy and imag- ing. In the rst part of thesis study, we have demonstrated the use of a custom- built ber laser-based microscope system for nanosurgery and tissue ablation experiments. Through the use of custom FPGA electronics acting through ber- coupled AOMs, we are able to generate custom pulse sequence. Using this system, we have made photodisruption experiments in tissue level, cellular level and sub- cellular level. In the second part of this thesis study, in collaboration with Bogazici Univer- sity, we have developed a nanosecond ber laser system that is able to generate wavelength components of 600 nm to 1300 nm, developed speci cally for pho- toacoustic excitation. Using this system, we have made photoacoustic signal excitation in a ceramic sample and prepared the system for further experiments to generate photoacoustic images from biological specimens. In the third part of thesis study, in collaboration with ODTU, the development of a THz-TDS system driven by a novel Yb-doped ber laser whose repetition rate can be tuned, speci cally for fast scan THz measurements, is realized. Char- acterization of the built laser system is done considering the necessities for the OSCAT technique as an alternative method for fast scan THz measurements. Stability of the oscillator is examined in terms of power, spectrum and pulse duration with the changing repetition rate of the laser. Using this system THz waveforms are generated at different wavelengths and the system is prepared for further research in spectroscopy. In the last part of the thesis study, a high-pulse energy femtosecond laser system is developed and utilized for transparent material processing. The laser output is coupled to a fast galvo-scanner system, and a synchronized translation stage such that very wide areas (10 cm x 10 cm) are able to be processed with very high speed (2 m/s). Using this system, glass samples are cut, engraved and photodarkened.Item Open Access Ytterbium doped all-fiber integrated high power laser systems and their applications(Bilkent University, 2013) Yılmaz, Saniye SinemFor 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.