Abstract
Recent progress on erbium-doped fiber amplifiers (EDFAs) has been very rapid since they show a great potential for opening many new fields in optical communication [1]~[4]. The main advantages of EDFA are polarization-insensitive high gain, low insertion loss, low-noise, and a wide bandwidth in the loss-minimum region of silica fibers. In order to evaluate the performance of the EDFAs, high output power, high gain coefficient, and low noise characteristics are important. From these, a high gain coefficient EDFA is attractive since it offers the possibility of a low-noise high gain EDFA with a very low pump power as a preamplifier. With a view to realizing a high gain coefficient EDFA, the gain characteristics of a high NA (Numerical Aperture) EDFA have been investigated [5]~[7]. The high NA EDFA has a small spotsize so that efficient pumping is realized, resulting in a high gain coefficient.
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