Erbium-doped fiber amplifier and its application to nonlinear optics

Gain characteristics of an Er³⁺-doped fiber for high-power picosecond input pulses are studied with an InGaAsP laser diode pump source at 1.46-1.48 µm. The output energy and peak power of the amplified pulses reach as high as 7.9 pJ and 792 mW for a repetition rate of 100 MHz and a pulse width of 10 ps. The gain saturation is so slow that the gain in high speed pulse transmission systems is determined by a steady-state saturated gain. With the Er³⁺-doped fiber amplifier, it is shown that solitons can be amplified and transmitted over a long dispersion-shifted fiber by using the dynamic range of an N = 1 soliton. Furthermore, optical solitons at wavelengths of 1.535 µm and 1.552 µm have been amplified and transmitted simultaneously over 30 km with an Er³⁺-doped fiber repeater for the first time. The collision experiments between these different wavelength solitons are described. It is shown that there is a saturation-induced cross talk between multi channel solitons, and the cross talk (the gain decrease) is determined by the aver­age input power in high bit-rate transmission systems. Subpicosecond soliton and 20 GHz soliton pulse amplifications with Er³⁺-doped fiber are also described, which indicate that Er fibers are very advantageous for short pulse soliton communication. Finally, a gain coefficient as high as 2.4 dB/mW is reported using InGaAsP laser diodes.