Heavily Er-doped fibers (EDFs) based on P(2)O(5)-Al(2)O(3)-SiO(2) (PAS) ternary glass have been studied. A unique feature of this glass is the formation of a AlPO(4) join having a structure similar to that of SiO(2) glass and a refractive index below it. It is found that the Er(3+) absorption and emission spectra in the PAS EDFs are defined by the dopant (Al(2)O(3) or P(2)O(5)) present in excess and are close to those of the corresponding binary glass (Al(2)O(3)-SiO(2) or P(2)O(5)-SiO(2)). The presence of the AlPO(4) join results in the enhancement of the pump-to-signal conversion efficiency in the PAS EDFs as compared with the EDFs based on the P(2)O(5)-SiO(2) and Al(2)O(3)-SiO(2) (with 1.5 mol. %Al(2)O(3) and less) binary glasses. The PAS host glass is advantageous in the case of large-mode-area active fibers.

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Radiation-induced absorption and lasing efficiency of a hermetically coated erbium-doped fiber saturated with molecular hydrogen are studied. It is shown that H 2 -loading of hermetically coated erbium-doped fibers prolongs their service time in space more than in 5 times, making such fibers promising for space applications.

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We discuss various Penrose limits of conformal and nonconformal backgrounds. In AdS 5 ×T 1,1 , for a particular choice of the angular coordinate in T 1,1 the resulting Penrose limit coincides with the similar limit for AdS 5 ×S 5 . In this case an identification of a subset of field theory operators with the string zero mode creation operators is possible. For another limit we obtain a light-cone string action that resembles a particle in a magnetic field. We also consider three different types of backgrounds that are dual to nonconformal field theories: The Schwarzschild black hole in AdS 5 , D3-branes on the small resolution of the conifold and the Klebanov-Tseytlin background. We find that in all three cases the introduction of nonconformality renders a theory that is no longer exactly solvable and that the form of the deformation is universal. The corresponding world sheet theory in the light-cone gauge has a τ = x + dependent mass term.

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