| 摘要 |
On the output side of an optical fiber which optically amplifies a wavelength division multiplexed signal light to output amplified signal light, a first optical divider is provided. The amplified signal light is partially divided by the divider and is converted to an electric signal by an photoelectric converter. The electric signal is compared with a prescribed reference value. The multiplexed signal light is controlled depending on a comparison result. Moreover, a second optical divider is provided. A part of the amplified signal light is made incident on a variable wavelength band-pass light filter which selectively passes signal light of a prescribed wavelength. The light filter is swept with a sweep signal whereby the number of channels of the signal light is detected. The foregoing reference value is set depending on the number of the channels of the signal light by a reference voltage generation circuit. The optical amplifier is controlled depending on the reference value. The foregoing variable wavelength band-pass light filter can be constituted by either an acoustooptic filter or an electric field optic filter.
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