Optical transmission method and apparatus and optical amplification method and apparatus for optical communication system

An optical amplification apparatus for an optical communication system, comprising:

• a doped fiber doped with ions of a type;
• an input port for inputting signal light into said doped fiber;
• an output port for outputting the signal light amplified by said doped fiber;
• an excitation light source for outputting excitation light;
• a wave combiner for inputting the excitation light from said excitation light source into said doped fiber;
• a polarization holding fiber interconnecting said excitation light source and said wave combiner for maintaining a polarization condition of the excitation light outputted from said excitation light source, and
• depolarization means provided for said polarization holding fiber for depolarizing the excitation light from said excitation light source into depolarized light,
• wherein said depolarization means includes delay time difference providing means for providing a delay time difference between two polarized light component of the excitation light outputted from said polarization holding fiber, and a wave combiner for inputting the excitation light having passed said delay time difference providing means.

An optical amplification apparatus for an optical communication system, comprising:

• an excitation light source for outputting excitation light;
• a first doped fiber doped with ions of a type;
• a first input port for inputting signal light to said first doped fiber;
• a first output port for outputting the signal light amplified by said first doped fiber;
• a first wave combiner for inputting the excitation light from said excitation light source to said first doped fiber;
• a second doped fiber doped with ions of the same type or another type;
• a second input port for inputting signal light to said second doped fiber;
• a second output port for outputting the signal light amplified by said second doped fiber;
• a second wave combiner for inputting the excitation light from said excitation light source to said second doped fiber;
• and
• excitation light distribution means interposed between said excitation light source and said first and second wave combiners for supplying the excitation light from said excitation light source simultaneously to said first and second doped fibers.

An optical amplification apparatus for an optical communication system, comprising:

• a doped fiber doped with ions of a type;
• an input port for inputting signal light into said doped fiber;
• an output port for outputting the signal light amplified by said doped fiber;
• an excitation light source for outputting excitation light;
• a wave combiner for inputting the excitation light from said excitation light source into said doped fiber;
• and
• a polarization holding fiber interconnecting said excitation light source and said wave combiner for maintaining a polarization condition of the excitation light outputted from said excitation light source,
• wherein said excitation light source has a variable oscillation wavelength so as to vary polarization of the excitation light to be outputted from said excitation light source.