Raman amplifier and optical relay transmission system

An optical relay transmission system comprising:

• a signal light transmitting unit that outputs a signal light that has a plurality of wavelengths, wavelength-multiplexes and transmits the signal light;
• a reference light output unit that outputs a reference light of a wavelength that is out of a wavelength band of the signal light;
• a Raman amplifier that amplifies, based on the reference beam, the wavelength-multiplexed signal light, wherein the Raman amplifier includes
• an optical fiber that propagates and amplifies a second signal light that is a wavelength-multiplexed signal of a first signal light and a reference light, wherein the first signal light includes a plurality of wavelengths and the reference light is out of a wavelength range of amplification;
• an excitation light source that outputs an excitation light for amplifying the second signal light;
• a beam splitter that splits a portion of the second signal light into the first signal light and the reference light;
• a signal light level detecting unit that detects a level of the first signal light;
• a reference light level detecting unit that detects a level of the reference light;
• and
• a signal level setting unit that, based on the level of the reference light, calculates a target value to control signal light level for constantly maintaining a Raman gain, and controls the output level of the excitation light in such a way that the first signal level matches with the target value;
• and
• a signal receiving unit that receives the signal light amplified by the Raman amplifier.