Optical amplification device and optical amplification method

1. An optical amplification device comprising:

a first Raman amplifier configured to output a first excitation light to a transmission line in a same direction as a signal light transmitted in the transmission line to amplify the signal light; and

a second Raman amplifier configured to output a second excitation light to the transmission line in an opposite direction to the signal light to amplify the signal light, wherein

a first wavelength band of the first excitation light includes a second wavelength band of the second excitation light and is wider than the second wavelength band,

the first Raman amplifier includes:

a first optical filter configured to cut off the second wavelength band from the second excitation light and/or a first reflected light of the first excitation light, which are input from the transmission line, and

a first detector configured to detect first power of a first transmitted light transmitted through the first optical filter,

the second Raman amplifier includes:

a second optical filter configured to cut off the second wavelength band from the first excitation light and/or a second reflected light of the second excitation light, which are input from the transmission line, and

a second detector configured to detect second power of a second transmitted light transmitted through the second optical filter,

the first Raman amplifier stops output of the first excitation light when the first power is higher than a threshold, and

the second Raman amplifier stops output of the second excitation light when the second power is reduced from power of the first excitation light transmitted through the second optical filter.

2. The optical amplification device according to claim 1 , wherein

the first Raman amplifier further includes:

a first excitation light source configured to output the first excitation light, and

a first controller configured to cause the first excitation light source to stop output of the first excitation light when the first power is higher than the threshold, and

the second Raman amplifier includes:

a second excitation light source configured to output the second excitation light, and

a second controller configured to cause the second excitation light source to stop output of the second excitation light when the second power is reduced from the power of the first excitation light transmitted through the second optical filter.

3. The optical amplification device according to claim 1 , wherein the second Raman amplifier stops output of the second excitation light when the power of the second transmitted light detected by the second detection unit becomes 0.

4. The optical amplification device according to claim 1 , wherein

the first Raman amplifier includes:

a light guide configured to guide the first excitation light to the transmission line, and guide the second excitation light and the first reflected light from the transmission line to the first optical filter.

5. The optical amplification device according to claim 1 , wherein

the second Raman amplifier further includes

a light guide configured to guide the second excitation light to the transmission line, and guide the first excitation light and the second reflected light from the transmission line to the second optical filter.

6. The optical amplification device according to claim 1 , wherein

the second wavelength band includes at least same bandwidth as the signal light on a short wavelength side of a wavelength band of the signal light.

7. An optical amplification method comprising:

outputting, by a first Raman amplifier, a first excitation light to a transmission line in a same direction as a signal light transmitted in the transmission line to amplify the signal light; and

outputting, by a second Raman amplifier, a second excitation light to the transmission line in an opposite direction to the signal light to amplify the signal light, wherein

a first wavelength band of the first excitation light includes a second wavelength band of the second excitation light and is wider than the second wavelength band,

the first Raman amplifier includes:

a first optical filter configured to cut off the second wavelength band from the second excitation light and/or a first reflected light of the first excitation light, which are input from the transmission line, and

a first detector configured to detect first power of a first transmitted light transmitted through the first optical filter,

the second Raman amplifier includes:

a second optical filter configured to cut off the second wavelength band from the first excitation light and/or a second reflected light of the second excitation light, which are input from the transmission line, and

a second detector configured to detect second power of a second transmitted light transmitted through the second optical filter,

the first Raman amplifier stops output of the first excitation light when the first power is higher than a threshold, and

the second Raman amplifier stops output of the second excitation light when the second power is reduced from power of the first excitation light transmitted through the second optical filter.

8. The optical amplification method according to claim 7 , wherein the second Raman amplifier stops output of the second excitation light when the power of the second transmitted light detected by the second detection unit becomes 0.

9. The optical amplification method according to claim 7 , wherein

the second wavelength band includes at least same bandwidth as the signal light on a short wavelength side of a wavelength band of the signal light.