Patent ID: 8174757

Claim:
An optical amplifier comprising: a first optical amplifying fiber; a second optical amplifying fiber connected in series with the first optical amplifying fiber; a multi-layer thin film filter connected between the first and second optical amplifying fibers; and a pumping light source connected so as to supply a pumping light to the first and second optical amplifying fibers; wherein the multi-layer thin film filter has a gain flattening transmission loss property in a signal wavelength band, and a flattening transmission property at a pumping light wavelength band of the pumping light source, wherein a predetermined wavelength band other than said signal wavelength band and said pumping light wavelength band is set as a cutting off band in the multi-layer thin film filter, wherein said cutting off band is between said signal wavelength band and said pumping light wavelength band or is set at a longer side of said signal wavelength band.