Wavelength-multiplexed optical communication system and optical amplifier used therefor

An optical communication system comprising:

• a plurality of optical communication apparatuses for communicating signals by using a light having a first wavelength;
• an optical transmitter for transmitting a light having a second wavelength different than the first wavelength, the second wavelength light being transmitted independently of any communicating from the plurality of communication apparatuses;
• an optical transmission line for interconnecting said optical transmitters and said plurality of optical communication apparatuses to transmit the lights having the first wavelength and the second wavelength;
• an optical amplifier for amplifying the lights having the first wavelength and the second wavelength transmitted over the optical transmission line;
• and
• control means for taking a part of the light having the second wavelength amplified by said optical amplifier so that the rest of the light having the second wavelength remains in said optical transmission line and controlling a gain of said optical amplifier to keep a light quantity of said part of the light having the second wavelength at a constant level.

An optical communication method in an optical communication system having a plurality of optical communication apparatuses, an optical transmission line for interconnecting said communication apparatuses and an optical amplifier arranged in said optical transmission line, comprising the steps of:

• communicating signals among said optical communication apparatuses over said optical transmission line by using a plurality of communicating lights having different wavelengths;
• transmitting a controlling light having a wavelength different than any of said plurality of wavelengths over said optical transmission line, the controlling light being transmitted independently of any communicating from the plurality of optical communication apparatuses;
• amplifying the communicating lights and the controlling light transmitted over said optical transmission line by said optical amplifier;
• taking a part of the controlling light amplified by said optical amplifier so that the rest of the controlling light remains in said transmission line;
• and
• controlling a gain of said optical amplifier to keep a light quantity of said part of the controlling light amplified by said optical amplifier at a constant level.

An optical amplifier apparatus used in an optical communication system having a plurality of optical communication apparatuses for communicating signals by using a plurality of communicating signals having different wavelengths, an optical transmitter for transmitting a controlling light having a wavelength different than any of said plurality of wavelengths, and an optical transmission line for interconnecting said optical transmitter and said plurality of optical communication apparatuses to transmit the communicating lights and the controlling light, comprising:

• an optical amplifier for amplifying the communicating lights and the controlling light transmitted over the optical transmission line;
• and
• control means for taking a part of the controlling light amplified by said optical amplifier so that the rest of the controlling light remains in said optical transmission line and controlling a gain of said optical amplifier to keep a light quantity of said part of the controlling light at a constant level,
• wherein the controlling light is transmitted independently of any communicating from the plurality of optical communication apparatuses.