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Short fiber lasers produce record power/length of 1.33W/cm

In this paper we present experimental results1 achieved with short (7 cm) phosphate fiber lasers which generate more than 9.3 W multimode and 4 W single mode output power. To our knowledge, the power generated per unit fiber length of 1.33W/cm represents a record in the fiber laser world. This research may enable the development of on-chip, wattlevel single-frequency light sources in the 1,550 nm wavelength band. High power fiber lasers are a hot topic.2 Most high power fiber lasers use silica fibers measuring several tens of meters in length. It is impractical for such lasers to be integrated into very compact devices, such as those produced on chips or small boards. What’s more, when single frequency output is needed, long fiber lasers are unsuitable because of the difficulties inherent in selecting a single frequency from their closely spaced longitudinal modes. Short fiber lasers, those with a cavity length measured in centimeters, promise single mode and single frequency operation. The maximum output power of a centimeters long fiber laser is limited for the most part to the milliwatt level3 because of the difficulty of increasing ion doping concentrations in the fibers and of achieving high pump absorption with a double-cladding pump scheme. By solving these problems, we were able to boost the output power of such fiber lasers by more than one order of magnitude. The schematic layout of the fiber laser is shown in the inset of Fig. 1. The phosphate glasses used for fiber fabrication have high solubility of rare-earth ions and low clustering effects, which allows us to increase the concentration of Er 3+ (1.1 10 26 ions/m3) and Yb3+(8.6 1026 ions/m3) ions without enhancing the detrimental quenching processes. To achieve the “chaotic propagation” of the pump which improves pump absorption, the fiber has a D-shaped clad and an off-center circular core. The core diameters of the multimode and single mode fiber lasers are 19 m and 13.5 m. The numerical apertures of the multimode and single mode fiber lasers are 0.17 and 0.07. The fibers have an inner clad diameter of about 125-130 m. For both single mode and multimode fiber lasers, we used fibers measuring 7 centimers in length. The fiber laser cavities were formed by dielectric coatings at the fiber ends and an output coupler. The performances of both lasers are plotted in Fig. 1. We obtained up to 9.3 W power from the multimode fiber laser with M2 < 3.5 and 4 W from the single mode fiber laser with M2 < 1.2. To our knowledge, these are the highest powers ever generated from multimode and single mode fiber lasers with fiber lengths shorter than 10 centimeters. In addition, we demonstrated a new side-pumping scheme compatible with short fiber lasers.4 We used six pump delivery fibers which provided inputs for 12 independent pump diodes at 976 nm. The side-pumped laser constitutes a 12 cm long circular fiber with 18 m core doped with the same Er-Yb concentrations as those of the end-pumped fiber lasers. The performance of this laser is shown in Fig. 1 by the triangular symbols. Up to 5 W output power with optical-to-optical slope efficiency of 24 percent was obtained from this laser. This work was supported by the MRI program under AFOSR contract.