Bernd Köhler

The elastodynamic finite integration technique for waves in cylindrical geometries

This paper deals with the elastodynamic finite integration technique for axisymmetric wave propagation in a homogeneous and heterogeneous cylindrical medium (CEFIT). This special variant of a finite difference time domain (FDTD) scheme offers a suitable method to calculate real three-dimensional problems in a two-dimensional staggered grid. In order to test the accuracy of the numerical CEFIT code, problems for which analytical solutions are available are presented. These solutions involve wave propagation in an elastic plate, the scattering of a plane longitudinal wave by a spherical obstacle, and ultrasound generation by a thermoelastic laser source. For the latter problem experimental results are included. The CEFIT code also allows the treatment of more complicated problems. Further possible applications are the investigation of elastic waves generated in an acoustic microscope, the simulation of impact-echo measurements in multi-layer systems, axisymmetric wave propagation in arbitrary bodies of revo...

Generation and detection of fluorescent color centers in diamond with submicron resolution

Fluorescent color-center patterns have been written on surfaces of synthetic type-Ib diamonds with spatial resolution below 180 nm via irradiation with focused ion and electron beams and subsequent annealing. The patterns are detected and spectroscopically analyzed using confocal optical microscopy. From the spatial extent of the color-center distributions, the activation energy for diffusion of vacancies in diamond is determined as (2.55±0.15) eV. Detailed information about the formation of color centers in diamond is obtained employing the three-dimensional spatial resolution of the confocal microscope combined with spectral resolution. In particular, the distributions of two color centers, ascribed to different charge states of the NV defect in diamond, have been spatially mapped and shown to depend strongly on the irradiation dose.

The acoustic finite integration technique for waves of cylindrical symmetry (CAFIT)

Many ultrasonic nondestructive testing applications have cylindrical geometries. Examples involve the excitation of ultrasound by cylindrical piezoelectic probes or by laser, x rays, electron beams [A. C. Tam, Rev. Mod. Phys. 58, 381–431 (1986)], or ion beams [L. Sulak et al., Nucl. Instrum. Methods 161, 203–217 (1979)]. Thus, calculations of cylindrical wave propagation are important for a better understanding and interpretation of many testing situations. This paper deals with the AFIT Code or finite volume method for numerical simulation of sound propagation in fluids adapted to cylindrical geometries (CAFIT). A comparison is made with standard difference-equations techniques also utilized for cylindrical geometries. Two examples are dealt with: (1) The sound generation by a high energy beam of heavy ions stopping in water; (2) the multimode sound propagation in a medical doppler injection device excited by a disk probe.

Feedback-induced catastrophic optical mirror damage (COMD) on 976nm broad area single emitters with different AR reflectivity

We have carried out a comprehensive study on 976nm single emitters with different AR coatings (1%, 3%, 4%, and 5%), which have been exposed to optical feedback to investigate damages caused by back-reflected light and how to prevent them. By observing the near-field pattern while varying the probe current, we got information about the influence on filamentation and on peak-power densities with and without external optical feedback. For constant feedback strength, filamentation became more pronounced and more dynamic with increasing current. We observed bistable and chaotic “jumping” of high-intensity filaments. For usual operation currents and external feedback strengths of ≥4%, single emitters with low AR coating show COMDs; their positions correlate with excessive peaking in the near-field pattern. Finally we found that an increasing AR reflectivity depletes the influence of feedback light on the near-field pattern as well as on the emission spectra and lowers the risk of COMD.

Multi-kW cw fiber oscillator pumped by wavelength stabilized fiber coupled diode lasers

High power Yb doped fiber laser sources are beside CO2- and disk lasers one of the working horses of industrial laser applications. Due to their inherently given robustness, scalability and high efficiency, fiber laser sources are best suited to fulfill the requirements of modern industrial laser applications in terms of power and beam quality. Pumping Yb doped single-mode fiber lasers at 976nm is very efficient. Thus, high power levels can be realized avoiding limiting nonlinear effects like SRS. However the absorption band of Yb doped glass around 976nm is very narrow. Therefore, one has to consider the wavelength shift of the diode lasers used for pumping. The output spectrum of passively cooled diode lasers is mainly defined by the applied current and by the heat sink temperature. Furthermore the overall emission line width of a high power pump source is dominated by the large number of needed diode laser emitters, each producing an individual spectrum. Even though it is possible to operate multi-kW cw single-mode fiber lasers with free running diode laser pumps, wavelength stabilizing techniques for diode lasers (e.g. volume holographic gratings, VHG) can be utilized in future fiber laser sources to increase the output power level while keeping the energy consumption constant. To clarify the benefits of wavelength stabilized diode lasers with integrated VHG for wavelength locking the performance of a dual side pumped fiber oscillator is discussed in this article. For comparison, different pumping configurations consisting of stabilized and free-running diode lasers are presented.

Multi-kW high-brightness fiber coupled diode laser

Fiber coupled diode laser devices are attractive light sources for applications in the area of solid-state laser pumping and materials processing. The ongoing improvement in the brightness of diode lasers, which means power per beam quality, makes more and more industrial applications accessible to diode lasers. For many applications in materials processing multi-kW output power with a beam quality of better than 30 mm x mrad is needed. Previously we have reported on a modular diode laser platform based on a tailored bar design (T-Bar) and have demonstrated an output power of up to 785 W out of a 200 μm NA 0.22 fiber at a single wavelength of 976 nm. We have now extended that tailored bar platform to different wavelengths in the range from 900 nm to 1100 nm. At each single wavelength efficient fiber coupling into a 200 μm NA 0.22 fiber will be demonstrated. One important concept for power scaling is coarse wavelength multiplexing with a spectral separation of typically about 40 nm. Combining of different wavelengths enables scalable multi-kW high-brightness diode laser units. Further power scaling can be achieved by dense wavelength multiplexing with a spectral separation of only about 5 nm. In this paper we report on a diode laser unit with 3.5 kW output power and a beam quality of 25 mm x mrad.

Grain structure visualization with surface skimming ultrasonic waves detected by laser vibrometry

Laser vibrometry was used to visualize the process of ultrasonic wave propagation on a metallic sample surface. We were able to image the material microstructure from a sequence of high resolution laser vibrometric snapshots of the wave field. This information is usually hidden by the large amplitude of the propagating wave, but we were able to extract it by appropriate evaluation of the measured data. The method was applied to an austenitic weld specimen; the grain structure information obtained by this approach strongly resembles metallographic micrographs.

3‐D IMAGE‐BASED SIMULATION FOR ULTRASONICWAVE PROPAGATION IN HETEROGENEOUS AND ANISOTROPIC MATERIALS

Time domain simulation tools for ultrasonic wave propagation in a target material with a complex outer surface or various inclusions are developed by combining the finite integration technique (FIT) and the finite element method (FEM) with an image‐based modeling. In our simulation, the geometry of the target is determined by a digital image such as X‐ray picture, CAD data, etc. and the processed voxel data is directly fed into the numerical calculation with the FIT or FEM. The accuracy of the two methods is discussed by comparison with the boundary element method. A simulation of the ultrasonic testing for a concrete material is shown with the 3‐D image‐based FIT.

Scalable and modular diode laser architecture for fiber coupling that combines high-power, high-brightness and low weight

The demand for high-power and high-brightness fiber coupled diode laser devices is mainly driven by applications for solid-state laser pumping and materials processing. The ongoing power scaling of fiber lasers requires scalable fibercoupled diode laser devices with increased power and brightness. In particular, applications and technologies that demand a high degree of mobility, such as airborne or field transportable systems, also require a robust and extremely lightweight design. We have developed a scalable and modular diode laser architecture that combines high-power, high-brightness, and low weight that fulfills these requirements for a multitude of applications. At the heart of the concept is a specially tailored diode laser bar with an epitaxial and lateral structure designed such that only standard fast- and slow-axis collimator lenses are required to couple the beam into a 200μm core fiber with a numerical aperture (NA) of 0.22. To fulfill the requirements of scalability and modularity, a reduced size heat sink populated with multiple tailored bars is used. This enables a compact and lightweight design with minimum beam path length. The design concept is capable of providing single wavelength, high-power laser diode modules, with optional volume holographic gratings for wavelength stabilization. Modules with output power levels of more than one kW at a power-to-weight ratio of less than 1 kg/kW are achievable. In this paper, two laser modules based on this concept are presented. The optical output power is above 500W at a module weight less than 500g and 300W at 300g. Both modules are coupled into a 200μm, 0.22NA fiber.

Low-loss smile-insensitive external frequency-stabilization of high power diode lasers enabled by vertical designs with extremely low divergence angle and high efficiency

Broad area lasers with narrow spectra are required for many pumping applications and for wavelength beam combination. Although monolithically stabilized lasers show high performance, some applications can only be addressed with external frequency stabilization, for example when very narrow spectra are required. When conventional diode lasers with vertical far field angle, ΘV 95% ~ 45° (95% power) are stabilized using volume holographic gratings (VHGs), optical losses are introduced, limiting both efficiency and reliable output power, with the presence of any bar smile compounding the challenge. Diode lasers with designs optimized for extremely low vertical divergence (ELOD lasers) directly address these challenges. The vertical far field angle in conventional laser designs is limited by the waveguiding of the active region itself. In ELOD designs, quantum barriers are used that have low refractive index, enabling the influence of the active region to be suppressed, leading to narrow far field operation from thin vertical structures, for minimal electrical resistance and maximum power conversion efficiency. We review the design process, and show that 975 nm diode lasers with 90 μm stripes that use ELOD designs operate with ΘV 95% = 26° and reach 58% power conversion efficiency at a CW output power of 10 W. We demonstrate directly that VHG stabilized ELOD lasers have significantly lower loss and larger operation windows than conventional lasers in the collimated feedback regimes, even in the presence of significant (≥ 1 μm) bar smile. We also discuss the potential influence of ELOD designs on reliable output power and options for further performance improvement.