Marwan Abdou-Ahmed

Very-large-mode-area, single-mode multicore fiber

A single-mode evanescently coupled multicore fiber consisting of 19 hexagonally arranged cores is investigated. Theoretical and experimental results are presented and compared to an equivalent hypothetical step-index fiber. A fundamental mode with an effective area of 465 microm(2) and a beam propagation factor M(2) of 1.02 was measured, showing the high potential of the developed fiber.

Efficient pump beam shaping for high-power thin-disk laser systems

We report a beam-shaping technique whereby the output power from a high-power laser-diode stack is efficiently coupled, reconfigured, and transmitted to a thin-disk laser by means of a compact optical fiber bundle. By using this technique, the power density is increased by a factor of 2 when compared to direct coupling with a octagonal fused silica rod while the numerical aperture is kept constant. Transmission efficiency of 80% was measured for the beam shaper without antireflection coating. The top-hat distribution is numerically calculated at the thin-disk laser crystal.

Experiments towards resolving the proton charge radius puzzle

We review the status of the proton charge radius puzzle. Emphasis is given to the various experiments initiated to resolve the conflict between the muonic hydrogen results and the results from scattering and regular hydrogen spectroscopy.

Application of the extended Jones matrix formalism for higher-order transverse modes to laser resonators.

The extension of the Jones matrix formalism to higher-order transverse modes using N x N matrices presented in a previous paper [8] is applied to laser resonators. The resonator discussed in detail has a TEM(01)* Hermite-Gaussian mode, an axially symmetric polarizer combined with an axially symmetric phase shifter as a rear mirror and a folding mirror with conventional polarization dependent reflectivity and phase shift. The analysis reveals some useful regimes, where the output polarization is close to radial or azimuthal and the sensitivity to variations in the phase shift of the folding mirror is minimized.

Intracavity beam shaping for high power thin-disk lasers

The thin-disk laser (TDL) is a promising solid state laser concept since it combines good power scalability with high efficiency and small thermal lensing effects. Therefore this concept is a good candidate for near diffraction limited multi-kilowatt operation. In addition, due to its low depolarization losses, the TDL can be operated polarized without sacrificing its efficiency. To exploit the full potential of the TDL concept, the small thermally induced wave front distortions of the disk have to be compensated. At the same time, the intensity distribution of the output beam can be formed according to the requirements of the application. The wave front can be either corrected by a static graded-phase mirror or by an adaptive mirror. For the TDL, a thermally driven adaptive mirror was developed. The polarization state of the radiation field is another important aspect which can be optimized for specific processes. For example, a radially polarized beam promises significantly increased process efficiency for cutting sheet metal. The polarization state oscillating inside the resonator can be controlled by means of a polarization selective resonant grating-waveguide mirror. We describe the successive steps of development towards optical elements optimized for the intra-cavity beam shaping of the TDL.

Very-large-mode-area, single-mode multicore fiber: erratum

In a previous Letter [Opt. Lett.34, 2876 (2009)] a small error concerning a label of a figure is corrected here.

Highly-efficient continuous-wave intra-cavity frequency-doubled Yb:LuAG thin-disk laser with 1 kW of output power

We report on the generation of continuous-wave, intra-cavity frequency-doubled, multi-mode laser radiation in an Yb:LuAG thin-disk laser. Output powers of up to 1 kW at a wavelength of 515 nm were achieved at an unprecedented optical efficiency of 51.6% with respect to the pumping power of the thin-disk laser. The wavelength stabilization and spectral narrowing as well as the polarization selection, which is necessary for a stable and efficient second-harmonic generation, was achieved by the integration of a diffraction grating into the dielectric end mirror of the cavity, which exhibits a diffraction efficiency of 99.8%. At a frequency-doubled output power of 820 W the peak-to-valley power fluctuations measured during 100 minutes of laser operation amounted to only 8.2 W (1.0%). The beam parameter product of the frequency-doubled output was 3.4 mm·mrad (M2 ≈ 20), which is suitable for standard beam delivery using fibers with a core diameter of 100 µm and a NA of 0.2.

Improved x-ray detection and particle identification with avalanche photodiodes

Avalanche photodiodes are commonly used as detectors for low energy x-rays. In this work, we report on a fitting technique used to account for different detector responses resulting from photoabsorption in the various avalanche photodiode layers. The use of this technique results in an improvement of the energy resolution at 8.2 keV by up to a factor of 2 and corrects the timing information by up to 25 ns to account for space dependent electron drift time. In addition, this waveform analysis is used for particle identification, e.g., to distinguish between x-rays and MeV electrons in our experiment.

Wavelength selection, spatial filtering and polarization control of an Er:YAG laser cavity by resonant-grating mirror

Summary form only given. Er:YAG crystals are good candidates for eye-safe solid-state lasers with output pulses energy in the mJ range, which are required for applications in atmospheric propagation such as active imaging, lidar and wind mapping. Er:YAG crystals can emit at 1645 nm or 1617 nm. The laser emission of Er:YAG naturally occurs at 1645 nm and is unpolarized. In addition, the required high incident pump powers in quasi-three-levels laser such as Er:YAG could lead to a poor beam quality factor (M2) because of well-known thermal effects in rod lasers. Some applications may require emission at 1617 nm with a good M2 factor for long range sensing, as well as linearly polarized output beams for pollutant probing [1]. Therefore, a basic Er:YAG cavity could be provided with an intra-cavity etalon for wavelength selection [2], a reflective polarizer for polarization control, and a pinhole for spatial filtering. In this contribution, we report on a resonant-grating mirror (Fig. 1 left) which can be used to fullfil these three functions, hence simplifying the laser setup [3].

440 W polarized single-transverse-mode CW fiber amplifier with thin disk laser seed source

Diode-pumped master oscillator fiber amplifiers (MOFA) are known as very flexible laser sources since their spectral, temporal and polarization properties are mainly determined by the seed source which can be controlled quite easily at comparatively low powers. Additionally, they exhibit several of the typical advantages of fiber-based sources such as good beam quality, high efficiency and excellent power scalability. In the last few years, polarized laser beams have shown a remarkably growing demand in material processing. In the present contribution we report on a high-power linearly polarized single-transverse-mode Yb-doped fiber amplifier seeded by a linearly polarized Yb:YAG thin disk laser with an M2 of < 1.1 and a degree of linear polarization (DOLP) of 99 % at a output power of 44.5 W and an optical efficiency of 53 %. The fiber amplifier consists of a 7 m long, highly Yb-doped and polarization maintaining double-clad fiber with a core and a cladding diameter of 20 μm and 400 μm, respectively. The active fiber used for the amplifier exhibits a V-parameter of 4.9 and a cladding absorption of 3 dB/m at 976 nm. It was pumped by a fiber-coupled pump diode. With a launched pump power of 717 W, an output power of 440 W with a DOLP of about 96.5 % was extracted from the thin disk master oscillator Yb-doped fiber amplifier. An optical efficiency of 58 % and a gain of 11.7 dB were reached at a seed power of 30 W. A detailed description of our system and the latest experimental results obtained with the fiber described above as well as with other types of active fibers (e.g. non polarization-maintaining fibers) will be presented.