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We report high power frequency conversion in a diamond Raman laser operating on the first and second Stokes shift. At the first Stokes, a maximum output power of 125 W at 1178 nm was achieved.

We report on the experimental realization of two different low-loss single- to multimode fiber coupler designs, each combining more than 5 kW of output power. The adiabatic mode evolution in the seven-fiber input bundle provides a brightness close to its theoretical maximum. The all-glass structures exhibit very low heat loads mainly due to small avoidable contamination, indicating room for improvement. To the best of our knowledge, regarding all-fiber geometrical combined power generation, beam quality and heat stability, the presented results had never been reported before.

The welding of transparent materials with ultrashort laser pulse at high repetition rates has attracted much attention due to its potential applications in fields such as optics, microfluidics, optofluidics and precision machinery. One demanding issue is the stable and reliable welding of different materials without the utilization of an intermediate layer or an optical contact. In this work, we maximized the size of the molten volume in order to generate a large pool of molten material which is able to fill an existing gap between the samples. To this end, we used bursts of ultrashort laser pulses with an individual pulse energy of up to $$10\,\upmu \hbox {J}$$ . The laser-induced welding seams exhibit a base area with a size of up to $$450\,\upmu \hbox {m}\,\times 160\,\upmu \hbox {m}$$ . Using these large modifications, we are able to overcome the requirement of an optical contact and weld even gaps with a height of about $$3\,\upmu \hbox {m}$$ . Bulging of the sample surface and ejection of molten material in the gap between the two samples allow to bridge the gap and enable successful welding. We also determined the breaking strength of laser-welded fused silica samples without an optical contact by a three-point bending test. The determined value of up to 73 MPa is equivalent to 85 % of stability of the pristine bulk material.

To further decrease feature sizes, the semiconductor industry heavily improved the photolithography. Photoresist layers are exposed through photomasks and structured accordingly. The underlying substrate is then selectively altered by adding or subtracting material. The unrivaled precision, resolution, and accuracy allow an unmatched level of miniaturization, and several thousand electrical components can be processed simultaneously. This technology was expanded to the fabrication of sensors and actuators, creating microelectromechanical systems. With silicon, the possibilities of further expanding this technology to new dimensions and applications are limited. Reasons are the stiffness, 2.5-D fabrication, and lack of optical transparency. To enable new applications, we present the successful translation of wafer-level fabrication to millimeter-scale actuators. This was achieved by changing the material to ultraviolet (UV)-curable polymers. Based on the electrostatic zipper actuator, avoiding the common pull-in-effect is essential for the successful realization of an acceptable microactuator. Therefore, the bottom electrodes are split and shaped to adjust the generated electrostatic forces, balancing the mechanical restoring forces. Besides the theoretical design, the fabrication technology based on classic photolithography is explained. Experimental results include voltage-dependent deflection measurements, achieving 110- $\mu $ m deflection with 70 V driving voltage. The dynamic response measurements show resonance frequencies of 1.89 kHz. [2014-0181]

The MERTIS instrument is a thermal infrared imaging spectrometer onboard of ESA's cornerstone mission BepiColombo to Mercury. MERTIS will provide detailed information about the mineralogical composition of Mercury's surface layer by measuring the spectral emittance in the spectral range from 7-14 μm with a high spatial and spectral resolution. Furthermore MERTIS will obtain radiometric measurements in the spectral range from 7-40 μm to study the thermo-physical properties of the surface material. Under the lead of the German Aerospace Center DLR (Dep. Optical Information Systems, Berlin) a development model (DM) is in development which integrates all MERTIS sub-units of later flight models. With the DM the general design and performance goals of the system shall be investigated and verified. Besides a general overview about the instrument principles the following topics are addressed: Optics setup with a Three Mirror Anastigmatic (TMA) telescope and Offner Spectrometer, Manufacturing techniques for the robust and high precision optics and Radiometer Concept and Design

A high-precision opto-mechanical breadboard for a lens mount has been assembled by means of a laserbased soldering process called Solderjet Bumping; which thanks to its localized and minimized input of thermal energy, is well suited for the joining of optical components made of fragile and brittle materials such as glasses. An optical element made of a silica lens and a titanium barrel has been studied to replicate the lens mounts of the afocal beam expander used in the LIDAR instrument (ATLID) of the ESA EarthCare Mission, whose aim is to monitor molecular and particle-based back-scattering in order to analyze atmosphere composition. Finally, a beam expander optical element breadboard with a silica lens and a titanium barrel was assembled using the Solderjet Bumping technology with Sn96.5Ag3Cu0.5 SAC305 alloy resulting in a low residual stress (

Fiber lasers entered numerous applications due to their high efficiency and superior stability. Er-doped fiber lasers emitting around 1550 nm optical wavelength are capable to produce hundreds of Watts [1] and Millijoule pulse energy [2,3].

In this contribution we demonstrate the generation of a frequency comb with sub-picosecond pulses by seeding an amplifier with two narrow linewidth laser diodes separated by 50 GHz. The results are achieved in the normal dispersion regime with parameters where the forming of the comb is dominated by self-phase modulation. A numerical simulation is performed with different nonlinear interaction lengths, whereas the experimental realization is done in a high power Yb-doped fiber amplifier to generate the necessary nonlinearity. An output power of 750 W with compression to sub-picosecond pulse durations is shown. Further perspectives towards higher power levels and larger comb-line spacing are discussed.

Analyzing and profiling the dopants in laser active materials, especially fibers and preforms is an important task. These information allows you to monitor the preform fabrication and drawn fibers and investigate their quality. It has been show that using micro-spectroscopy and fluorescence lifetime images (FLIM) can be used to detect and quantify erbium ions in silica samples [1]. The gathered lifetimes and spectra are not only used for monitoring the dopants profile, but also to gather data like emission cross-sections for simulations and so predict the fibers behavior for running a laser.

Frequency combs with large line spacing have many applications and are under intense research. Methods for generating such combs either start with initial frequency spaced lasers (like combined single-frequency lasers, phase- and amplitude modulated lasers or mode-locked-laser) or employ nonlinear effects to start or extend its spectral bandwidth in fibers or micro cavities [1-3]. Here we demonstrate the generation of a frequency comb of several nm widths with arbitrary frequency spacing from ∼50 GHz to 500 GHz by four-wave-mixing (FWM) of two external cavity lasers (EDCL) combined amplified in a high power large-mode area fiber amplifier. The power levels demonstrated are in the range of several 100 W to 4 kW. We also demonstrate that in the temporal picture, pulses of multiple kW peak power can be generated by compressing the output beam.

Fiber amplifiers are representing one of the most promising solid state laser concepts, due to the compact setup size, a simple thermal management and furthermore excellent beam quality. In this contribution, we report on the latest results from a low-NA, large mode area single mode fiber with a single mode output power beyond 4 kW without any indication of mode instabilities or nonlinear effects and high slope efficiency. Furthermore, we quantify the influence of the bending diameter of our manufactured low NA fiber on the average core loss by an OFDR measurement and determine the optimal bending diameter in comparison to a second fiber with a slightly changed NA. The fibers used in the experiments were fabricated by MCVD technology combined with the solution doping technique. The investigation indicates the limitation of the step index fiber design and its influence on the use in high power fiber amplifiers. We demonstrate, that even a slightly change in the core NA crucially influences the minimum bending diameter of the fiber and has to be taken into account in applications. The measured output power represents to the best of our knowledge the highest single mode output power of an amplifier fiber ever reported on.

A method to simulate induced stresses for a laser crystal packaging technique and the consequent study of birefringent effects inside the laser cavities has been developed. The method has been implemented by thermo-mechanical simulations implemented with ANSYS 17.0. ANSYS results were later imported in VirtualLab Fusion software where input/output beams in terms of wavelengths and polarization were analysed. The study has been built in the context of a low-stress soldering technique implemented for glass or crystal optics packaging's called the solderjet bumping technique. The outcome of the analysis showed almost no difference between the input and output laser beams for the laser cavity constructed with an yttrium aluminum garnet active laser crystal, a second harmonic generator beta-barium borate, and the output laser mirror made of fused silica assembled by the low-stress solderjet bumping technique.

The High-Power Focus Mirror we present in this paper gives access to dynamic focus position adaptation along 3.6 mm in high-power laser manufacturing. We developed and tested a new thermo-mechanical design for a unimorph deformable mirror that provides an extensive focal length range down to -2 m focal length. Moreover, the mirror’s unique thermal characteristics enable high-power applications up to 6.4 kW (2000 W/cm²) with stable optical beam quality as thermal lensing is successfully suppressed. Thus, the laser’s optical beam quality M² is stable over the entire actuation and thermal range. We will describe the design and the characterization of the High-Power Focus Mirror. The mirror setup is based on a unimorph concept using a piezoelectric actuator and a thin glass substrate with a highly reflective multilayer coating. An integrated copper layer improves the heat dissipation. Providing maximum stroke, as well as excellent dynamic properties, the deformable mirror substrate is mounted by our established compliant cylinders [1]. Furthermore, we investigate the incorporation of the High-Power Focus Mirror into a commercial laser-cutting system. We set up a laser-cutting test bench including a multimode laser source, the focus mirror, a commercial laser processing head, and measuring instruments. In this assembly, we measure the achievable focus position range as well as the laser beam quality. With this focus mirror, we want to encourage new, innovative high-power application fields in 3D laser processing such as laser cutting, welding, and structuring.

We report on detailed in situ distributed temperature measurements inside a high power fiber amplifier. The deducted thermal load and the transversal mode instability (TMI) threshold of a commercial large mode area fiber with 25 mu m core and 400 mu m cladding were measured at various seed wavelengths. By matching these results with detailed simulations we show that photodarkening has a negligible impact on the thermal load and, therefore, on the TMI threshold in this fiber.

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. 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.

We present our recent results of scaling low-NA fibers to high average power by overcoming the TMI-limitations. We obtained a record output power of 4.4kW with a higher stability and smaller bending compared to our previous result.

Mechanico-optical simulations performed with ANSYS and VirtualLab Fusion software to analyze stress-induced birefringence for the packaging of a laser ignition device by a low-stress soldering technique showed almost no influence on the laser output beam.

All inkjet printed piezoelectric actuators based on poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF–TrFE)) for applications as pump actuators in microfluidic lab-on-a-chip systems (LOC) are manufactured and investigated in terms of their morphology and actuator performance. Furthermore, a pump demonstrator with an all-printed P(VDF–TrFE) actuator is characterized here for the first time. The actuators are manufactured in a fully additive and flexible way by successive inkjet printing of a P(VDF–TrFE) film sandwiched between two silver electrodes on a polyethylene terephthalate (PET) substrate. Different from most current micropumps where actuator elements are fabricated separately, no additional joining step is required in the manufacturing approach employed here. Actuator performance is investigated by measurements of piezoelectric d31 coefficients as well as remanent polarization Prem for different thermal treatments of the as-printed P(VDF–TrFE) films. A strong dependence of the device performance on the annealing temperature is found with maximum values for d31 and Prem of approximately 10 pm V−1 and 5.8 μC cm−2, respectively. Morphology investigations of the printed films by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Atomic Force Microscopy (AFM) indicate an increased degree of crystallinity of the piezoelectric β-phase for samples annealed at temperatures above 110 °C, which coincides with improved device performance. A basic pumping function with pump rates of up to 130 μL min−1 is demonstrated, which is promising for future applications in LOC. Furthermore, the process chain and characterization presented here can be employed to design and manufacture also other P(VDF–TrFE)-based devices and allows the combination with additional printed on-chip functionalities in future LOC.

We present the amplification of a narrow-bandwidth signal at a wavelength of 1018 nm to a power exceeding 600 W with a stable output polarization state. The beam showed an excellent, nearly diffraction-limited beam quality. The high-power output could be realized using an in-house designed and fabricated fiber with a core-cladding diameter ratio of 32/260, ultra-low NA of 0.041, and ring-up doping. A seed source with high amplified spontaneous emission (ASE) suppression was also required, which was realized by a double-pass pre-amplifier with 13 W output power.

Advanced optical systems of telescopes and scientific instrumentation require high accuracy mounting and joining of components. Applications for deep UV, under high energetic radiation, for vacuum operation, or assemblies subjected to environmental loads (e.g. humidity and temperature) require a replacement of organic adhesives or optical cement by a more robust bonding agent. Soldering allows the bonding of different materials with an inorganic filler material. We present the optimization of the laser-based Solderjet Bumping for the mounting of optical components and the parameters of the bonding process for fused silica and LAK9G15 (radiation resistant glass) with thermally matched metal mounts. The investigation covers the experimental determination and optimization of solder wetting to the respective base materials and the bond strengths achieved.

We report on a newly designed and fabricated ytterbium-doped large mode area fiber with an extremely low NA (~0.04) and related systematic investigations on fiber parameters that crucially influence the mode instability threshold. The fiber is used to demonstrate a narrow linewidth, continuous wave, single mode fiber laser amplifier emitting a maximum output power of 3 kW at a wavelength of 1070 nm without reaching the mode-instability threshold. A high slope efficiency of 90 %, excellent beam quality, high temporal stability, and an ASE suppression of 70 dB could be reached with a signal linewidth of only 170 pm.

We report on the measurement of the longitudinal temperature distribution in a fiber amplifier fiber during high power operation. The measurement signal of an optical frequency domain reflectometer is coupled to an ytterbium doped amplifier fiber via a wavelength division multiplexer. The longitudinal temperature distribution was examined for different pump powers with a sub mm resolution. The results show even small temperature variations induced by slight changes of the environmental conditions along the fiber. The mode instability threshold of the fiber under investigation was determined to be 480W and temperatures could be measured overall the measured output power values.

We present measurements of the temperature increase inside the active fiber of a thulium fiber amplifier during high power operation. At a pump power of over 100 W at a wavelength of 793 nm, we measure the core temperature distribution along the first section of a large mode area (LMA) highly thulium doped active fiber by use of an optical backscatter reflectometer. A mode field adaptor is used to maintain single mode operation in the LMA fiber. An increase in temperature of over 100 K can be observed in spite of conductive cooling, located at the pumped fiber end and jeopardizing the fiber coating. The recoated splice can be clearly identified as the hottest fiber region. This allows us to estimate the maximum thermally acceptable pump power for this amplifier. We also observe that the temperature can be decreased by increasing the seed power, which is in agreement with theoretical predictions on the increase of cross relaxation efficiency by depletion of the upper laser level. This underlines the role of power scaling of the respective seed power of a thulium amplifier stage as a means of thermal management.

Microfluidic lab-on-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 scientific analysis on a microfluidic chip. When using such devices, cost and space-intensive lab equipment is no longer necessary. However, in order to make a monolithic and cost-efficient/disposable microfluidic sensing device, direct integration of the excitation light source for fluorescent sensing is often required. To achieve this, we introduce a fully solution processable deviation of OLEDs, organic light-emitting electrochemical cells (OLECs), as a low-cost excitation light source for a disposable 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. Thanks to this doping effect, work function adaptation is not necessary and air-stable electrode can be used. An ambient manufacturing process for fully solution-processed OLECs is presented, which consist of a spin-coated blue light-emitting polymer plus dopants on an ITO cathode and an inkjet-printed PEDOT:PSS transparent top anode. A fully transparent blue OLEC is able to obtain light intensity > 2500 cd/m 2 under pulsed driving mode and maintain stable after 1000 cycles, which fulfils requirements for simple fluorescent on-chip sensing applications. However, because of the large refractive index difference between substrates and air, about 80% of emitted light is trapped inside the device. Therefore, inkjet printed micro-lenses on the rear side are introduced here to further increase light-emitting brightness.

We report on quantitative modal content measurements by S2 measurements in a Kagome type hollow core fiber depending on bending and in-coupling optimization. The higher order mode content could not be completely suppressed impacting important applications.

A high power ultraviolet (UV) light emitting diode (LED) multi-chip module package based on aluminum nitride (AlN) and alumina (Al2O3) is presented. The AlN substrate with a high thermal conductivity of up to 180 W/(m· K) and LED chips based on a copper alloy provide superb thermal management and heat extraction. Efficient cooling is an important prerequisite for the increase of extractable optical power and decrease of thermally induced wavelength shift. A design of a stackable module featuring arrays of 7×2 indium-gallium-aluminum- nitride UV LED chips at 395 nm is developed. This configuration of sub-modules allows for the scalable assembly of line sources with different lengths. Applications using UV LEDs cover market segments such as curing of adhesives, inks and coatings, sterilization of medical equipment and treatment of potable water, as well as various uses in chemical detection, biochemical analytics and spectroscopy. Thermal and thermo-mechanical modelling of the sub-mount is conducted using finite elements analysis. Die attach using eutectic gold-tin solder, lower melting tin-lead solder and silver-filled adhesive are compared with respect to optical output power and wavelength drift. Mechanical strength and structure of the resulting joints are investigated using shear force measurements, cross-sectioning and micro-tomography. An optical output power of 7.7 W is achieved using a cluster of 14 LED chips at 1050 mA resulting in a peak irradiance of 30.8 W/cm2 at the LED surface with respect to the footprint and pitch of the attached chips.

A high-power tunable Raman Amplifier is presented. The seed signal (varying from 1118 nm to 1130 nm in wavelength) was generated in a tunable Raman oscillator and fed into the Raman amplification stage. A conversion efficiency of up to 86 % was achieved and a maximum output power of over 200 W was measured. The Raman gain coefficient for the amplifier fiber was measured to be 0.76×10−14 m/W. Furthermore, the measured output power was compared with values obtained from simple mathematical model and a good agreement up to the highest output power of amplified signal was achieved.

Using linear gas bearing guides in a high vacuum environment, the common method to keep the vacuum quality is to exhaust the gas emitted by the bearing pads before leaking into the vacuum chamber. Thereby the exhaust tubes between the guide and the exhaust pumps should interfere with the guide as little as possible while maintaining a flexible connection and a highly effective exhaustion rate. A novel exhaust system that implements these requirements is described within this paper. The major achievement was the realization of two exhaust tubes slidable into one another combined with the known method of non-contact clearance seals, thus enabling an highly efficient and yet disturbance free exhaustion. This setup was developed and characterized at static and dynamic conditions. An analytical model for dimensioning the non-contact seal was worked out and experimentally verified. The number of seal stages and the clearance height were identified as the major impact factors on the leakage rate of the setup. It is concluded that the investigated approach is very suitable for vacuum compatible gas bearing guides since a vacuum level in the order of 10 −4 Pa was maintained during the experiments.

Multi-Shaped electron beam lithography is considered a promising approach for high throughput mask and direct writing. Providing multiple apertures and individually controlled electrodes it allows for massive parallelization of exposure shots, thus significantly decreasing write time. A silicon-based micro-structured MEMS multi-beam deflection array (MDA) featuring 8×8 apertures is presented. The hybrid integration of MDA devices in ceramic system carriers utilizing a laser-based Solderjet Bumping process is demonstrated. This flux-free soldering process provides adhesive-free, long term stable and vacuum compatible joints and is used for both mechanical fixation and electrical connection. Electron beam deflection in two perpendicular directions requires the highly accurate placement of two crossed MDA devices, which is carried out by three degrees of freedom alignment procedures and solder joining. Electrical signal routing within the electron optical column using flexible printed circuit boards and flux-free soldering is also reported. The precision adjustment of two carriers is accomplished by fiducial mark detection using image processing. Results on alignment accuracy in the sub-micron range, mechanical and electrical testing of such assemblies are reported.

We report on investigations of direct bonding of glass materials for application as optical devices and high precision mechanical stages under vacuum conditions. Thin SiO2 (fused silica) wafers of about 1mm thickness were bonded to massive SiO2 substrates of up to 20mm thickness at diameters up to 200mm. Low Temperature Glass Bonding (LTGB) was performed under moderate vacuum, using a commercial bonding equipment of EVG (Austria). We report on our experience with suitable cleaning and low pressure plasma surface activation technologies to achieve high quality (optically transparent) bonds with a very low fraction of arial defects. Wetting angles were measured to monitor surface conditions during various steps of cleaning and low pressure plasma activation. Lowest defect levels were achieved with a combination of wet cleaning and N2-plasma processing, immediately proceeding the bonding process. Successful bonding was achieved in a moderate vacuum by exerting compressive forces of several kN to the glass stack at temperatures of about 250°Celsius. In the sandwich composite, essentially two classes of defects were discernible from interference fringes or haze: small circular defects –apparently from finite particle contamination– and slightly more extended edge related features –apparently from insufficient compressive forces– during the bonding process at these locations. The remaining bonding defects were analyzed by transmitting polarized light and measuring stress birefringence with equipment from ILIS (Germany). Particle related internal defects in the bonding area revealed a typical stress level of the order of 1 MPa. The unbounded edge regions showed no particular stress above the noise level. An exemplary 200mm diameter glass bond is documented in Figs. 1 and 2 below. Wafer (substrate) thickness was about 1mm (15mm), respectively, in this particular bond. This work was supported by DLR/Germany under contract No. 50YB0814.

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.

Micro Lens Arrays (MLA) containing thousands of lenses with an aspheric shape and a precise position on large substrates are used in sensor devices directly or as master moulds for the low-cost replication of micro lenses in an array. They have to fulfill the high requirements in wafer scale manufacturing of small optics. This high volume manufacturing method for low-cost but effective micro optics relies on the sandwich-like assembly of the sensors and optical components like lenses as well as mechanical components like apertures and spacers on wafer level. After joining all components the dicing results in a batch of wafer level cameras for the use in cellular phones or webcams. Micro Lens Arrays are also a centrepiece of today's sensor products, either to raise the fill factor and collect more light on each pixel or to deflect the incoming beam to measure the aberrations of the wavefront, the working principle of the Hartmann-Shack sensor. Within illumination optics, MLA are commonly used for beam homogenization in projection systems. In all fields of application ranging from automotive, medical, consumer and industrial optics to high-end sensors for space instrumentation, high quality lens arrays for direct use or replication must be provided.

We review the technique of incoherent beam combining and show experimentally the combination of four continuous wave fiber amplifiers to an average power of 2 kW and four pulsed 2 ns fiber amplifiers to an average power of 187 W (pulse energy 3.7 mJ) using binary dielectric gratings. The scaling potential and limitations are discussed in detail.

This paper presents an analytic model for the calculation of circular high-vacuum compatible gas-bearing pads with arbitrary feedings. The algorithms afford the reliable dimensioning of load-bearing properties as well as the required seal structures and the exhaust equipment. Theoretical and experimental investigations on orifice-bearing pads with micro-channel structure and porous bearing pads identify differences of characteristics in a high-vacuum environment compared to ambient conditions. At vacuum conditions, stiffness and gas load of the bearing pads are significantly degraded and the operating point of the orifice-bearing pad is shifted to larger gap heights. Gap height and gas load are identified as essential impact parameters for the gas leakage flow. Thus, the bearing pad design parameters have to be adapted for application in vacuum. Both investigated bearing pad types are comparably suitable for application at vacuum conditions. The gas leakage flow is analysed depending on the number of exhaust stages. With a single-stage exhaust system, high vacuum is already maintained in the test chamber. A second exhaust stage further reduces the gas leakage flow by two orders of magnitude.

The first rare-earth-doped fiber lasers were operated in the early 1960s, and produced a few milliwatts at a wavelength around 1 mu m [1], 12]. Since the beginning of the decade, an enormous increase of fiber laser output power has been reported, the realm of kilowatt power has been entered, and power levels as high as 100 kW are envisaged. Apart from the power, fiber laser systems are renowned for their inherent compactness, monolithic architecture, and a power-independent beam quality. This paper reviews the challenges, achievements, and perspectives of highpower continuous-wave (CW) laser generation and amplification in fibers.

The assembly effort of an optical system naturally relies on the degrees of freedom and the maximum allowable tolerances each optical surface introduces into the overall budget. Snap-together approaches traditionally can be regarded as attractive solutions for IR systems having moderate tolerances, where the required precision is achieved by simultaneously machining optical surfaces and mounting interfaces in a single machine setup. Recent improvements in manufacturing and metrology enable a transfer of the assembly approach to shorter wavelength applications, where sub-aperture figuring techniques are used in combination with suitable amorphous polishing layers to achieve the increased requirements on figure and finish. A further decrease of the assembly effort is gained by machining several optical surfaces on common mechanical substrates and fixing the relative position with uncertainties as low as the machine precision. The article presents the fabrication of large electroless nickel coated aluminum mirror modules having two functional freeform surfaces and references for metrology and system integration. The modules are part of an all metal anamorphic imaging telescope operating in the visual spectral range. Presented methods open up a rapid and reliable assembly of metal mirror based VIS telescopes to be used in ground and space based astronomy or remote sensing applications.

We introduce and experimentally validate a pulse picking technique based on a travelling-wave-type acousto-optic modulator (AOM) having the AOM carrier frequency synchronized to the repetition rate of the original pulse train. As a consequence, the phase noise characteristic of the original pulse train is largely preserved, rendering this technique suitable for applications requiring carrier-envelope phase stabilization. In a proof-of-principle experiment, the 1030-nm spectral part of an 74-MHz, carrier-envelope phase stable Ti:sapphire oscillator is amplified and reduced in pulse repetition frequency by a factor of two, maintaining an unprecedentedly low carrier-envelope phase noise spectral density of below 68 mrad. Furthermore, a comparative analysis reveals that the pulse-picking-induced additional amplitude noise is minimized, when the AOM is operated under synchronicity. The proposed scheme is particularly suitable when the down-picked repetition rate is still in the multi-MHz-range, where Pockels cells cannot be applied due to piezoelectric ringing.

Diamond machining of metal optics is a flexible way to manufacture structured elements on different surface geometries. Especially curved substrates such as spheres, aspheres, or freeforms in combination with structured elements enable innovative products like headlights of automobiles or spectrometers in life science or space applications. Using diamond turning, servo turning, milling, and shaping, different technologies for arbitrary geometries are available. The addressed wavelengths are typically in the near- infrared (NIR) and infrared (IR) spectral range. Applying additional finishing processes, diamond machining is also used for optics applicable down to the EUV spectral range. This wide range of applications is represented in the used materials, too. However, one important material group for diamond machining is metal substrates. For diamond machining of structured surfaces, it is important to consider the microstructure of the utilized materials thoroughly. Especially amorphous materials as nickel-phosphorus alloys or fine-grained copper allow the fine structuring of refractive and diffractive structures. The paper analyzes the influence variables for diamond machining of structured surfaces and shows the use of this research for applications in the spectral range from IR to EUV.

A novel laser-based soldering technique – Solderjet Bumping – using liquid solder droplets in a flux-free process with only localized heating is presented. We demonstrate an all inorganic, adhesive free bonding of optical components and support structures suitable for optical assemblies and instruments under harsh environmental conditions. Low strain bonding suitable for a following high-precision adjustment turning process is presented, addressing components and subsystems for objectives for high power and short wavelengths. The discussed case study shows large aperture transmissive optics (diameter approx. 74 mm and 50 mm) made of fused silica and LAK9G15, a radiation resistant glass, bonded to thermally matched metallic mounts. The process chain of Solderjet Bumping – cleaning, solderable metallization, handling, bonding and inspection – is discussed. This multi-material approach requires numerical modelling for dimensioning according to thermal and mechanical loads. The findings of numerical modelling, process parametrization and environmental testing (thermal and vibrational loads) are presented. Stress and strain introduced into optical components as well as deformation of optical surfaces can significantly deteriorate the wave front of passing light and therefore reduce system performance significantly. The optical performance with respect to stress/strain and surface deformation during bonding and environmental testing were evaluated using noncontact and nondestructive optical techniques: polarimetry and interferometry, respectively. Stress induced surface deformation of less than 100 nm and changes in optical path difference below 5 nm were achieved. Bond strengths of about 55 MPa are reported using tin-silver-copper soft solder alloy.

Lasers used for diverse applications from industry to fundamental science tend to increasing output powers. Some applications require frequency conversion via nonlinear optical crystals, which suffer from the formation of temperature gradients at high power operation which causes thermal lensing or destruction of the crystal due to tensile stresses. To avoid these unwanted effects we joined a beta barium borate (BBO) crystal with sapphire disks serving as effective heat spreaders due to their high thermal conductivity (thermal conductivity κ = 42 W/Km). Therefore, smooth and flat crystal surfaces were joined by plasma-activated bonding. The joining relies on covalent bonds, which are formed via a condensation reaction of the surfaces which are first connected by Van der Waals forces. The cleaned surfaces are activated by plasma and brought into contact, pressed together and heat treated at a temperature of about 100°C. Special attention has been paid to the cleaning of the surfaces. Therefor the surfaces have been evaluated before and after treatment by means of atomic force microscopy. A stable connection has been formed successfully, which has been tested in a proof of principle experiment and demonstrated efficient second harmonic generation at up to 253 W of input power. Compared to a bare single BBO crystal it could be shown that the temperature within the crystal compound is significantly reduced. Such hybrid structures pave the way for frequency conversion at kilowatts of average power for future high power lasers.

Solder-joining using metallic solder alloys is an alternative to adhesive bonding. Laser-based soldering processes are especially well suited for the joining of optical components made of fragile and brittle materials such as glasses, ceramics and optical crystals due to a localized and minimized input of thermal energy. The Solderjet Bumping technique is used to assemble a miniaturized laser resonator in order to obtain higher robustness, wider thermal conductivity performance, higher vacuum and radiation compatibility, and better heat and long term stability compared with identical glued devices. The resulting assembled compact and robust green diode-pumped solid-state laser is part of the future Raman Laser Spectrometer designed for the Exomars European Space Agency (ESA) space mission 2018.

Systematic experimental investigations toward the mode instability (MI) threshold in low-NA fibers are performed. By testing several fibers with varying V-parameters drawn from the same preform, a high degree of reproducibility of the experimental conditions could be achieved. This allows for systematic investigations on isolated parameters influencing the complex behavior of MI. A maximum MI threshold of 2 kW could be demonstrated for the tested fibers, which represents a new record output power for narrow linewidth fiber amplifiers. The MI threshold was found to sensitively depend on the V-parameter for large V-parameters (> 2), but to be robust for smaller V-parameters. Furthermore, the fiber bending diameter and the seed excitation conditions were identified to sensitively influence the MI threshold.

In this contribution we demonstrate an approach to determine the longitudinal resolved thermal load in the core of an ytterbium-doped amplifier-fiber. The temperature distribution was measured by an OFDR in a single-mode amplifier in operation.

We demonstrate an alternative approach facilitating the use of a travelling-wave type acousto-optic modulator to pick pulses from a high-repetition-rate carrier-envelope phase stable pulse train where Pockels cells cannot be applied due to piezoelectric ringing.

The fabrication of Yb doped fibers with low-NA (0.04) for high-power fiber laser applications in the multi-kW class is presented and discussed. Laser tests with ~3 kW output power and excellent beam quality are demonstrated.

A modified solution doping approach for incorporating appropriate amounts of Yb, Al and P in fused silica is shown. The mitigation of phosphorus evaporation is intended by the initial formation of AlPO4 from solution.