| 摘要 |
An optical amplifier amplifies and outputs wavelength multiplexed signal lights and a pilot light. The pilot light has a wavelength different from that of the wavelength multiplexed signal lights and a low frequency Tone signal is superposed thereon. The amplified lights are converted into electric signals, and then a Tone signal is selected. A controlling circuit controls an excitation light so that level of the Tone signal will be fixed. A pilot light generating circuit outputs a pilot light having a Tone signal of a constantly fixed level superposed thereon.
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