Microstructured fiber lasers

Summary form only given. Microstructured fibers and fiber lasers are providing new approaches for achieving single transverse mode guiding in large core fibers. We have successfully fabricated microstructured fibers from phosphate glass that allows the same high doping levels as in our step index fibers. We developed a two step draw-stack-redraw process that enables the realization of various microstructured cladding and multiple core designs. With our first active microstructured fibers we obtained the following important results. We fabricated and tested the first phosphate glass microstructured fiber lasers with large Er-Yb-codoped cores. For an 11-cm-long cladding-pumped fiber laser, more than 3 W continuous wave output powers has been demonstrated and single-mode beam quality was obtained for an active core area larger than 400 μm². We performed, to the best of our knowledge, the first systematic study on how a negative core-cladding index difference influences microstructured optical fiber´s modal behavior. Single-mode lasing has been realized for short-length cladding-pumped phosphate glass fibers with large depressed-index Er-Yb-codoped cores. Large core microstructured fibers allowed us to construct even shorter fiber lasers at Watt level output power. A 3.5 cm short microstructured fiber laser produced 5 W cw output power at a pump-to-signal conversion efficiency of 20%. To scale the output power of compact fiber lasers systems to even higher levels, we are working on phase-locked arrays of single mode fiber lasers. This work was initiated through theoretical modeling. Only recently, we were able to use the flexibility of our two-step fiber drawing process to fabricate fibers with multiple cores that can be used to test phase-locking techniques. Initial experiments in this direction has been started. As an example, a microscope image of our microstructured fiber with 12 active cores is shown. Another image demonstrates the lasing action of all 12 cores just above threshold indicating quality and homogeneity of the multi-core fiber. To avoid thermal issues, the first 2 fibers feature a 4 times lower (2 wt%) Yb₂O₃ doping level compared to our record breaking step index fibers leading to reduced pump absorption and efficiency.- However, even with these test fibers we achieved more than 2 W of combined output power with more than 10% conversion efficiency in a 10 cm fiber laser.