Optical amplifier and optical amplification method

Even when wavelength number of input optical signals is changed, or when the optical signals are intermittently transmitted e.g. in the case of the optical packet signals, amplification with constant output intensities for respective wavelengths is provided. An optical amplifier of the invention comprises a semiconductor laser which outputs light having a constant intensity and a given wavelength λ L, a semiconductor optical amplifier which amplifies part of the input optical signals and output light from the semiconductor laser, an optical filter which extracts only an optic element having a wavelength of λ L among the output light therefrom, a means which provides a rare earth ion-doped optical fiber with other part of the input optical signals and the extracted optic element having a wavelength of λ L. By utilizing nonlinear saturation phenomenon of the semiconductor optical amplifier, when a total optical intensity of the input optical signals having wavelengths from λ s1 to λ sn becomes large, an optical intensity of the optical signal having a wavelength of λ L becomes small. Meanwhile, when a total optical intensity of the optical signals having wavelengths from λ s1 to λ sn becomes small, an optical intensity of the optical signal having a wavelength of λ L becomes large. Therefore, a total intensity of the whole light including light having a wavelength of λ L is always constant, and a gain of the optical amplifier is always constant.