SMALL-SIGNAL ERBIUM-DOPED FIBER AMPLIFIERS PUMPED AT 980 NM - A DESIGN STUDY

A model for the erbium-doped fibre amplifier (EDFA) is presented and discussed. The model is used in a detailed analysis of the waveguide design of 980-nm pumped EDFAs operated in the small-signal gain region. A comparison of results for signals at wavelengths 1532 and 1552 nm is given. The influence of the amplified spontaneous emission on the optimum cut-off wavelength for the LP 11-mode of step index fibres is shown to be significant for gains exceeding approximately 40 dB. For lower gain levels, the optimum cut-off wavelength is 800 nm. An analysis of EDFAs used as preamplifiers indicates that significant improvements of the noise figures may be obtained by cut-back of the fibre to lengths that are approximately 50% of that for maximum gain. Very low noise figures of approximately 3.1 dB are only obtainable if a reduction of approximately 10 dB in the gain is acceptable.