Andreas Popp

Thin-disk-laser-pumped ytterbium-doped fiber laser with an output power in the kW range

We report on an ytterbium-doped fiber laser (FL) pumped by an Yb:YAG thin disk laser [1]. The FL essentially acts as a brilliance converter suitable for applications requiring a diffraction limited kilowatt-class laser source. The FL can be positioned close to the application at the end of a multimode delivery fiber, which solves the length limitation of highpower single-mode delivery caused by non-linear effects in the fibers. The 11 m long fiber used in the experiments has a highly doped (1.3•1026 ions/m3) core with a diameter of 30 μm surrounded by a circular cladding with a diameter of 100 μm. The core has a N.A. of 0.06 and a V-parameter of 5.2. A commercial thin disk laser with a BPP of 4 mm•mrad is used as pump source. At the wavelength of 1030 nm, the pump light absorption is 1.4 dB/m, which is 77 times lower than the peak value at 976 nm. So far, a maximum cw output power of 925 W has been achieved at a pump power of 1203 W [2]. The output power was limited by the incomplete conduction cooling of the fiber and the temperature stability of the acrylate coating. With single pass pumping, an absolute efficiency of 77 % was achieved in good agreement with the simulation result of 81 %. Double-pass pumping will further increase the efficiency. The M2 was measured to be 2.6 with the fiber coiled on a 120 mm diameter cylinder. With a fully optimized fiber, having a smaller V-number and an octagonal pump cladding, an efficiency of 85 % and a beam quality of M2 < 1.5 are expected. We present the preliminary results achieved with a commercially available fiber as well as the recent results obtained with an improved active fiber as described above.

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.

Thin-disk laser-pumped high-power ytterbium-doped fiber laser

We report on what we believe is the first demonstration of an ytterbium-doped fiber laser pumped by an Yb:YAG thin-disk laser at 1030 nm [1]. This fiber laser essentially acts as an efficient brilliance converter suitable for all applications requiring a near-diffraction limited kilowatt-class solid-state laser source. Since it can be realized in a compact set-up without much supply, the fiber laser can be positioned close to the application at the end of long-distance fiber-optic beam-delivery cables, which solves the problem of non-linear effects usually occurring with high-brightness radiation in passive fibers. For a new customized fiber, which will be available soon, an efficiency of 85 % is predicted by reliable simulations and validated by experiments.

Investigation of bending effects on evanescent-field coupled waveguides designed for high-power fundamental-mode delivery systems

The objective of the present work is to develop a fiber suitable for high-power fundamental-mode beam delivery over a useful length for material processing (~100 m). The investigated design is based on evanescent-field coupled waveguides, also called multi-core fibers (MCFs) [6]-[8]. The investigated MCF consists of a hexagonal array of 7 cores in which the fundamental mode, the so-called in-phase supermode, has an effective mode area (Aeff) of 348 μm2 and a numerical aperture (N.A.) of 0.035. We show how the bending-induced losses and the mode-mixing depend on the bending radius and on the structure of the waveguide. The experimental results on the behavior of the near-field and the far-field for different bending radii will also be reported. Additionally, we will show another method to reproduce experimental results with multi-mode fibers supporting few modes where the conventional approach leads to irreproducible results due to the mode-mixing effect.

Novel multicore fibers for large-mode-areas and high beam quality

A passive multicore fiber with a mode field area of 465 μm2 at 1050 nm delivering virtually diffraction limited output beam quality and a few-mode multicore fiber are investigated and compared to suitable step-index fibers.

Multicore fibers for high-brilliance laser beam delivery

A passive few-mode multicore fiber consisting of 7 coupled cores is investigated. The fiber is compared to a largemode- area step-index fiber, with the same number of modes and a similar mode field area. Based on the 7-core fiber results a single transverse mode multicore fiber, with a mode field area of 465 μm2 at 1050 nm, delivering virtually diffraction limited output beam quality is demonstrated. Stimulated Raman threshold measurements are presented and a fundamental mode high-power beam transport with more than 350 W is shown.

Cw-operation of an Ytterbium doped 19-core fiber laser

The multi-core fiber laser is a promising fiber laser concept since it combines good beam quality and a high mode field diameter to reduce nonlinear effects especially for pulsed laser operation. Therefore this concept is a good candidate for high power fiber laser operation with a good beam quality. In the present paper we report on the characterization and the laser operation of a fiber laser with a hexagonal array of 19-cores. The near and the far field intensity distribution of the emitted beam as well as the bending-induced transversal mode selection have been investigated. The obtained experimental and simulation results and show a good agreement.