Abstract
The efficiency of unsaturated amplifiers scales as k = 1/σaτ, where σa is the absorption cross-section of the pump and τ is the lifetime of the metastable excited level. For μm direct pumping of praseodymium (DPP), k ≤ 2.5 10−29 m−2s−1’ which is to compare to 4. 10−26 m−2s−1 for 980 nm pumping of 1.55 μm erbium-doped fiber amplifiers (EDFA). This small value of k is both due to a short lifetime (non radiative decay is 30 times more efficient than radiative decay) and a weak absorption. The other consequence of the weak absorption is the use of cither long fibers or high praseodymium concentration. High NA monomode ZBLAN fibers are still lossy (0.05 to 1 dB/m) which reduces the gain of long fibers. Upper lasing level lifetime of praseodymium is quenched by cross-relaxations for praseodymium concentrations beyond 1000 ppm wt (ref. 1) which reduces the efficiency of these fibers. Optimization of direct pumping of praseodymium (DPP) results in a trade-off between fiber loss and 1G4 level lifetime.
© 1993 Optical Society of America
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