By Yuan Zhu // Martin Leich // Martin Lorenz //Tina Eschrich //Anka Schwuchow // Jens Kobelke // Hartmut Bartel // Matthias Jäger
The use of short laser pulses of very high peak pulse power and high beam quality is essential for several laser applications, in particular remote processing and structuring of large surfaces such as solar modules. Fiber lasers and amplifiers are ideally suited for this purpose, notably because of their flexibility, compactness, and good thermal management.
At Leibniz IPHT, innovative materials and structural concepts are being developed to make robust fiber laser sources suitable for industrial applications as cost effectively as possible. One current example is large-mode-area (LMA) fibers with a “classical” step-index profile, the beam quality of which can be significantly improved by local tapers. The main component of such an LMA fiber is ytterbium-doped core glass, which can be produced in large volumes using in-house REPUSIL technology.
In order to suppress nonlinear processes in high-power amplifiers, short fiber lengths and large fiber cores are required. The high ytterbium/aluminum doping in the fiber core required for this does not normally coincide with the low numerical aperture (NA) of the fiber core to achieve excellent beam quality. With a refractive index-adapted pedestal made of pure aluminum-doped REPUSIL glass, high core doping and low NA of the core can be carried out simultaneously in one fiber. Such a pedestal cannot be manufactured using classic fiber technology (MCVD process). At the same time, it serves as an inner pump light cladding, via which pump light is coupled into the amplifier fiber and the signal light is amplified in the laser core (see Fig. 1). A short taper is used to generate excellent beam quality from the highly multimodal fiber in an amplifier. In the process, a part of the amplifier fiber that is only a few centimeters long is subsequently adiabatically tapered to about 1/4 of its initial diameter. This taper makes it possible to couple a single-mode signal light into the fundamental mode of the amplifier fiber and thus creates the prerequisite for high beam quality and at the same time a monolithic and thus robust signal path.
Figure 2 schematically shows the entire refractive index profile of the complex LMA amplifier fiber. The core/pedestal structure is achieved by stacking many individual rods that have been carefully manufactured from REPUSIL materials. This automatically gives the pedestal area (pump cladding) of the fiber a noncircular, hexagonal structure, which is advantageous for good mixing of the cladding modes and thus ensures high pump absorption (see photo in Fig. 2). The pedestal is enclosed by a highly fluorine-doped cladding tube (F520), which produces a high numerical aperture of >0.2 for the pedestal (pump cladding).
The tapering and subsequent splicing of the pedestal fiber to the signal fiber of the preamplifier takes place at very high temperatures and can trigger local changes in dopant concentrations by diffusion processes. This “smears” the refractive index profile of the fiber and changes its beam properties, since an ideal signal coupling into the fundamental mode of the main amplifier is impaired. In the present newly developed fiber design, diffusion is able to be almost completely suppressed by two strategies:
- By using similar dopant concentrations of 2.5 or 3 mol% Al2O3for core and pedestal glass, the material diffusion between the two is greatly slowed down.
- By using a highly F-doped and thus lower viscous cladding tube material, which accounts for the majority of the fiber volume, the process temperatures during drawing, tapering, and splicing of the fiber were able to be significantly reduced.
Finally, a fiber with a core diameter of 45 µm and a pump cladding diameter of 200 µm was produced at Leibniz IPHT, thus creating a tapered amplifier including an end cap. This was tested within a monolithic, multi-stage nanosecond amplifier system (MOPA) at a pulse duration of 2 ns.
Up to an average power of 15.5 W and a corresponding peak pulse power of 375 kW, an almost diffraction-limited beam quality with a measured M2of 1.3 to 1.7 was able to be demonstrated (see Fig. 3). In contrast, the untapered fiber with M2≈3.5 delivers a significantly lower beam quality, which highlights the success of the taper concept in conjunction with the new step-index pedestal fiber. There were no signs of increased nonlinear processes with the signal power achieved. The use of picosecond pulses promises a further increase in peak pulse power.
Funded by: BMBF, China Scholarship Council
Yuan Zhu, Martin Leich, Martin Lorenz, Tina Eschrich, Claudia Aichele, Jens Kobelke, Hartmut Bartelt, and Matthias Jäger, "Yb-doped large mode area fiber for beam quality improvement using local adiabatic tapers with reduced dopant diffusion," Opt. Express 26, 17034-17043 (2018)