Showing papers by "Andreas Tünnermann published in 2018"

Watt-scale super-octave mid-infrared intrapulse difference frequency generation

Abstract: The development of high-power, broadband sources of coherent mid-infrared radiation is currently the subject of intense research that is driven by a substantial number of existing and continuously emerging applications in medical diagnostics, spectroscopy, microscopy, and fundamental science. One of the major, long-standing challenges in improving the performance of these applications has been the construction of compact, broadband mid-infrared radiation sources, which unify the properties of high brightness and spatial and temporal coherence. Due to the lack of such radiation sources, several emerging applications can be addressed only with infrared (IR)-beamlines in large-scale synchrotron facilities, which are limited regarding user access and only partially fulfill these properties. Here, we present a table-top, broadband, coherent mid-infrared light source that provides brightness at an unprecedented level that supersedes that of synchrotrons in the wavelength range between 3.7 and 18 µm by several orders of magnitude. This result is enabled by a high-power, few-cycle Tm-doped fiber laser system, which is employed as a pump at 1.9 µm wavelength for intrapulse difference frequency generation (IPDFG). IPDFG intrinsically ensures the formation of carrier-envelope-phase stable pulses, which provide ideal prerequisites for state-of-the-art spectroscopy and microscopy. A table-top-sized, coherent light source that offers a compact and bright alternative to a synchrotron in the 4−18 µm spectral range has been developed by a German-US research team. The team used a novel ultrashort (16 fs) pulse, high power Tm-doped fiber laser operating at 1.9 µm to induce a nonlinear frequency downconversion process called intrapulse difference frequency generation in a crystal of GaSe. The broad spectral coverage and high brightness render this mid-infrared source a unique tool for state-of-the art spectroscopy and microscopy. The team says that the compactness and simplicity of the presented approach brings exciting prospects for the future accessibility, in particular for emerging applications that are currently addressed only with mid-infrared beamlines in large-scale synchrotron facilities.

5D hyperspectral imaging: fast and accurate measurement of surface shape and spectral characteristics using structured light

Abstract: Measuring the shape (coordinates x, y, z ) and spectral characteristics (wavelength-dependent reflectance R (λi)) of macroscopic objects as a function of time (t) is of great interest in areas such as medical imaging, precision agriculture, or optical sorting Here, we present an approach that allows to determine all these quantities with high resolution and accuracy, enabling measurement in five dimensions We call this approach 5D hyperspectral imaging We describe the design and implementation of a 5D sensor operating in the visible to near-infrared spectral range, which provides excellent spatial and spectral resolution, great depth accuracy, and high frame rates The results of various experiments strongly indicate the great benefit of the new technology

GOBO projection for 3D measurements at highest frame rates: a performance analysis

Abstract: Aperiodic sinusoidal patterns that are cast by a GOBO (GOes Before Optics) projector are a powerful tool for optically measuring the surface topography of moving or deforming objects with very high speed and accuracy. We optimised the first experimental setup that we were able to measure inflating car airbags at frame rates of more than 50 kHz while achieving a 3D point standard deviation of ~500 µm. Here, we theoretically investigate the method of GOBO projection of aperiodic sinusoidal fringes. In a simulation-based performance analysis, we examine the parameters that influence the accuracy of the measurement result and identify an optimal pattern design that yields the highest measurement accuracy. We compare the results with those that were obtained via GOBO projection of phase-shifted sinusoidal fringes. Finally, we experimentally verify the theoretical findings. We show that the proposed technique has several advantages over conventional fringe projection techniques, as the easy-to-build and cost-effective GOBO projector can provide a high radiant flux, allows high frame rates, and can be used over a wide spectral range.

Experimental investigations on the TMI thresholds of low-NA Yb-doped single-mode fibers

Abstract: In this contribution we investigate the transversal mode instability behavior of a ytterbium-doped commercial 20/400 fiber and obtain 2.9 kW of output power after optimizing the influencing parameters. In this context, we evaluate the influence of the bend diameter and the pump wavelength within the scope of the absorption length and the length of the fiber. Furthermore, with a newly developed fiber we report on 4.4 kW of single-mode output power at 40 cm bend diameter.

Carbon chloride-core fibers for soliton mediated supercontinuum generation.

Abstract: We report on soliton-fission mediated infrared supercontinuum generation in liquid-core step-index fibers using highly transparent carbon chlorides (CCl4, C2Cl4) By developing models for the refractive index dispersions and nonlinear response functions, dispersion engineering and pumping with an ultrafast thulium fiber laser (300 fs) at 192 μm, distinct soliton fission and dispersive wave generation was observed, particularly in the case of tetrachloroethylene (C2Cl4) The measured results match simulations of both the generalized and a hybrid nonlinear Schrodinger equation, with the latter resembling the characteristics of non-instantaneous medium via a static potential term and representing a simulation tool with substantially reduced complexity We show that C2Cl4 has the potential for observing non-instantaneous soliton dynamics along meters of liquid-core fiber opening a feasible route for directly observing hybrid soliton dynamics

Coherently combined 16-channel multicore fiber laser system.

Abstract: We present a coherently combined laser amplifier with 16 channels from a multicore fiber in a proof-of-principle demonstration. Filled-aperture beam splitting and combination, together with temporal phasing, is realized in a compact and low-component-count setup. Combined average power of up to 70 W with 40 ps pulses is achieved with combination efficiencies around 80%.

Coherent Beam Combination of Ultrafast Fiber Lasers

Abstract: The performance of fiber laser systems has drastically increased over recent decades, which has opened up new industrial and scientific applications for this technology. However, currently a number of physical effects prevents further power scaling. Coherent combination of beams from multiple emitters has been established as a power scaling technique beyond these limitations. It is possible to increase the average power, and for pulsed laser systems, also parameters such as the pulse energy and the peak power. To realize such laser systems, various aspects have to be taken into account that include beam combination elements, stabilization systems, and the output parameters of the individual amplifiers. After an introduction to the topic, various ways of implementing coherent beam combination for ultrashort pulses are explored. Besides the spatial combination of beams, the combination of pulses in time will also be discussed. Recent experimental results will be presented, including multidimensional (i.e., spatial and temporal) combination. Finally, an outlook on possible further developments is given, focused on scaling the number of combinable beams and pulses.

Modal energy transfer by thermally induced refractive index gratings in Yb-doped fibers

Abstract: Thermally induced refractive index gratings in Yb-doped fibers lead to transverse mode instability (TMI) above an average power threshold, which represents a severe problem for many applications. To obtain a deeper understanding of TMI, the evolution of the strength of the thermally induced refractive index grating with the average output power in a fiber amplifier is experimentally investigated for the first time. This investigation is performed by introducing a phase shift between the refractive index grating and modal interference pattern, which is obtained by applying a pump power variation to the fiber amplifier. It is demonstrated that the refractive index grating is sufficiently strong to enable modal energy coupling at powers that are significantly below the TMI threshold if the induced phase shift is sufficiently large. The experiments indicate that at higher powers, the refractive index grating becomes more sensitive to such phase shifts, which will ultimately trigger TMI. Furthermore, the experimental results demonstrate beam cleaning above the TMI threshold via the introduction of a positive phase shift. This finding paves the way for the development of a new class of mitigation strategies for TMI that are based on controlling the phase shift between the thermally induced refractive index grating and modal interference pattern.

Pump-modulation-induced beam stabilization in high-power fiber laser systems above the mode instability threshold.

Abstract: A new way of stabilizing the output beam of a fiber laser system operating above the mode instability threshold is described and the first proof-of-principle experimental results are presented. This technique, which relies on a modulation of the pump power, works by washing the thermally-induced refractive index grating out, which weakens the coupling efficiency between transverse modes. One of the main advantages of this simple, yet powerful, approach is that it can be easily incorporated in already existing fiber laser systems since it does not require any additional optical elements. Using this beam stabilization strategy, a significant pointing stability and beam quality improvement has been demonstrated up to an average power of ~600W, which is a factor of 2 above the mode instability threshold.

Precision manufacturing of a lightweight mirror body made by selective laser melting

Abstract: This article presents a new and individual way to generate opto-mechanical components by additive manufacturing, embedded in an established process chain for the fabrication of metal optics. The freedom of design offered by additive techniques gives the opportunity to produce more lightweight parts with improved mechanical stability. The latter is demonstrated by simulations of several models of metal mirrors with a constant outer shape but varying mass reduction factors. The optimized lightweight mirror exhibits 63.5% of mass reduction and a higher stiffness compared to conventional designs, but it is not manufacturable by cutting techniques. Utilizing selective laser melting instead, a demonstrator of the mentioned topological non-trivial design is manufactured out of AlSi12 alloy powder. It is further shown that – like in case of a traditional manufactured mirror substrate – optical quality can be achieved by diamond turning, electroless nickel plating, and polishing techniques, which finally results in

Thermodynamic control of soliton dynamics in liquid-core fibers

Abstract: Liquid-core fibers offer local external control over pulse dispersion due to their strong thermodynamic response, offering a new degree of freedom in accurate soliton steering for reconfigurable nonlinear light generation. Here, we show how to accurately control soliton dynamics and supercontinuum generation in carbon disulfide/silica fibers by temperature and pressure tuning, monitored via the spectral location and the onset energy of non-solitonic radiation. Simulations and phase-matching calculations based on an extended thermodynamic dispersion model of carbon disulfide confirm the experimental results, which allows us to demonstrate the potential of temperature detuning of liquid-core fibers for octave spanning recompressible supercontinuum generation in the near-infrared.

Spatio-temporal analysis of glass volume processing using ultrashort laser pulses

Abstract: Ultrashort laser pulses allow for the in-volume processing of glass through non-linear absorption, resulting in permanent material changes and the generation of internal stress. Across the manifold potential applications of this technology, process optimization requires a detailed understanding of the laser–matter interaction. Of particular relevance are the deposition of energy inside the material and the subsequent relaxation processes. In this paper, we investigate the spatio-temporal evolution of free carriers, energy transfer, and the resulting permanent modifications in the volume of glass during and after exposure to femtosecond and picosecond pulses. For this purpose, we employ time-resolved microscopy in order to obtain shadowgraphic and interferometric images that allow relating the transient distributions to the refractive index change profile. Whereas the plasma generation time is given by the pulse duration, the thermal dynamics occur over several microseconds. Among the most notable features is the emergence of a pressure wave due to the sudden increase of temperature and pressure within the interaction volume. We show how the structure of the modifications, including material disruptions as well as local defects, can be directly influenced by a judicious choice of pulse duration, pulse energy, and focus geometry.

Time-resolved tomography of ultrafast laser-matter interaction

Abstract: We demonstrate time-resolved tomography with 200 fs resolution for the three-dimensional analysis of the non-linear dynamics of ultrafast laser-matter interaction inside the volume of transparent materials. We reconstruct as an example the three-dimensional spatial distribution of the transient extinction coefficient induced by focusing higher-order Bessel-Gaussian-beams into Gorilla glass. This approach can be employed to gaseous, liquid and transparent solid state matter which interact with laser light.

Phase-shift evolution of the thermally-induced refractive index grating in high-power fiber laser systems induced by pump-power variations

Abstract: A phase shift between the modal interference pattern and the thermally-induced refractive index grating is most likely the ultimate trigger for the damaging effect of transverse mode instabilities (TMI) in high-power fiber laser systems By using comprehensive simulations, the creation and evolution of a thermally-induced phase shift is explained and illustrated in detail It is shown that such a phase shift can be induced by a variation of the pump power The gained knowledge about the generation and evolution of the phase shift will allow for the development of new mitigation strategies for TMI

Femtosecond-written long-period gratings in fluoride fibers

Abstract: Long-period gratings induced in fluoride glass fibers using femtosecond laser pulses at 800 nm are, to the best of our knowledge, demonstrated for the first time. By means of tightly confined ultrashort laser pulses, smooth periodic lines of refractive index changes are induced along the fiber core. Taking advantage of heat accumulation effects in the focal volume, attenuation peaks down to −24 dB, with sharp and predictable spectral resonances, were obtained. Thermal annealing of the grating up to 250°C yielded a significant reduction of the induced refractive index change. The gratings could find applications in various integrated mid-infrared optical devices, such as optical notch filters in fiber amplifiers.

87-W 1018-nm Yb-fiber ultrafast seeding source for cryogenic Yb: yttrium lithium fluoride amplifier.

Abstract: We demonstrate a compact and robust Yb-fiber master-oscillator power-amplifier system operating at 1018 nm with 2.5-nm bandwidth and 1-ns stretched pulse duration. It produces 87-W average power and 4.9-μJ pulse energy, constituting a powerful seed source for cryogenically cooled ultrafast Yb: yttrium lithium fluoride (Yb:YLF) amplifiers.

High resolution XUV Fourier transform holography on a table top.

Abstract: Today, coherent imaging techniques provide the highest resolution in the extreme ultraviolet (XUV) and X-ray regions. Fourier transform holography (FTH) is particularly unique, providing robust and straightforward image reconstruction at the same time. Here, we combine two important advances: First, our experiment is based on a table-top light source which is compact, scalable and highly accessible. Second, we demonstrate the highest resolution ever achieved with FTH at any light source (34 nm) by utilizing a high photon flux source and cutting-edge nanofabrication technology. The performance, versatility and reliability of our approach allows imaging of complex wavelength-scale structures, including wave guiding effects within these structures, and resolving embedded nanoscale features, which are invisible for electron microscopes. Our work represents an important step towards real-world applications and a broad use of XUV imaging in many areas of science and technology. Even nanoscale studies of ultra-fast dynamics are within reach.

Mitigation of stimulated Raman scattering in high power fiber lasers using transmission gratings

Abstract: The average output power of fiber lasers have been scaled deep into the kW regime within the recent years. However a further scaling is limited due to nonlinear effects like stimulated Raman scattering (SRS). Using the special characteristics of femtosecond laser pulse written transmission fiber gratings, it is possible to realize a notch filter that mitigates efficiently this negative effect by coupling the Raman wavelength from the core into the cladding of the fiber. To the best of our knowledge, we realized for the first time highly efficient gratings in large mode area (LMA) fibers with cladding diameters up to 400 μm. The resonances show strong attenuation at design wavelength and simultaneously low out of band losses. A high power fiber amplifier with an implemented passive fiber grating is shown and its performance is carefully investigated.

Angular resolved power spectral density analysis for improving mirror manufacturing.

Abstract: Ultra-precise diamond turning is the method of choice for manufacturing freeform optics. Analyzing surface errors in different spatial frequency ranges has mainly been performed in a one-dimensional representation of the power spectral density function. However, the advanced machine dynamics at the fabrication of freeform mirrors result in highly anisotropic surfaces with regular ripples in different orientations. To properly analyze the entire surface in the frequency regime, a new way of representing the two-dimensional power spectral density is introduced in this paper. This novel tool is utilized for the evaluation of an example freeform mirror.

Thermal analysis of Yb-doped high-power fiber amplifiers with Al:P co-doped cores

Abstract: It has been recently shown that photodarkening can significantly reduce the mode instability threshold in high power Yb-doped fiber amplifiers, thus resulting in an even more severe limitation to the scaling of the output average power of these systems. Therefore, an efficient reduction of photodarkening in an Yb-doped active fiber will lead to very significant gains in the output average power delivered by such systems. In this context, it has been reported that photodarkening can be significantly mitigated when co-doping a fiber core with Al and P, which makes this approach potentially appealing to increase the TMI threshold. Unfortunately co-doping the fiber core with Al and P also alters the effective cross-sections of the fiber, which has repercussion in the amplification efficiency. Thus, a fiber with a higher P concentration will exhibit lower cross-sections, therefore requiring a higher Yb-ion concentration to reach a certain desired amplification efficiency. However, increasing the Yb-ion concentration leads to higher photodarkening losses, which might potentially counteract the benefits of using P co-doping. In this paper we present a comparative analysis of the expected performance of different fiber amplifiers for a given constant average heat-load and amplification efficiency as a function of the ratio of Al:P concentration in the fiber core. This study indicates which core compositions are more beneficial for increasing the mode instability threshold in Yb-doped high-power fiber amplifier systems.

Modelling the refractive index behavior of Al,P-doped SiO2, fabricated by means of all-solution doping, in the vicinity of Al:P = 1:1

Abstract: A novel and simple solution doping technique is used to explore the refractive index behavior of Al,P-doped SiO2 in the vicinity of the Al:P-ratio of 1:1 at low doping concentrations (0.4 up to 2.0 mol% Al2O3 and/or P2O5). It is found that even if Al:P = 1:1 is matched precisely, an index increase is observed. This is in contradiction to previous findings in the literature and the already sophisticated models need to be refined in this region. In the proposed model, an incomplete formation of AlPO4 is assumed and solves the contradiction. Furthermore, the presented model can be combined with previous literature models.

Line-edge roughness as a challenge for high-performance wire grid polarizers in the far ultraviolet and beyond.

Abstract: High-performance nano-optical elements for application wavelengths in the ultraviolet spectral range often require feature sizes of only a few tens of nanometers where line edge roughness (LER) becomes a critical parameter for the optical performance. In this contribution, we explore the influence of LER on the optical performance of wire grid polarizers (WGP) in the far ultraviolet range. Therefore, we present a method, which uses the finite difference time domain method in combination with a comprehensive spatial frequency dependent LER model. The measured LER of 3.6 nm (standard deviation) reduces the WGP's extinction ratio by a factor of 3.6 at a wavelength of 248 nm. We identify a critical range of the correlation length, which maximizes the detrimental effect of LER. The presented method and the results provide the basis for future fabrication technology optimization of WGPs and other optical meta-surfaces in the ultraviolet spectral region or at even shorter wavelengths.

Sequential phase locking scheme for a filled aperture intensity coherent combination of beam arrays.

Abstract: We present a novel phase locking scheme for the coherent combination of beam arrays in the filled aperture configuration. Employing a phase dithering mechanism for the different beams similar to LOCSET, dithering frequencies for sequential combination steps are reused. By applying an additional phase alternating scheme, this allows for the use of standard synchronized multichannel lock-in electronics for phase locking a large number of channels even when the frequency bandwidth of the employed phase actuators is limited.

Transverse mode instabilities in burst operation of high-power fiber laser systems

Abstract: We propose, to the best of our knowledge, the first mitigation strategy for TMI based on controlling the phase shift between the thermally-induced index grating and the modal intensity pattern. In particular, in this work we present a study of transverse mode instabilities in burst operation in a high-power fiber laser system. It is shown that, with a careful choice of the parameters, this operation regime can potentially lead to the mitigation of TMI by forcing an energy transfer from the higher-order-modes into the fundamental mode during the burst.

Comparison Between Bidirectional Pumped Yb-doped All-fiber Single-mode Amplifier and Oscillator Setup up to a Power Level of 5 kW

Abstract: We present and compare highly robust Yb-doped monolithic amplifier and -oscillator setups in 20/400 μm geometry achieving signal powers of 3.5 kW and 5 kW in a bidirectional pumping scheme while maintaining single mode beam quality of M2 ~ 1.3.

1.8-kW 16-channel ultrafast fiber laser system

Abstract: An ultrafast ytterbium-doped fiber laser system based on coherent beam combination of 16 amplifier channels is presented. The system delivers 1.83 kW average power at 2.3 mJ pulse energy and 240 fs pulse duration. The combining efficiency of 82% and the beam M2-value of 1.8 currently is limited by thermal lensing in some optical components, which were identified and are to be replaced.

Growth of Atomic Layer Deposited Ruthenium and Its Optical Properties at Short Wavelengths Using Ru(EtCp)2 and Oxygen

Abstract: High-density ruthenium (Ru) thin films were deposited using Ru(EtCp)2 (bis(ethylcyclopentadienyl)ruthenium) and oxygen by thermal atomic layer deposition (ALD) and compared to magnetron sputtered (MS) Ru coatings. The ALD Ru film growth and surface roughness show a significant temperature dependence. At temperatures below 200 °C, no deposition was observed on silicon and fused silica substrates. With increasing deposition temperature, the nucleation of Ru starts and leads eventually to fully closed, polycrystalline coatings. The formation of blisters starts at temperatures above 275 °C because of poor adhesion properties, which results in a high surface roughness. The optimum deposition temperature is 250 °C in our tool and leads to rather smooth film surfaces, with roughness values of approximately 3 nm. The ALD Ru thin films have similar morphology compared with MS coatings, e.g., hexagonal polycrystalline structure and high density. Discrepancies of the optical properties can be explained by the higher roughness of ALD films compared to MS coatings. To use ALD Ru for optical applications at short wavelengths (λ = 2–50 nm), further improvement of their film quality is required.

Advances in laser concepts for multiplex, coherent Raman scattering micro-spectroscopy and imaging

Abstract: In this contribution, we want to review recent advances and innovations in laser sources and detection concepts for coherent Raman scattering microscopy with special emphasis on hyperspectral and multiplex imaging methods for biomedical applications. Groundbreaking advances have been made to increase the chemical sensitivity while maintaining video-rate imaging speeds. Excitation and detection schemes have been further improved to enhance the image contrast as well as to increase the spectral coverage and speed of multi-spectral data acquisition along with the ongoing quest to develop both compact and robust but also powerful and rapidly tunable laser sources suitable for integration into biomedical equipment.

Controlling mechanical, structural, and optical properties of Al2O3 thin films deposited by plasma-enhanced atomic layer deposition with substrate biasing

Abstract: Complex interference multilayer systems typically implemented in high-performance optics consists of several layers of low and high refractive index materials. Low mechanical stress of the coatings is desired to avoid cracking and delamination of the film or a deformation of the substrate. It is known that the ion energies in plasma-assisted deposition can be employed to control material properties and thereby mechanical stress. In this study, we evaluate the influence of substrate biasing on mechanical stress and optical properties of alumina (Al2O3) coatings deposited by plasma enhanced atomic layer deposition (PEALD). Substrate biasing up to -300 V was applied during O2 plasma exposure in the second step of a two-step PEALD process. To distinguish the physical effect of ion bombardment from the physico-chemical effect, a substrate bias of -100 V was applied separately and only during Ar plasma exposure that constituted the third step of a three-step PEALD process. Al2O3 films were characterized using spectroscopic ellipsometry, spectrophotometry, xray photoelectron spectroscopy (XPS), x-ray diffractometry (XRD), x-ray reflectometry (XRR), Fourier transform infrared spectroscopy (FT-IR), wafer-curvature measurement and atomic force microscopy (AFM).

Manufacturing of aluminum mirrors for cryogenic applications

Abstract: Several mirrors for the upgrade of the CRyogenic high-resulution InfraRed Echelle Sprectrograph (CRIRES) at the Very Large Telescope, were manufactured by diamond turning and polishing. These mirrors will be used in the crossdispersion unit (CDU) and the fore optics of the instrument. For background level reasons, the operational temperature of the CDU is set to 65 K. Therefore, the flat and spherical mirrors used in the CDU, which are made of melt-spun aluminum alloy Al6061, had to be artificially aged, to improve the dimensional stability at cryogenic temperatures. After diamond turning, magnetorheological finishing (MRF) was used for a deterministic shape correction and to remove the turning marks of the RSA6061 mirrors. To reduce the micro-roughness, a further smoothing step was necessary. A micro-roughness between 1 nm RMS and 5 nm RMS as well as shape deviations below 35 nm RMS were achieved. The mirrors were coated by inline magnetron sputtering with a high-reflective gold layer or protected silver, respectively.