Optically amplifying fiber and optical fiber amplifier

An optical fiber amplifier suitable for amplification of wavelength division multiplexed signal light, comprising:

• an optical fiber having a center core region doped with Er, Al, P, and a ring region doped with Er and Al, said ring region being radially spaced a given distance from said center core region;
• a first pumping source for outputting first pump light having a first wavelength band;
• a second pumping source for outputting second pump light having a second wavelength band;
• a first multiplexer for coupling said first pump light to said optical fiber;
• a second multiplexer for coupling said second pump light to said optical fiber;
• a first coupler for branching amplified signal light output from said optical fiber to obtain monitor light;
• a second coupler for branching said monitor light into first and second monitor lights;
• a first filter for filtering said first monitor light to obtain first signal light having a wavelength .lambda..sub.1 ;
• a second filter for filtering said second monitor light to obtain second signal light having a wavelength .lambda..sub.2 ;
• a first photodetector for detecting said first signal light;
• a second photodetector for detecting said second signal light;
• and
• control means for controlling power levels and a power ratio of said first and second pump lights to be output from said first and second pumping sources so that outputs from said first and second photodetectors become equal to each other to reach a given level.