Optimum design of Er/sup 3+/-Yb/sup 3+/ codoped fibers for large-signal high-pump-power applications

A comprehensive numerical fiber amplifier model has been used to optimize Er/sup 3+/-Yb/sup 3+/ codoped active fiber for maximum gain and quantum conversion efficiency (QCE) at large signal operation. The optimum cutoff wavelength of the LP/sub 11/ mode has been found to increase from 800 mm at low pump powers (/spl ap/50 mW) to 1400 mn at pump powers higher than 500 mW. While at low pump powers fibers with higher numerical aperture give higher QCE, at high pump levels better large signal performance is achieved with fibers having lower numerical aperture.