PROBLEM TO BE SOLVED: To notify an uncontrollable state by comparing output signal light detected by an output light monitor and a target value thereof to control excited light to be supplied from excited light supplying means to a light amplification optical fiber so as to reduce its difference, and outputting an abnormal signal if its difference is greater than a given value. ;SOLUTION: An output light level detected by an output light monitor 7 and an error signal from a comparator 11 to which a target value 10 thereof is inputted controls excited light of an excitation semiconductor laser 2 driven by a laser driving circuit 3. The excited light is injected into a light amplification optical fiber 1, and natural emitted light in which an output is constantly controlled is outputted. A control state signal of a control state detecting circuit 9 having a comparator and a resistor or the like where an error signal from a comparator 11 is an output of a high level when a control state is normal and an output of a low level when it is abnormal. Therefore, if a state of an output constant control is abnormal, danger of continuing operation in an unstable output state is prevented.;COPYRIGHT: (C)1999,JPO
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