To investigate the effect of high-power laser heating on the floating zone (FZ) method, a crystal of beta-Ga 2 O 3 was grown by the LDFZ method using a newly developed optical system equipped with a 20-kW laser diode (LD) system as a heat source. The growth started from a twin-free seed crystal with a diameter of 7 mm and the diameter of the crystal was increased gradually up to 30 mm. With increasing the diameter of the crystal, three patterns of the beam intensity profile were switched to heat the whole of the molten zone locally and intensively. The intensity profile of the five beamlets irradiating the sample was adjusted with a combination of the zooming by the optical system and the partial blocking by the alumina cylinder. By the proper tuning of the beam profile and the proper selection of the diameter of the feed rod, the molten zone was kept stable against gravity. The obtained crystal has a layer texture without clear facets or cracks. It almost conserves the crystallographic direction of the seed crystal but is twinned except for a twin-free region near the seed crystal.

Laser diode pump sources are a key technology for laser-driven inertial fusion energy (IFE). On the development path to pump modules for actual power plants significant progress regarding multiple performance parameters as well as the overall cost of the modules is required. In this paper, we focus on the reliability of laser diodes and present a multiphysics model to predict the threshold of catastrophic optical damage (COD).

For the satellite-based methane lidar instrument MERLIN a reliable laser source is needed that emits laser pulses at two wavelengths of around 1645 nm to measure the methane concentration of earth’s atmosphere with an Integrated Path Differential Absorption LIDAR (IPDA). To generate those pulses, the laser (LASO) consists of a seeded, actively qswitched, diode pumped Nd:YAG master oscillator power amplifier (MOPA) and a subsequent seeded and frequencycontrolled optical parametric oscillator (OPO). Due to the passive thermal control of the instrument the laser has to withstand a large non-operational and operational temperature range and also high mechanical loads while at the same time a compact envelope is required. Together with the demanding requirements on the laser performance a very robust optical design is needed. To meet those requirements, Fraunhofer Institute for Laser Technology (ILT) uses optomechanical mounts that were developed in a previous project and show very low tilting over a large operational temperature range, even after nonoperational temperature cycling and applying mechanical loads. The mounts are soldered and free of organic substances as the LASO is enclosed in a pressurized housing (LASH). Any outgassing could lead to a decay or damage of the optics and thus a failure of the laser. During the development of the optomechanical mounts many tests were performed to quantify the statistical behavior under mechanical and thermal loads. Based on those results and additional mechanical simulations, Monte-Carlo-Analyses have been performed to analyze the performance of the laser and to verify the fulfilment of the requirements.

In this work we present results of a GPU-accelerated wave-optical model for external-cavity diode lasers containing a beam transformation system to symmetrize the beam qualities in fast and slow axis. We show the rotation of the beam profile and analyze the feedback field into the diode. We further consider the effect of misaligned optical elements on the feedback field.

In this work a diode-pumped Q-switched Alexandrite laser operating in single longitudinal mode (SLM) at the potassium resonance line is presented. The self-developed laser diode pump device is fiber-coupled ( = 400μm, NA=0.22) and delivers a pump energy of 18 mJ at 636 nm with a pulse duration of 120 μs and a repetition rate of 500 Hz. Pump light not absorbed in single-pass through the 7 mm long crystal is recollimated, polarization adjusted and refocused into the crystal. The Alexandrite laser yields a pulse energy of 1.7 mJ at a repetition rate of 500 Hz with a high pulse-to-pulse stability of 0.2 % (rms) and a beam quality of M2 < 1.1 in both spatial directions. The output beam is round and stigmatic without further beam shaping. The electro-optical efficiency of the laser system is 2 % which is approximately two magnitudes higher than of comparable flashlamp-pumped Alexandrite laser systems. By seeding the resonator with a SLM diode laser and actively stabilizing the cavity length, SLM-operation at the resonance line of potassium at 769.898 nm with a linewidth of approximately 10 MHz is achieved. Thereby the laser fulfills all the requirements for a resonance-lidar system. The investigations presented in this publication show the feasibility for pumping a complex ring resonator with a fibercoupled pump module in the red spectral region. This presents an important step to compact lidar systems for autonomous measurements under rough environmental conditions.

Based on established short pulse lasers with an output wavelength around 1 μm optical parametric frequency converters open up the spectral range between 1.4 and 4.0 μm for the first time in a power range of interest to laser material processing. The systems can be flexibly adapted as regards wavelength, pulse parameters and spectral properties to the requirements of various applications. We will discuss technical implementation and characterization of different optical parametric generators (OPG) based on periodically poled Lithium Niobate (PPLN) to show the parameter flexibility of this approach as well as current technical limits. Actual design examples will address output wavelengths between 1.6 μm and 3.4 μm with output powers ranging from several watts to tens of watts. The pulse parameters of these lasers range from a pulse duration of 9 ps with a repetition rate of 86 MHz to 1.5 ns and 100 kHz. The spectral bandwidth of the OPG examined can be very large. In particular, spectral bandwidths of about 100 nm are measured at the degenerated point, where the output wavelength is equal to twice the pump wavelength. Even beyond this point, a spectrum of typically a few tens of nanometers width generally accompanies a large conversion efficiency (>50 %). For applications that require a narrower spectrum, the OPG can be operated in a seeded mode, where only a few milliwatts of power from a continuously emitting laser diode are sufficient to seed a pulsed high power OPG efficiently and reduce the bandwidth to few nanometers.

We present a direct diode laser with an optical output power of more than 800 W ex 100 mu m with an NA of 0.17. The system is based on 6 commercial pump modules that are wavelength stabilized by use of VBGs. Dielectric filters are used for coarse and dense wavelength multiplexing. Metal sheet cutting tests were performed in order to prove system performance and reliability. Based on a detailed analysis of loss mechanisms, we show that the design can be easily scaled to output powers in the range of 2 kW and to an optical efficiency of 80%.

An Nd:YAG-MOPA system consisting of a stable oscillator and two subsequent Innoslab-based amplifier stages has been designed and built as a technology demonstrator for future lidar applications. Overall, the authors demonstrate that it generates more than 500 mJ of pulse energy at a 1064 nm wavelength and 100 Hz pulse repetition frequency at about 30 ns pulse duration in the single longitudinal mode. Seeded with 75 mJ pulses, the second amplifier stage achieved an optical efficiency (extracted energy to pump energy) of more than 23% while preserving excellent beam quality. To address the 500 mJ regime while retaining the basic system properties, an established Innoslab design was scaled geometrically.

A single-frequency q-switched Ho:YLF laser oscillator with a bow-tie ring resonator, specifically designed for high-spectral stability, is reported. It is pumped with a dedicated Tm:YLF laser at 1.9 mu m. The ramp-and-fire method with a DFB-diode laser as a reference is employed for generating single-frequency emission at 2051 nm. The laser is tested with different operating modes, including cw-pumping at different pulse repetition frequencies and gain-switched pumping. The standard deviation of the emission wavelength of the laser pulses is measured with the heterodyne technique at the different operating modes. Its dependence on the single-pass gain in the crystal and on the cavity finesse is investigated. At specific operating points the spectral stability of the laser pulses is 1.5 MHz (rms over 10 s). Under gain-switched pumping with 20 % duty cycle and 2 W of average pump power, stable single-frequency pulse pairs with a temporal separation of 580 mu s are produced at a repetition rate of 50 Hz. The measured pulse energy is 2 mJ (< 2 % rms error on the pulse energy over 10 s) and the measured pulse duration is approx. 20 ns for each of the two pulses in the burst.

The potential of diamond as an optical material for high-power laser applications in the wavelength regime from the visible spectrum (VIS) to the near infrared (NIR) is investigated. Single-crystal diamonds with lateral dimensions up to 7×7mm2 are grown with microwave plasma assisted chemical vapor deposition (MPACVD) in parallel with up to 60 substrates and are further processed to spherical optics for beam guidance and shaping. The synthetic diamonds offer superior thermal, mechanical and optical properties, including low birefringence, scattering and absorption, also around 1 μm wavelength. We present dielectric (AR and HR) coated single-crystal diamond optics which are tested under high laser power in the multi-kW regime. The thermally induced focal shift of the diamond substrates is compared to the focal shift of a standard collimating and focusing unit for laser cutting made of fused silica optics. Due to the high thermal conductivity and low absorption of the diamond substrates compared to the fused silica optics no additional focal shift caused by a thermally induced refractive index change in the diamond is observed in our experiments. We present experimental results regarding the performance of the diamond substrates with and without dielectric coatings under high power and the influences of growth induced birefringence on the optical quality. Finally, we discuss the potential of the presented diamond lenses for high-power applications in the field of laser materials processing.

In the field of atmospheric research lidar is a powerful technology to measure remotely different parameters like gas or aerosol concentrations, wind speed or temperature profiles. For global coverage, spaceborne systems are advantageous. To achieve highly accurate measurements over long distances high pulse energies are required.A Nd:YAG-MOPA system consisting of a stable oscillator and two subsequent InnoSlab-based amplifier stages was designed and built as a breadboard demonstrator. Overall, more than 500 mJ of pulse energy at 100 Hz pulse repetition frequency at about 30 ns pulse duration in single longitudinal mode were demonstrated. When seeded with 75 mJ pulses, the 2nd amplifier stage achieved an optical efficiency (pump energy to extracted energy) of more than 23 % at excellent beam quality.Recently, different MOPA systems comprising a single InnoSlab amplifier stage in the 100 mJ regime were designed and built for current and future airborne and spaceborne lidar missions. Amplification factors of about 10 at optical efficiencies of about 23 % were achieved. In order to address the 500 mJ regime the established InnoSlab design was scaled geometrically in a straight forward way. Hereby, the basic design properties like stored energy densities, fluences and thermal load densities were retained.The InnoSlab concept has demonstrated the potential to fulfill the strong requirements of spaceborne instruments concerning high efficiency at low optical loads, excellent beam quality at low system complexity. Therefore, it was chosen as baseline concept for the MERLIN mission, currently in phase B.

We present a compact, modular and cross talk free approach for dense wavelength division multiplexing of high power diode lasers based on ultra-steep dielectric filters. The mini bars consist of 5 narrow stripe broad area emitters with a beam parameter product in the range of 2 mm mrad and a wavelength spacing of 2.5 nm between 2 adjacent emitters. Experimental results for fiber coupling (35 μm core diameter, NA < 0.2) of internally and externally stabilized diode lasers are presented. Optical losses are analyzed and alternative optical designs to overcome the current limitations of the setup are discussed.

We present a flexible, modular and cross-talk free concept for dense wavelength beam combining of HPDL by use of ultra-steep dielectric filters. Five emitters of one mini bar are coupled into a 35 µm fiber. Experimental results of internally and externally stabilized HPDL are presented.

A high brightness diode laser module based on single emitters is presented, offering a maximum output power of 15.5 W at 35 Hz repetition rate and 100 mu s pulse length. While the presented module (6xx nm) is designed and used for longitudinally pumping an alexandrite laser for atmospheric measurements, the conceptual design is suited for the whole range of diode laser wavelengths from UV to NIR as well as for fiber coupling. The beam propagation factor has been measured to be M-2 = 34. To the authors' knowledge this is the highest brilliance ever reported at the specific wavelength.

A Tm:YLF laser in INNOSLAB design is reported. It produces 200 W of output power at an optical efficiency of 24 % and a slope efficiency of 27 % with respect to incident pump power. The laser crystal is partially end-pumped in a tophat line focus with a width of 12 mm and a height of about 1 mm. It is placed in a stable, spherical laser resonator, which results in a highly elliptical output beam. The beam is near diffraction limit and Gaussian in shape in one axis and contains very high order transversal modes and is Top-Hat-like in shape in the other axis. The beam shape is ideal for pumping a Ho:YLF laser crystal in INNOSLAB design for a pulsed amplifier.

We report on a single-frequency laser oscillator based on a new Er:YLuAG laser crystal which is spectrally suitable for application as a transmitter in differential absorption lidar measurements of atmospheric CH4. The laser emits single-frequency laser pulses with 2.3 mJ of energy and 90 ns duration at a repetition rate of 100 Hz. It is resonantly pumped by two linearly polarized single-mode cw fiber lasers at 1532 nm. A scan of the CH4-absorption line at 1645.1 nm was performed and the shape of the line with its substructure was reproduced as theoretically predicted. A 2.5-dimensional performance model was developed, in which pump absorption saturation and laser reabsorption is included. Also the spectral output of the laser oscillator longitudinal multimode operation could be predicted by the laser model.

The high power optical technologies enter a new era: The age of DIGITAL PHOTONIC PRODUCTION. Very recently new lasers in new time- and wavelength domains with high average powers have been developed for new processes like Laser Additive Manufacturing, Laser Ablation by ultrafast lasers or Laser Polishing.

Integrating volume holographic gratings into micro-optical components such as cylindrical fast-axis collimation lenses (VHG-FAC) for diode lasers constitutes a promising concept for wavelength stabilization by forming an external cavity laser. Compared to standard wavelength stabilization configurations the integrated element reduces the alignment complexity and is furthermore insensitive to the smile-error of diode laser bars. In order to configure and optimize these components the diffraction of the divergent field distribution of a broad area semiconductor laser must be calculated.The present paper presents the extension of the coupled-mode theory in order to calculate the spectral distribution of the diffracted field and the coupling efficiency within the external cavity. The model was extended to three-dimensional space and supplemented to include surface effects, polarization dependency and wave-optical propagation.The asymmetric spectral distribution emitted by an external cavity laser with VBG-FAC is tracked back to the feedback of highly divergent radiation diffracted at the holographic grating. Power losses due to the coupling efficiency within the cavity are also calculated for various field distributions and compared with experimental data. In summary the mathematical model allows to estimate the minimum spectral width and the losses using a VHG-FAC in an external cavity. Thus the injection locking concept using the VHG-FAC can be compared to the spectral characteristics and estimated power losses of standard wavelength stabilization configurations, e. g. the alignment of the grating in the collimated beam.

190 W output power at 515 nm were achieved by extracavity frequency conversion of sub700 fs pulses at 76 MHz. Numerical simulations considering thermal effects and group velocity dispersion are applied to model the conversion.