Browsing by Subject "Fiber laser"

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Open Access
3.5-GHz Intra-Burst Repetition Rate Ultrafast Yb-Doped Fiber Laser
(Elsevier, 2016) Kerse, C.; Kalaycioʇlu, H.; Elahi, P.; Akçaalan, Ö.; Ilday, F. Ö.
We report on an all-fiber Yb laser amplifier system with an intra-burst repetition rate of 3.5 GHz. The system is able to produce minimum of 15-ns long bursts containing approximately 50 pulses with a total energy of 215μJ at a burst repetition rate of 1 kHz. The individual pulses are compressed down to the subpicosecond level. The seed signal from a 108 MHz fiber oscillator is converted to approximately 3.5 GHz by a multiplier consisting of six cascaded 50/50 couplers, and then amplified in ten stages. The highly cascaded amplification suppresses amplified spontaneous emission at low repetition rates. Nonlinear interactions between overlapping pulses within a burst is also discussed.
Open Access
Atomic layer deposition of zirconium oxide thin film on an optical fiber for cladding light strippers
(TÜBİTAK, 2020) Karatutlu, Ali
Cladding light strippers are essential components in high-power fiber lasers used for removal of unwanted cladding light that can distort the beam quality or even damage the whole fiber laser system. In this study, an Atomic Layer Deposition system was used for the first time to prepare the cladding light stripper devices using a 40 nm thick zirconia layer grown on optical fiber. The thickness of the zirconia coating was confirmed using the Scanning Electron Microscopy (SEM) and the Ellipsometry techniques. The elemental analysis was also performed using the wavelength dispersive X-ray spectroscopy technique. The Raman spectroscopy and XRD data confirm the structure of the atomic layer deposition-grown zirconia thin films to be predominantly amorphous. The cladding light stripper devices formed using the zirconia thin films with the lengths of 8.5 and 15.5 cm were able to strip approximately 30% (~1.5 dB) and 40% (~2.3 dB) of the unwanted cladding light.
Open Access
Development of a rapid-scan fiber-integrated terahertz spectrometer
(Springer New York LLC, 2014) Keskin, H.; Altan, H.; Yavas, S.; Ilday, F. O.; Eken, K.; Sahin, A. B.
Scientists in terahertz (THz) wave technologies have benefited from the recent developments in ultrafast laser technologies and RF technologies and applied these new gained techniques into characterizing a wide variety of phenomena. Undoubtedly, the most successful of these applications has been in the development of time-domain terahertz spectroscopic and imaging systems which has been utilized in the characterization of dielectrics and semiconductors. This pulsed technique has allowed users to characterize dynamical behavior inside materials under illumination with picosecond resolution. Typically pump/probe or similar dynamical measurements require the use of amplified pulses derived from free-space solid state lasers in the μJ-mJ range and since interferometric techniques are typically used in pulsed measurements the measurement time of a THz spectrum can last at least tens of minutes. Better systems can be realized based on fiber laser technologies. Here we discuss the advantages of a THz spectrometer driven by an ultrafast Ytterbium doped fiber laser whose repetition rate can be tuned rapidly allowing for rapid dynamical measurements. The efficient gain medium, robust operation and compact design of the system opens up the possibility of exploring rapid detection of various materials as well as studying dynamical behavior using the high brightness source.
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
Intracavity optical trapping with fiber laser
(2019-06) Kalantarifard, Fatemeh
After Ashkin's seminal works, optical trapping has been a powerful technique for capturing and manipulating sub micro particles not only in physics research fields but also in biology and photonics. Standard optical tweezers consists of a single beam with Gaussian or profile which focused by a high numerical aperture (NA) water or oil immersion microscope objective. Typically, objective with NA>1.2 is used to provide strong enough gradient forces being able to overcome Brownian uctuations and gravity and trap the particle stably. On the other hand, compare with high NA, trapping with low NA, has its own advantage and among all the advantages, low local heating of the sample has a particular interest in molecular biology and manipulating living cells. The main concern is that the interaction of trapping laser beam and biological object induces a damage on the specimen which is mainly due to light absorption of the sample. It is, therefore, recommended to use NIR (near infrared ) wavelength due to its minimal absorption by water and biological objects. Other important factors that must be considered, to secure the viability of the cell, are spot size of the focused beam and laser power at the sample plane. Thus, it deserves an effort to look for new configurations with low NA with the capability of creating 3D confinement. Standard optical tweezers rely on optical forces that arise when a focused laser beam interacts with a microscopic particle: scattering forces, which push the particle along the beam direction, and gradient forces, which attract it towards the high-intensity focal spot. Importantly, the incoming laser beam is not affected by the particle position because the particle is outside the laser cavity. Here, we demonstrate that intracavity nonlinear feedback forces emerge when the particle is placed inside the optical cavity, resulting in orders-of-magnitude higher confinement per unit laser intensity on the sample. We first present a toy model that intuitively explains how the microparticle position and the laser power become nonlinearly coupled: The loss of the laser cavity depends on the particle position due to scattering, so the laser intensity grows whenever the particle tries to escape. We describe a simple toy model to clarify how the nonlinear feedback forces emerge as a result of the interplay between the particle's motion and the laser's dynamics. It also quantifies how and to what extent this scheme reduces the average laser power to which a trapped particle is exposed. In this model, the power and hence trapping force are considered to be zero for small particle displacements. However, in reality they have small values that do operate the trap even when the particle is near the equilibrium position. Thus, we need an accurate description of the coupling between the laser and the trapped particle thermal dynamics at equilibrium to compare with experiments. In particular, accurate simulations can help to associate an effective harmonic potential to the optical trap for small displacements from the equilibrium position, and hence to define a meaningful stiffness using the standard calibration methods based on the thermal uctuations of a trapped particle. We therefore present a series of numerical simulations based on an extended theoretical model, including highly realistic descriptions of the laser dynamics, optical losses incurred by the particle, and the particle's Brownian motion in order to gain a quantitative understanding of the dynamics of intracavity optical trapping and to guide the experiments. Finally, guided by the simulation results, we have built an experimental setup to prove the operational principle of intracavity optical trapping and experimentally realize this concept by optically trapping microscopic polystyrene and silica particles inside the ring cavity of a fiber laser. One of the major advantages of the intracavity optical trapping scheme is that it can operate with very low-NA lenses, with a consequent large field-of-view, and at very low average power, resulting in about two orders of magnitude reduction in exposure to laser intensity compared to standard optical tweezers. When compared to other low-NA optical trapping schemes, positive and negative aspects can be considered, such as in terms of trap stiffness and average irradiance of the sample. These features can yield advantages when dealing with biological samples. Ultra-low intensity at our wavelength can grant a safe, temperature controlled environment, away from surfaces for micro uidics manipulation of biosamples. Accurate studies on Saccharomices cerevisiae yeast cells in near-infrared counterpropagating traps and standard optical tweezers have found no evidence for a lower power threshold for phototoxicity. We observed that we can 3D trap single yeast cells with about 0:47 mW, corresponding to an intensity of 0:036 mW m􀀀2, that is more than a tenfold less intensity than standard techniques.
Open Access
Optical trapping of microparticles and yeast cells at ultra-low intensity by intracavity nonlinear feedback forces
(SPIE, 2020) Kalantarifard, A.; Elahi, P.; Makey, Ghaith; Ünlü, B.; Marago, O. M.; İlday, Fatih Ömer; Volpe, G.; Dholakia, K.; Spalding, G. C.
In standard optical tweezers optical forces arise from the interaction of a tightly focused laser beam with a microscopic particle. The particle is always outside the laser cavity and the incoming beam is not affected by the particle position. Here we describe an optical trapping scheme inside the cavity of a fiber laser where the laser operation is nonlinearly influenced by the displacement of trapped particle and there is a coupling between laser operation to the motion of the trapped particle and this can dramatically enhances optical tweezers action and gives rise to nonlinear feedback forces. This scheme operates using an aspheric lens at low numerical aperture (NA=0.125), NIR wavelength (λ = 1030 nm), and very low average power which results in about two orders of magnitude reduction in exposure to laser intensity compared to standard optical tweezers. Ultra-low intensity at our wavelength can grant a safe, temperature-controlled environment, away from surfaces for microfuidics manipulation of biosamples that are sensitive to light intensity. As the main advantage of our approach and highly relevant application, we observed that we can trap single yeast cells at a very low power, corresponding to an intensity of 0.036 mW μm-2, that is more than a tenfold less intensity than standard techniques reported in the literature.
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
Pulse shaping for a long-distance optical synchronization system
(Denshi Jouhou Tsuushin Gakkai, Institute of Electronics Information and Communication Engineer, 2007) Ilday, F. O.; Winter, A.; Kärtner, F. X.; Danailov, M. B.
Next generation free electron lasers aim to generate x-ray pulses with pulse durations down to 30 fs, and possibly even sub-fs. Synchronization of various stages of the accelerator and the probe laser system to the x-ray pulses with stability on the order of the pulse width is necessary to make maximal use of this capability. We are developing an optical timing synchronization system in order to meet this challenge. The scheme is based on generating a train of short optical pulses, with a precise repetition frequency, from a mode-locked laser oscillator and distributed via length-stabilized optical fibers to points requiring synchronization. The timing information is embedded in the repetition frequency and its harmonics. A significant advantage of the optical synchronization system is that multiple mode-locked Ti:sapphire seed oscillators typically present in an accelerator facility can be replaced by the master mode-locked fiber laser. In this paper, we briefly review progress on the development of the synchronization system and then discuss the implementation of this new possibility. Several technical issues related to this approach are analyzed.
Open Access
Semi-analytic theory self-similar optical propagation and mode-locking using a shape-adaptive model pulse
(American Physical Society, 2014-01-21) Jirauschek, C.; Ilday, F. O.
A semianalytic theory for the pulse dynamics in similariton amplifiers and lasers is presented, based on a model pulse with adaptive shape. By changing a single parameter, this test function can be continuously tweaked between a pure Gaussian and a pure parabolic profile and can even represent sech-like pulses, the shape of a soliton. This approach allows us to describe the pulse evolution in the self-similar and other regimes of optical propagation. Employing the method of moments, the evolution equations for the characteristic pulse parameters are derived from the governing nonlinear Schrodinger or Ginzburg-Landau equation. Due to its greatly reduced complexity, this description allows for extensive parameter optimization, and can aid intuitive understanding of the dynamics. As an application of this approach, we model a soliton-similariton laser and validate the results against numerical simulations. This constitutes a semianalytic model of the soliton-similariton laser. Due to the versatility of the model pulse, it can also prove useful in other application areas.
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.