Rare-Earth-doped Laser Materials: Spectroscopy and Laser Properties
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Rare-Earth-doped Laser Materials: Spectroscopy and Laser Properties Larry D. Merkle US Army Research Laboratory, Attn RDRL-SEE-M, 2800 Powder Mill Rd, Adelphi, MD 20783, U.S.A. ABSTRACT Trivalent rare earth ions in crystalline or fiber hosts are among the most successful of laser materials, but new dopant-host combinations and more detailed understanding of existing materials continue to be needed. This paper presents a few examples from the work of our team at the Army Research Laboratory, highlighting the interrelation between spectroscopic properties and laser behavior. It focuses on bulk solids, though rare-earth-doped fiber lasers are also extremely important. One system discussed is Nd:YAG, particularly concentration quenching in heavily doped ceramic YAG. Spectroscopic properties of Yb:Y2O3 and Yb:Sc2O3 help to elucidate their laser performance. Spectra indicate that Er:YAG is more promising than Er:Sc2O3 for room temperature laser operation, but that the reverse is true for operation at and somewhat above liquid nitrogen temperature. INTRODUCTION Since nearly the beginning of the laser, some of the most successful lasers have relied on rare earth ions, principally in the form of trivalent cations in crystalline or fiber hosts [1]. Most trivalent lanthanides have several strong emission lines, and the wavelengths of these lines differ substantially among the lanthanides, providing some flexibility in laser wavelength selection. Their well-shielded 4fN states result in narrow absorption and emission lines, making it relatively easy to achieve large cross sections (for gain at the laser wavelength and for absorption of pump light) in systems with upper state lifetimes that are attractively long due to parity-forbidden transitions. Lasers based on these ions are studied by far too many groups to cite, and used in many applications from research to communications to cutting and welding to range finders. Knowledge of at least the basic spectra is required to optimize a given rare earth impurity-host combination for a given task, and often more detailed spectroscopic understanding is required. To illustrate this, a few examples of laser-related spectroscopic studies of bulk polycrystalline rare-earth-doped materials at the Army Research Laboratory are summarized here. CERAMIC Nd:YAG In the last 10-15 years, polycrystalline ceramic YAG (Y5Al3O12) has become available with optical quality very comparable – and in some ways superior – to the best single-crystalline YAG [2]. Higher concentrations of large rare earths like Nd3+ can be accommodated in these ceramics, permitting more extended investigations of concentration quenching in Nd:YAG than previously possible. We have measured and analyzed the fluorescence decay of this material for Nd concentrations as high as 9 % atomic. The decay is non-exponential for all but the lowest concentrations, and both the initial decay rate and the later exponential tail lifetime vary strongly with concentration. The tail decay rate varies proportionally to the square of the Nd
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