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 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 is described. It is shown that there is a saturation-induced cross talk between multichannel solitons, and the cross talk (the gain decrease) is determined by the average input power in high bit-rate transmission systems.