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We report on an opto-mechanical metal mirror design for highly dynamic, diffraction-limited focus shifting. Here, the mechanical geometry of the membrane is of crucial interest as it must provide sufficient optical performance to allow for diffraction limited focussing and have a high mechanical eigenfrequency to provide dynamic motions. The approach is the analytical consideration of the plate theory and provides the basis for a parameterized finite element model. By means of an finite element analysis (FEA), essential steps for the optimization of the mirror design with respect to a wide range of optical power and a high operating frequency are shown. To verify the results of the FE analysis, the deformed surface is decomposed into Zernike coefficients. An analysis of the point spread function is performed to evaluate the optical performance. For dynamic evaluation a modal and a harmonic vibration analysis are conducted. The opto-mechanical design allows a biconical deformation of the mirror surface, enabling the generation of a diffraction-limited spot diameter in the adjustment range of ±1.2 dpt. The surface shape error in this range is 53 nm. The dynamic analysis shows the first excited eigenfrequency at 21.6 kHz and a diffraction-limited operation frequency at 9.5 kHz. This paper provides an alternative design approach for highly dynamic beam oscillation in the Z direction, forming a complement to highly dynamic X-Y scanning systems.

Anti-Stokes fluorescence cooling of a Yb-doped silica glass optical fiber preform is achieved using a high-power laser in a double-pass configuration. The coherent laser beam illuminates the silica glass preform in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. The high-purity Yb-doped silica glass preform has low parasitic absorption and is codoped with modifiers to mitigate the quenching-induced non-radiative decay for sufficiently high concentrations of Yb ions in silica glass. Therefore, sufficiently large laser absorption could be achieved to observe cooling while maintaining a near-unity external quantum efficiency.

Laser cooling of a solid is achieved when a coherent laser illuminates the material, and the heat is extracted by annihilation of phonons resulting in anti-Stokes fluorescence. Over the past year, net solid-state laser cooling was successfully demonstrated for the first time in Yb-doped silica glass in both bulk samples and fibers. Here, we report more than 6 K of cooling below the ambient temperature, which is the lowest temperature achieved in solid-state laser cooling of silica glass to date to the best of our knowledge. We present details on the experiment performed using a 20 W laser operating at a 1035 nm wavelength and temperature measurements using both a thermal camera and the differential luminescence thermometry technique.

In this contribution, we report high-power Raman frequency downconversion based on an Yb-doped fiber amplifier and a linear external diamond Raman cavity. A maximum output power of 136 W with nearly diffraction-limited beam quality was achieved by pumping in quasi-continuous-wave mode with 10% duty cycle and 10 ms on-time duration. For continuous-wave operation, we achieved record average power of 46 W centered at 1178 nm. The emergence of stimulated Brillouin scattering in diamond is further investigated. This technology shows the potential to extend the spectral range of fiber lasers to reach uncommon wavelengths at high power levels.

We report the observation of anti-Stokes fluorescence cooling of Yb-doped silica glass by 0.7 degrees Celsius. We conduct a detailed investigation of the cooling parameters of this glass, including the wavelength dependence of the cooling efficiency as a function of the wavelength and also the parasitic absorption of the pump laser. The measurements are performed on three different glass samples with different compositions and cooling is observed in all samples to varying degrees. The results highlight the possibility of using Yb-doped silica glass for radiation-balancing in fibers. Radiation-Balancing is a viable technique for heat mitigation in lasers and amplifiers.

A laser based soldering technique – Solderjet Bumping – using liquid solder droplets in a flux-free process with localized thermal impact demonstrates the all inorganic, adhesive free attachment of optical components and support structures made of heterogeneous materials for a high-resolution optical filter under harsh environmental conditions. Space applications demand an attachment technology which maintains the precise alignment of bonded components and overcomes challenges of common adhesives such as being more radiation resistant and appropriate for vacuum environments. Besides, stress and strain induced into optical components can deteriorate the wavefront of passing light and therefore reduce the system performance significantly. The presented case study shows the mandatory changes in the design of an optical filter instrument according to the boundary conditions of Solderjet Bumping for different bonding issues. First, a filter window made of N-BK10, covering the optical sensor beneath, is soldered into a frame of DilverP1®. Second, this sub-assembly is aligned w.r.t. to fiducials on a support structure and is attached in this state by soldering as well. The process chain of Solderjet Bumping including cleaning, wettable metallization layer, handling, soldering and inspection is discussed. This multi-material approach requires well-defined reflow energies to melt the spherical shaped solder preforms to create a media-fit joint and to prevent damages on the fragile filter window simultaneously. The findings of process parametrization and environmental testing are presented. The optical performance with respect to stress/strain before and after soldering as well as the alignment state are evaluated using non-contact optical techniques.

In order to achieve the excellent seismic isolation necessary for the operation of gravitational wave detectors, very sensitive inertial sensors are required to measure and counteract the motion of the ground. This paper presents work on a lightweight, compact, and vacuum compatible inertial sensor based on an optomechanical design. It aims to achieve comparable noise performance to the sensors used in current gravitational wave detectors across the frequency region used for seismic isolation control, from 0.1 to 100 Hz. The quasi monolithic design of the device, along with initial measurements of the device quality factor of 28000 and the design philosophy underpinning it, are presented.

Nonlinear effects and transverse mode instabilities (TMI) limit power scaling of single-mode fiber lasers. To overcome these limitations not only the fiber design but also laser relevant properties of the actively doped material itself need to be optimized. By being able to fabricate Yb-doped fibers for high power applications in-house, we have direct access to laser relevant material parameters.We fabricated fibers using three different co-doping systems, namely Yb:Al:P, Yb:Al:F, and Yb:Al:F:Ce. Afterwards we characterized and compared their laser relevant properties. All three co-doping systems showed nearly identical background losses and absorption cross-sections. In contrast, we found that the PD losses and the factor between PD losses @633nm and the laser wavelength range (1μm) to be significantly different. The retrieved characterization results were implemented into our simulations tool in order to improve the reliability of predictions. Finally, we characterized the fibers in kW-amplifier setups according to their power scaling limits, especially the TMI threshold. This cycle of fiber fabrication, characterization, and simulation enabled us to identify the impact of individual fiber parameters on the TMI threshold. We demonstrated that the impact of PD loss leads to a reductions of the TMI threshold for Yb:Al:F co-doping system of 13% to 23% (depending on the Yb-concentration). The PD loss for the two other systems was proved to be significantly lower and was found to have no impact on the TMI threshold. We experimentally proved that your in-house Yb:Al:P and Yb:Al:F:Ce fibers performed like PD-free fibers.

Supported by both experimental and simulated results, this contribution demonstrates the heat load distribution in a co-pumped, ytterbium (Yb)-doped fiber amplifier seeded with two different wavelengths can be significantly changed depending on the seed power ratio. Longitudinal temperature measurements in a Yb-doped 10.5 m 20/400 μm fiber confirm a significant shift of the heat load maximum by 3.5 m towards the fiber output when decreasing the seed power ratio from P1030nm/P1080nm = 1.7 to 20. In single-tone operation with a seed power of P1080nm = 3.5 W, the amplifier is limited by the onset of transverse mode instabilities at a power-level of 1950 W. However, dual-tone seeding with a seed power ratio up to P1030nm/P1080nm = 10 reduces the TMI-threshold dramatically down to 1050 W. Additionally we show, that the modal instability threshold is very susceptible to 1030 nm seed noise in the frequency regime up to 10 kHz.

Laser cooling of a solid is achieved when a coherent laser illuminates the material in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. Solid-state laser cooling has been successfully demonstrated in several materials, including rare-earth-doped crystals and glasses. Here we show the net cooling of high-purity Yb-doped silica glass samples that are fabricated with low impurities to reduce their parasitic background loss for fiber laser applications. The non-radiative decay rate of the excited state in Yb ions is very small in these glasses due to the low level of impurities, resulting in near-unity quantum efficiency. We report the measurement of the cooling efficiency as a function of the laser wavelength, from which the quantum efficiency of the Yb-doped silica is calculated. Laser cooling of rare-earth-doped silica may provide a route to vibration-free refrigeration of integrated photonic circuits and quantum sensors to reduce the thermal noise. Here, cooling of high-purity Yb-doped silica to 0.7 K below ambient temperature is demonstrated using sub-optimal laser parameters, and in spite of a substantial extra thermal load.

The measured fiber coupling efficiency (FCE) in atmospheric laser communication is not fully explored. We experimentally study the FCE for tip-tilt aberrations. Finally, we explain the FCE with the tip-tilt phase response.

For the manufacturing of space-qualified spectrometric applications, the benefits of direct bonding are highly promising. While there are some recent spectrometric design concepts which rely on the usage of optical coatings, the direct bonding of optical glasses with added optical coatings is still challenging. This work presents investigations on plasma-activated direct bonding of optical glasses without and with added optical coatings. As a result, it is highly beneficial to add an additional SiO2 final layer onto the coating and to perform an additional chemical-mechanical polishing process prior to direct bonding in order to achieve high bonding strengths.

We report the successful laser cooling of a Ytterbium-doped silica optical fiber glass preform by 0.7K relative to the room temperature. The observed temporal behavior of cooling agrees well with the theoretical predictions.

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 $< 150$~nm peak-to-valley shape deviation and a roughness of $< 1$~nm rms in a measurement area of $140 \times 110$ $��$m${}^2$. Negative implications from the additive manufacturing are shown to be negligible. Further it is shown that surface form is maintained over a two year storage period under ambient conditions., 13 pages, 19 figures, online version (corrected proof)

Solution processing manufacturing is the key method to reduce the OLED device production cost. Furthermore, digital printing techniques such as inkjet printing provide the advantages of mask-free patterning, non-contact manufacturing and direct chip-integration. However, a complex layer structure consisting of ultrathin layers is typically required to obtain high device performance, which is difficult for inkjet printing manufacturing due to interlayer mixing, limited layer uniformity, and limited material availability. In this contribution, we introduce an ITO-free, inkjet-printed transparent OLED with a simple 4-layer structure via an inkjet-printable Al:ZnO:PEI interlayer. This is the first ITO-free, vacuum-free, fully solution-processed transparent OLED using the inkjet printing technique as the exclusive technique for electrode deposition and patterning. High emission brightnesses of 50 000 cd m−2 and 16 000 cd m−2 were achieved in a voltage-pulsed driving mode on ITO and silver nanowire embedded PEDOT:PSS bottom electrodes, respectively. In order to use the OLEDs for on-chip fluorescence sensing applications, the operational lifetime was also tested under a high emission brightness of ∼10 000 cd m−2. LT85 was achieved after 3000 pulses.

In this contribution, two approaches of the solution processing of novel spectral light filters are demonstrated. The first approach shows the combination of light filtering capability with electrical conductivity in a “light filter electrode” by mixing colorants into poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) ink. Different colorants are investigated concerning their influence on the workfunction and the electrical conductivity of the resulting PEDOT:PSS layers. In addition, these electrodes could be integrated into organic photodiodes, opening up the possibility of using the whole device for specific light detection. Within the second approach, the surface plasmon resonance and the scattering of metal nanoparticles is used for light filtering. Light scattering (and therefore the reflection of excitation light back into the microfluidic channel) increases the excitation efficiency, thus increasing the intensity of the fluorescent light. With this approach, the sensitivity of the demonstrated light detectors can effectively be increased while simultaneously decreasing the detection limit of the substance of interest.

In this contribution in situ emission filter generation for, e.g. fluorescence light detection by morphology tailoring of silver nanoparticles within a polymer layer, is presented for the first time. After depositing a pretreated film of polyethylenimine containing spherical silver nanoparticles the composite layer was irradiated with green LEDs (peak wavelength 530 nm). This leads to a morphology change of the silver nanoparticles and therefore to a change in absorption properties of the resulting polymer film with an absorption maximum at 550 nm. This tailoring of silver nanoparticles embedded in a transparent polymer matrix could address many novel applications such as adaptable light filters for future lab-on-a-chip applications. A LED for fluorescence light excitation was used to illuminate the silver nanoparticles, which led to an increasing absorption of that particular light spectrum, and therefore in situ produced a fluorescence emission filter. The effect of the treatment time, the silver ion concentration, and the temperature at which the “light-shaping” was done was studied. The silver nanoparticles were investigated using UV/Vis-spectroscopy and scanning electron microscopy. The observed nanoparticle shapes give reasons for a novel hypothesis of nanoparticle evolution. The forced plasmon oscillation seems to cause excited regions at the silver nanoparticles where adsorbed silver ions are preferably photoreduced. This results in the possibility of tailoring the morphology and therefore the optical properties of the growing nanoparticles.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Though long anticipated, adaptive optical (AO) pre-compensation is now experiencing a surge in interest thanks to technological advances and the identification of a real-world application with the advent of satellite laser communication as a viable alternative to radio-frequency. Here, we present an overview of our activities to date in pre-compensation, from the development of our AO system in laboratory experiments using artificial turbulence [1], to the integration into a transportable "AO-box" and testing in first proof-of-principle experiments over horizontal links using real turbulence [2] to the verification in horizontal tests of pre-compensation under large point-ahead-angles beyond the estimated isoplanatic range [3]. We describe the concept and realization of our experimental setup which provides simultaneous bidirectional correction of the incoming beam (post-compensation) and outgoing beam (pre-compensation). We then detail the transfer of this setup into two optical terminals representing a "ground terminal" and "satellite terminal". We describe the results of the horizontal measurement campaigns which have been undertaken to date in which a 7.1 dB increase in the received intensity was demonstrated under a small PAA over 0.5 km and in which a decline in the received laser power over increasingly large point-ahead-angle was demonstrated over a 1.0 km test range. We conclude with the current outlook for pre-compensation technology and indicate future planned experiments.

Here, we report on the approach of realizing an all-fused-silica PGP disperser (prism + grating + prism) by low temperature direct bonding. A surface relief grating with period 660nm and overall depth of approximately 2000nm is sandwiched between two equal prisms. Direct bonding of glass relies on the formation of covalent bonds between hydrophilic silicon-oxide surfaces. Compared to other joining technologies, like adhesive bonding or optical contacting, the established connection is stiff, shows no outgassing, is highly resistant against chemical and radiative degradation and the established optical interface is intrinsically impedance-matched. In summary, two prototypes were realized, optically characterized and successfully underwent environmental testing. The overall diffraction efficiency of the PGP is larger than 90%.

Adaptive optical pre-compensation is seen as crucial for free-space laser communication in order to overcome the influence of atmospheric turbulence, particularly with respect to Earth-to-GEO feederlinks. This paper presents an experimental investigation into adaptive optical pre-compensation under large point-ahead-angles. We detail the design and realization of a free-space laser communication experiment over a 1.0 km horizontal path using a divergent beacon beam and a focussed signal beam, propagating in opposite directions. We describe the design and development of our experimental setup and measurement campaign using real turbulence. The median isoplanatic angle was calculated to be 0.16 mrad, while an increase in the received optical power through pre-compensation could be demonstrated for point-ahead-angles in the range of 0.13 mrad to 0.27 mrad.

Currently, the achievable average power from single-mode Yb-doped fiber lasers is in the multi-kW range [1]. However, the transmission range or the spectral gain of the active medium often limits the output power of highly demanded laser sources at uncommon wavelengths. Nonlinear frequency conversion represents a versatile approach to extend the spectral range at high power levels and is thus beneficial for wavelength-dependent applications. Compared to optical parametric oscillators (OPOs) the Raman laser technology is less complex and does not require management of phase matching. Furthermore, the spatial-hole-burning-free characteristic of the Raman gain provides an advantage for narrow band, high power operation [2]. Industrial CVD-diamond is an attractive medium for Raman frequency conversion and especially suitable for high power regions due to its unique combination of thermal and optical properties [3]. Using a second-Stokes setup lowers the threshold and increases the obtainable wavelength shift ever further [4]. Here, we employ this technology to achieve record power at 1478 nm in cw-operation.

1960 is the birth year of both the laser and the Mars exploration missions. Eleven years passed before the first successful landing on Mars, and another six before the first rover could explore the planet&rsquo, s surface. In 2011, both technologies were reunited with the first laser landing on Mars as part of the ChemCam instrument, integrated inside the Curiosity Rover. In 2020, two more rovers with integrated lasers are expected to land on Mars: one through the National Aeronautics and Space Administration (NASA) Mars 2020 mission and another through the European Space Agency (ESA) ExoMars mission. The ExoMars mission laser is one of the components of the Raman Spectrometer instrument, which the Aerospace Technology National Institute of Spain (INTA) is responsible for. It uses as its excitation source a laser designed by Monocrom and manufactured in collaboration with the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF). In this paper, we present for the first time the final flight module laser that has been installed in the rover&rsquo, s onboard laboratory and validated to be shipped to Mars in 2020. Particular emphasis is given to mechanical considerations and assembly procedures, as the ExoMars laser assembly has required soldering techniques in contrast to the standard adhesive technologies used for most laser assembly processes in order to fulfill the environmental and optical requirements of the mission.

We present and compare the performance of bidirectionally pumped Yb-doped monolithic amplifier and oscillator setups in 20/400 μm geometry tested up to signal powers of 3.5 kW and 5 kW without the occurrence of transverse mode instabilities and maintaining a single mode beam quality of M2 ~ 1.3. The scaling was primarily limited by the nonlinear effect of Stimulated Raman Scattering. This contribution contains detailed analysis of the temporal and spectral behavior of both configurations. The results show the excellent feasibility of monolithic oscillators and FBG for high power operation, even outperforming the amplifier pendant in terms of output power.

We present highly efficient three-channel dual-grating spectral beam combining with a combined signal output power of 5.5 kW at an excellent beam quality of M2 = 1.5. Three 2-kW all-fiber narrow-linewidth continuous-wave Ytterbium-doped fiber amplifiers at 1050 nm, 1070 nm and 1090 nm were combined using in-house fabricated polarizationindependent dielectric reflection gratings. The total combining efficiency was 94% at full power level, which is close to the expected value referred to the incorporated grating’s efficiency in a dual-grating setup.

Droplet based microfluidic technology is a miniaturized platform for microbial analysis on picoliter scale. With its costefficiency, high-throughput and feasibility of complex handling protocols, droplet microfluidics is a favorable platform for applications such as microorganism screening or synthetic biology. Scattered-light-based microbial detection, in comparison to the widely used fluorescent-label-based approach, provides a contact-free and label-free, yet sensitive measuring solution. The angular dependency of scattered light delivers an elaborate information about the morphology and the physical properties, e.g. size and refractive index, of microbial samples. Due to the complexity and ambiguity of the droplet contents, an angle resolved scattered light detection system could provide powerful method for a label-free identification and quantification of the microbes in droplets. In this paper, a novel approach of light scattering measurement in Polydimethylsiloxane (PDMS) microfluidic chips is presented, engaging optical fibers for a light-scattering-based on-chip microbial detection. Optical fibers, with their fast readout and compact size, are very suitable for easier system integration towards flexible and versatile lab-on-a-chip applications.

In this contribution the influence of fabrication technique (solution doping, gas-phase doping) and the choice of suitable material systems (Al, P, Yb:SiO2 and Al, F, Yb:SiO2) for high power fiber laser materials on their optical properties is analyzed. The materials under analysis contain low amounts of codopants (Yb < 0.15 mol%, other

The low-stress Solderjet Bumping technique was employed to assemble the optical components of an increased-robustness laser spark-plug ignition device using the low melting alloys 96.5Sn3Ag0.5Cu and 80Au20Sn. A finite-element-method analysis, optical simulations, and a soldering parametrization test were performed to prove that different optical materials (sapphire, ECO-550, D-ZLaF52LA, TAC4, and N-SF11 glasses) could be fastened to the stainless steel body. The assembled spark-plug device featured a passively Q-switched Nd : YAG/Cr4 + : YAG composite ceramic medium and delivered laser pulses with energy variable between 2.40 and 4.70 mJ, with 0.8 ns duration, suitable for inducing air breakdown phenomenon and engine combustion.

We report the highest output-power from an external cavity diamond Raman oscillator in continuous-wave operation mode at 1478 nm. An output power of 34 W with 11 % conversion efficiency was achieved. Furthermore, an effective facility suppression of parasitic Brillouin scattering is proposed.

This paper describes an opto-mechanical concept of a deformable metal mirror membrane, which can shift the focus position over a large range by use of a single actuator. The core element of the mirror is a diamond turned tulip-shaped membrane, the design is optimized to correct astigmatic aberrations which arise from the use of a curved mirror under a deflection angle. For this purpose, the target mirror surface is biconic. The manufactured mirror was tested with a maximum central deflection of 28 µm and, when used in combination with a 200 mm focal lens, is capable of producing a focus shift of up to 17.9 mm with a resulting wavefront aberration of 238.7 nm RMS error .

Microfluidic lab-on-a-chip devices can be used for chemical and biological analyses such as DNA tests or environmental monitoring. Such devices integrate most of the basic functionalities needed for the analysis on a microfluidic chip. When using such devices, cost and space intensive lab equipment is thus not necessary. However, in order to make a monolithic and cost-efficient/disposable sensing device, direct integration of excitation light source for fluorescent sensing is often required. Organic light emitting diodes (OLEDs) have the advantages of self-emitting property, high luminous efficiency, full-color capability, wide viewing angle, high contrast, low power consumption, low weight and flexibility. All these capacitate OLEDs to be a suitable optical source for microfluidic devices. However, low work function cathode and/or electron injection layer like Ba, LiF are indispensable for high bright OLEDs, which require vacuum deposition and inert fabrication atmosphere. Hereby we introduce a fully solution processable deviation of OLEDs, organic light-emitting electrochemical cells (OLECs) as a low-cost excitation light source for a microfluidic sensing platform. By mixing metal ions and a solid electrolyte with light-emitting polymers as active materials, an in-situ doping and in-situ PN-junction can be generated within a three layer sandwich device. Because of this doping effect, work function adaption is not necessary and air-stable cathodes like silver can be used. A manufacturing process for fully solution-processed OLECs is presented, which consist of an inkjet-printed silver cathode, spin-coated blue light-emitting polymer plus dopants and an inkjet-printed PEDOT:PSS transparent top anode. Furthermore, by replacing silver with ITO, a fully transparent blue OLEC is able to emit > 2500 cd/m 2 light under pulsed driving mode, which fulfils requirements for simple fluorescent on-chip sensing applications.

Manufacturing telescopes with 4, 8 or 16 meter apertures is the most effective way to gather the light of faint exoplanets or look back towards the Big Bang. However, ultra-high optical quality large mirrors drastically increase the mass of such instruments - if made conventionally. Thinner, and hence lighter, primary mirrors suffer from gravity release, temperature changes and misalignment during launch. The resulting surface distortions as well as inherent surface errors which arise during manufacturing can be reduced by the implementation of active optics. We designed an active metal mirror as a key element for active optics in space. Our goal was to develop an ultra-stable, set-and-forget, lightweight active mirror with good wavefront correction performance. A simulation routine was developed to investigate the dependency between geometric parameters of the Deformable Mirror (DM) and the residual surface error after correction of Zernike modes. With the final 25 actuator mirror design we can achieve residual errors of less than 10 nm RMS for individual Zernike modes for an optical pupil of 103 mm diameter. It is able to withstand quasi-static launch loads, is insensitive to temperature changes and we can limit the overall weight to 2500 g including actuators and mirror mount.

In this contribution we investigate the transversal mode instability behavior of an 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.

With their advantages like good beam quality, easy thermal management, high robustness and compact size, fiber lasers are one of the most promising solid state laser concepts for high power scaling with excellent beam quality. One issue of further power scaling is the reduction of nonlinear effects, especially Raman scattering, which consequently led to increased mode field areas. However, for large mode area fibers, new challenges, namely transversal mode instabilities (TMI) have to be taken into account. Beside our investigations in the power scaling of ytterbium doped fiber amplifiers up to 4.4kW output power, we present our investigations of the TMI threshold in dependence on bend diameter and absorption length of a well-known, commercial fiber. Within this scope, we used a 13m piece of the fiber and gradually reduced the bend diameter from 60cm slightly below 14cm within a pump wavelength of 976nm. Furthermore, we increased the fiber length to 30 m, presuming the bend diameter of 14 cm and all experimental conditions. However, in a next step, we detuned the pump wavelength up to 980 nm in order to increase the pump absorption length As a result, we achieved 2.9kW of single mode output at a bend diameter of 14cm. The 4.4kW result was obtained with a separately manufactured low-NA fiber, allowing for a slope efficiency of 90% with regards to the absorbed pump light and an extremely temporal stability.

We present investigations on the seed source dependence of stimulated Raman scattering (SRS) created in a high power fiber amplifier. It is shown that fiber oscillators are much worse in terms of SRS than other seed sources. The longitudinal mode composition was found to be of less importance. We reinforce the experimental observations by a numerical investigation, which shows that temporal power variations on the ps-scale and their propagation along the fiber are crucial for the SRS creation in high-power fiber systems, extending the well-known but simplified SRS threshold description.

We present the fabrication and properties of active fiber laser materials fabricated by a newly developed solution doping technique. The contribution focusses on Aluminum, Phosphorus, Ytterbium as well as Boron doped SiO2 for the use as fiber laser material. More specifically low doping concentration in the vicinity of the molar ratio of Al2O3:P2O5 = 1:1 will be elucidated. The effect of fabrication parameters on optical properties like refractive index, absorption and emission properties will be covered. Currently it is possible to achieve cw output powers greater than 4 kW using Al, P, Yb doped fibers fabricated with this method. Fibers additionally codoped with Boron are as well suitable for kW class applications as well.

The amount of cladding light is important to ensure longevity of high power fiber components. However, it is usually measured either by adding a cladding light stripper (and thus permanently modifying the fiber) or by using a pinhole to only transmit the core light (ignoring that there may be cladding mode content in the core area). We present a novel noninvasive method to measure the cladding light content in double-clad fibers based on extrapolation from a cladding region of constant average intensity. The method can be extended to general multi-layer radially symmetric fibers, e.g. to evaluate light content in refractive index pedestal structures. To effectively remove cladding light in high power systems, cladding light strippers are used. We show that the stripping efficiency can be significantly improved by bending the fiber in such a device and present respective experimental data. Measurements were performed with respect to the numerical aperture as well, showing the dependency of the CLS efficiency on the NA of the cladding light and implying that efficiency data cannot reliably be given for a certain fiber in general without regard to the properties of the guided light.

We present our recent advances on power scaling of a high-power single-pass pumped CVD-diamond Raman oscillator at 1.2 μm. The single pass scheme reduced feedback to the high gain fiber amplifier, which pumps the oscillator. The Yb-doped multi-stage fiber amplifier itself enables up to 1 kW output power at a narrow linewidth of 0.16 nm. We operate this laser in quasi-cw mode at 10% duty cycle and on-time (pulse) duration of 10 ms. With a maximum conversion efficiency of 39%, a maximum steady-state output power of 380 W and diffraction limited beam quality was achieved.

We present an amplified mode-locked erbium fiber oscillator reaching high average powers of 20 W combined with an ultra-short pulse length of 305 fs and well-controlled peak powers by monolithic divided pulse amplification.

We present the application of a confocal fluorescence microscope to the analysis of Yb-doped solid-state laser materials, with examples of Yb-doped crystals, photonic crystal fibers and fiber preforms made with different manufacturing processes. Beside the fluorescence lifetime image itself, a microscopic spectral fluorescence emission analysis is presented and spatially resolved emission cross sections are obtained. Doping concentration and its distributions and other laser optical parameters are measured, which help to analyze manufacturing steps. Further properties like photodarkening and saturation are addressed.

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.

We present our latest test of hollow-core fibers for applications in high-power beam delivery. A negative-curvature hollow-core fiber has been tested with a single mode fiber laser, with up to 1.2 kW average power transmission.