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DIRECTED
ENERGY
PROFESSIONAL
SOCIETY
Abstract: 24-Symp-052
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UNCLASSIFIED, PUBLIC RELEASE
Improving the quantum efficiency in erbium doped fibers with high doping concentrations via nanoparticle doping: A path towards power scaling to greater than 1 kW
Erbium doped fibers are well known for applications in telecommunications, forming the basis for optical amplifiers in the C and L wavelength bands (1530 nm to 1565 nm and 1565 nm to 1625 nm, respectively). Two flavors of erbium doped fiber serve this application: 1) those lightly doped with erbia (< 0.25 weight %) and 2) those sensitized with ytterbia. The former group of fibers is lightly doped to prevent deleterious ion-ion interactions that lead to unacceptable decreases in quantum efficiency; this is an infamous characteristic of the erbium ion in silicate glasses. Yb-sensitized fibers, on the other hand, have found use where the pump absorption is required to be larger, such as in double clad fibers in wavelength division multiplexed systems. This comes courtesy of the large absorption cross-sections characteristic of the trivalent ytterbium ion. While these fibers have found great commercial success, power scaling for directed energy applications has been muted relative to Yb-doped, and even Tm-doped, fiber laser systems, both of which, as is well known, have already achieved over kW level lasing with a near diffraction limited beam. There are several obstacles to power scaling. First, pumping at 976 nm and subsequent lasing near the “eye safe” wavelength of 1550 nm leads to a rather large quantum defect and significant fiber heating that must be managed. This is true of erbium doped fibers with or without Yb sensitization. As a result, in-band pumping has attracted much attention since the quantum defect can be significantly reduced. However, high-brightness semiconductor pumps near 1500 nm are still lacking. Second, the quantum efficiency must be high (preferably > 90%) since quenching only serves to greatly exacerbate the problem of heat generation. Following hints from the telecom industry, the erbium doping concentration can be kept low to maintain high quantum efficiency. However, in high power applications, or where the fibers are double-clad, this requires the use of a long gain fiber. Although this indeed helps to reduce the thermal loading in the fiber, it also comes with unwanted effects, such as reduced power thresholds for nonlinearities, increased total background loss, among other parasitic processes related to Rayleigh scattering. In fact, on the topic of scattering, a key problem with Yb-sensitized fibers is self-oscillation by the Yb ion near 1000 nm. To overcome the obstacle of scaling the erbium concentration while maintaining high quantum efficiency, we present a new paradigm in Yb-free, Er:nanoparticle fibers. The fibers are drawn from preforms fabricated with erbium-doped, alkaline earth fluoride nanoparticles. Due to the chemistry associated with the fabrication process, the fluoride nanoparticles oxidize leaving a glass core heavily doped with alkaline earth oxides in the vicinity of the erbium ion. As a result of this chemical configuration, ion-ion interactions, as exemplified for example by cooperative upconversion and its characteristic visible luminescence, are greatly suppressed compared to other silicate hosts. Accordingly, these fibers exhibit extraordinary efficiency with high doping concentration. Amplifier data and detailed spectroscopy of these fibers will be presented at the meeting. Other topics discussed at the meeting will include fiber fabrication, compositional optimization of the glass, feasibility of scaling erbia concentrations to 2% by weight, and modeling suggesting that scaling to over a kW is possible.
UNCLASSIFIED, PUBLIC RELEASE
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