Showing papers by "Andreas Tünnermann published in 2013"
TL;DR: An overview of the state of the art in the field can be found in this paper, where the authors discuss present challenges and the future outlook of high-power fiber laser applications.
Abstract: High-power fibre lasers are in demand for industrial, defence and scientific applications. This review provides an overview of the present state of the art in the field and discusses present challenges and the future outlook.
781 citations
TL;DR: In this paper, an inhomogeneous strain in photonic band structures of a honeycomb lattice of waveguides was applied experimentally and theoretically to induce a pseudomagnetic field at optical frequencies.
Abstract: Magnetic effects are fundamentally weak at optical frequencies. Now, by applying inhomogeneous strain in photonic band structures of a honeycomb lattice of waveguides, scientists show experimentally and theoretically that it is possible to induce a pseudomagnetic field at optical frequencies. The field yields 'photonic Landau levels', which suggests the possibility of achieving greater field enhancements and slow-light effects in aperiodic photonic crystal structures than those available in periodic structures.
353 citations
TL;DR: It is shown that by broadening the spectral band in the rest of the waveguide array it is possible to suppress exponentially localized modes, while the Fano state having the eigenvalue embedded in the continuum is preserved.
Abstract: We describe theoretically and observe experimentally the formation of a surface state in a semi-infinite waveguide array with a side-coupled waveguide, designed to simultaneously achieve Fano and Fabry-Perot resonances. We demonstrate that the surface mode is compact, with all energy concentrated in a few waveguides at the edge and no field penetration beyond the side-coupled waveguide position. Furthermore, we show that by broadening the spectral band in the rest of the waveguide array it is possible to suppress exponentially localized modes, while the Fano state having the eigenvalue embedded in the continuum is preserved.
192 citations
TL;DR: In this article, the authors demonstrate that the ionization-limited attainable intracavity peak intensity increases with decreasing pulse duration, which can be used for high-order harmonic generation in a gas, with repetition rates around 100 MHz.
Abstract: Coherently enhancing laser pulses in a passive cavity provides ideal conditions for high-order harmonic generation in a gas, with repetition rates around 100 MHz (refs 1,2,3). Recently, extreme-ultraviolet radiation with photon energies of up to 30 eV was obtained, which is sufficiently bright for direct frequency-comb spectroscopy at 20 eV (ref. 4). Here, we identify a route to scaling these radiation sources to higher photon energies. We demonstrate that the ionization-limited attainable intracavity peak intensity increases with decreasing pulse duration. By enhancing nonlinearly compressed pulses of an Yb-based laser and coupling out the harmonics through a pierced cavity mirror, we generate spatially coherent 108 eV (11.45 nm) radiation at 78 MHz. Exploiting the full potential of the demonstrated techniques will afford high-photon-flux ultrashort-pulsed extreme-ultraviolet sources for a number of applications in science and technology, including photoelectron spectroscopy, coincidence spectroscopy with femtosecond to attosecond resolution5,6 and characterization of components and materials for nanolithography7. Spatially coherent 11.45 nm radiation is produced by outcoupling the harmonics of cavity-enhanced nonlinearly compressed pulses from a Yb-based laser through a pierced cavity mirror. This technique may lead to high-photon-flux ultrashort-pulse extreme-ultraviolet sources for use in a wide range of applications.
172 citations
TL;DR: A femtosecond fiber laser system comprising four coherently combined large-pitch fibers as the main amplifier and an excellent beam quality and efficiency have been obtained.
Abstract: We report on a femtosecond fiber laser system comprising four coherently combined large-pitch fibers as the main amplifier. With this system, a pulse energy of 1.3 mJ and a peak power of 1.8 GW are achieved at 400 kHz repetition rate. The corresponding average output power is as high as 530 W. Additionally, an excellent beam quality and efficiency of the combination have been obtained. To the best of our knowledge, such a parameter combination, i.e., gigawatt pulses with half a kilowatt average power, has not been demonstrated so far with any other laser architecture.
153 citations
TL;DR: In this article, a fiber-laser-pumped optical parametric amplifier for high-harmonic generation has been used to realize a megahertz-repetition-rate source of extreme-ultraviolet continua, with evidence of isolated attosecond pulses at 0.6 MHz.
Abstract: A fibre-laser-pumped optical parametric amplifier for high-harmonic generation has been used to realize a megahertz-repetition-rate source of extreme-ultraviolet continua, with evidence of isolated attosecond pulses at 0.6 MHz. This technique could potentially enable a vast array of new applications, such as attosecond-resolution coincidence and photoelectron spectroscopy.
141 citations
TL;DR: In this article, an ultrashort pulse laser induced formation of birefringent structures in the volume of different glasses: Borofloat 33, BK7 and ULE™.
Abstract: We report on the ultrashort pulse laser induced formation of birefringent structures in the volume of different glasses: Borofloat 33, BK7 and ULE™. Using polarization contrast and scanning electron microscopy we could prove that this birefringence is induced by nanogratings. We were able to identify the pulse duration as a crucial process parameter for the generation of nanogratings in these glasses. The achieved birefringence in ULE is comparable to fused silica, while borosilicate glasses show much less birefringence (only about 12%). Remarkably, the period of the nanogratings is also dependent on the type of the glass, being 250 nm for ULE and only 60 nm in case of Borofloat 33.
96 citations
TL;DR: In this paper, all inkjet-printed piezoelectric polymer actuators are presented based on polyvinylidene fluoride trifluoroethylene (P(VDF-TrFE)) and electrodes printed from silver nanoparticle dispersions.
92 citations
TL;DR: Experimental approaches to control mode instabilities at average output powers higher than 330 W, which corresponds to three times the typical power threshold of the fiber laser system in use, is presented.
Abstract: Mode instabilities have quickly become the most limiting effect when it comes to scaling the output average power of fiber laser systems. In consequence, there is an urgent need for effective strategies to mitigate it and, thus, to increase the power threshold at which it appears. Passive mitigation strategies can be classified into intrinsic, which are related to the fiber design, and extrinsic, which require a modification of the setup. In order to evaluate the impact of mitigation strategies, a means to calculate its power threshold and predict its behavior is required. In this paper we present a simple semi-analytic formula that is able to predict the changes of the mode instability threshold by analyzing the strength of the thermally-induced waveguide perturbations. Furthermore, we propose two passive mitigation strategies, one intrinsic and one extrinsic, that should lead to a significant increase of the power threshold of mode instabilities.
88 citations
TL;DR: A high pulse energy and high average power Q-switched Tm-doped large-pitch fiber oscillator, which allows for large core diameters in combination with effective single-mode operation.
Abstract: We report on a high pulse energy and high average power Q-switched Tm-doped fiber oscillator. The oscillator produces 2.4 mJ pulses with 33 W average power (at a repetition rate of 13.9 kHz) and nearly diffraction-limited beam quality. This record performance is enabled by a Tm-doped large-pitch fiber, which allows for large core diameters in combination with effective single-mode operation.
85 citations
TL;DR: An approach to actively stabilize the beam profile of a fiber amplifier above the mode instability threshold is demonstrated and the beam quality and the pointing stability are significantly increased at power levels of up to three times the mode instabilities threshold.
Abstract: We demonstrate an approach to actively stabilize the beam profile of a fiber amplifier above the mode instability threshold. Both the beam quality and the pointing stability are significantly increased at power levels of up to three times the mode instabilities threshold. The physical working principle is discussed at the light of the recently published theoretical explanations of mode instabilities.
TL;DR: This Letter reveals serious challenges originating from thermal load in the nonlinear crystal caused by absorption in high average power OPAs based on beta barium borate, indicating a high risk of crystal fracture under such conditions.
Abstract: Optical parametric amplifiers (OPAs) have the reputation of being average power scalable due to the instantaneous nature of the parametric process (zero quantum defect). This Letter reveals serious challenges originating from thermal load in the nonlinear crystal caused by absorption. We investigate these thermal effects in high average power OPAs based on beta barium borate. Absorption of both pump and idler waves is identified to contribute significantly to heating of the nonlinear crystal. A temperature increase of up to 148 K with respect to the environment is observed and mechanical tensile stress up to 40 MPa is found, indicating a high risk of crystal fracture under such conditions. By restricting the idler to a wavelength range far from absorption bands and removing the crystal coating we reduce the peak temperature and the resulting temperature gradient significantly. Guidelines for further power scaling of OPAs and other nonlinear devices are given.
TL;DR: In this paper, a photonic crystal L3 cavity was used for second harmonic generation in a thin self-suspended lithium niobate membrane with a quality factor of around 500.
Abstract: We report on second harmonic generation in a photonic crystal L3 cavity drilled in a thin self-suspended lithium niobate membrane. The cavity, resonant for the pump beam in the telecom wavelength range, exhibits a quality factor of around 500. Second harmonic generation has been measured with a low power continuous laser. A conversion efficiency of 6.4×10−9 has been estimated with an input coupled power of 53 μW.
TL;DR: A high precision, low cost, three-layer 3D artificial compound eye consisting of a 3D microlens array, a freeform lens array, and a field lens array was constructed to mimic an apposition compound eye on a curved substrate.
Abstract: Artificial compound eyes are typically designed on planar substrates due to the limits of current imaging devices and available manufacturing processes. In this study, a high precision, low cost, three-layer 3D artificial compound eye consisting of a 3D microlens array, a freeform lens array, and a field lens array was constructed to mimic an apposition compound eye on a curved substrate. The freeform microlens array was manufactured on a curved substrate to alter incident light beams and steer their respective images onto a flat image plane. The optical design was performed using ZEMAX. The optical simulation shows that the artificial compound eye can form multiple images with aberrations below 11 μm; adequate for many imaging applications. Both the freeform lens array and the field lens array were manufactured using microinjection molding process to reduce cost. Aluminum mold inserts were diamond machined by the slow tool servo method. The performance of the compound eye was tested using a home-built optical setup. The images captured demonstrate that the proposed structures can successfully steer images from a curved surface onto a planar photoreceptor. Experimental results show that the compound eye in this research has a field of view of 87°. In addition, images formed by multiple channels were found to be evenly distributed on the flat photoreceptor. Additionally, overlapping views of the adjacent channels allow higher resolution images to be re-constructed from multiple 3D images taken simultaneously.
TL;DR: A novel near-infrared (NIR) fiber laser of 1 MHz repetition rate, 65 ps pulse duration, and 1 cm(-1) spectral resolution is reported to realize an efficient but experimentally simple SGH/TPEF/multiplex CARS multimodal imaging approach for a label-free characterization of composition of complex tissue samples.
Abstract: Over the past years fast label-free nonlinear imaging modalities providing molecular contrast of endogenous disease markers with subcellular spatial resolution have been emerged. However, applications of these imaging modalities in clinical settings are still at the very beginning. This is because single nonlinear imaging modalities such as second-harmonic generation (SHG) and two-photon excited fluorescence (TPEF) have only limited value for diagnosing diseases due to the small number of endogenous markers. Coherent anti-Stokes Raman scattering (CARS) microscopy on the other hand can potentially be added to SHG and TPEF to visualize a much broader range of marker molecules. However, CARS requires a second synchronized laser source and the detection of a certain wavenumber range of the vibrational spectrum to differentiate multiple molecules, which results in increased experimental complexity and often inefficient excitation of SHG and TPEF signals. Here we report the application of a novel near-infrared (NIR) fiber laser of 1 MHz repetition rate, 65 ps pulse duration, and 1 cm(-1) spectral resolution to realize an efficient but experimentally simple SGH/TPEF/multiplex CARS multimodal imaging approach for a label-free characterization of composition of complex tissue samples. This is demonstrated for arterial tissue specimens demonstrating differentiation of elastic fibers, triglycerides, collagen, myelin, cellular cytoplasm, and lipid droplets by analyzing the CARS spectra within the C-H stretching region only. A novel image analysis approach for multispectral CARS data based on colocalization allows correlating spectrally distinct pixels to morphologic structures. Transfer of this highly precise but compact and simple to use imaging approach into clinical settings is expected in the near future.
TL;DR: Incorporation of coherent combination into a state-of-the-art fiber-chirped pulse amplification system obtains 1.1 mJ, 340 fs pulses with up to 280 W of average power at 250 kHz repetition rate, which opens up new possibilities in multidimensional surface science and coherent soft x-ray generation.
Abstract: Incorporation of coherent combination into a state-of-the-art fiber-chirped pulse amplification system obtains 1.1 mJ, 340 fs pulses with up to 280 W of average power at 250 kHz repetition rate. Propagation of this laser pulse inside a krypton-filled hollow-core fiber results in significant spectral broadening. Chirped mirrors are used to compress the pulses to 26 fs, 540 μJ (135 W) leading to a peak power of more than 11 GW. This unprecedented combination of high peak and average power ultrashort pulses opens up new possibilities in multidimensional surface science and coherent soft x-ray generation.
TL;DR: By optimizing the burst frequency and repetition rate, the welding of fused silica with bursts of ultrashort laser pulses is able to achieve a breaking resistance of up to 96% of the bulk material, which is significantly higher than conventional high repetition rate laser bonding.
Abstract: We report on the welding of fused silica with bursts of ultrashort laser pulses. By optimizing the burst frequency and repetition rate, we were able to achieve a breaking resistance of up to 96% of the bulk material, which is significantly higher than conventional high repetition rate laser bonding. The main reason for this stability increase is the reduced stress in the surroundings of the laser induced weld seams, which is proven by measurements of the stress-induced birefringence. A detailed analysis of the shape of the molten structures shows elongated structures in the burst regime. This can be attributed to stronger heating, which is supported by our thermodynamic simulations of the laser melting and bonding process.
TL;DR: A compact light-scatter sensor is used to characterize the scattering and nanostructures of surfaces and to identify the origins of anisotropic scattering features.
Abstract: Light scattering measurement and analysis is a powerful tool for the characterization of optical and nonoptical surfaces. A new 3D scatter measurement system based on a detector matrix is presented. A compact light-scatter sensor is used to characterize the scattering and nanostructures of surfaces and to identify the origins of anisotropic scattering features. The results from the scatter sensor are directly compared with white light interferometry to analyze surface defects as well as surface roughness and the corresponding scattering distributions. The scattering of surface defects is modeled based on the Kirchhoff integral equation and the approach of Beckmann for rough surfaces.
TL;DR: Implementing a novel compact fiber laser system into a tailored designed laser scanning microscope results in a small footprint easy to use multimodal imaging platform enabling simultaneously highly efficient generation and acquisition of second harmonic generation, two-photon excited fluorescence, and coherent anti-Stokes Raman scattering signals for lipid imaging for label-free investigation of tissue samples.
Abstract: The past years have seen increasing interest in nonlinear optical microscopic imaging approaches for the investigation of diseases due to the method's unique capabilities of deep tissue penetration, 3D sectioning and molecular contrast. Its application in clinical routine diagnostics, however, is hampered by large and costly equipment requiring trained staff and regular maintenance, hence it has not yet matured to a reliable tool for application in clinics. In this contribution implementing a novel compact fiber laser system into a tailored designed laser scanning microscope results in a small footprint easy to use multimodal imaging platform enabling simultaneously highly efficient generation and acquisition of second harmonic generation (SHG), two-photon excited fluorescence (TPEF) as well as coherent anti-Stokes Raman scattering (CARS) signals with optimized CARS contrast for lipid imaging for label-free investigation of tissue samples. The instrument combining a laser source and a microscope features a unique combination of the highest NIR transmission and a fourfold enlarged field of view suited for investigating large tissue specimens. Despite its small size and turnkey operation rendering daily alignment dispensable the system provides the highest flexibility, an imaging speed of 1 megapixel per second and diffraction limited spatial resolution. This is illustrated by imaging samples of squamous cell carcinoma of the head and neck (HNSCC) and an animal model of atherosclerosis allowing for a complete characterization of the tissue composition and morphology, i.e. the tissue's morphochemistry. Highly valuable information for clinical diagnostics, e.g. monitoring the disease progression at the cellular level with molecular specificity, can be retrieved. Future combination with microscopic probes for in vivo imaging or even implementation in endoscopes will allow for in vivo grading of HNSCC and characterization of plaque deposits towards the detection of high risk plaques.
TL;DR: In this article, the authors observed and investigated vortex light bullets, solitarily propagating spatiotemporal waves in a two-dimensional waveguide array, which are complex solitons.
Abstract: Light bullets, solitarily propagating spatiotemporal waves recently observed in a two-dimensional waveguide array, are complex solitons. For the first time, scientists observe and investigate vortex light bullets---stably bound and propagating triplets of light bullets with an energy vortex at their centers.
TL;DR: This platform provides a miniaturized optical test-bed for the physics of Dirac fermions with variable mass, as well as of antiferromagnetic spin systems, and suggests the occurence of long-range correlations in an integrated optical device, despite the exclusively short-ranged interactions between the constituting channels.
Abstract: Photonic lattices provide a useful platform for simulating quantum dynamics and systems. Keil et al. fabricate coupled waveguides on-chip and use them to simulate the one-dimensional random mass Dirac model, a test-bed for both Dirac fermions and antiferromagnetic spin systems.
TL;DR: In this paper, the authors experimentally observe anomalous wavepacket evolution in a realization of a one-dimensional finite binary Anderson model in the presence of short-range correlations and employ weakly-coupled optical waveguides with propagation constants e1 and e2.
Abstract: We experimentally observe anomalous wavepacket evolution in a realization of a one-dimensional finite binary Anderson model in the presence of short-range correlations. To this end, we employ weakly-coupled optical waveguides with propagation constants e1 and e2. The correlations enforce the creation of dimers, i.e. two adjacent waveguides with the same e, randomly placed along the lattice. A transition from a ballistic to a superdiffusive wavepacket expansion and, eventually, to localization is observed as the contrast between the two propagation constants increases.
TL;DR: This paper reports on the characterization of SSD in ground and polished optical surfaces, using different light scattering measurement techniques in the visible and extreme ultraviolet spectral ranges, for fused silica, borosilicate glass, and calcium fluoride.
Abstract: Subsurface damage (SSD) in optical components is almost unavoidably caused by mechanical forces involved during grinding and polishing and can be a limiting factor, in particular for applications that require high laser powers or an extreme material strength. In this paper, we report on the characterization of SSD in ground and polished optical surfaces, using different light scattering measurement techniques in the visible and extreme ultraviolet spectral ranges. The materials investigated include fused silica, borosilicate glass, and calcium fluoride. The scattering results are directly linked to classical destructive SSD characterization techniques, based on white light interferometry, optical microscopy, and atomic force microscopy of the substrate topography and cross sections obtained after etching in hydrofluoric acid and fracturing.
TL;DR: The energy scaling of ultrashort-p Pulse systems employing simultaneously the techniques of chirped-pulse amplification and passively combined divided-p pulse amplification is analyzed both experimentally and numerically.
Abstract: The energy scaling of ultrashort-pulse systems employing simultaneously the techniques of chirped-pulse amplification and passively combined divided-pulse amplification is analyzed both experimentally and numerically. The maximum achievable efficiency is investigated and fundamental limitations originating from gain saturation, self-phase modulation and depolarization are discussed. A solution to these limitations could be an active stabilization scheme, which would allow for the operation of every single fiber amplifier at higher pulse energies.
TL;DR: In this article, all optical inscription of quasi-phase matching structures in an x-cut LiNbO3 crystal was demonstrated and the structures were used to frequency double 1.55 µm light.
Abstract: All optical inscription of quasi phase matching structures in an x-cut LiNbO3 crystal is demonstrated. Quasi phase matching is obtained by periodically lowering the nonlinear refractive index of the crystal using focussed ultrashort pulses. The structures were used to frequency double 1.55 µm light. The converted signal could be enhanced by a factor of 70 with respect to the unmodified material. From these measurement it could be deduced that the nonlinearity has been periodically damped up to 20%.
TL;DR: A model is suggested explaining the formation of these disruptions as a results of a fast quenching process of the molten material after the laser irradiation has stopped.
Abstract: The absorption and heat accumulation of successive ultrashort laser pulses in fused silica leads to melting of the material. We analyze the structure and formation of disruptions that occur within the trace of the molten material. We employed focused ion beam (FIB) milling to reveal the inner structure of these disruptions. The disruptions consist of several small voids which form a large cavity with a diameter of several tens of micrometer. Based on the observations, we suggest a model explaining the formation of these disruptions as a results of a fast quenching process of the molten material after the laser irradiation has stopped. In addition, we analyzed the periodic and non-periodic formation of disruptions. The processing parameters strongly influence the formation of disruptions.
TL;DR: The nonlinear pulse compression of temporally divided pulses, which is presented in a proof-of-principle experiment, holds promise for overcoming fundamental limitations of the pulse peak power that lead to destruction of the fiber or ionization limitations in high-energy hollow-core compression.
Abstract: We report on the nonlinear pulse compression of temporally divided pulses, which is presented in a proof-of-principle experiment. A single 320 fs pulse is divided into four replicas, spectrally broadened in a solid-core fiber, and subsequently recombined. This approach makes it possible to reduce the nonlinearities in the fiber and therefore to use total input peak power of about 13.3 MW, which is more than three times higher than the self-focusing threshold. Finally, the combined output pulse could be compressed to sub-100 fs pulse duration. This general and universal approach holds promise for overcoming fundamental limitations of the pulse peak power that lead to destruction of the fiber or ionization limitations in high-energy hollow-core compression.
TL;DR: In this article, the authors performed an in-situ analysis of the percussion drilling process with ultrashort laser pulses in silicon, which acts as a model system for drilling opaque materials like metals.
TL;DR: In this article, a simple linear all-fiber cavity without dispersion compensation is realized using a uniform fiber Bragg grating (FBG), different cavity lengths are investigated and repetition rates down to 0.7 MHz are obtained.
Abstract: A SESAM-mode-locked, all-polarization-maintaining Ytterbium fiber laser producing picosecond pulses with narrow spectral bandwidth is presented. A simple linear all-fiber cavity without dispersion compensation is realized using a uniform fiber Bragg grating (FBG). Different cavity lengths are investigated and repetition rates down to 0.7 MHz are obtained. Bandwidth and pulse duration of the output pulses are mainly determined by the choice of FBG. Pulses between 30 and 200 ps are generated employing different FBGs with bandwidths between 17 and 96 pm. The experimental results are in good agreement with numerical simulations. The laser holds great potential for simple amplification setups without pulse picking.
TL;DR: In this article, femtosecond laser pulses were focused at the interface of two optically contacted transparent samples, and strong bonds between the samples were achieved using nonlinear absorption processes and heat accumulation of successive pulses.
Abstract: We report on the joining of different glass types with dissimilar optical, thermal and mechanical properties by ultrashort laser welding at high repetition rates. Femtosecond laser pulses were focused at the interface of two optically contacted transparent samples. Using nonlinear absorption processes and heat accumulation of successive pulses, we achieved strong bonds between the samples. We used a three-point bending test to determine the breaking strength. With this technique, we achieved for instance for a borosilicate glass a breaking strength of up to 95 % of the bulk material. In addition, we even welded different material combinations. Although the welded glasses exhibit different thermal and mechanical properties, we obtained breaking strengths which are comparable to the utilized bulk materials. Using Raman spectroscopy we mapped the laser-processed material along the welded interface. Thereby, we determined that the welds consist of a mixture of both species, which is formed during the laser induced melting of the materials.