| 摘要 |
An ambient light (LS) as separated by a wavelength selecting element (12) is inputted to, via a feedback optical path (18), and amplified by a second optical amplification element (44), and then combined with an input signal light (LIN) by an optical coupler (46), being inputted to a first optical amplification element (14). A part or the whole of the ambient light (LS) included in an output light of the first optical amplification element (14) is opposed in intensity to an output signal light (LOUT) of a first wavelength (.lambda.1) included in that output light, further amplified by the second optical amplification element (44), and inputted to the first optical amplification element (14). Therefore, the greater the intensity of the outp ut signal light (LOUT) is, the smaller the intensity of the negative feedback signal is. As a result, the gain, waveform and base line of the output signa l light (LOUT) are stabilized. In addition, there can be obtained such a characteristic that the greater the intensity of the input signal light (LIN ) of the first wavelength (.lambda.1) is, the smaller the gain of the amplification is. Therefore, even if an input signal light (LIN) of an excessive magnitude is inputted, an output signal light having a suppressed magnitude can be obtained.
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