Moritz M. Vogel

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.

Multilayer polarizing grating mirror used for the generation of radial polarization in Yb:YAG thin-disk lasers

The design, fabrication and characterization of a multilayer polarizing grating mirror developed for an Yb:YAG thin-disk laser resonator are reported. The potential of the proposed solution is discussed together with the first demonstration of a radially polarized Yb:YAG thin-disk laser.

High-power radially polarized Yb:YAG thin-disk laser with high efficiency

Radially polarized beams with an output power of 275 W, M2=2.3 and an efficiency of about 52.5% were generated from an Yb:YAG thin-disk laser. An intra-cavity circular resonant waveguide grating was used as a polarization selective mirror inside the laser cavity. We report on the design and the fabrication using a scanning beam interference lithography system and discuss the calculated and measured performances of the presented polarizing grating mirrors.

Single-layer resonant-waveguide grating for polarization and wavelength selection in Yb:YAG thin-disk lasers.

A single-layer resonant-waveguide grating consisting of a sub-wavelength grating coupler etched into a waveguide is proposed in order to achieve high polarization and high spectral selectivity inside an Yb:YAG thin-disk laser resonator. The designed structure was fabricated with the help of a Lloyds-mirror interference lithography setup followed by reactive ion beam etching down to the desired grating groove depth. The wavelength and polarization dependent reflectivity is measured and compared to the design results. The behaviour of the device at higher temperatures is also investigated in the present work. The device is introduced as the end mirror of an Yb:YAG thin-disk laser cavity. Output powers of up to 123 W with a spectral bandwidth of about 0.5 nm (FWHM) is demonstrated in a multimode configuration (M2~6). In fundamental-mode operation (TEM00 with M2~1.1) 70 W of power with a spectral bandwidth of about 20 pm have been obtained. Moreover, the degree of linear polarization was measured to be higher than 99% for both multimode and fundamental mode operation.

Radially polarized high-power lasers

In the recent years high-power laser beams with radial polarization have attracted an increasing interest because of their interesting properties in material processing. We present an overview of the current activities and different techniques to generate such beams in CO2 and solid-state lasers. With a polarising end-mirror which comprises a resonant grating on a dielectric multilayer Bragg-structure we recently demonstrated a 3-kW radially polarised CO2 laser. Current investigations are also focused especially on the application of this technology to thin-disk lasers. The specific requirements and the whole development from the design and fabrication to the characterization and test are illustrated with the example of a multilayer polarizing grating mirror developed to generate a radially polarised beam in an Yb:YAG thin-disc laser resonator. The potential of this kind of beams are discussed with a number of first application results, which largely confirm the predictions presented by Niziev et al1.

A 1-kW radially polarized thin-disk laser

Radially and azimuthally polarized beams have attracted significant attention and increasing interest in various fields of physics and engineering because of their specific and very attractive properties and their promising applications [1, 2]. At high power levels, the most interesting application of such beams is material processing where it was reported that the process efficiency can be significantly enhanced by using radial or azimuthal polarization in comparison to circular or linear polarization [1, 2]. This encouraged several scientific groups to investigate and to develop different approaches to generate such modes [3, 4]. In the present contribution we discuss two techniques which were applied to high power Yb:YAG thin-disk laser systems for the generation of radially polarized beams.

Delivery of 800 W of nearly diffraction-limited laser power through a 100 m long multi-mode fiber

We present the efficient propagation of a nearly diffraction-limited laser beam with a continuous wave power of 800 W through a multi-mode step-index delivery fiber with a core diameter of 30 µm and a numerical aperture of 0.056. The M2-value was measured to be 1.35 after 100 m of this passive fiber. This is an important advance as the delivery fiber length for high-brightness beams in the kilowatt range is usually limited to a few meters by the onset of nonlinear effects. For this demonstration a single-mode MOPA system was set-up consisting of a fiber oscillator and two amplifier stages. This source was coupled into the delivery fiber through a 500 mm long mode field adapter.

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.

Radially polarized Yb:YAG thin-disc laser

Beams with radial polarization have attracted an increasing interest during the past few years because of their attractive properties. An overview of the different intra- or extra-cavity techniques to generate such beams is given in the present paper. The design, fabrication and characterization of a multilayer polarizing grating mirror developed for an Yb:YAG thin-disc laser resonator are reported. The potential of the proposed solution is discussed together with the first demonstration of a radially polarized Yb:YAG thin-disc laser.

800 W cw nearly diffraction-limited beam delivery through a 100 m long multi-mode fiber

We present the nearly diffraction-limited propagation of 800 W of cw laser power through 100 m of delivery fiber having a core diameter of 30 μm. The laser source was a single-mode MOPA consisting of a fiber oscillator and two amplifier stages and was matched to the delivery fiber through a 500 mm long taper. At a maximum power of 800 W, a M2 value of 1.35 was measured after 100 m of passive fiber. A minor Stokes-shifted spectral content was observed above 780 W.