Optical Fiber Developments at Fraunhofer IOF

In many areas of modern society, such as telecommuni-cations, industrial manufacturing technology, and medical technology, it is hard to imagine life without glass fibers and their enabling technologies. Their use has already revolutionized several fields of application, such as data transmission in telecommunication, imaging in endoscopic systems, and laser material processing. Fraunhofer IOF has been developing a process chain for the production of speciality fibers since 2012. This year, this process chain was finalized with the completion of the fiber competence center, in particular with the commissioning of the fiber drawing tower. This means that the fabrication of laser-doped materials, preform technology, and fiber drawing technology is now established, thus speciality fibers can be made in-house. With the 16 m high fiber drawing tower for speciality fibers, consisting of two complementary drawing lines, the available options are extended by precise pressure control, the application of optical and mechanical functional layers, and the extrusion of thermoplastics with the produced fibers.

Based on basic research in the field of glass chemistry for the refractive index adjustment of doped quartz glass, step-index fibers with precise control of the refractive index distribution could be realized. The understanding of further scaling of fiber lasers to output powers above 5 kW could be extended by novel fiber Bragg gratings written through the polymer-based fiber cladding. For short wavelengths, a special fiber was developed that has a higher laser-active doped ring around the central, also actively doped fiber core, with the refractive index of the ring adapted to that of the surrounding material (Fig. 2). In addition to the performance scaling of single core fibers, the technology for the production of multicore fibers is also being developed. The new infrastructure put into operation this year also opens the possibility of producing micro-structured fibers. Current investigations focus on large-pitch fibers (Fig. 1), which are essential for the power scaling of ultra-short pulsed laser systems. These special fibers require a refractive index accuracy and homogeneity in the range of significantly less than 10-4. In order to realize these high requirements and produce a homogeneous material on a wavelength scale, a nanostructuring process of the actively doped material has been established at Fraunhofer IOF. In the future, the range of speciality fibers will be expanded, e.g. to address hollow core fibers for applications in sensor technology, power transport, and the future field of quantum technologies.

We acknowledge the following funding: BMBF (13N13652, 03WKCV02D); State of Thuringia 2015-0020; State of Thuringia supported by EU programs EFRE and ESF (2015FOR0015, 2015FOR0017, 13030-715, 2011FGR0104, 2015FGR0107, 2015FGR0108, B715-11011, B715-11005).

Authors: Johannes Nold, Thomas Schreiber, Nicoletta Haarlammert

Continuous performance improvements of femtosecond fiber laser systems have allowed new applications in industry and science to open up. Through the coherent combination of multiple laser amplifiers, physical performance limitations could be overcome and the most powerful femtosecond fiber laser systems to date could be realized. However, parallelization of the amplifier architecture leads to an increase in complexity and cost, which again sets an effective boundary for the maximum performance.

To decouple performance and complexity, a new integrated amplifier architecture is required. It is based on multicore fibers, which contain multiple channels in a single element. This setup also allows a single laser diode to be used to generate optical inversion. The beam-splitting and -combination as well as the phase stabilization are realized with compact and preferably monolithic components. In addition, a novel phase stabilization algorithm is used, which can determine and minimize phase deviations between the beams even at high channel counts. Hence, the number of discrete components is basically independent of the number of parallel amplifier channels and opens up a high scaling potential.

A first prototype based on this laser architecture was realized using an in-house designed step-index fiber with 4x4 cores in rectangular orientation. A novel and cost-effective production process was employed. A combination of picosecond and stretched femtosecond pulses was achieved with a combination efficiency of 80 %, demonstrating the viability of this concept.

Further development of this technology will offer the possibility to move fiber laser technology to the high pulse energy regime without sacrificing the intrinsic advantages of optical fibers.

Authors: Arno Klenke, Andreas Tünnermann, Jens Limpert